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Article

Synergistic Promotion of Particulate and Mineral-Associated Organic Carbon Within Soil Aggregates After 10 Years of Organic Fertilization in Wheat-Maize Systems

by
Jing Li
1,2,
Huijun Wu
2,
Xiaojun Song
2,
Shengping Li
2,
Xueping Wu
2,*,
Ya Han
1,
Zhiping Liu
3,
Na Yang
4,
Ke Wang
4,
Zhiguo Yang
4 and
Jiancheng Zhang
4,*
1
Hebei Center for Ecological and Environmental Geology Research, Hebei GEO University, Shijiazhuang 050031, China
2
State Key Laboratory of Efficient Utilization of Arid and Semi-Arid Arable Land in Northern China, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China
3
College of Resources and Environment, Shanxi Agricultural University, Taiyuan 030031, China
4
State Key Laboratory of Sustainable Dryland Agriculture, Shanxi Agricultural University, Taiyuan 030031, China
*
Authors to whom correspondence should be addressed.
Land 2024, 13(10), 1722; https://doi.org/10.3390/land13101722
Submission received: 12 September 2024 / Revised: 14 October 2024 / Accepted: 19 October 2024 / Published: 20 October 2024
(This article belongs to the Topic Greenhouse Gas Emission Reductions and Carbon Sequestration in Agriculture)
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Versions Notes

Abstract

:
How different fertilization practices modify soil organic carbon (SOC) sequestration is still unclear. Our study aimed to evaluate the changes in SOC stocks and their physical fractions after 10 years of organic and inorganic fertilization. Five treatments were established under a wheat-maize system in Northern China: control (CK), chemical fertilizer (F), straw plus chemical fertilizer (SF), manure plus chemical fertilizer (MF), and straw and manure plus chemical fertilizer (SMF). The results showed that the SOC sequestration rate at 0–20 cm depth decreased in the following order: SMF (1.36 Mg C/ha/yr) > MF (1.13 Mg C/ha/yr) > SF (0.72 C/ha/yr) > F (0.15 Mg C/ha/yr) > CK (−0.25 Mg C/ha/yr). The values indicated that straw returning and manure application were important measures to achieve the “4 per 1000” target, and the application of manure was a more effective strategy. The high input of chemical fertilizer only maintained the initial SOC level and was not a powerful C-farming practice. A minimum input of 4.93 Mg C/ha/yr was required to keep the initial SOC storage. The SOC associated with small macroaggregate (0.25–2 mm) was the most sensitive indicator for the changes of bulk SOC. In addition, the accumulation of SOC under SMF, MF, and SF treatments mainly occurred in the occluded particulate organic C (oPOC) in small macroaggregates, indicating that the physical protection of macroaggregates played a predominant role in SOC sequestration. The SMF, MF, and SF treatments also displayed higher mineral organic C (mSOC) in soil aggregates than the CK and F treatments. A transformation of oPOC towards the mSOC fraction indicated that exogenous C further shifted into stable C pools under the physical protection of soil aggregates. In conclusion, these findings confirmed the important role of straw returning and manure application in SOC accumulation and stabilization, highlighting that a combination strategy of straw + manure + chemical fertilizer had the best effect.

1. Introduction

Soil is the largest carbon (C) reservoir in terrestrial ecosystems. Even minor soil organic C (SOC) changes may dramatically influence atmospheric CO2 emissions [1,2]. The “4 per 1000” initiative highlights the significance of SOC sequestration in mitigating global climate change [3]. The goal set by this initiative is to increase the global SOC stock by 0.4% annually to counterbalance current global anthropogenic CO2 emissions [3]. Additionally, SOC is the core factor directly influencing soil fertility and productivity [4,5]. The SOC sequestration can be divided into two parts: SOC accumulation and SOC stabilization. The former represents the SOC stock, while the latter represents the passive pool storage [6,7]. Both the accumulation and stabilization of SOC are largely dependent on agricultural practices, such as crop rotation [8], tillage method [9], and fertilization practices [10]. It is imperative to elucidate the quantity and potential stabilization mechanisms of SOC sequestration under different agricultural practices.
Fertilization is one of the most important agricultural practices that influence crop yields and soil properties [10]. Chemical fertilizer could enhance stubble residues and underground biomass by promoting crop growth, thereby increasing the C inputs [11,12]. However, chemical fertilizer also enhanced the decomposition of both crop residues and initial SOC by promoting the soil priming effect, and thus, it increased the C outputs [13]. In this sense, the effects of chemical fertilizers on SOC stocks are inconsistent, with positive [12], negative [13], or neutral effects having been reported [14]. Conversely, organic fertilizers (e.g., manure or straw) have greater effects on SOC than chemical fertilizers and are considered key strategies for increasing SOC stocks [15,16,17]. Organic fertilizers directly increase exogenous C inputs, thereby improving soil physicochemical properties and microbial community characteristics, which further promotes transfer rates of exogenous C to SOC [11]. However, the results of SOC promotion by organic fertilizers varied greatly depending on the types of organic materials [18], soil textures [19], crop rotation systems [19], and climatic conditions [20]. Therefore, the effects of different fertilization practices on SOC sequestration should be reevaluated in specific soil-crop-environmental systems.
Fertilization practices can influence SOC sequestration by changing the mass distribution of soil aggregates and the turnover of aggregate-associated C [6,14]. Aggregate protection is recognized to be the key mechanism of SOC stabilization and sequestration [21,22]. A total of 90% of soil SOC is stored in different-sized aggregates [21]. In general, the C associated with macroaggregates (>0.25 mm) is more susceptible to management practices, while C associated with microaggregates (<0.25 mm) is more stable, and the C is retained for longer periods than in macroaggregates [23,24,25]. Kamran et al. reported that green manure enhanced soil macroaggregate formation and aggregate stability [23]. Sodhi et al. indicated that the manure applied with chemical fertilizers improved the C concentrations in aggregates, with a higher degree in water-stable macroaggregates than in microaggregates [26]. The investigation of soil aggregation and aggregate-associated C is essential for understanding the physical protection mechanisms across different fertilization practices.
Particulate organic C (POC) and mineral-associated soil organic C (mSOC) have been frequently used to explore the mechanisms and dynamics of aggregate-associated SOC stabilization [27,28,29]. The POC is determined by the inputs of organic materials and the turnover of soil aggregates [7,30]. However, the mSOC is recognized as a ‘stabilized pool’ and is determined by the physical protection of aggregates and the chemical adsorption of minerals fraction [31,32,33]. Gunina et al. reported that 20% of C was accumulated as the POC fraction, while 80% was in the mSOC fraction in cropland soil [33]. Previous investigations reported that organic amendments increased POC and mSOC contents by increasing aggregate formation compared to chemical fertilizers [34,35]. However, several investigations indicated that manure or crop residues did not increase the mSOC when the soils reached C saturation [23,36]. Thus, more investigations regarding these two SOC categories are needed to deeply understand the effects of fertilization strategies on long-term SOC stabilization.
The wheat-maize rotation covers approximately 16 Mha in Northern China, contributing to approximately 25% of the total national grain production [37,38]. Traditional agricultural management, such as frequent tillage, excessive application of inorganic fertilizers, and straw burning, have badly affected the soil quality [39], crop productivity [40], and the agricultural environment [41,42]. In response, the government has advocated the utilization of manure and straw as organic fertilizers to promote sustainable agricultural practices in the region [14,43]. Previous researches demonstrated that manure or straw effectively stimulated SOC accumulation [14,24]. However, it remains unclear how different exogenous organic materials and combined inorganic fertilization application methods result in SOC storage and different SOC fractions in the wheat-maize system. Thus, this study aimed to (1) assess whether organic or inorganic fertilization increases SOC stocks at a rate of 0.4% and (2) assess the effects of organic or inorganic fertilization on SOC contents in soil aggregate and in aggregate-associated density fractions. We hypothesize that (1) the appropriate application of organic fertilizers is an effective strategy to enhance SOC sequestration, and (2) the physical C fractions can provide important information for accessing the quantity and stability of SOC sequestration resulting from the application of organic fertilizers.

2. Materials and Methods

2.1. Field Site Description

The field experiment was carried out at the Agricultural Experimental Farm of Shanxi Agricultural University in Yuncheng City (35°11′ N, 111°05′ E), Shanxi Province, China (Figure 1). This area exhibits features of a typical temperate continental monsoonal climate (with a temperature of 13.3 °C and 525 mm precipitation). The soils are formed from the loess-like parent material and classified as Calcaric Cambisol (IUSS Working Group WRB, 2015) with a silty loam texture (28.0% sand, 54.5% silt, and 17.5% clay). The initial surface soil (0–10 cm) contained 8.2 g/kg of SOC, 1.2 g/kg of total nitrogen (TN), along with a 1.4 g/cm3 bulk density (BD), 8.2 pH, and 65.0 g/kg of CaCO3.

2.2. Experimental Design

The in situ experiment was started in 2007 with five treatments: control (CK), chemical fertilizer (F), straw plus chemical fertilizer (SF), manure plus chemical fertilizer (MF), and straw and manure plus chemical fertilizer (SMF). Every treatment was replicated three times, and the replication plot was 60 m2. Wheat (Triticum aestivum L.) grows in winter, and maize (Zea mays L.) grows in the summer season. Chemical N fertilizer was applied to each crop at 225 kg/ha/yr, and phosphate fertilizer (P2O5) was applied at 74.25 kg/ha/yr. Composted chicken manure was added to the soil at a rate of 9 t/ha/yr. Before wheat sowing, the maize straw was returned to the soil in SF and SMF, and the straw was removed manually from CK, F, and MF. The rotary tillage was applied to mix the manure and straw to the soil at a 0–10 cm depth. Additional information regarding these procedures can be found in our previous publication [11]. Annual amounts of total C and N inputs for different treatments are presented in Table 1, and the calculation is presented in Supplementary Methods S1.
In March 2016, soil samples were collected from the 0–10 and 10–20 cm layers. In every plot, five random cores were chosen and mixed into one. Fresh samples were placed in rigid plastic containers, taken to the laboratory, and crushed along natural crevices.

2.3. Aggregate Sieving and Physical Fractionation

The dry-sieving method with “optimal moisture” was performed to obtain soil aggregates. Dry sieving was preferable because it avoided the loss of dissolved C, fine particles, POC, and their redistribution across aggregates [44]. Soil samples (10–12% moisture) at the 0–10 cm depth were manually shaken on sieves with 2 mm and 0.25 mm mesh openings. Three sizes of aggregates were obtained using the above sieving procedure. The >2 mm ones were named by large macroaggregates (LM), the 0.25–2 mm ones were small macroaggregates (SM), and <0.25 mm ones were microaggregates (MI).
The three groups of aggregates were separated by a density fractionation method [45]. Specifically, the fPOC was isolated by 1.6 g/cm3 of a sodium polytungstate (SPT) solution, aspirated by vacuum suction, and collected on a Millipore unit with a 0.45 µm filter. The oPOC was isolated by 2.0 g/cm3 of SPT. The heavy fraction was dispersed and sieved through a 53 μm mesh. These steps isolated two fractions: mSOC>53 and mSOC<53. The processes used to isolate the fPOC, oPOC, mSOC>53, and mSOC<53 fractions are illustrated in Figure 2.

2.4. SOC Analysis and Calculations

The SOC concentrations of bulk soils and aggregates, as well as the density and size fractions, were determined using an element analyzer (Vario MACRO Elementar, Germany). Prior to the organic C analysis, inorganic C was removed from the samples by digestion in 1.0 mol/L of HCl for 24 h [46].
The SOC stock (SOCstock, Mg C/ha), annual SOC sequestration rate (SOCSR, Mg C/ha/yr), annual SOC increase rate (SOCIR, % per year), and SOC content of the density and size fractions (SOCD, g/kg aggregate) was calculated as follows:
SOCstock = SOCc × BD × H × 10
where SOCc is the SOC concentration (g/kg), BD is the bulk density (g/cm3), H is soil depth (m), and 10 is a factor to adjust the units.
SOCSR = (SOCstock-2016 − SOCstock-2007)/10
SOCIR = (SOCstock-2016 − SOCstock-2007)/SOCstock-2007/10 × 100
where SOCstock-2016 (Mg C/ha) and SOCstock-2007 (Mg C/ha) are the SOCstock in 2016 and 2007, respectively, and 10 is the experiment duration (year).
SOCD = CD × MD
where CD is the C concentration of the density and size fractions (g/kg fraction), and MD represents the mass percentage of density and size fractions in particular aggregate particles (% aggregate).

2.5. Statistical Analysis

A two-way ANOVA with Duncan’s multiple range test was employed to assess the significant effects of fertilizer treatments, aggregate sizes, and their interaction on parameters. Linear regression models were performed to explore relationships between the C input and SOCSR and between the bulk SOC and its fractios. All statistical analysis was performed using R 4.2.0. The cluster heatmap was conducted to assess the similarities between fertilizer treatments and SOC parameters. The cluster heatmap was performed using the package “pheatmap”.

3. Results

3.1. SOC Sequestration

Compared to CK, the SOC concentrations were significantly increased by 133.56%, 113.17%, and 70.42% in the SMF, MF, and SF at the 0–10 cm depth and by 35.65%, 21.06%, and 29.45% at the 10–20 cm depth (p < 0.05; Table 2). Similarly, the SOCstock was higher in the SMF, MF, and SF than in the CK and F, with increases of 74.45%, 63.79%, and 44.96% relative to the CK, and 46.74%, 37.78%, and 21.94% relative to F across the entire 0–20 cm depth soils.
The SOCSR under the SMF, MF, and SF was 1.36, 1.13, and 0.72 Mg C/ha/yr, respectively (Table 2). The F also increased the SOC by a rate of 0.15 Mg C/ha/yr. However, the CK reduced the SOC by 0.25 Mg C/ha/yr. A positive linear association was shown between the SOCSR and annual C input (R2 = 0.83 **, p < 0.01) (Figure 3). The fitting equation indicated that a 4.93 Mg C/ha/yr input was necessary to maintain the initial C level.
According to the “4 per 1000” initiative, the SOCIR (%) was calculated at a depth of 0–20 cm (Table 2). The SMF, MF, and SF increased the SOC by 5.63%, 4.47%, and 2.97% per year, respectively. The F also increased the SOC at a rate of 0.64% per year.

3.2. Aggregate Size Distribution and Aggregate-Associated SOC

Dry-sieving results showed that SM (0.25–2 mm) fractions were the most abundant (52.38–58.71%), while MI (<0.25 mm) fractions were the least abundant (11.37–22.96%) in the soils (Figure 4a). The mass percentages of SM fractions were decreased in the following order: CK > F > SMF > SF > MF. Correspondingly, lower proportions of MI fractions were found in unfertilized CK soils than in other treatments.
According to the two-way ANOVA results, both fertilizer treatments and aggregate sizes exerted significant effects on the aggregate-associated SOC (p < 0.05; Figure 4b). The SOC in the SM fractions was higher than in aggregates in the other two size classes. In comparison to the CK, the SMF, MF, and SF treatments resulted in increases in the SOC of 106.72%, 62.02%, and 59.33% in LM fractions, 151.76%, 122.76%, and 82.73% in SM fractions, and 134.86%, 71.57%, and 68.01% in MI fractions, respectively. No significant differences in the SOC in aggregates were found between the F and CK.
The aggregate-associated SOC was more strongly related to annual C input than the mass percentages of aggregates (Figure 4c,d). The increase in C input significantly increased the aggregate-associated SOC in all size levels (p < 0.05; Figure 4d). Moreover, the SOC associated with the SM fractions was more sensitive to annual C input than those in the LM and MI fractions.

3.3. Intra-Aggregate SOC Fractions

In general, the SOCD (g/kg aggregate) of all density fractions was higher in macroaggregates than in microaggregates (Figure 5). In the LM fractions, the SOCD followed in the order of mSOC<53 > oPOC > mSOC>53 > fPOC, contributing 37.58%, 28.63%, 22.98%, and 10.82% to the total aggregate-associated SOC, respectively. In the SM fractions, the SOCD was in the order of oPOC > mSOC<53 > mSOC>53 > fPOC, contributing 39.75%, 31.15%, 15.90%, and 13.21% to the aggregate-associated SOC, respectively. In the MI fractions, the SOCD was in the order of mSOC<53 > oPOC > mSOC>53 > fPOC, contributing 43.79%, 36.26%, 12.89% and 7.06% to the aggregate-associated SOC, respectively.
Organic fertilizer treatments significantly increased the SOCD of all density fractions (p < 0.01, Figure 5). Specifically, the fPOC, oPOC, mSOC>53, and mSOC<53 contents were 270.10–591.63%, 171.01–205.87%, 160.72–354.60%, and 19.25–56.83% higher in the SF, MF, and SMF than in the CK, respectively. The highest values of the SOCD were observed in oPOC-SM under the MF treatments. Chemical fertilization only significantly increased mSOC>53 in SM fractions compared to CK (p < 0.01).

3.4. Relationships Among SOC Fractions

The bulk SOC was positively correlated with the aggregate-asscoated SOC (p < 0.05, Figure 6a). The SOC in the SM fraction had the steepest slope value of regression equations (1.09, R2 = 0.93 **), followed by the MI fraction (0.88, R2 = 0.91 **), and the LM fraction (0.77, R2 = 0.92 **), suggesting that SOC associated with SM fraction was most sensitive to fertilizer practices. The bulk SOC was also positively correlated with the SOCD of all density fractions within aggregates (p < 0.05, Figure 6b–d). The oPOC within the SM fractions had the highest slope value (0.55, R2 = 0.63 **), indicating that oPOC within SM was the main form of SOC accumulation.
The clustering analysis indicated that the CK and F were categorized together, and the SF, MF, and SMF were categorized together, demonstrating that non-organic fertilizers and organic fertilizers had different effects on SOC fractions (Figure 7). The SOC and oPOC-SM were clustered together, indicating that sequestered SOC in the bulk soil was mainly stored in SM fractions in the form of oPOC. The oPOC was linked with mSOC<53 within the SM and MI fractions, reflecting the distinguished transfer bridge between oPOC and mSOC<53.

4. Discussion

4.1. Organic Fertilization Increases SOC Sequestration

Our research showed that straw returning and manure application increased SOC stocks (Table 2). From a former investigation, Li et al. noted that an annual 0.7% increase in top soils (0–15 cm) was equivalent to a 0.4% increase in deeper soil profiles (0–40 cm) [14]. Here, straw returning combined with chemical fertilization increased SOC with a rate of 2.97% per year (0.72 Mg C/ha/yr) in 0–20 cm soils, indicating that continuous (10 years) straw returning enhanced SOC sequestration in the wheat-maize system. A meta-analysis indicated that straw returning increased SOC content by 16.1% in China compared to straw removal, and the Northern region had higher increase rates than the Southern region [47]. Wang et al. noted that SOC improvement effects of straw returning were more obvious under conditions with moderate chemical N input and deep tillage [48]. The manure application with chemical fertilizer or straw plus chemical fertilizer increased SOC by 4.47–5.63% (1.13–1.36 Mg C/ha/yr). Similarly, Li et al. reported that the SOC increased at 5.4–10.1% (0.89 to 1.66 Mg C/ha/yr) after 9 years of manure application in the wheat-maize system [14]. This indicated that manure application might be more feasible in sequestering SOC than straw returning. This aligned with previous studies, in which the quality of organic materials resulted in variations in SOC sequestration, and manure (low C/N ratio) increased soil storage more efficiently than plant residues [49,50,51,52]. High inputs of chemical fertilizer resulted in an increase in SOC stock with a rate of 0.64% (0.15 Mg C/ha/yr) in 0–20 cm (Table 2). The application of chemical fertilizer only enhances C inputs by promoting crop growth [53,54] while increasing the decomposition of SOC by increasing microbial N availability [55]. In summary, our results suggested the application of manure and/or straw in the wheat-maize system could be feasible for achieving the 0.4% target, while the sole application of chemical fertilizer only maintained the initial SOC level and was not a powerful C-farming practice.
We observed a significant linear correlation between the SOCSR and the C input (Figure 3), indicating the studied soils were not C-saturated [14]. This was similar to the findings of Li et al., who also discovered a linear relationship between SOCSR and the C input in silt loam soils in northern China (R2 = 0.674 **, p < 0.01). Long-term experiments at the Lausanne Station reported that SOC still grew at a rate of 0.7% after 40–60 years of continuous application of organic fertilizer [56]. Georgiou et al. further pointed out that agricultural soils showed a large undersaturation of mineral-associated C and a high capacity to store C [57]. The regression equation also indicated that a C input of 4.93 Mg C/ha/yr was necessary to maintain SOC stock (Figure 3). This value was higher than that reported by Kong et al. [58] (3.2 Mg C/ha/yr), Wang et al. [59] (1.17 Mg C/ha/yr), Xiang et al. [13] (1.4 Mg C/ha/yr) and by Fan et al. [43] (2.04 Mg C/ha/yr). The soil at our study site, which was derived from the loess material, had a higher sand plus silt content than those developed from other sediments. Several studies have indicated that soil with a light texture generally exhibited low transformation rates from organic materials to soil organic matter [14,24]. Additionally, frequent tillage in the double cropping system could result in a higher C decomposition rate [60]. These factors suggested that more C input was required to keep the initial SOC level in this study.

4.2. Size Distribution of Soil Aggregates and Aggregate-Associated SOC Under Different Fertilization Strategies

The dry-sieved SM fractions (0.25–2 mm) were predominant in soils (52.38–58.71%) for all fertilization treatments (Figure 4a), in line with those of Xiao et al., who also found smaller macroaggregates (0.25–1 mm) were dominated in cropland soils, while larger macroaggregates (2–10 mm) prevailed in forest or grassland soils [61]. Frequent tillage management was observed to disintegrate large macroaggregates in cropland soils [62]. Multiple studies have reported that organic fertilizers enhanced the mass proportion of macroaggregates compared to inorganic fertilizers [23,50,63]. However, the long-term application of organic fertilizers did not promote the mass proportion of macroaggregates in our study (Figure 4a). This might be due to the fact that CaCO3 was the primary binding agent for aggregation rather than organic matter in loessial soils [37]. Huang et al. observed that CaCO3 concentration in the surface soils decreased following 10 years of application of non-organic and organic fertilizers [64]. Moreover, Li et al. reported that simultaneous addition of Na+ stemmed from manure might disperse soil aggregates [14]. In addition, drying-wetting cycles accelerated macroaggregate formation [65]. The drying-wetting cycles might be pronounced in unfertilized soils due to the lack of enough residues and manure on the topsoil. Finally, the reduced number of macroaggregates resulting from lower CaCO3 levels, higher Na+ contents, and weaker drying-wetting cycles largely offset the increased number of macroaggregates due to greater C inputs under organic fertilization treatments.
Notably, the aggregate-associated SOC was greater under the SMF, MF, and SF compared to the F and CK, and a greater SOC was observed in macroaggregates than in microaggregates (Figure 4b). This was in line with previous publications [6,26,66]. One explanation could be that macroaggregates were formed through the cementing effect of labile organic matter, such as fine roots, plant residues, manure particles, and fungal hyphae [22,67]. Another explanation could be that macroaggregates provided greater physical protection for organic C compared to microaggregates [68]. Taking into account the high abundance of SM fractions, we can confidently conclude that SM fractions were the major reservoir of SOC; in other words, C dynamics inside would play crucial roles in regulating SOC sequestration under organic fertilization.

4.3. Organic Fertilization Increased SOC Contents of Physical Fractions in Aggregates

Soil POC was sensitive to global change and agricultural management practices and could be used as a short-term diagnostic indicator of SOC dynamics [30]. Overall, the increases in POC from organic fertilizers were greater than those of mSOC (Figure 5). The oPOC in aggregates was generally considered to be a physically protected organic C fraction [45,69]. Compared to the control, the solo chemical fertilizer had no significant change in the oPOC, while chemical fertilizer plus straw and/or manure significantly increased the oPOC in aggregates (Figure 5b). The promoted oPOC could be mainly attributed to the direct effects of straw and manure input and the indirect effects of promoted crop yields [50]. Some studies also reported that organic amendments promoted the formation of microaggregates within macroaggregates, thus accelerating the accumulation of new oPOC [22,24]. The highest oPOC was observed in manure treatments (Figure 5b). Mustafa et al. noted that manure provided a lot of different-sized organic particles, thus directly participating in the turnover of soil aggregation [35]. Compared to plant residues, manure had more lignin and was difficult to decompose by microorganisms, directly improving soil POC storage [52]. In addition, recent studies reported that manure provided more N and P to the soil, thereby alleviating the nutrient limitation of microorganisms and reducing the decomposition of SOC within the aggregates [51]. The improvement of physical protection C indicated a greater SOC sequestration efficiency of manure [52]. Moreover, our results found that oPOC in the SM fractions explained the largest variants of bulk SOC (Figure 6). Similarly, Huang et al. noted that the microaggregate-associated C occluded in macroaggregates determined SOC levels in the wheat-maize system [24]. This indicated that the physical protection of macroaggregates played a predominant role in SOC sequestration under organic fertilization.
The mSOC represented a biochemically protected C pool and was important for SOC stability [70]. Organic fertilizers increased mSOC (mSOC>53 and mSOC<53) compared to CK across all aggregate fractions (Figure 5c,d). Similarly, Wang et al. also demonstrated that manure application plus chemical fertilizer increased the C storage of mineral organic matter by 25.6–45.7% in soils of the Loess Plateau [34]. The higher mSOC contents might be attributed to the greater C transferred from POC, as indicated by the hierarchical dendrogram heatmap analysis (Figure 7). The newly organic C in the soil first turned into physically protected C and eventually became mineral-associated C with a higher degree of stability [44]. A 13C labeled litter incubation experiment showed that POC could act as a functional substrate for persistent SOC [71]. An increasing number of studies proved that microbial C use efficiency and microbial necromass played essential roles in mSOC [72,73,74]. Xiao et al. indicated that the application of organic fertilizers improved microbial C use efficiency by neutralizing pH and alleviating microbial N limitation, thus facilitating soil C stabilization [74]. In addition, solo chemical fertilizer only significantly increased mSOC>53 in SM fractions compared to CK (p < 0.05, Figure 5c) and had no significant effects on other C fractions. Zhang et al. demonstrated that the addition of N to low-fertility soils increased soil microbial community diversity and the formation of mSOC [75]. However, Lan et al. showed that NPK treatment reduced the alkyl C content and significantly increased the carboxyl C content of mSOC compared to CK treatment [76]. This suggested that the mSOC increased by chemical fertilizers might not be as stable as expected [77].

5. Conclusions

Our research clarified the effects of long-term incorporation of organic fertilizers on SOC sequestrate rates, aggregate-associated SOC, and physical C fractions, providing useful knowledge for evaluating C-farming practices in the wheat-maize system. Straw returning and manure application were important measures to achieve the “4 per 1000” target, and the manure application was a more effective strategy. The high input of chemical fertilizer only maintained the initial SOC level and was not a powerful C-farming practice. The organic fertilizers increased all aggregate-associated SOC compared to control or chemical fertilizers. In addition, the SOC associated with small macroaggregate (0.25–2 mm) was the most sensitive indicator for the changes of bulk SOC. The accumulation of SOC under organic fertilization mainly occurred in the oPOC fraction in small macroaggregates, indicating that the physical protection of macroaggregates played a predominant role in SOC sequestration. Organic fertilization also increased the mSOC fraction in soil aggregates. A transformation of oPOC towards the mSOC fraction indicated that occluded POC further shifted into stable C pools under the physical protection of soil aggregates. Based on these results, we recommend a combined strategy of straw + manure + chemical fertilizer as the optimal strategy for enhancing SOC sequestration in the wheat-maize system. Further studies are needed to ascertain SOC sequestration in deep soils.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/land13101722/s1. References [11,53,78,79] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, J.L, H.W. and X.W.; methodology, H.W., X.S. and S.L.; software, J.L.; validation, Y.H. and N.Y.; formal analysis, Z.L.; investigation, K.W. and Z.Y.; resources, J.Z.; data curation, Y.H.; writing—original draft preparation, J.L.; writing—review and editing, H.W. and X.S.; visualization, S.L.; supervision, X.W.; project administration, N.Y.; funding acquisition, X.W. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Natural Science Foundation of Hebei Province (D20224030002); the National Key Research and Development Program of China (2023YFD1500301); the Open Project of State Key Laboratory of Efficient Utilization of Arid and Semi-arid Arable Land in Northern China (EUAL-2023-07; EUAL-2023-09); and the PhD Research Startup Foundation of Hebei GEO University (BQ2024053).

Data Availability Statement

All the data are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Land 13 01722 g001
Figure 1. Location of the study site.
Figure 1. Location of the study site.
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Figure 2. Procedure of size and density fractionation.
Figure 2. Procedure of size and density fractionation.
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Figure 3. Relationships between annual C input and SOCSR. ** indicates p < 0.01.
Figure 3. Relationships between annual C input and SOCSR. ** indicates p < 0.01.
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Figure 4. Mass proportion (a), aggregate-associated SOC (b), and relationships between annual C input and mass proportion (c) and SOC (d) of aggregates after 10 years of fertilization. Error bars represent the standard deviation (n = 3). Different lowercase letters indicate the significant differences between treatments in same aggregate fraction at p < 0.05. * indicates p < 0.05 and ** indicates p < 0.01.
Figure 4. Mass proportion (a), aggregate-associated SOC (b), and relationships between annual C input and mass proportion (c) and SOC (d) of aggregates after 10 years of fertilization. Error bars represent the standard deviation (n = 3). Different lowercase letters indicate the significant differences between treatments in same aggregate fraction at p < 0.05. * indicates p < 0.05 and ** indicates p < 0.01.
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Figure 5. Contents of fPOC (a), oPOC (b), mSOC>53 (c), mSOC<53 (d) in aggregates after 10 years fertilization. Error bars represent the standard deviation (n = 3). Different lowercase letters indicate the significant differences between treatments in same aggregate fraction at p < 0.05.
Figure 5. Contents of fPOC (a), oPOC (b), mSOC>53 (c), mSOC<53 (d) in aggregates after 10 years fertilization. Error bars represent the standard deviation (n = 3). Different lowercase letters indicate the significant differences between treatments in same aggregate fraction at p < 0.05.
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Figure 6. Regression relationships between bulk SOC and aggregate-associated SOC (a), and SOC fractions in LM (b), and SOC fractions in SM (c), and SOC fractions in MI (d). * indicates p < 0.05 and ** indicates p < 0.01.
Figure 6. Regression relationships between bulk SOC and aggregate-associated SOC (a), and SOC fractions in LM (b), and SOC fractions in SM (c), and SOC fractions in MI (d). * indicates p < 0.05 and ** indicates p < 0.01.
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Figure 7. The double hierarchical dendrogram heatmap based on the correlation matrix of the soil SOC fractions. The yellow colors represent a positive (+), and the blue colors represent a negative (–) correlation.
Figure 7. The double hierarchical dendrogram heatmap based on the correlation matrix of the soil SOC fractions. The yellow colors represent a positive (+), and the blue colors represent a negative (–) correlation.
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Table 1. The annual total C and N inputs into soils under different fertilization strategies.
Table 1. The annual total C and N inputs into soils under different fertilization strategies.
TreatmentC input (Mg C/ha/yr)N input (Mg N/ha/yr)
ManureStrawRootTotal C InputChemical FertilizerManureStrawRootTotal N Input
CK02.11.63.7000.020.030.05
F03.32.45.70.4500.030.040.52
SF06.82.59.30.4500.090.040.59
MF4.23.62.610.40.450.360.030.050.89
SMF4.27.42.814.40.450.360.100.050.96
Table 2. SOC concentration, SOCstock, SOCSR, and SOCIR at 0–20 cm depth in soil after 10 years fertilization.
Table 2. SOC concentration, SOCstock, SOCSR, and SOCIR at 0–20 cm depth in soil after 10 years fertilization.
Layer (cm)TreatmentSOC Concentration (g/kg)SOCstock (Mg C/ha)SOCSR (Mg C/ha/yr)SOCIR (%)
0–10CK7.44 (0.75) c10.96 (1.08) c
F9.03 (0.46) c13.16 (0.61) c
SF12.68 (1.24) b17.10 (2.29) b
MF15.86 (2.90) ab22.26 (3.63) ab
SMF17.37 (3.65) a22.69 (5.14) a
10–20CK7.2 (0.38) c10.66 (0.67) c
F8.37 (0.24) bc12.53 (0.33) b
SF9.32 (0.03) ab14.24 (0.44) ab
MF8.72 (0.5) ab13.14 (0.68) ab
SMF9.77 (1.05) a15.01 (1.56) a
0–20CK 21.61 (1.11) c−0.25 (0.11) c −1.05 (0.32) c
F 25.7 (0.91) c0.15 (0.09) c 0.64 (0.42) c
SF 31.33 (1.9) b0.72 (0.19) b 2.97 (0.63) b
MF 35.40 (2.95) ab 1.13 (0.29) ab 4.47 (1.36) ab
SMF 37.71 (3.63) a 1.36 (0.36) a 5.63 (1.71) a
Different lowercase letters indicate that significant differences between treatments at p < 0.05.
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Li, J.; Wu, H.; Song, X.; Li, S.; Wu, X.; Han, Y.; Liu, Z.; Yang, N.; Wang, K.; Yang, Z.; et al. Synergistic Promotion of Particulate and Mineral-Associated Organic Carbon Within Soil Aggregates After 10 Years of Organic Fertilization in Wheat-Maize Systems. Land 2024, 13, 1722. https://doi.org/10.3390/land13101722

AMA Style

Li J, Wu H, Song X, Li S, Wu X, Han Y, Liu Z, Yang N, Wang K, Yang Z, et al. Synergistic Promotion of Particulate and Mineral-Associated Organic Carbon Within Soil Aggregates After 10 Years of Organic Fertilization in Wheat-Maize Systems. Land. 2024; 13(10):1722. https://doi.org/10.3390/land13101722

Chicago/Turabian Style

Li, Jing, Huijun Wu, Xiaojun Song, Shengping Li, Xueping Wu, Ya Han, Zhiping Liu, Na Yang, Ke Wang, Zhiguo Yang, and et al. 2024. "Synergistic Promotion of Particulate and Mineral-Associated Organic Carbon Within Soil Aggregates After 10 Years of Organic Fertilization in Wheat-Maize Systems" Land 13, no. 10: 1722. https://doi.org/10.3390/land13101722

APA Style

Li, J., Wu, H., Song, X., Li, S., Wu, X., Han, Y., Liu, Z., Yang, N., Wang, K., Yang, Z., & Zhang, J. (2024). Synergistic Promotion of Particulate and Mineral-Associated Organic Carbon Within Soil Aggregates After 10 Years of Organic Fertilization in Wheat-Maize Systems. Land, 13(10), 1722. https://doi.org/10.3390/land13101722

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