The Effects of Different Tillage Techniques and N Fertilizer Rates on Nitrogen and Phosphorus in Dry Land Agriculture

Abstract

Processes governing soil organic matter (SOM) decomposition and mineralization are important for soil fertility and ecosystem sustainability. However, in the sub-Saharan region, limited work has been conducted on SOM dynamics; therefore, there was an imperative need for this study. The objective of this study was to determine the phosphorus (P) and nitrogen (N) dynamics in soil under different tillage and fertilizer management practices. The field trial was arranged as a randomized split plot design, with tillage forming the whole plot and the fertilizer application rate being the sub-plot. The tillage techniques were no-till (NT), annual tillage (CT-ANNUAL) and conventional tillage every fifth season (CT-Y5), whereby NT was practiced for four consecutive seasons, and in the fifth season, conventional tillage was employed. For all these tillage techniques, urea fertilizer was applied in amounts of 60, 120 and 240 kg N ha⁻¹ with a control for each tillage treatment. Ammonium and nitrate levels were determined calorimetrically. Sulfuric acid, at 1 M, along with 0.057 M ascorbic acid and molybdate reagent were used to extract the organic P from the soil and extractable P. The total N, ammonium, total P, organic P and extractable P in the surface soil under NT were higher (p < 0.05) compared to the values of both CT-Y5 and CT-ANNUAL. The high levels of total N, N mineralization, total P, organic P and extractable P under NT, compared to CT-Y5 and CT-ANNUAL, at the depth of 0–10 cm may be attributed to the accumulation of crop residues on the surface and less soil disturbance. Furthermore, nitrate was found in higher (p < 0.05) concentrations under CT-ANNUAL compared to CT-Y5 at all the application rates and depths. The ploughing of soil under CT-ANNUAL improves the aeration, which accelerates the decomposition of organic material and mineralization of organic N and P into soluble forms. No-till, at 60 kg N ha⁻¹ and a 0–10 cm soil depth, had optimum total N, nitrate, ammonium, total P and organic P values, thus showing its salient impact on the maintenance of soil fertility. However, the conclusion drawn from this study is that CT-Y5, due to its average N and P pools, can be recommended to under-resourced farmers in sub-Saharan preferentially over both NT and CT-ANNUAL, because it does not require advanced technology and equipment and it sustains an average soil fertility.

1. Introduction

Nitrogen (N) and phosphorus (P) are major elements that are essential for plant growth and development. N is a constituent of proteins and nucleic acids and P is an essential component of ATP, DNA and RNA, with most soil N found in organic matter (OM), where it is continuously mineralized into ammonium (NH₄⁺) and nitrate (NO₃⁻), the available mineral forms. There is need for the constant assessment of the amounts and mineralization patterns of NH₄⁺ and NO₃⁻ in soils during cropping in order to offer informed recommendations regarding the application rates of N fertilizers and organic inputs. P, in soil, especially highly weathered soils, is limited; hence, supplementation with P is required in order to sustain crop productivity [1].

Tillage and fertilizer applications alter the soil profile in terms of the nutrient distribution, thus impacting the availability, adsorption, leaching, decomposition and mineralization of nutrients. The authors of [2] observed increased N mineralization under the condition of conventional tillage (CT) compared to no-till (NT). This is because the ploughing of soil increases aeration and water movement, thereby enhancing its decomposition, mineralization and nitrification. Increases in N mineralization following soil disturbance by CT are largely due to the process of more organic N substrates becoming available to support greater microbial activity [3]. The authors of [4] explained that, although the rate constant for organic N mineralization is often greater in CT systems, the gradual or long-term accumulation of a larger organic N pool in NT systems may compensate for this.

No-till involves an abundance of crop residues which continuously add organic N after mineralization [5]. N fertilizer application, on the other hand, stimulates N uptake and turnover by providing microbes to the substrates [6]. N fertilizers are required by both plants and microbes; however, excessive application may have a detrimental effect on microbial activity. According to [7], N fertilization had little effect on N mineralization, while changes in active N pools of soil organic matter (SOM) depended more on tillage for their distribution within the surface soil than on N fertilization. However, in a study of negative pressure irrigation reported in [8], it was observed that NO₃⁻ becomes distributed according to the movement of water. While NH₄⁺ movement in the soil is limited because it is adsorbed by negative soil colloids, both have a high potential to be taken up by crops and be immobilized by microbes [8]. Hence, the decrease in organic C decomposition and mineralization lowers the NH₄⁺ and NO₃⁻ concentrations.

P remains to be a major limitation in regard to agricultural productivity [9]. Residue accumulation and less soil disturbance under NT helps to sustain both N and P. The authors of [10] reported findings of more extractable P in NT compared with CT below the till-zone, rather than near the surface alone, as reported in other studies, and suggested that this may be due to the accumulation of P in senescent roots. Globally, critical P values have been reported for different soils, varying from 5 to 26 mg kg⁻¹ for wheat [11] and 6.9 to 28 mg kg⁻¹ for maize [11]. Since P requirement rates differ between soil types, crop types and climates, the application requirements differ from one environment to another [12]. Therefore, it is crucial to determine the available P and organic and total P for each specific environment and cropping system. Compared to other major nutrients, P is firmly bound to soils due to the precipitation of P with calcium ions in calcareous soil and adsorption of P by Fe and Al- oxides in acidic soils [13]. However, this fixation is largely reversible over time, as plant roots can take up P accumulated in soil through the application of fertilizers and organic sources over many years [1]. By differentiating between fertilizer application rates, one can determine their suitability for crop uptake. The authors of [14] found that in acid soils, the use of basic N fertilizers such as sodium nitrate and calcium nitrate increased the solubility and plant utilization of P, while acidic fertilizers, such as ammonium sulphate and urea, had the opposite effect.

Though P is immobile in soil, its distribution profile may be influenced by the type of tillage applied [15]. The P cycle, including total P, organic P and inorganic P, should be well researched, especially in arable farms with poor resources, in order to sustain crop and soil productivity. Furthermore, many studies have demonstrated that P leaches to subsoil in both coarse-and fine-textured soils [16]. In a study by the authors of [17], NT showed a greater concentration of P at soil depths of 0–5 and 15–30 cm compared to CT. Therefore, tillage systems which improve the P availability for uptake through the elimination of leaching, adsorption and precipitation should be promoted. According to [18], improving the P availability may be an added advantage as an aspect of conservation agriculture in the case of weathered soils, since reduced tillage and residue retention could reduce P fixation and increase labile P and organic P accumulation and its mineralization by phosphatases. Residue retention can reduce P fixation by increasing organic anion competition for P binding sites [19]. Thus, knowledge of the magnitude of changes in the active soil N and P pools is crucial for understanding how tillage systems can be better managed in order to increase the N and P sequestration of soil. The processes governing SOM decomposition and mineralization are important for soil fertility, GHGs and ecosystem sustainability; however, they are not fully understood. Thus, studies on N and P pools in under-resourced areas are highly essential. Many studies, especially in the sub-Saharan region, only focus on the effect of tillage management on either P or N rather than the interaction effects of the tillage and N application. The objective of this study was to determine the P and N dynamics in soil under different tillage and N fertilizer management practices. Therefore, the null hypothesis of this study was that NT would have greater N and P pools compared to other tillage techniques at all depths.

2. Material and Methods

2.1. Site

The study was conducted in Loskop, KwaZulu Natal Province, South Africa, which is in the sub-Saharan region, located at a latitude of 28°55′26.83″ S and longitude of 29°33′38.64″ E. The trial started in the 2003/2004 season; however, the data presented here are only from the 2017 season. The site had previously been utilized for dry-land maize and soybean cultivation in rotation and has been used for NT since 1990. Furthermore, N fertilizer was applied uniformly under all application rates. The experimental site was used for dry-land agriculture, with maize being planted in the summer and fallowing practiced in winter. It was managed under zero tillage (NT) conditions, with annual conventional tilling (CT-ANNUAL) ploughed to a depth of 30 cm using a moldboard plough, and conventional tillage every 5th season (CT-Y5), whereby NT was practiced for four consecutive seasons, and in the 5th season it was ploughed to a depth of 30 cm. The trial has been running since the 2003/2004 season. In this season, the sowing date was 21 November 2017, which was determined according to the commencement of rainfall, and the harvesting was carried out in mid-May 2018. The Pannar SC 701 seed was used, with the plant population per hectare estimated to be 25,000, with a row spacing of 90 cm, interrow of 43.5 cm and 2.3 seeds per meter. The soil was classified as Hutton [20], within the clay loam and loam textural class. The area receives approximately 643 mm of rainfall per annum, which occurs mostly during summer (November–January) and has a mean average midday temperature ranging between 19.3 °C in June (winter) and 28 °C in January (summer) [21].

The trial was carried out under a split plot arrangement using a randomized complete block design (RCBD) with three replications. The tillage techniques were assigned to the main plot, with N fertilizer (urea) assigned to the sub-plot. The treatments were three tillage techniques, CT-ANNUAL, CT-Y5 and NT, and four N fertilizer application rates, namely 0, 60, 120 and 240 kg N ha⁻¹. Although the study focused on both P and N pools, there no P application was carried out in the experimental area. The NT was characterized by a lack of soil disturbance (no ploughing), while a planter was utilized for the planting. CT-Y5 comprised leaving the soil un-ploughed for four seasons followed by tilling every 5th season. CT-ANNUAL involved an annual disturbance of the soil by tilling using a tractor at a depth of 30 cm. Soil was sampled in the year 2018 using a bucket auger after harvest to avoid plant disturbance at three depths of 0–10, 10–20 and 20–30 cm. Sampling was performed during the ploughing cycle of CT-Y5. The collected soil samples were spread out to air-dry under a shed and then gently crushed with a mortar to be passed through a 2 mm sieve prior to analysis. Cores for the bulk density measurements were sampled using a core sampler at three depths of 0–10, 10–20 and 20–30 cm.

2.2. Laboratory Analyses

2.2.1. N Pools

The total N was analyzed using a LECO TruMac CNS/NS Carbon/Nitrogen/Sulfur Determinator [22]. The colorimetry method, using 0.5 mol L⁻¹ K₂SO₄ extracting solution, was used for the analysis of NH₄⁺ from the soil [23]. Absorbance was measured at 655 nm using a spectrophotometer (SPECTRO UV-11) in μgNH4⁺−N/kg. Nitrate from the soil was determined calorimetrically using 0.5 mol L⁻¹ K₂SO₄ extracting solution [24]. The absorbance of NO₃⁻ was measured at a 419 nm wavelength [25] using a spectrophotometer. The anaerobic N mineralization rate was measured using 10 g of fresh soil and 50 mL 1 mol L⁻¹ KCI, and NH₄⁺−N was determined using the spectrophotometer at a wavelength of 655 nm, referred to as time_o NH₄⁺−N [26]. Meanwhile, 2 mol L⁻¹ KCI was used and applied to 5 g of fresh soil to determine NH₄⁺−N using the spectrophotometer at a 655 nm wavelength, referred to as time₁ NH₄⁺−N [26]. The 5 g of fresh soil was supplemented with 12.5 mL distilled water, gently swirled to remove air bubbles and placed in an incubator for 7 days at 40 °C. After incubation, the contents were filtered using Whatman No. 42 filter paper and transferred into a clean 50 mL bottle, and then 25 mL of 2 mol L⁻¹ KCI was added. The time_o NH₄⁺−N was obtained immediately on the first day of the experiment, whereas time₁ NH₄⁺−N was obtained after 7 days of incubation. The calculation of the anaerobic N mineralization was performed as follows [26]:

Anaerobic N mineralization rate (ugN/g soil/day) = [(time₁ NH₄⁺−N) − (time₀ NH₄⁺−N)]/7

(1)

2.2.2. P Pools

The digestion of total P by the soil was assessed using 5N H₂SO₄ as a digesting agent and 0.057 mol L⁻¹ ascorbic acid as a reducing agent [27]. The contents were allowed to stand for 1 h to permit full color development and the absorbance was measured using a spectrophotometer set at an 880 nm wavelength. A total of 0.057 mol L⁻¹ ascorbic acid and molybdate reagent were used for both the P total and bicarbonate extractable P determinations. The absorbance was observed using a SPECTRO UV-11 spectrophotometer set at an 880 nm wavelength. Organic P was analyzed using two samples of the same soil, with one ignited on the furnace and the other unignited. Sulfuric acid, at 1 mol L⁻¹, with 0.057 mol L⁻¹ ascorbic acid and molybdate reagent were used to extract the organic P from the soil [28]. Ignited soil was washed at 550 °C for 1 h and allowed to cool, and then extracting reagent, 1 M H₂SO₄, 0.057 mol L⁻¹ ascorbic acid and molybdate reagent were added [29]. The contents were left for an hour to allow a blue color to develop [30]. The absorbance was read using a spectrophotometer set at an 880 nm wavelength, and the organic P calculated as follows [28]:

$${\mathrm{P}}_{\mathrm{ignited}} (\%)=\frac{c\ast 0.2}{w}$$

(2)

$${\mathrm{P}}_{\mathrm{unignited}} (\%)=\frac{c\ast 0.2}{w}$$

(3)

Organic Phosphorus (%) = P_ignited (%) − P_unignited (%)

(4)

where w = the weight of the sample, and c = the P concentration of a sample solution. All samples assessed using Equation (2) were analyzed after they were ignited in the furnace.

2.2.3. Soil Organic C/Soil Organic Matter

The level of soil organic carbon (SOC) was determined using the Walkley–Black method [31]. Furthermore, the SOC and bulk density were used to calculate the soil C stock per the depth. The soil was digested using 0.167 mol L⁻¹ K₂Cr₂O₇, and H₂SO₄ was added rapidly with continuous swirling, and concentrated ortho-phosphoric was added with barium diphenylamine sulphonate used as an indicator. The dichromate was back-titrated with 0.5 mol L⁻¹ ferrous ammonium sulphate solution. At the end point of the process, the color abruptly changed to green [31]. The Formulas (5) and (6) below, defined by the authors of [31], was used to calculate the organic C and bulk density, respectively.

$$\mathrm{Organic} \mathrm{C} (\%)=\frac{0.003g\ast N\ast 10 mL\ast \left(1-\frac{T}{S}\right)\ast 100}{ODW}=\frac{3\left(1-\frac{T}{S}\right)}{ODW}$$

(5)

where N = the normality of the K₂Cr₂O₇ solution, T = the volume of FeSO₄ used in the sample titration (mL), S = the volume of FeSO₄ used in the blank titration (mL), and ODW = the oven-dried sample weight (g).

Bulk density (kg m⁻³) = Mass of soil/volume of cylinder

(6)

2.3. Statistical Analyses

An analysis of variance (ANOVA) was performed to determine the interaction effects of the tillage and fertilizer application rate on the soil P and N availability at different soil depths. The treatment factors were the tillage practices, N fertilizer application rates and soil depths. Fisher’s protected LSD test was used as a post hoc test for multiple comparisons in order to compare the treatment means and their interactions. The p value (<0.05) was used to determine significant differences between treatment factors. All tests were performed with GenStat 14.1 software for Windows [32].

3. Results

3.1. Total N Contents

The total N under NT was higher than the values obtained for CT-Y5 and CT-ANNUAL at all application rates at the soil depth of 0–10 cm (Figure 1). At 240 kg N/ha, the total N under CT-Y5 (0.46 g kg⁻¹) was higher than that of CT-ANNUAL (0.42 g kg⁻¹) at the depth of 0–10 cm, while the 60 and 240 kg N ha⁻¹ rates generally produced higher levels of total N than the control and the 120 kg N ha⁻¹ rate for most of the tillage techniques. At a 10–20 cm soil depth, NT produced a higher total N than CT-Y5 and CT-ANNUAL in the control, while at 60 kg N ha⁻¹, NT produced a higher level of total N than CT-ANNUAL. However, at 120 kg N ha⁻¹, at a depth of 10–20 cm, CT-ANNUAL produced a higher total N than CT-Y5, while at 240 kg N ha⁻¹, CT-ANNUAL produced more total N than NT. At the depth of 20–30 cm, at 60 and 120 kg N ha⁻¹, NT produced a higher total N than CT-ANNUAL, whereas CT-Y5 produced more total N than NT at a 240 kg N/ha application rate.

3.2. Nitrate Contents

Figure 2 shows that, at 0, 60 and 120 kg N ha⁻¹, CT-Y5 produced the lowest nitrate-N at the 0–10 cm soil depth, while at 120 kg N ha⁻¹, CT-ANNUAL produced higher levels of nitrate-N than NT. However, at 240 kg N ha⁻¹, NT (9.96 mg kg⁻¹) produced a higher nitrate-N than CT-ANNUAL (5.27 mg kg⁻¹) and CT-Y5 (5.22 mg kg⁻¹). At the 10–20 cm soil depth, CT-ANNUAL produced a higher level of nitrate-N than NT and CT-Y5 at 60 kg N ha⁻¹, while at 120 and 240 kg N ha⁻¹, NT produced the highest nitrate-N (Figure 2). At the 20–30 cm soil depth, CT-Y5 produced the lowest nitrate-N compared to CT-ANNUAL at 0 and 60 kg N ha⁻¹; however, at 60 kg N ha⁻¹, CT-ANNUAL produced a higher nitrate-N than NT. Again, at 120 and 240 kg N ha⁻¹, NT produced a higher level of nitrate-N than CT-Y5 at the 20–30 cm soil depth, while at 120 kg ha⁻¹, NT also produced higher levels than CT-ANNUAL.

3.3. Ammonium Contents

Ammonium-N was highest in the case of NT at 0 and 60 kg N ha⁻¹ and at the 0–10 cm soil depth, while no notable differences were recorded using the other application rates (Figure 3). At the depth of 10–20 cm, NT also produced more ammonium-N (1.77 µg kg⁻¹) than CT-Y5 (0.58 µ kg⁻¹) and CT-ANNUAL (0.41 µ kg⁻¹) at 60 kg N ha⁻¹. At the 20–30 cm depth, CT-Y5 produced more (p < 0.05) ammonium-N than CT-ANNUAL and NT at 120 kg N ha⁻¹.

3.4. N Mineralisation

N mineralization was highest in the case of NT in the control at the 0–10 cm soil depth but did not vary at other N rates (Figure 4). At the 10–20 cm soil depth, CT-Y5 produced higher N mineralization than NT at 120 and 240 kg N ha⁻¹. Furthermore, CT-Y5, at 120 kg N ha⁻¹, produced higher N mineralization, with 0.044 µg N/kg of soil, compared to CT-ANNUAL, which had a value of 0.017 µg N kg⁻¹ of soil at a depth of 10–20 cm. Moreover, at the 20–30 cm soil depth, CT-ANNUAL produced higher N mineralization than NT at 60 and 240 kg N ha⁻¹, while CT-Y5 at 240 kg N ha⁻¹ produced higher N mineralization than NT.

3.5. Total P Contents

The total P at the 0–10 cm soil depth was higher under NT for all treatments (Figure 5). At 120 kg N ha⁻¹, CT-ANNUAL produced higher total P than NT at the 10–20 cm soil depth, whereas at 240 kg N ha⁻¹, CT-Y5 produced higher total P than NT. At the 20–30 cm soil depth, the total P was highest under NT at all application rates.

3.6. Organic P Contents

Figure 6 shows that organic P was generally higher under NT compared to the other tillage techniques at all application rates at the 0–10 cm soil depth. CT-ANNUAL, at 0 and 240 kg N ha⁻¹, also produced higher organic P than CT-Y5 at this depth. At the 10–20 cm soil depth, NT produced higher organic P than CT-ANNUAL in the control (p < 0.05). Meanwhile, at 60 kg N ha⁻¹, both NT and CT-ANNUAL produced higher organic P than CT-Y5, with NT also producing higher levels of organic P than CT-ANNUAL at the 10–20 cm depth. Again, at 60 kg N ha⁻¹ and at the 20–30 cm soil depth, NT produced higher organic P than CT-Y5.

3.7. Extractable Phosphorous Contents

Figure 7 shows that extractable P at the 0–10 cm soil depth was higher under NT for all treatments. At 60 kg N ha⁻¹ and at the 10–20 cm soil depth, CT-Y5 produced higher extractable P (0.008 µg g⁻¹) than CT-ANNUAL (0.001 µg g⁻¹), while at 120 kg N ha⁻¹, NT (0.007 µg g⁻¹) produced higher extractable P than CT-Y5 (0.003 µg g⁻¹), and at 240 kg N ha⁻¹, CT-ANNUAL produced higher extractable P than CT-Y5. In the control, at the 20–30 cm depth, CT-Y5 and CT-ANNUAL produced higher extractable P than NT, while the other N rates did not differ in regard to the amount of extractable P.

3.8. Soil Organic C

NT produced higher SOC/SOM than both CT-Y5 and CT-ANNUAL at all fertilizer application rates at the 0–10 cm depth. There was no significant difference in SOC/SOM at the subsoil level according to either the tillage technique or fertilizer application rate.

4. Discussion

The high levels of total N obtained under NT compared to CT-Y5 and CT-ANNUAL at the 0–10 cm depth may be attributed to the accumulation of crop residues which are left on the soil surface and less soil disturbance. The high SOM obtained under NT at a depth of 0–10 cm proves that crop residues add C to the soil surface, a result which agrees with the findings of [5]. Meanwhile, the ploughing of soil through CT-ANNUAL improves aeration, which accelerates the decomposition of organic material and mineralization of organic N into soluble forms that are easily lost through leaching. The reduced oxygen availability below the surface in NT systems also reduces decomposition rates, causing SOM to be retained in conservation tillage systems [33]. Meanwhile, buried residues in CT also decompose at 3.4 times the rate of residues left on the soil surface [34]. The authors of [35] observed that NT and minimum tillage promoted greater concentrations of N at the soil surface level, but this was uniformly distributed in terms of depth under CT. This explains the observed increase in the total N under CT-ANNUAL at 120 and 240 kg N ha⁻¹ due to the incorporation of mineral N fertilizer at deeper soil layers at the 10–20 cm depth. The authors of [36] also observed that buried residues and roots experience more extensive and rapid decomposition than surface residues, causing the more slowly decomposed surface residues to supply nutrients to crops in the long term. In the current study, tillage alone determined the variations in the N concentrations on the surface layer, whereas in the subsoil, N fertilization also had a significant effect. Thus, the high total N at the 20–30 cm depth and 240 kg N ha⁻¹ observed in the case of CT-Y5 may be attributed to both the increased decomposition of organic material through residue incorporation during tillage and the high amount of N fertilizer.

Soil NO₃⁻ and NH₄⁺ concentrations were inversely related in all treatments. Thus, NO₃⁻ had its peak at 0–10 cm under CT-ANNUAL at a rate of 120 kg N ha⁻¹, whereas NH₄⁺ peaked under NT at 0 kg N ha⁻¹ at the same depth. Moreover, NO₃⁻ was the predominant available N ion in all treatments. This can be attributed to the clay loam texture of the soil, with its good drainage and aeration, which enhances the nitrification of NH₄⁺ to NO₃⁻. According to [37], NH₄⁺ levels are usually relatively low compared to NO₃⁻ in arable soils, as nitrification is predominant. This is contrary to the findings of the authors of [38], who observed that NH₄⁺ was more prevalent than NO₃⁻ and attributed this to the high activity of the ammonifying bacteria, in contrast to nitrifiers, which can be retarded by the low water content of dryland soil. However, the authors [39] observed lower soil NO₃⁻ in the context of minimum tillage compared to both ploughed and NT treatments due to the decreased mineralization of organic N under minimum tillage compared to CT and reduced accumulation of residues compared to NT. This concurs with the findings for CT-Y5 (minimum tillage) in the current study, which produced the lowest NO₃⁻ concentrations at all depths. The authors of [40] also reported lower soil NO₃⁻ under minimum tillage compared to CT and attributed this to lower levels of nitrification and mineralization, as well as a larger loss of NO₃⁻ through denitrification due to less aerobic soil conditions under minimum tillage compared to CT [41].

The high NO₃⁻ under CT-ANNUAL at 0–10 cm may be attributed to the increased mineralization of organic N during ploughing, while at 60 kg N ha⁻¹, in the sub-surface layer, it may be attributed to the enhanced mineralization of immobilized N because of the incorporation of surface materials. High N application rates of 120 and 240 kg N ha⁻¹ were not beneficial, as they led to reduced NO₃⁻, since they stimulate an increase in the microbial population which can assimilate and immobilize N, and excessive NO₃⁻ is susceptible to leaching losses. The authors of [42] found that CT at a 20 cm soil depth increased NO₃⁻ leaching losses by 21%, mainly because of the enhanced mineralization of SOM. However, minimum tillage increased NO₃⁻ leaching losses following fertilizer application [43], probably because of the greater bypass (macropore) flow in minimum tillage compared to CT, as ploughing disrupts the continuity and connectivity between macropore channels involved in solute transportation.

The high NO₃⁻ levels of NT compared to those of CT-Y5 were attributed to higher microbial activity due to the organic surface residues of NT. This was contrary to the findings of the authors of [44], who found no observable differences in NO₃⁻ accumulation at any depth when the N rate was <90 kg N ha⁻¹. According to [44], excess NO₃⁻ that is not consumed by microbes is assimilated by the crop, fixed on soil exchange sites, denitrified, volatilized and/or immobilized via other pathways. In the current study, the control was characterized by low NO₃⁻ levels compared to 60 kg N ha⁻¹ in most treatments, and this may be attributed to a reduced biomass production due to the low N supply. The authors of [45] also observed greater NO₃⁻ (by 45–66%) in the N-fertilized plots. At a dryland longitudinal research site, the authors of [46] reported an 88% increase in inorganic N after the application of 67 kg N ha⁻¹ fertilizer. Again, the disturbance caused by cultivation did not always increased the soil mineral N level [47], and the fate of any additional mineral N depends on the soil texture, which controls its susceptibility to leaching.

The higher NH₄⁺ under NT and CT-Y5 may be attributed to a high degree of ammonification due to abundant OM substrates and high adsorption of NH₄⁺ by organic soil colloids. The authors of [48] found that the application of N fertilizers contributed to producing large amounts of NH₄⁺, which are eventually converted into ammonia that volatilizes in a gaseous form. However, we cannot explicitly verify the concentration of ammonia that is volatilized, since there no measurements were performed, but the soil conditions, such as high soil moisture and temperature levels, and the accumulation of soil residues favored volatilization under NT. A study by the authors of [44] found no treatment differences in NH₄⁺ accumulation at any depth or N application rate. In the present study, the higher NH₄⁺ levels under NT at low application rates at the 0–10 cm depth are explained by the high organic N that can easily be decomposed to produce available N, whereas at higher application rates, ammonia is volatilized. NH₄⁺ is normally generated by the hydrolysis of inorganic fertilizers and is either adsorbed on clay surfaces or nitrified [49].

N mineralization generally decreased in line with the fertilizer application. Again, it was higher at the 0–10 cm depth compared to the subsurface soil, especially in the control for NT. The authors of [50] also observed high N mineralization at 0–10 cm in NT soil, whereas for CT, it was greater at the 10–20 cm depth, which is similar to the findings of the current study at 240 kg N ha⁻¹. Long-term residue retention in NT systems can increase the size and turnover of microbial biomass and, hence, OM mineralization compared with CT due to improved soil moisture retention induced by surface residues [51]. However, the authors of [38] reported that practices that disrupt the soil structure, such as cultivation, may accelerate N mineralization and nitrification. This is due to their positive effects on the soil porosity, aeration and hydraulic conductivity, and because the physical disruption may bring microbial populations into contact with fresh, previously unavailable organic substrates [38].

CT-Y5 generally produced higher N mineralization for most treatments at 10–20 cm. This might be attributed to the combined effects of residue retention and ploughing over four seasons. The authors of [52] also observed that ploughing after 4–5 years of NT enhanced N mineralization in a comparison of NT and CT. The results also showed that NT produced the least N mineralization at the 20–30 cm depth. According to [53], the tillage effect differs with depth, with the N mineralization of NT soils being 34% higher on the surface layer (0–7.5 cm) than that of ploughed soils, while the opposite was observed at the 7.5–15 cm depth. Cultivation generally leads to a temporary increase in soil mineral N levels and the availability of a larger pool of C substrates that supports greater microbial activity [37]. Without CT, OM and nutrients such as N tend to accumulate at or near to the surface, and this may restrict mineralization in the soil beneath [54]. An oversupply of N, at 240 kg N ha⁻¹, under NT may lead to immobilization, N assimilation and a reduction in N mineralization of the subsurface soil. This is because microbes assimilate the excess N, thus immobilizing it and causing it to become fixed. Thus, excessive N fertilization causes a shift in the decomposer community composition, where microbes with a high N-assimilation efficiency outcompete decomposers [55].

No-till produced higher total P at the 0–10 and 20–30 cm soil depths. This may be attributed to high OM under NT due to the accumulation of crop residues and a lack of soil disturbance. The low P under NT at 10–20 cm may be due to the accumulation of P in senescent roots under CT-Y5 and CT-ANNUAL, because roots can easily be distributed to open pores through tillage. Similar findings were observed by the authors of [17], where the soil under NT had greater P at the 0–5 cm and 15–30 cm depths compared to CT, which produced higher P at the 5–15 cm depth. The authors of [56] observed an accumulation of total P at the surface three to five times greater under NT compared to CT. According to [1], the rate of translocation of P down the soil profile was much less than that of N and K, as P was fixed to mineral and organic compounds, which resulted in its accumulation at the plough layer. However, many field studies have demonstrated that P leaches to subsoil in both coarse and fine-textured soils [16]. Indeed, continuous long-term fertilization, more so than crop requirements, has been shown to result in P leaching in many soils [1]. In a study comparing P levels between irrigated and dryland agriculture, the authors of [57] observed greater concentrations of P under dryland agriculture due to leaching and increased plant growth at irrigated site. The authors of [58] highlighted that greater total soil P at 0–10, but not 15–30, cm under reduced tillage reflected a lower degree of the mixing and distribution of P compared to CT.

An increase in P availability under NT could be attributed to its release from crop residues [59]. The authors of [60], however, observed that crop residues with low P concentrations (because of the translocation of most stubble P into grain), such as cereal stubble, do not make a significant contribution to the soil P levels. The authors of [61] highlighted that the difference in the total P could also be attributed to higher erosive losses under CT compared to NT. In the current study, NT produced higher total P at all application rates and all sampled depths, except at 10–20 cm, which may be attributed to higher ammonium, which fixed P, at the same depths relative to other tillage systems. NH₄⁺ is often superior to NO₃⁻ in stimulating the P uptake because of its effect in transferring P across the root symplast [62]. The uptake of NO₃⁻, on the other hand, causes plant roots to exude anions into the soil solution, balancing the excess negative charge, which can displace the P adsorbed onto soil colloids, therefore enhancing P desorption. The P sorption capacity of soils is governed by the concentrations, types and surfaces of Al and iron oxides, even in calcareous soils [63]. Meanwhile, P’s removal from the soil solution results from its adsorption, precipitation and immobilization [64]. However, P replenishment results from a combination of desorption, dissolution and mineralization [64]. The immobilization of soil P occurs when the total P content of residues is insufficient for meeting the P requirements of the microbial biomass as it proliferates in response to new C substrates [60]. Meanwhile, the availability of organic P adsorbed by microorganisms or enzymes can increase with the increasing residue cover of the soil [65], as well as the exudation of organic anions [66].

A slower decomposition of plant residues may prevent the rapid mineralization of organic P and its translocation through the soil profile, which is more likely to occur when plant residues accumulate on the soil surface under NT because of less aeration and a high soil moisture content. The authors of [67] highlighted that the sorption and stabilization of organic P is greatest in highly weathered soils with high P-fixing capacities because weathering results in the prevalence of 1:1 clay, such as kaolinite, and in Al and Fe sesquioxides that effectively sorb P. This agrees with the current study, since ploughing breaks down soil aggregates and increases the number of soil pores, which in turn may accelerate chemical weathering, causing a high level of organic P to be sorbed under CT-ANNUAL compared to CT-Y5. The sorption and subsequent stabilization of organic P in soil remains to be a major factor in the dynamics of P release from crop residues, causing it to deviate from that of C and N [60]. Furthermore, under CT, there is rapid mineralization of organic P, supplying plant and microbial P, compared to conservation tillage. On the other hand, under NT, organic P in plant residues is decomposed slowly by microbes and has less opportunity to be fixed by soil colloids compared to CT [68]. Thus, high levels of organic P under NT, compared to CT-Y5 and CT-ANNUAL, were observed in the current study, especially in the surface soil. Under NT, organic P remains protected against fixation, whereas in CT, the residues are decomposed, and organic P is adsorbed immediately [68].

In the present study, the level of extractable P was less than both the total and organic P, which concurs with many previous literature findings. Again, the effect of the N application on extractable P at 0–10 and 20–30 cm soil depths was minimal, with visible fluctuations at the 10–20 cm depth. However, the amount of extractable P decreased with depth. This result is similar to the findings of the authors of [17], who observed no effect of N fertilization on extractable P. The high extractable P under NT at the 0–10 cm depth can also be attributed to residue accumulation and low soil disturbance compared to CT-Y5 and CT-ANNUAL. This is because the higher total and organic P under NT at 0–10 cm enabled sufficient levels of available P for the plant within the soil solution. According to [18], reduced tillage appeared to favor P immobilization, while CT favored its mineralization, a finding which is contrary to the current study. Higher mineralization under CT compared to NT was not observed in the current study because of the high OM under NT, which compensated for the higher concentration of mineralized P. Thus, over a longer time, more P was mineralized under NT than under CT due to the sufficient OM supply. In CT, ploughing exposes adsorption sites, thereby increasing the adsorption of P by ligand exchange in comparison to NT [69]. Soluble P accumulation in NT is attributed to a decrease in the adsorption of P onto mineral surfaces due to the fact that there are less adsorption sites and to the release of P from the mineralization of OM [59]. The authors of [17] reported similar findings of greater extractable P under NT in surface soil prior to a rapid decrease with depth. This agrees with current findings, especially at the 20–30 cm depth in the control treatment, where extractable P was extremely low under NT.

OM additions under residue retention can reduce P fixation by increasing organic anion competition for P binding sites [19]. Furthermore, NT improves soil aggregation, which can reduce P fixation by decreasing the outer soil surface area and, therefore, number of potential sorption sites [70]. This shows that P availability is influenced by the physicochemical properties of soil, such as the moisture, organic C, bases and pH, which change with the depth and tillage technique. However, the authors of [17] reported that more extractable P obtained with NT rather than CT was also present below the till-zone, which could be due to the accumulation of P in senescent roots [10]. In a study by the authors of [71], there were no differences in extractable P in the subsoil between tillage systems. The elongation of plant roots under CT-ANNUAL and their decomposition later may increase the levels of available P for plants in the subsoil through P release from the roots. The authors of [72] proposed that the transfer of P can be caused by the leaching of dissolved and colloidal P from the top- to the subsoil, bioturbation by soil-dwelling animals and redistribution due to root activity.

5. Conclusions

Soil tillage influences the magnitude and dynamics of mineralization by altering the distribution of both N and P according to depth. The accumulation of crop residues and minimal soil disturbance under NT enhanced the organic matter and, consequently, led to a high buffering capacity of either plants or microbes to replenish diminished nutrients. The low contents of N and P under CT-ANNUAL was attributed to significant mineralization due to ploughing and the mixing of soil with organic materials and fertilizer, respectively. The interaction between the tillage and N fertilizer under dryland conditions accentuates NT at 60 kg N ha⁻¹, as a management technique that can sustain the soil quality. The findings of this study explicitly show the great difference in N and P pools between different tillage techniques, subsequently leading us to regard NT and optimum N application more highly. It is worth noting that CT-Y5 produced average results compared to NT and CT-ANNUAL, which leaves a great scientific gap for further investigation. Moreover, under-resourced farmers can rely on CT-Y5 to efficiently maximize their production while curbing soil damage, since it does not require expensive and well-advanced equipment and knowledge. A greater weight can be placed upon further investigations if the amount of biomass obtained in each treatment can be incorporated into the findings. CT-ANNUAL, with its high cost, should be avoided, since it has less benefits in terms of soil quality. Therefore, it is crucial to disseminate knowledge about the benefits of the interaction effect of NT and N fertilizer application, especially in the case of dryland agriculture. A high level of nutrient retention and distribution, especially on the surface, indicate its salient impact on the maintenance of soil fertility. Thus, the null hypothesis, which presumed that NT would lead to higher N and P pools at all depths, was accepted. Therefore, NT at 60 kg N ha⁻¹ is mostly highly recommended due to its role in sustaining soil productivity.