Co-Composted Chicken Litter Biochar Increases Soil Nutrient Availability and Yield of Oryza sativa L.

Abstract

Intensified cultivation of rice has accelerated weathering of most tropical acid soils leading to significant loss of base cations. In most developing countries, rice yield is low and this results in its production being costly because productivity versus labor is low. The objectives of this study were to (i) enhance soil chemical properties, nutrient uptake, and grain yield of rice grown on a mineral tropical acid soil using agro-wastes; (ii) determine the agro-waste (chicken manure, cow dung, forest litter, and Leucaena) that has the potential to significantly increase rice yield; and (iii) determine the residual effects of the organic soil amendments produced from the agro-wastes on soil and rice productivity. The treatments used in this three-cycle field study were (i) soil without amendments (S0); (ii) prevailing recommended rates for fertilizers (NPK-Mg); (iii) biochar–forest litter compost (OSA1); (iv) biochar–chicken litter compost (OSA2); (v) biochar–cow dung compost (OSA3); (vi) biochar–Leucaena compost (OSA4); and (vii) biochar–Leucaena–chicken litter compost (OSA5). Standard procedures were used to determine the plants’ rice growth, grain yield, plant nutrient concentrations and uptake, and selected soil chemical properties. The use of organic soil amendments (OSA1 to OSA5) significantly improved the soil chemical properties, rice plant growth, nutrient uptake, and grain yield compared with the prevailing method of cultivating rice (NPK-Mg). The application of organic soil amendments reduced the use of inorganic N, P, K, MgO, and trace elements fertilizers up to 25%, 100%, 64%, 100%, and 100%, respectively. The organic soil amendments with Leucaena significantly increased rice grain yield of OSA5 at 11.17, 13.11, and 10.06 t ha⁻¹ in the first, second, and third cropping cycles, respectively. The residual effect of the organic soil amendments also improved rice plant growth, nutrient uptake, and rice grain yield although these were slightly reduced as compared to those of the two previous cropping cycles, the afore-stated treatments were superior to the prevailing method of cultivating rice (NPK-Mg). Transforming agro-wastes into organic soil amendments can improve tropical mineral acid soils and rice productivity.

1. Introduction

Southeast Asian soils are dominated by Oxisols and Ultisols [1]. Ultisols are among the highly weathered cultivated soils which are generally infertile because of the accumulation of high quantities of Fe and Al in these soils [2,3]. Ultisols in particular are low in silt but high in sand, resulting in low water holding capacity. This makes them less suitable for rice production [4]. Apart from being acidic because of their parent materials are mainly feldspars and micas, these soils are low in cation exchange capacity (CEC) and base saturation [5]. High Al:Ca ratios which resulted from subsoil acidity makes root extension to this zone difficult for most crops [6].

The aforementioned problems are further deteriorated because most farmers excessively use chemical fertilizers to improve soil productivity. For example, the current rice yield in Malaysia is considered low in a range of 3.5 to 4 t ha⁻¹ [7]. The low yield and increasing fertilizer costs cause the rice production to become an expensive venture in most developing countries [8]. Intensified cultivation of rice because of increasing human population has accelerated weathering of most tropical soils leading to significant loss of base cations [2,9]. These processes significantly reduce soil fertility; thus, restoration is done through the use of chemical fertilization to increase rice production [9]. However, the impacts of increasing use of chemical N, P, and K fertilizers is not just causing an increase in rice production costs, but it also leads to environmental pollution and poor grain quality [10,11].

Sustainable and cost-effective methods of producing rice using organic soil amendments are gaining recognition. The quality of organic soil amendments depends mainly on the type and source of wastes used [12]. This is because animal manure and plant residues such as cow dung, chicken litter, forest litter, and Leucaena have different nutrient contents. The assignment of a general or fixed percentage for nutrients in animal manures is not feasible due to the variations of such components according to animal type, age, ration, and feed consumption of the animals [13]. Plant residues are high in organic matter and can be used to restore soil organic matter in an intensive rice field. Some plant residues such as Leucaena leucocephala are nutritious and are used for fodder, controlling soil erosion, and mulching [14]. Meena et al. [15] demonstrated that total N and P contents of the leaves of Leucaena leucocephala were 4.2% and 0.23%, respectively. From the foregoing rationale, organic soil amendments which were produced from cow dung, poultry manure, forest litter, and Leucaena co-composted with chicken litter biochar were used to (i) improve soil chemical properties, rice plant growth, nutrient uptake, and grain yield on a tropical mineral acid soil; (ii) determine the type of agro-waste (chicken manure, cow dung, forest litter, and LLeucaena) that has the potential to significantly increase rice yield; and (iii) determine the residual effects of the organic soil amendments produced from the agro-wastes in this present study on soil and rice productivity.

2. Materials and Methods

2.1. Field Study Area

This research was conducted at a rice plot at Universiti Putra Malaysia Bintulu Campus Sarawak, Malaysia, which has the geographical coordinates of latitude 3° 12′ 54.48″ N and longitude 113° 05′ 39.03″ E (Figure 1). The elevation of the rice plot is 31 m and the soil used for the field study is the Nyalau series (Typic Paleudults).

2.2. Experimental Plots Preparation

The entire research area was 24 m long and 23 m wide (552 m²). The design used for this research was a Randomized Complete Block Design with four blocks. The size of each plot was 2 m × 2 m (4 m²). The distance between plots was 1 m and the distance between blocks was 2 m. The well fenced experimental site was initially cleared from plant debris. Afterwards, the edges of the plots were covered with silver shine to mitigate weed infestation and soil run-off. The plots were netted to prevent birds from getting into the plots to feed on rice grains.

2.3. Experimental Plots Fertilization

The organic soil amendments used in this field study were based on the most promising treatments selected from our pot study (

Table 1. Codes and description of the treatments used in the field study.

Treatment	Code	Description
T1	S0	Soil without amendments
T2	NPK-Mg	Full chemical fertilizers rate (N, P, K, and Mg) *
T3	OSA1	Chicken litter biochar–forest litter compost (1:1) **
T4	OSA2	Chicken litter biochar–chicken litter compost (1:1) **
T5	OSA3	Chicken litter biochar–cow dung compost (1:1) **
T6	OSA4	Chicken litter biochar–Leucaena compost (1:1) **
T7	OSA5	Chicken litter biochar–Leucaena–chicken litter compost (2:1:1) **

* MADA [7]; ** Maru [17].

). The chemical properties of the organic soil amendments used in this present study are presented in

Table 2. Initial chemical properties of organic soil amendments.

	OSA1	OSA2	OSA3	OSA4	OSA5
pH in water	7.37 ± 0.52	9.32 ± 0.72	8.47 ± 0.63	8.57 ± 0.28	8.68 ± 0.53
pH in KCl	6.87 ± 0.47	8.79 ± 1.04	8.10 ± 0.56	8.04 ± 0.57	8.26 ± 0.87
	(%)
Organic matter	67.00 ± 5.23	55.00 ± 3.15	37.66 ± 4.78	53.00 ± 2.45	57.33 ± 5.44
ash content	33.00 ± 3.21	45.00 ± 6.75	62.33 ± 4.16	47.00 ± 4.72	42.66 ± 5.39
Total N	0.20 ± 0.05	0.30 ± 0.01	0.13 ± 0.01	0.64 ± 0.02	0.29 ± 0.01
Total P	1.65 ± 0.29	3.04 ± 0.34	1.81 ± 0.13	1.98 ± 0.13	2.02 ± 0.31
C/N ratio	192.19 ± 26.24	106.27 ± 6.78	164.00 ± 9.41	48.26 ± 4.33	113.92 ± 9.93
C/P ratio	23.69 ± 1.68	10.56 ± 1.58	12.18 ± 4.87	15.66 ± 1.38	16.75 ± 1.92
	(g kg−1)
Total K	426.53 ± 23.21	393.23 ± 27.47	372.80 ± 32.41	361.97 ± 13.54	389.13 ± 34.12
Total Ca	70.93 ± 3.43	72.36 ± 2.48	64.06 ± 3.11	63.87 ± 1.45	78.60 ± 4.82
Total Na	11.96 ± 0.67	16.88 ± 2.42	12.30 ± 0.7.4	8.73 ± 0.25	15.26 ± 1.78
Total Mg	14.46 ± 0.46	18.13 ± 2.84	14.56 ± 1.67	12.50 ± 0.83	15.53 ± 2.58
Total Zn	0.51 ± 0.034	0.85 ± 0.06	0.49 ± 0.02	0.59 ± 0.023	0.64 ± 0.08
Total Cu	0.06 ± 0.004	0.15 ± 0.008	0.08 ± 0.002	0.10 ± 0.004	0.15 ± 0.007
Total Mn	0.66 ± 0.03	0.84 ± 0.12	0.68 ± 0.07	0.54 ± 0.014	0.65 ± 0.11
Total Fe	1.85 ± 0.21	0.92 ± 0.08	1.38 ± 0.18	4.02 ± 0.10	3.10 ± 0.48

. The application rate of the organic soil amendments was 5 t ha⁻¹ [16] and the quantity used was scaled down based on 100 hills plot⁻¹ (

Table 3. Application rates of organic soil amendments and fertilizers used in the field study.

Plant Growth Stages		Early Tillering	Active Growth	Stalk Formation	Grain Filling
Days after Transplanting		15 to 20	35 to 40	50 to 55	70 to 75
Code	Amount of Organic Soil Amendment	Amount of Chemical Fertilizers
	g Plot−1
S0	0	0	0	0	0
NPK-Mg	0	Mix NPK 1	40 urea	Mix NPK-Mg 2	Mix NPK-Mg 2
OSA1	2000 *	40.3 urea	30 urea + 24 MOP	18 urea	18 urea
OSA2	2000 *	40.3 urea	30 urea + 24 MOP	18 urea	18 urea
OSA3	2000 *	40.3 urea	30 urea + 24 MOP	18 urea	18 urea
OSA4	2000 *	40.3 urea	30 urea + 24 MOP	18 urea	18 urea
OSA5	2000 *	40.3 urea	30 urea + 24 MOP	18 urea	18 urea

¹ Mix NPK = 55 g Urea + 50 g TSP + 24 g MOP. ² Mix NPK-Mg = 18.3 g Urea + 18.7 g TSP + 19.8 g MOP + 1.4 g MgO. Note: TSP = Triple Super Phosphate, MOP = Muriate of Potash. * Application of organic soil amendments was skipped in the third cropping cycle.

). In the first and second cropping cycles, 2 kg of each organic amendment was applied to the respective plots labeled T3 (OSA1), T4 (OSA2), T5 (OSA3), T6 (OSA4), and T7 (OSA5) (Table 1). The organic soil amendments were spread on the soil surface and subsequently mixed well with the aid of a shovel. Thereafter, the plots were watered. In the third cropping cycle, application of the organic soil amendments was skipped because this aspect of the present study was to assess the residual effect of the organic soil amendments used in the two previous cropping cycles. Chemical fertilizers were broadcast on the soil surface of each plot on 15, 35, 50, and 70 days after transplanting (Table 3). The fertilization regime adopted in this study was in accordance with the recommendation by MADA (Table 3). Nitrogen, P, K, and trace elements were reduced by 25%, 100%, 64%, and 100 %, respectively, in the first and second cropping cycles of the present study. However, in the third planting cycle, N was applied at 100 % of the recommended fertilization due to the lower N in the organic residues.

2.4. Experimental Plots Irrigation

A day before transplanting, the 28 plots were irrigated up to 1.5 cm depth using tap water. Afterwards, the water level in the plots was maintained at 1.5 cm above the soil surface to mimic waterlogged conditions until the rice plants were established (14 days after transplanting). Thereafter, the water level was increased and maintained to approximately 2.5 cm to 4 cm throughout the study.

2.5. Transplanting of Fifteen-Day Seedlings

The nursed rice seedlings (15-day old) of the MR219 variety in a plastic basin were transported to the rice plot to enable the rice seedlings to adapt to the field condition. The following day, 100 hills (3 seedlings per hill) of the rice seedlings were transplanted into each rice plot (4 m²) as demonstrated in Figure 2. Afterwards, they were monitored until maturity (98 days to 120 days after transplanting). Figure 2 demonstrates the growth of rice plants at one, forty, and eighty days as affected by treatments, respectively.

2.6. Weed and Pest Control

Manual weeding was carried out to control weeds from outcompeting the rice plants’ growth, whereas Halex Malathion 84 EC was used to control insect pests such as grasshoppers, stem borers, and caterpillars.

2.7. Harvesting of Rice Plants at Maturity

A day before harvest, 10 rice plant hills were randomly selected (excluding border plants) and harvested using a knife for the rice plant growth and grain yield measurements. The rice plant height was measured from the soil surface to the tip of the tallest leaf using a measuring tape. The number of tillers and number panicles per hill were counted from the 10 harvested hills and subsequently 10 panicles were randomly harvested and placed into separate plastic bags for total grain per panicle, percentage of grain filling, and weight of 1000 grains determination. Thereafter, another 10 rice plant hills were harvested for determination of dry matter weight and nutrient contents. The following day, the remaining panicles were harvested using a pair of scissors, followed by air-drying at room temperature and grain removal.

2.8. Soil Sampling and Analysis

The soils in each rice plot were sampled immediately after harvesting at five points into separate plastic containers using the diagonal method, after which the soil samples were air-dried, manually crushed, and sieved to pass through a 2 mm sieve for chemical analyses. Soil pH was determined in a 1:2.5 (soil: distilled water or 1 mol dm⁻³ KCl) suspension using a digital pH meter [18]. Soil total organic matter was determined using the loss on ignition method, and soil total C was calculated as 58% of the organic matter [19]. Soil cation exchange capacity (CEC) was determined using the leaching method [20]. Soil total N was determined using the Kjeldahl method [21,22], whereas inorganic N (${NO}_3^{-}$ and ${NH}_4^{+}$) were determined using the method described by Keeney and Nelson [23]. Soil total P was extracted using the aqua regia method [22], whereas soil available P was extracted using Mehlich’s No.1 double-acid method [24]. Thereafter, soil total and available P were determined using Ultraviolet-visible Spectrophotometry (Lambda 25, Perkin Elmer, Shelton, CT, USA) after blue color development [25]. Soil exchangeable acidity, H⁺, and Al³⁺ were determined using acid–base titration method [26]. The initial chemical properties of Nyabau series soil (Typic Paleudults) are presented in

Table 4. Initial chemical properties of Nyalau series (Typic Paleudults).

Soil Properties	Value Determined (Mean ± S.E. 1)
pHwater	5.11 ± 0.02
pHKCl	3.97 ± 0.02
(%)
Total Organic Matter	4.40 ± 0.12
Total Organic Carbon	2.55 ± 0.07
Total N	0.10 ± 0.01
(mg kg−1)
Exchangeable ${NH}_4^{+}$	9.34 ± 0.47
Available ${NO}_3^{-}$	2.57 ± 0.23
Total P	58.29 ± 0.51
Available P	3.00 ± 0.19
(cmol(+) kg−1)
Exchangeable K+	0.19 ± 0.003
Exchangeable Ca2+	1.55 ± 0.02
Exchangeable Mg2+	0.001 ± 0.000
Exchangeable Na+	1.40 ± 0.03
Exchangeable Fe2+	48.31 ± 0.06
Exchangeable Mn2+	0.083 ± 0.001
Exchangeable Cu2+	0.52 ± 0.01
Exchangeable Zn2+	2.25 ± 0.10
Cation exchange capacity	4.40 ± 0.12
Exchangeable acidity	1.42 ± 0.01
Exchangeable Al3+	0.75 ± 0.01
Exchangeable H+	0.67 ± 0.01

¹ S.E. = Standard error.

.

2.9. Organic Soil Amendments and above Ground Biomass Analysis

The organic soil amendments and above ground biomass samples were digested using the single dry ashing method [20] to extract P and cations such as K, Ca, Mg, Mn, Zn, Fe, and Cu. The cation concentrations were determined using Atomic Absorption Spectrometry (AAS), whereas P was determined using the blue method [25]. Total N was determined using the Kjeldahl method followed by steam distillation [21,22]. The nutrient concentrations were multiplied by the dry weight of above ground biomass for nutrient uptake determination.

2.10. Total Grain Yield Determination

Total grain yield was determined using the following formula which is described by Matsushirna and Tanaka [27]:

$$\mathrm{Yield} (\mathrm{t} {\mathrm{ha}}^{-1})=\frac{\mathrm{weight} \mathrm{of} 1000 \mathrm{grains} \times \mathrm{spikelet} \times \% \mathrm{filled} \mathrm{grain} }{10000{ \mathrm{m}}^2\times 1000 \mathrm{grains}}$$

(1)

where the area of one hectare = 10,000 m² was used to express the yield per hectare basis.

2.11. Weight of 1000 Grains Determination

A total of 1000 matured rice grains from the 10 selected panicles were placed in a clean crucible after separating filled and unfilled grains. The grains were oven dried at 60 °C until constant weight was attained, followed by keeping in a desiccator to enable the samples to equilibrate with room temperature. The dried grains were weighed and the dry weight of one grain was determined using the following formula:

$$\mathrm{Dry} \mathrm{weight} \mathrm{of} \mathrm{one} \mathrm{grain} \left(\mathrm{g}\right)=\frac{\mathrm{dry} \mathrm{weight} \mathrm{of} 1000 \mathrm{rice} \mathrm{grains}}{1000 \mathrm{grains}}$$

(2)

2.12. Percentage of Total Grain Filling and Number of Spikelet

The percentage total grain filling was determined by dividing the number of filled grains with total number of grains from the 10 selected panicles. Thereafter, the number of spikelets was determined by multiplying the number of panicles per hill with percentage of total grain filling, followed by dividing with the area per hill.

$$\mathrm{Percentage} \mathrm{of} \mathrm{total} \mathrm{grain} \mathrm{filling} (\%)=\frac{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{filled} \mathrm{grains}}{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{grain} }\times 100$$

(3)

$$\mathrm{Number} \mathrm{of} \mathrm{spikelet}=\frac{\mathrm{number} \mathrm{of} \mathrm{panicles} \mathrm{per} \mathrm{hill} \times \% \mathrm{total} \mathrm{grain} \mathrm{filling}}{\mathrm{area} \mathrm{per} \mathrm{hill} }$$

(4)

2.13. Statistical Analysis

The collected data were subjected to analysis of variance (ANOVA) to detect the treatment effects, followed by mean comparisons using Tukey’s HSD test at p ≤ 0.05. The statistical software used was Statistical Analysis System version 9.2 [28].

3. Results and Discussion

3.1. Effects of Organic Soil Amendments on Soil Chemical Properties

The organic soil amendments such as biochar, animal manures, and plant residuals do not only improve soil productivity but they also stabilize yields over time and encourage farmers to increase rice cultivation. Effects of the organic soil amendments on the soil chemical properties after the first cropping cycle of rice cultivation are presented in

Table 5. Effects of organic soil amendments on chemical properties of Nyalau series soil after the first cropping cycle of rice cultivation.

Soil Chemical Properties	Treatments
	S0	NPK-Mg	OSA1	OSA2	OSA3	OSA4	OSA5
pH in water	3.97 b ± 0.12	4.18 ab ± 0.04	4.78 a ± 0.09	4.77 a ± 0.13	4.60 ab ± 0.19	4.39 ab ± 0.30	4.73 a ± 0.10
pH in KCl	3.38 b ± 0.09	3.71 ab ± 0.08	3.83 a ± 0.04	3.80 a ± 0.04	3.72 ab ± 0.05	3.64 ab ± 0.11	3.87 a ± 0.08
	%
Total C	2.49 bc ± 0.07	2.18 bc ± 0.11	2.46 ab ± 0.04	2.39 abc ± 0.08	2.56 ab ± 0.12	2.40 abc ± 0.07	2.80 a ± 0.05
Total N	0.10 ab ± 0.01	0.11 a ± 0.01	0.07 ab ± 0.01	0.06 b ± 0.01	0.08 ab ± 0.01	0.09 ab ± 0.01	0.08 ab ± 0.01
	mg kg−1
Avail. ${NO}_3^{-}$	2.50 b ± 0.17	4.58 a ± 0.11	3.85 a ± 0.20	4.03 a ± 0.18	4.55 a ± 0.20	4.38 a ± 0.18	4.55 a ± 0.20
Exch. ${NH}_4^{+}$	2.13 c ± 0.03	2.73 ab ± 0.07	2.28 bc ± 0.18	2.90 a ± 0.10	2.13 c ± 0.03	2.10 c ± 0.01	2.68 ab ± 0.13
Total P	143.13 d ± 7.81	249.96 c ± 9.57	415.25 a ± 6.93	336.13 b ± 18.17	341.5 b ± 11.11	347.00 b ± 19.37	420.74 a ± 15.13
Available P	25.34 f ± 0.17	129.63 e ± 7.20	333.00 ab ± 8.75	314.50 b ± 9.38	161.71 d ± 5.66	248.77 c ± 6.64	349.54 a ± 3.28
	cmol(+) kg−1
Exch. acidity	1.53 a ± 0.06	1.27 b ± 0.04	0.60 de ± 0.03	0.56 e ± 0.03	0.77 c ± 0.03	0.78 c ± 0.02	0.73 cd ± 0.03
Al3+	0.81 a ± 0.02	0.69 a ± 0.07	n.d.	n.d.	n.d.	n.d.	n.d.
H+	0.73 abc ± 0.04	0.57 cd ± 0.06	0.60 bcd ± 0.03	0.56 d ± 0.03	0.77 a ± 0.04	0.76 ab ± 0.02	0.73 abc ± 0.03
CEC	1.70 d ± 0.18	1.65 d ± 0.17	2.90 b ± 0.06	2.35 c ± 0.17	2.37 c ± 0.10	2.43 bc ± 0.13	3.75 a ± 0.16

Means with different letters within a row indicate significant difference using Tukey’s test at p ≤ 0.05. Data are presented as mean of four replicates ± standard error. Note: n.d. = not detectable.

. Amending the acidic soil with organic soil amendments in the first cropping cycle had significantly increased pH in water of OSA1 (4.78), OSA2 (4.77), and OSA5 (4.73) compared with that of S0 (3.97), NPK-Mg (4.18), OSA3 (4.60), and OSA4 (4.39). Similarly, the pH in KCl of OSA5 (3.87), OSA1 (3.83), and OSA2 (3.80) was significantly higher compared with that of S0 (3.38), NPK-Mg (3.71), OSA3 (3.72), and OSA4 (3.64). This was due to the alkaline nature of the organic amendments (Table 2). The use of organic soil amendments significantly increased soil total C of OSA5 compared with that of NPK-Mg and S0. The application of organic soil amendments did not increase soil total N compared with that of S0 and NPK-Mg. This was due to the low N in the organic amendments. There were no significant differences in the soil available ${NO}_3^{-}$ between the soils with the organic amendments (OSA1 to OSA5) compared with that of NPK-Mg, except for soil without amendment (S0). The exchangeable ${NH}_4^{+}$ of NPK-Mg, OSA2, and OSA5 were similar but significantly higher than that of S0, OSA3, and OSA4, whereas the soil total P of OSA1 and OSA5 were significantly higher than those of other treatments. After the first cropping cycle, the incorporation of organic soil amendments (OSA1 to OSA5) significantly improved available P to a range of 161.71 mg kg⁻¹ to 349.54 mg kg⁻¹ compared with that of S0 (25.34 mg kg⁻¹) and NPK-Mg (129.63 mg kg⁻¹). This was due to the residual effect of the applied soil organic amendments in addition to that of the second application. The use of organic soil amendments significantly suppressed soil exchangeable acidity and Al³⁺ of OSA1 to OSA5 compared with that of S0 and NPK-Mg. This was due to the high affinity of the organic amendments towards Al and Fe. After the first cropping cycle, OSA5 had the highest soil CEC (3.75 cmol₍₊₎ kg⁻¹), followed by OSA1 (2.90 cmol₍₊₎ kg⁻¹), OSA2 (2.35 cmol₍₊₎ kg⁻¹), OSA3 (2.37 cmol₍₊₎ kg⁻¹), and OSA4 (2.43 cmol₍₊₎ kg⁻¹), whereas S0 and NPK-Mg demonstrated the lowest CEC of 1.70 cmol₍₊₎ kg⁻¹ and 1.65 cmol₍₊₎ kg⁻¹, respectively. These findings indicate that the organic soil amendments improved the chemical properties of Nyalau series soil compared with the prevailing fertilization method (NPK-Mg), except for soil total C, total N, available ${NO}_3^{-}$, and exchangeable ${NH}_4^{+}$.

The soil chemical properties preceding the second cropping cycle of MR219 rice cultivation through the use of organic soil amendments are demonstrated in

Table 6. Effects of organic soil amendments on chemical properties of Nyalau series soil after the second cropping cycle of rice cultivation.

Soil Chemical Properties	Treatments
	S0	NPK-Mg	OSA1	OSA2	OSA3	OSA4	OSA5
pH in water	4.41 b ± 0.14	4.57 b± 0.20	5.60 a± 0.17	5.84 a ± 0.09	5.86 a ± 0.07	5.74 a ± 0.17	5.85 a ± 0.05
pH in KCl	3.78 b ± 0.08	3.88 b ± 0.07	4.93 a ± 0.65	5.07 a ± 0.01	5.07 a ± 0.05	4.96 a ± 0.05	5.15 a ± 0.06
	%
Total C	2.18 cd ± 0.06	2.00 d ± 0.03	2.73 ab ± 0.07	2.90 a ± 013	2.93 a ± 0.09	2.49 bc ± 0.07	2.70 ab ± 0.06
Total N	0.06 b ± 0.002	0.06 b ± 0.003	0.09 a ± 0.002	0.09 a ± 0.003	0.08 a ± 0.001	0.09 a ± 0.002	0.08 a ± 0.001
	mg kg−1
Avail. ${NO}_3^{-}$	3.40 bc ± 0.10	3.08 c ± 0.17	3.33 bc ± 0.18	3.68 abc ± 0.18	4.03 abc ± 0.18	4.58 a ± 0.45	4.45 ab ± 0.17
Exch. ${NH}_4^{+}$	3.33 b ± 0.18	3.33 b ± 0.34	3.33 b ± 0.18	4.85 a ± 0.26	4.28 ab ± 0.07	4.48 a ± 0.17	4.45 a ± 0.17
Total P	140.51 d ± 9.33	349.58 c ± 14.71	426.81 bc ± 15.02	437.17 bc ± 14.76	509.42 b ± 27.61	347.50 c ± 19.10	668.50 a ± 35.21
Available P	34.85 e ± 1.20	143.21 d ± 10.75	331.13 b ± 12.65	277.00 c ± 13.26	328.38 b ± 10.14	353.75 b ± 8.29	448.77 a ± 12.00
	cmol(+) kg−1
Exch. acidity	1.69 a ± 0.07	1.52 a ± 0.03	0.71 b ± 0.02	0.73 b ± 0.05	0.74 b ± 0.03	0.75 b ± 0.05	0.51 c ± 0.04
Al3+	1.01 a ± 0.04	0.74 b ± 0.03	n.d.	n.d.	n.d.	n.d.	n.d.
H+	0.68 a ± 0.04	0.79 a ± 0.03	0.71 a ± 0.02	0.73 a ± 0.05	0.74 a ± 0.03	0.75 a ± 0.05	0.51 b ± 0.04
CEC	3.45 b ± 0.13	3.80 b ± 0.32	5.20 a ± 0.18	5.03 a ± 0.31	3.78 b ± 0.28	5.80 a ± 0.15	5.80 a ± 0.20

Means with different letters within a row indicate significant difference using Tukey’s test at p ≤ 0.05. Data are presented as mean of four replicates ± standard error. Note: n.d. = not detectable.

. The results showed that soil pH (in both water and KCl) and total N of all the soils following the application of organic soil amendments (OSA1 to OSA5) were significantly higher than those of S0 and NPK-Mg. On the other hand, organic soil amendments (OSA1 to OSA5) significantly lowered soil total acidity and exchangeable Al³⁺. Soil total C of OSA1 (2.73%), OSA2 (2.90%), OSA3 (2.93%), and OSA5 (2.70%) was significantly higher compared with that of OSA4 (2.49%), S0 (2.18%), and NPK-Mg (2.00%). The use of organic soil amendments in the second cropping cycle also contributed to the improved availability of ${NO}_3^{-}$ in OSA4 compared with that of other treatments (S0, NPK-Mg, OSA1, OSA2, OSA3, and OSA5). Enhanced exchangeable ${NH}_4^{+}$ availability after the second cropping cycle was observed in OSA2, OSA4, and OSA5, compared with that of S0, NPK-Mg, OSA1, and OSA3. The soil total and available P of OSA5 were significantly higher than those of other treatments. Notably, exchangeable Al³⁺ was negligible in the soils with the organic soil amendments. Application of the organic soil amendments significantly increased the soil CEC of OSA1, OSA2, OSA4, and OSA5 compared with those of S0, NPK-Mg, and OSA3. The pattern of these results was similar and consistent with those in the first cropping cycle. However, the soil chemical properties in the second cropping cycle were slightly higher than those of the first cropping cycle because of the residual effects of the organic soil amendments. This suggests that continued application of these organic soil amendments can improve the chemical properties of Nyalau series soil over time.

The residual effects of organic soil amendments on chemical properties of Nyalau series after the third cropping cycle are summarized in

Table 7. Residual effects of organic soil amendments on chemical properties of Nyalau series soil after the third cropping cycle of rice cultivation.

Soil Chemical Properties	Treatments
	S0	NPK-Mg	OSA1	OSA2	OSA3	OSA4	OSA5
pH in water	4.42 c ± 0.05	4.51 bc ± 0.16	4.87 abc ± 0.06	5.00 a ± 0.06	5.22 a ± 0.20	4.92 ab ± 0.08	4.80 abc ± 0.01
pH in KCl	3.94 b ± 0.05	4.01 ab ± 0.05	4.04 ab ± 0.05	4.19 a ± 0.06	4.14 ab ± 0.04	4.08 ab ± 0.05	4.12 ab ± 0.03
	%
Total C	1.98 b ± 0.08	2.09 b ± 0.11	2.58 a ± 0.06	2.53 a ± 0.06	2.52 a ± 0.10	2.58 a ± 0.07	2.64 a ± 0.10
Total N	0.04 d ± 0.003	0.09 ab ± 0.004	0.07 c ± 0.002	0.07 c ± 0.003	0.11 a ± 0.005	0.08 bc ± 0.002	0.11 a ± 0.004
	mg kg−1
Avail. ${NO}_3^{-}$	2.10 b ± 0.04	2.73 a ± 0.05	2.83 a ± 0.02	2.15 b ± 0.05	2.68 a ± 0.07	2.70 a ± 0.07	2.93 a ± 0.08
Exch. ${NH}_4^{+}$	4.38 b ± 0.10	4.98 a ± 0.08	5.13 a ± 0.13	4.25 b ± 0.05	4.91 a ± 0.05	4.88 a ± 0.03	5.00 a ± 0.10
Total P	139.10 d ± 6.16	372.08 b ± 9.06	309.05 c ± 5.85	501.13 a ± 7.27	500.63 a ± 7.95	399.69 b ± 3.33	400.38 b ± 9.73
Available P	28.95 e ± 0.79	232.39 d ± 6.27	236.77 cd ± 4.38	276.19 bc ± 15.74	370.75 a ± 8.73	255.79 cd ± 8.09	313.58 b ± 9.16
	cmol(+) kg−1
Exch. acidity	1.56 a ± 0.08	1.58 a ± 0.06	0.71 b ± 0.04	0.68 b ± 0.04	0.68 b ± 0.02	0.69 b ± 0.02	0.75 b ± 0.01
Al3+	0.82 a ± 0.11	0.69 b ± 0.08	n.d.	n.d.	n.d.	n.d.	n.d.
H+	0.74 a ± 0.14	0.89 a ± 0.13	0.71 a ± 0.04	0.68 a ± 0.04	0.68 a ± 0.02	0.69 a ± 0.02	0.75 a ± 0.01
CEC	3.03 c ± 0.17	3.60 c ± 0.11	5.83 a ± 0.14	4.64 b ± 0.23	5.00 b ± 0.16	5.40 ab ± 0.11	6.05 a ± 0.18

Means with different letters within a row indicate significant difference using Tukey’s test at p ≤ 0.05. Data are presented as mean of four replicates ± standard error. Note: n.d. = not detectable.

. The soil pH in water of OSA2 and OSA3 was higher compared with that of other treatments. It is observed that OSA2 demonstrated the highest pH in KCl of 4.19, which was higher than that of other treatments. Soil total N of OSA3 (0.11%), OSA5 (0.11%), and NPK-Mg (0.09%) were statistically similar, followed by OSA4 (0.08%), OSA1 (0.07%), and OSA2 (0.07%), whereas S0 had the lowest soil total N of 0.04%. Soil total Cand CEC of the soils treated with organic soil amendments (OSA1 to OSA5) were significantly higher compared with those of S0 and NPK-Mg. OSA2 demonstrated lower soil available ${NO}_3^{-}$ and exchangeable ${NH}_4^{+}$ compared with those of the other treatments with organic soil amendments (OSA1, OSA3, OSA4, and OSA5) and the prevailing method (NPK-Mg). Similar to the two preceding cropping cycles, the organic soil amendments (OSA1 to OSA5) consistently reduced the soil total acidity and exchangeable Al³⁺. The soil total P of OSA2 (501.13 mg kg⁻¹) and OSA3 (500.63 mg kg⁻¹) were the highest, followed by OSA5 (400.38 mg kg⁻¹), OSA4 (399.69 mg kg⁻¹), NPK-Mg (372.08 mg kg⁻¹), and OSA1 (309.05 mg kg⁻¹), whereas soil without amendments (S0) had the lowest total P of 139.10 mg kg⁻¹. The application of organic soil amendments (OSA1 to OSA5) significantly increased available P to a range of 236.77 mg kg⁻¹ to 370.75 mg kg⁻¹ compared with S0 (28.95 mg kg⁻¹). Consistent with the two previous cropping cycles, it is noteworthy that the exchangeable Al³⁺ was negligible in the soils amended with the organic soil amendments (OSA1 to OSA5). Our findings demonstrate that application of the organic soil amendments in the two preceding cropping cycles consistently improved the soil fertility when no organic soil amendments were applied in the third cropping cycle. This is because the organic soil amendments consistently improved soil total C, available ${NO}_3^{-}$, exchangeable ${NH}_4^{+}$, and CEC in addition to reducing exchangeable acidity and Al³⁺, which can result in optimum rice plant growth, nutrient uptake, and improved rice grain yield.

Apart from the buffering capacity of the amendments, the improvement in the soil chemical properties after amending the soil with the organic soil amendments was due to replacement of leached cations, especially Mg and Ca [29]. This was possible because the organic soil amendments formed complexes with Al and Fe [30]. There was an increase in soil pH because of proton exchange between the soil and organic soil amendments. The organic soil amendments have phenolic and humic-like compounds which are capable of increasing soil pH because of the occurrence of deprotonation [31]. The humic compounds of the organic soil amendments were absorbed onto the hydrous surfaces of Al oxides thereby releasing OH^- to increase soil pH [32]. Additionally, the organic matter especially in the biochar, animal manures, and plants residues are able to increase soil’s ability to adsorb and desorb essential plant nutrients because of increased negatively charged functional groups [29].

The organic soil amendments produced from LLeucaena, forest litter, chicken manure, and cow dung co-composted with chicken litter biochar increased the soil pH. Moreover, the organic soil amendments increased the soil nutrient availability because of reduction in the soil P and K fixing capacity. This explains why skipping or reducing P and K applications, respectively, in this present study did not adversely affect the productivity of the soil and rice plants [16,33]. The results of this present study further suggest that most of the Al in the soil were neutralized following the application of the organic soil amendments. The increase in pH by NPK-Mg in the first cropping cycle was due to urea hydrolysis which resulted in more OH^- ions. The use of these amendments reduced the soil acidity. This confirms the findings of several studies that organic soil amendment use increases soil pH [33,34,35]. The variation of the pH among the soil organic soil amendments used was due to their unique properties [35]. During the first and second cropping cycles, the organic soil amendments improved the soil exchangeable acidity and buffering capacity. This resulted in the increase in the soil pH. However, this increase in soil pH was temporary because the soil pH decreased in this cropping cycle. This process is, however, slow and depends largely on the amount of organic amendment used and its physicochemical properties [36].

The soil available nutrients at harvest (for the cropping cycles), especially after the third cropping cycle, were due to slow release of the nutrients and decomposition of the organic soil amendments [37]. Unlike N, which is mobile in soils, the mobility of P is significantly reduced in mineral soils particularly the highly weathered soils such as Utisols and Oxisols which are commonly high in Al and Fe [38]. The higher affinity of the organic soil amendments for Al and Fe reduced iron and aluminum-bound P and this reaction increases P availability [39]. Maru et al. [16] found that the use of 5 t ha⁻¹ chicken litter biochar alone provided sufficient amount of P for MR219 rice plants and this finding is confirmed in this present study because P, MgO, and trace elements were 100% reduced in all the three cropping cycles. Generally, using local organic agro-wastes through co-composting LLeucaena, forest litter, chicken manure, and cow dung with chicken litter biochar as amendments can be cost-effective. In addition, the use of these soil organic amendments can mitigate environmental pollutions such as forest residue slash pile burning and facilitate suitable disposal of animal wastes.

3.2. Effects of Organic Soil Amendments on Total Nitrogen and Nutrient Uptake of Rice Plants

The effects of the organic soil amendments on total N and nutrient uptake of the rice plants during the first cropping cycle are presented in

Table 8. Total nitrogen and nutrients uptake by MR219 rice plants as affected by organic soil amendments during the first cropping cycle.

Nutrient	Treatments
	S0	NPK-Mg	OSA1	OSA2	OSA3	OSA4	OSA5
N (%)	1.15 d ± 0.06	3.53 c ± 0.21	5.55 c ± 0.30	8.65 b ± 0.47	8.30 b ± 1.14	11.70 a ± 0.44	8.42 b ± 0.38
Nutrient uptake	mg hill−1
P	0.87 d ± 0.06	5.21 c ± 0.32	7.12 c ± 0.29	17.35 b ± 0.36	15.33 b ± 1.78	20.74 a ± 0.58	15.54 b ± 0.24
K	18.06 d ± 1.93	109.46 c ± 10.31	163.60 c ± 6.71	413.64 ab ± 19.08	346.25 b ± 40.67	477.05 a ± 9.21	428.76 ab ± 16.65
Ca	18.25 c ± 1.41	99.18 b ± 3.94	148.91 a ± 8.98	142.81 a ± 2.09	139.43 a ± 2.44	152.02 a ± 5.99	136.22 a ± 4.45
Na	3.19 c ± 0.16	10.79 b ± 1.04	16.87 a ± 2.07	19.23 a ± 0.77	16.03 a ± 0.92	17.68 a ± 0.38	17.94 a ± 0.35
Mg	10.95 d ± 0.48	51.51 c ± 1.73	80.40 b ± 4.10	84.72 b ± 3.07	79.70 b ± 2.38	88.51 ab ± 1.97	98.05 a ± 2.96
Fe	2.08 d ± 0.22	7.02 c ± 0.29	8.44 abc ± 0.75	8.58 ab ± 0.32	7.21 bc ± 0.20	8.04 bc ± 0.11	9.78 a ± 0.11
Cu	0.02 e ± 0.001	0.10 d ± 0.002	0.13 cd ± 0.011	0.15 bc ± 0.007	0.14 bc ± 0.004	0.17 ab ± 0.003	0.19 a ± 0.011
Mn	3.79 c ± 0.13	14.60 b ± 0.59	24.70 a ± 1.68	16.68 b ± 0.66	14.83 b ± 0.60	18.30 b ± 0.88	24.66 a ± 0.54
Zn	0.36 f ± 0.03	1.13 e ± 0.04	2.09 cd ± 0.14	2.66 a ± 0.15	1.89 d ± 0.06	2.28 bc ± 0.08	2.46 ab ± 0.06

Means with different letters within a row indicate significant difference using Tukey’s test at p ≤ 0.05. Data are presented as mean of four replicates ± standard error.

. During the first cropping cycle, total N and P uptake by the rice plants of OSA4 were significantly higher compared with those of other treatments. Additionally, the rice plants which were harvested on the soil without amendments (S0) had the lowest N content and P uptake. The K uptake of rice plants in OSA4 (477.05 mg hill ⁻¹) was significantly higher compared with OSA5 (428.76 mg hill ⁻¹), OSA2 (413.64 mg hill ⁻¹), OSA3 (346.25 mg hill ⁻¹), OSA1 (163.60 mg hill ⁻¹), and NPK-Mg (109.46 mg hill ⁻¹), whereas S0 had the lowest K uptake (413.64 mg hill ⁻¹). The rice plants with the organic soil amendments (OSA1 to OSA5) improved Ca and Na uptake compared with the rice plants with the prevailing fertilization method (NPK-Mg) and without application of amendments (S0). The Mg, Fe, and Cu uptake of rice plants with OSA5 were higher compared with those of other treatments, whereas the rice plants with OSA1 and OSA5 had the higher Mn uptake compared with those of other treatments. The Zn uptake in rice plants of OSA2 (2.66 mg hill ⁻¹) was the highest, followed by OSA5 (2.46 mg hill ⁻¹), OSA4 (2.28 mg hill ⁻¹), OSA1 (2.09 mg hill ⁻¹), and OSA3 (1.89 mg hill ⁻¹) whereas S0 (0.36 mg hill ⁻¹) had the lowest Zn uptake. Our findings suggest that the nutrient uptake of the rice plants was enhanced by the application of organic soil amendments. The nutrient variation among the treatments during the first cropping cycle was due to translocation of the nutrients for grain yield.

The effects of organic soil amendments on total N and nutrient uptake of rice plants during the second cropping cycle are demonstrated in

Table 9. Total nitrogen and nutrients uptake by MR219 rice plants as affected by organic soil amendments during the second cropping cycle.

Nutrient	Treatments
	S0	NPK-Mg	OSA1	OSA2	OSA3	OSA4	OSA5
N (%)	0.70 d ± 0.03	1.83 d ± 0.13	4.28 bc ± 0.35	3.79 bc ± 0.21	3.5 c ± 0.19	4.97 ab ± 0.40	6.10 a ± 0.36
Nutrient uptake	mg hill−1
P	7.01 d ± 0.40	40.69 c ± 0.47	78.35 b ± 3.91	88.19 ab ± 5.86	81.46 b ± 0.320	87.62 ab ± 4.01	96.06 a ± 4.42
K	169.16 e ± 5.85	291.34 d ± 17.43	608.04 c ± 7.71	742.35 b ± 26.52	604.48 c ± 11.82	688.68 bc ± 24.55	903.78 a ± 18.64
Ca	12.72 d ± 0.89	33.73 c ± 1.67	78.23 ab ± 2.20	87.12 ab ± 3.81	83.66 b ± 3.94	80.04 b ± 0.32	97.77 a ± 2.52
Na	4.82 d ± 0.20	16.74 c ± 1.22	27.35 a ± 0.97	22.85 b ± 0.79	20.49 bc ± 0.76	21.28 b ± 0.51	21.40 b ± 0.74
Mg	24.05 d ± 1.57	91.43 c ± 3.67	155.33 ab ± 5.95	164.97 a ± 3.09	147.10 ab ± 1.95	137.81 b ± 1.85	162.78 a ± 8.62
Fe	0.13 e ± 0.01	0.40 d ± 0.02	0.51 cd ± 0.03	0.59 bc ± 0.03	0.66 ab ± 0.03	0.76 a ± 0.03	0.77 a ± 0.02
Cu	0.06 c ± 0.005	0.12 bc ± 0.009	0.27 a ± 0.035	0.27 a ± 0.014	0.33 a ± 0.050	0.23 ab ± 0.030	0.26 a ± 0.011
Mn	0.04 d ± 0.002	0.08 c ± 0.004	0.13 b ± 0.005	0.14 b ± 0.005	0.13 b ± 0.005	0.14 b ± 0.004	0.23 a ± 0.011
Zn	0.41 d ± 0.02	1.15 c ± 0.02	1.52 b ± 0.05	1.77 a ± 0.03	1.84 a ± 0.03	1.22 c ± 0.04	1.84 a ± 0.04

Means with different letters within a row indicate significant difference using Tukey’s test at p ≤ 0.05. Data are presented as mean of four replicates ± standard error.

. During the second cropping cycle, the addition of organic soil amendments (OSA1 to OSA5) also increased total N and nutrient uptake in rice plants. The N content, P, K, Ca, and Mn uptakes of the rice plants for OSA5 were significantly higher compared with those of OSA1, OSA2, OSA3, OSA4, NPK-Mg, and S0. These results can be due to higher nutrients in the chicken manure and Leucaena co-composted with chicken litter biochar. The Na uptake of the rice plants with OSA1 was the highest, followed by the other organic soil amendments (OSA2 to OSA5) and the prevailing fertilizer method (NPK-Mg). The rice plants in S0 had the lowest Na uptake. The Fe uptake of the rice plants with NPK-Mg, OSA1, and OSA2 were lower compared with those of OSA3, OSA4, and OSA5 whereas the rice plants in S0 had the lowest Fe uptake. The Mg and Cu uptakes of the rice plants with the organic soil amendments (OSA1 to OSA5) were significantly higher than those grown on the soil with the chemical fertilizers (NPK-Mg) and without the amendments (S0). The Zn uptake of rice plants with OSA2, OSA3, and OSA5 was significantly higher compared with those of OSA1, OSA4, NPK-Mg, and S0.

Effects of the organic soil amendments on total N and nutrient uptake of the rice plants during the third cropping cycle are summarized in

Table 10. Total nitrogen and nutrients uptake by MR219 rice plants as affected by residual organic soil amendments during the third cropping cycles.

Nutrient	Treatments
	S0	NPK-Mg	OSA1	OSA2	OSA3	OSA4	OSA5
N (%)	0.67 b ± 0.08	7.94 a ± 0.83	8.71 a ± 0.43	9.28 a ± 1.49	7.96 a ± 0.79	9.01 a ± 0.85	10.84 a ± 0.52
Nutrient uptake	mg hill−1
P	3.72 b ± 0.25	29.61 a ± 2.18	28.20 a ± 2.82	41.69 a ± 4.12	36.33 a ± 3.54	29.76 a ± 1.55	35.71 a ± 3.10
K	166.01 e ± 11.48	203.44 de ± 5.33	306.95 bc ± 21.07	269.71 cd ± 25.93	216.32 ed ± 18.50	374.85 b ± 20.06	511.62 a ± 38.99
Ca	12.98 e ± 0.62	43.80 d ± 1.09	60.33 cd ± 5.63	84.62 ab ± 5.70	62.64 cd ± 8.64	71.24 bc ± 5.42	98.48 a ± 5.24
Na	6.79 d ± 0.29	17.56 cd ± 0.27	28.02 bc ± 1.90	57.31 a ± 3.56	46.15 a ± 3.64	31.20 b ± 1.82	54.57 a ± 4.54
Mg	26.86 d ± 0.48	113.24 c ± 8.81	121.16 bc ± 1.76	168.47 a ± 7.32	149.93 ab ± 6.11	147.66 ab ± 11.31	154.05 ab ± 10.04
Fe	0.15 b ± 0.01	0.53 a ± 0.02	0.56 a ± 0.02	0.66 a ± 0.05	0.55 a ± 0.05	0.51 a ± 0.03	0.56 a ± 0.03
Cu	0.07 c ± 0.00	0.24 b ± 0.01	0.24 b ± 0.02	0.30 ab ± 0.04	0.34 ab ± 0.04	0.25 b ± 0.02	0.42 a ± 0.03
Mn	0.05 e ± 0.00	0.11 d ± 0.01	0.16 bc ± 0.01	0.20 b ± 0.01	0.17 bc ± 0.01	0.15 cd ± 0.01	0.24 a ± 0.02
Zn	0.47 d ± 0.05	1.18 c ± 0.07	1.20 c ± 0.09	1.77 b ± 0.03	1.67 bc ± 0.16	1.37 bc ± 0.15	2.40 a ± 0.19

Means with different letters within a row indicate significant difference using Tukey’s test at p ≤ 0.05. Data are presented as mean of four replicates ± standard error.

. After the second cropping cycles, the residual effects of the organic soil amendments (OSA1 to OSA5) significantly increased total N and P uptake of the rice plants compared with those on S0. In addition, the N and P uptake of the rice plants with the organic soil amendments (OSA1 to OSA5) were similar to those of the prevailing fertilization regime (NPK-Mg). This suggests that the organic soil amendments used in this study have the potential to replace the use of chemical fertilizers in rice cultivation. The uptakes of K, Ca, Cu, Mn, and Zn of rice plants in OSA5 were significantly higher than those of other treatments. The highest Na uptake in rice plants was observed in OSA2 (57.31 mg hill ⁻¹), OSA5 (54.57 mg hill ⁻¹), and OSA3 (46.15 mg hill ⁻¹), followed by OSA4 (31.20 mg hill ⁻¹), and OSA1 (28.02 mg hill ⁻¹), whereas NPK-Mg (17.56 mg hill ⁻¹) and S0 (6.79 mg hill ⁻¹) had the lowest Na uptake. The highest Mg uptake of the rice plants was observed in OSA2 compared with that of the other treatments.

Nitrogen and P uptake under OSA4 were higher in the first cropping cycle due to the higher rates of Leucaena used. However, in the second cropping cycle, the N content and P uptake of the rice plants in OSA5 increased but were similar to those of OSA4. This demonstrates that the organic soil amendments produced from the chicken litter biochar with Leucaena and chicken manure can enhance N and P for uptake. The Leucaena and chicken litter (OSA5) stimulated N and K availability and their absorption by the rice plants. For S0 and NPK-Mg, the higher Al and Fe concentrations impeded the rice plants’ growth is because of the reduction in the rice plants” roots elongation. This is related to the direct and indirect effects on the rice plant metabolism [40]. The stress significantly reduced the rice plants’ roots nutrient adsorption thereby reducing the growth of rice plants [40]. For example, higher concentrations of Al impede P availability because Al binds or prevents P from being absorbed and this results in lower plant nutrient uptake and rice yields [40]. However, the soil data in this present study suggest that the adverse effects of Al and Fe were averted.

Generally, the quality of the organic soil amendments is related to lignin content and C:N ratio. The lower C:N ratio of materials (less than 20:1) leads to higher decomposition because of higher microbial growth [41,42]. Degradation of these organic soil amendments is important in terms of release of plant nutrients originally bound in organic biomass [36]. In mineralization, materials which have simpler nutrients, especially N compounds, are mineralized more rapidly than complex materials with higher lignin content [43,44]. Lignin-rich materials such as chicken litter biochar, chicken litter, and forest litter are resilient to microbial attack. Additionally, these materials may cause physical protection of soil, improve rice plant roots development, and enhance chemical fertilizer use efficiency [45,46].

Among the organic soil amendments used in this study, the treatments with Leucaena significantly increased the rice plants’ nutrients uptake. However, addition of the chicken manure to Leucaena resulted in the release of more nutrients for the rice plants. These nutrients might have been released from the chicken manure because of the reduction of high C:N ratio following the addition of Leucaena [47]. The lower C:N ratio of the chicken litter biochar and the chicken manure with Leucaena might have increased the decomposition of the chicken manure. Carbon-rich materials such as chicken litter biochar, chicken manure, forest litter, cow dung, leaves, straw, and wood chips tend to be dry and difficult to decompose [48]. Lower decomposition results in a minimum release of important nutrients for plants uptake. Nitrogen rich materials such as Leucaena, fresh grass clippings, and food waste cause release of excess ammonia if they are used without treating them with other materials. Thus, the use of organic soil amendments (OSA1, OSA2, and OSA3) had the capacity to significantly improve total N and nutrients uptake of the rice plants compared with those of the rice plants with the prevailing fertilization recommendation (NPK-Mg) and without amendments (S0), but not as high as the addition of Leucaena (OSA4 and OSA5). Hence, mixing Leucaena with other carbon-rich materials can optimize the release of nutrients in C rich materials such as biochar and chicken manure.

3.3. Effects of Organic Soil Amendments on Rice Growth and Grain Yield

The effects of organic soil amendments on rice growth and grain yield after the first cropping cycle are presented in Figure 3. The organic soil amendments used in this study enabled the rice plants to translocate most of the absorbed nutrients for desirable growth and yield. Application of the organic soil amendments (OSA1 to OSA5) significantly improved grain yield, dry matter, plant height, and total grain filling per panicle compared with those of NPK-Mg and S0 (Figure 3a–d). The number of panicles and number of tillers per hill of the rice plants with organic soil amendments (OSA1 to OSA5) were similar to those of the prevailing fertilization method (NPK-Mg) but significantly higher than the rice plants without the amendments (S0) (Figure 3e,f). These results were consistent with the higher soil nutrients availability and uptake of rice plants.

Effects of the organic soil amendments on rice plants’ growth and grain yield after the second cropping cycle are demonstrated in Figure 4. It is observed that the continued application of the organic soil amendments during the second cropping cycle further improved the translocation of most of the absorbed nutrients and it resulted in enhanced growth and yield of the rice plants. Similar to the first cropping cycle, the use of the organic soil amendments (OSA1 to OSA5) significantly improved grain yield, dry matter, plant height, and total grain filling per panicle compared with those of NPK-Mg and S0 (Figure 4a–d). Notably, the number of panicles and number of tillers per hill of the rice plants in OSA3 and OSA5 significantly increased compared with those of the prevailing fertilization method (NPK-Mg) (Figure 4e,f). The MR219 rice plants grown on the soil without amendments (S0) revealed the lowest numbers of panicles and tillers per hill. These results suggest that the rice plant growth and grain yield during the second cropping cycle were generally higher than those of the first cropping cycle because of the residual effects of organic soil amendments.

Effects of the organic soil amendments on the rice plants growth and grain yield after the third cropping cycle are summarized in Figure 5. Although application of the organic amendments was skipped in the third cropping cycle, the residual effects of the organic soil amendments (OSA1 to OSA5) significantly improved the rice grain yield (Figure 5a). The number of tillers per hill of the rice plants with the organic soil amendments (OSA1 to OSA5) were similar to those of the prevailing fertilization method (NPK-Mg) but significantly higher than the rice plants grown on the soil without the amendments (S0) (Figure 5f). The dry matter yield and rice plant height of OSA5 were the highest compared with those of other treatments (Figure 5b,c). The total filled grain per panicle of OSA1 OSA2, and OSA5 were significantly higher than those of OSA3, OSA4, S0, and NPK-Mg (Figure 5d). The pattern of these results was similar to those in the second cropping cycle. However, the rice grain yield slightly decreased compared with the two earlier cropping cycles.

The rice grain yields at the end of the first and second cropping cycles indicate that the continued application of the co-composted chicken litter biochar and the chemical fertilization (75% N and 34% K) significantly increased rice yield [16]. The organic soil amendments used in this study reduced the inorganic fertilizers application of N, P, K, MgO, and trace elements by 25%, 100%, 64%, 100%, and 100%, respectively [16]. The organic soil amendments used in this study also improved rice grain yields in the first (9 t ha⁻¹ to 11 t ha⁻¹), second (11 t ha⁻¹ to 13 t ha⁻¹), and third cropping cycles (8 t ha⁻¹ to 10 t ha⁻¹). Although the rice yield in the third cropping cycle decreased relative to those of the first and second cropping cycles, the residual effects of the organic soil amendments significantly improved the rice grain yield compared with the existing rice yield in Malaysia [48].

Among the treatments with the organic soil amendments, OSA5 significantly improved the rice grain yield. This finding is related to the integrated application of organic soil amendments and inorganic fertilizer (N and K), which enhanced stomata conductance and the photosynthetic rate of the rice plants [49]. These primary physiological processes are responsible for the production of rice plant dry matter yield, number of tillers, and number of panicles [50,51]. The similarity in the yields of the treatments with different organic soil amendments suggests the efficient utilization of nutrients [52]. The results of S0 and NPK-Mg on the other hand suggest that continued rice cultivation without organic soil amendments leads to yield reduction.

The improved rice grain yield over three cropping cycles through the use of the organic soil amendments was also due to the balanced fertilization. This suggests that our approach facilitates effective translocation of nutrients to improve grain formation and filling, thus, resulting in the improved rice grain yield [53]. Herencia et al. [54] reported that the use of organic soil amendments provides a conducive environment for optimum rice growth, thus resulting in a higher grain yield. Sathish et al. [55] also reported that the combined use of organic and inorganic fertilizers can increase rice yield over time because the gradual decomposition of organic soil amendments could slowly release nutrients for crop uptake throughout the cropping cycle.

4. Conclusions

The combined use of co-composted Leucaena, forest litter, chicken manure, and cow dung with chicken litter biochar (OSA1 to OSA5) as organic soil amendments with chemical fertilizers can improve soil chemical properties, nutrients content and uptake, rice growth, and grain yield compared with those of the prevailing fertilization method (NPK-Mg). The organic soil amendments used in this study improved the rice grain yields in the first (9 t ha⁻¹ to 11 t ha⁻¹), second (11 t ha⁻¹ to 13 t ha⁻¹), and third cropping cycles (8 t ha⁻¹ to 10 t ha⁻¹). Among organic soil amendments used, OSA5 (Chicken litter biochar: Leucaena: chicken litter compost = 2:1:1) is the most recommended organic soil amendment because it demonstrated the highest rice grain yields at the first (11.17 t ha⁻¹), second (13.11 t ha⁻¹), and third (10.06 t ha⁻¹) cropping cycles in addition to significantly improved soil chemical properties, nutrients content and uptake, and rice growth. The application of inorganic fertilizers N, P, K, MgO, and trace elements were reduced by 25%, 100%, 64%, 100%, and 100%, respectively because Leucaena improves mineralization, soil nutrient availability, and nutrient uptake of the rice plants especially when chicken litter biochar, chicken manure, and Leucaena are co-composted (OSA5). The residual effects of the organic soil amendments (OSA1 to OSA5) suggest that the nutrients content and uptake, rice growth, and grain yield were slightly reduced compared with those of the two earlier cropping cycles but significantly higher than those of prevailing fertilization method (NPK-Mg). Reduction of P, K, MgO, and trace elements by 100%, 64%, 100%, and 100%, respectively, were maintained; however, the application of N could not be reduced because of higher demand of N by rice plants and lower N supply by the residual organic soil amendments. Generally, the use of organic soil amendments from Leucaena, forest litter, chicken manure, and cow dung co-composted with chicken litter biochar does not only improve sustainable productivity of rice on tropical acid soil, but it also mitigates other environmental pollutions such as forest residue slash pile burning and facilitates suitable disposal of animal wastes. Thus, it is possible to transform organic agro-wastes into organic soil amendments to sustainably improve mineral acid soil and rice productivity.