Rebuilding soil organic carbon (SOC) on agricultural lands plays a pivotal role in mitigating climate change and improving soil function, yet its accumulation is constrained by both management practices and inherent soil properties. Scientists and farm advisors recommend that farmers plant cover crops, reduce tillage, and add organic amendments to increase SOC, yet the effectiveness of practices intended to improve soil health may be limited by underlying edaphic controls such as mineralogy, texture, and pH. Given that SOC consists of two distinct fractions – particulate organic matter (POM) and mineral-associated organic matter (MAOM) – which differ in their stability and response to management, a critical question emerges: How do soil health practices compare to edaphic factors in influencing carbon fractions and overall stocks? Despite extensive research in controlled field settings, real-world farming contexts remain less understood, limiting our ability to predict SOC gains across diverse soil conditions. Here, we evaluate how in-season and recent (< 5 years) implementation of soil health management systems on working farms relates to SOC fractions and stocks across 28 organic fields growing leafy greens in the Central Coast of California. Continuous living cover was associated with higher free particulate organic matter (fPOM; +3.16 Mg C ha−1 between representative high- and low-cover sites, 95 % CI: 0.82–5.92), with weaker, borderline associations with MAOM carbon concentration (p=0.07–0.11 depending on model specification) and total carbon stock (p=0.068). Reduced tillage showed a similar pattern, with a clear association with fPOM (+1.58 Mg C ha−1, 95 % CI: 0.39–2.93) and a weaker association with MAOM (p=0.07–0.09). Crop diversity was associated with higher occluded POM (+1.35 Mg C ha−1, 95 % CI: 0.11–2.53). We found no association between organic amendments and any measured carbon pool. On average, management variables explain 3.7 times more variance than edaphic variables across carbon fractions, while edaphic variables explained roughly 2.1 times more variance for total carbon stock. These findings suggest soil health management is most clearly linked to particulate carbon accumulation, with more tentative evidence for mineral-associated carbon and total stock, across a wide range of edaphic conditions on working organic farms. We studied how soil-building practices like cover cropping, reduced tillage, and crop diversity affect soil carbon on farms in California. We find that maintaining living plants throughout the year and reducing tillage can boost multiple forms of soil carbon with distinct functions and longevity. While soil type matters, how the land is managed often had a greater effect. This suggests that improving farming practices can meaningfully increase carbon in the soil and draw down atmospheric carbon.
In the last decade, the conceptual framework that characterizes soil organic carbon (SOC) into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) fractions has gained traction in studies of C dynamics. This SOC characterization is useful for developing empirical studies and for parsimonious model parameterizations. However, rigorous testing of model structures incorporating the POC-MAOC framework is still lacking, particularly tests evaluating whether this framework can adequately reproduce simultaneous measurements of changes in C pool contents and respiration fluxes. We conducted an incubation experiment using control and litter-addition treatments, measuring changes in SOC fraction contents and respiration fluxes throughout the incubation. Then, we applied an inverse modelling approach to compare the performance of 2-pool (POC-MAOC) and 3-pool models (which also included a faster-cycling litter C pool) to reproduce the observed data. We then calculated the C ages and transit times to explore the predicted C persistence. Finally, we performed simulations to evaluate the effects of different model structures and parameterizations on SOC persistence. For both treatments, we observed that 2-pool models were unable to simultaneously reproduce the changes in C pool contents and respiration, while the 3-pool models adequately predicted both variables and yielded lower C ages and transit times. 2-pool models collapsed POC dynamics operating across different timescales into a single one, failing to capture the distinct respiration phases and gradual C pool changes. Instead, 3-pool models distributed these processes among compartments: the Litter C pool captured fast-cycling dynamics, allowing POC and MAOC to better represent intermediate- and long-term dynamics, respectively. The fact that 3-pool models outperformed 2-pool models -even in control soils- indicates that, in our soils, POC is a heterogeneous pool that cannot be adequately represented as a single compartment. We also found that both model structure and changes in key parameters affected C persistence estimations: models that included shorter pathways to MAOC, or allowed faster transfers into more persistent pools, predicted higher C age and transit time, showing how model structure shapes SOC contents and persistence estimates. This study highlights that the POC-MAOC framework, which frames SOC dynamics using only two time scales, may not always be sufficient to fully characterize SOC processes. Rather than advocating for a specific model configuration, we argue that the conceptual simplification of soil C into POC and MAOC might fail to capture the multiple timescale responses frequently observed in experimental studies. Furthermore, as transfer rates play a key role in determining SOC persistence, it is important to better understand and quantify how C is transferred toward MAOC and how these processes can be represented in models.
Intensive agricultural practices have degraded soil fertility and polluted natural resources in Spanish citrus orchards, highlighting the need for more sustainable management strategies. Composting rice straw (RS) and sewage sludge (SS), two residues that are difficult to manage in Mediterranean regions, offers an environmentally sound alternative for residue valorisation and soil fertility restoration. This study assessed the agronomic performance of two industrial-scale composts produced from pruning residues (PR)/SS and RS/SS in a commercial Mediterranean citrus orchard. The effects of compost application at two rates were evaluated through analyses of soil physical, chemical, and biological properties, as well as foliar nutrient concentrations, yield, and fruit quality. RS/SS compost contained higher nutrient levels, particularly P, suggesting its potential as a P-rich organic fertiliser. Compost application reduced soil pH and slightly increased electrical conductivity without exceeding critical thresholds. Soil organic matter, organic N, available P and K, and Zn contents generally increased following compost application, while improvements in soil biological indicators were mainly observed during the first growing season. Effects on foliar nutrient status, yield, and fruit quality were limited. These results indicate that industrial-scale RS/SS compost application represents a sustainable strategy for residue valorisation and soil fertility improvement in Mediterranean citrus systems. Moreover, successive compost applications improved several soil fertility indicators and support the partial replacement of mineral P fertilisation under similar conditions.
Degradation of soil structure and function is a significant obstacle to sustainable agriculture, yet using biochar is known to be an effective way of increasing soil organic carbon and boosting structure. Aggregate stability is greatly influenced by soil internal forces (SIFs), such as van der Waals, hydration, and electrostatic forces. However, it is still not clear which specific organic carbon components play a significant role in the stability of aggregates and the action of soil internal forces. In this study, a field simulation experiment was conducted using maize straw and wheat straw biochars to investigate how biochar addition affects soil aggregate stability. The findings showed that as the rate of biochar application increased from 0 % to 10 % over 180 and 365 d, the contents of soil light fraction organic carbon (LFOC), particulate organic carbon (POC), dissolved organic carbon (DOC), readily oxidized organic carbon (ROC), as well as soil specific surface area and surface charge number, increased. The content of different carbon fractions decreased with prolonged incubation time, while soil electrochemical parameters continued to increase throughout the incubation period. Pearson correlation and redundancy analyses showed that soil active organic carbon fractions were strongly correlated with soil surface electrochemical properties, with ROC and POC being the primary factors. Soil aggregate stability increased with higher biochar application rates, and the beneficial effect of biochar on aggregate stability became increasingly pronounced over time. Based on the quantitative estimates of SIFs, the addition of biochar facilitated soil aggregate stability by increasing van der Waals attractive pressure and decreasing electrostatic repulsive pressure, and finally lowering net repulsive pressure between soil particles. The soil aggregate breaking strength increased rapidly initially and then leveled out as the electrolyte content decreased; 10−2 mol L−1 marked the tipping point. The study identified the active organic carbon (ROC and POC) fractions which could mediate the soil surface electrochemical properties and soil internal forces, thereby enhancing soil aggregate stability. Our research provides a theoretical basis for the targeted use and modification of biochar to efficiently improve soil structure.
Biochar is widely used to remediate heavy metal-contaminated soils; however, it can increase carbon (C) limitation in soil microorganisms, potentially compromising soil ecological sustainability. Effective strategies are, therefore, needed to reduce heavy metal bioavailability while maintaining microbial activity and soil fertility. To this end, we aimed to evaluate the effects of different straw–biochar ratios on heavy metal availability, microbial nutrient limitation, C cycling and soil fertility and to identify an optimal remediation strategy for cadmium (Cd) and zinc (Zn) contaminated soil. Cd- and Zn-contaminated soils were amended with varying straw–biochar ratios (S4C0, S3C1, S2C2, S1C3, S0C4 and control) and incubated for 90 d. Soil heavy metal bioavailability, microbial nutrient limitation, CO2 emissions, enzyme activities and organic C stability were analysed. Compared with the control, the straw–biochar treatment at a 1:3 ratio (S1C3) reduced diethylenetriaminepentaacetic acid–extractable Cd and Zn by 30.22 %–32.81 % and 21.31 %–23.06 %, respectively. S1C3 significantly improved soil organic C content and stability, enhanced available nutrients (nitrogen (N), phosphorus (P) and potassium), increased microbial biomass and elevated enzyme activities related to C, N and P acquisition and alleviated microbial C limitation. The optimized straw–biochar ratio (1:3) provides a novel integrated approach for remediating heavy metal-contaminated agricultural soils while simultaneously enhancing soil C stability, nutrient availability and microbial activity. This strategy offers a practical foundation for sustainable farmland management.
