Abstract :
[en] Increasing soil organic carbon (SOC) storage is essential for improving soil quality and mitigating climate change. Conservation tillage, which involves minimal or no soil disturbance and crop residue retention, is an effective carbon management strategy that regulate soil physical, chemical and microbial properties. However, the mechanisms underlying mineral-mediated chemical and physical protection of SOC under different tillage practices and soil types remain unclear.
We conducted long-term field experiments at three sites representing conservation tillage systems, including no-tillage with straw mulching (NT), subsoiling tillage with straw mulching (ST), and reduced tillage with straw mulching (RT); and conventional tillage without straw return, including conventional tillage (CT) and plow tillage (PT). These trials were established in Phaeozem, Calcaric Cambisol and Calcaric Regosol soils to assess regional differences in SOC storage and the physicochemical drivers regulated by minerals, aggregates and microbes. In Phaeozem, we applied density fractionation at the aggregate level to investigate the iron (Fe) oxides distribution across aggregate size classes and their association with SOC chemical composition. In Calcaric Cambisol, we measured calcium (Ca) contents, microbial biomass and community structure to investigate organo-Ca associations influencing SOC stability. Finally, isotope-labeled soil incubation experiments were conducted to clarify the role of microbial nutrient limitation and mineral interactions in regulating SOC mineralization, priming effects, and carbon seuqestration under different tillage practices.
(1) In three sites, NT increased the annual average C sequestration rate by 15.3–76.7% and SOC storage by 10.2–28.4% compared to CT. Furthermore, NT significantly increased mean aggregate diameter (MWD) and macroaggregate-associated SOC and total nitrogen contents by 17.8%–28.3% across the three soil types. Notably, the amorphous iron (Feo) content, Ca2+ concentration, and specific surface area all increased with the increasing aromatic-C/aliphatic-C ratio under NT in bulk soil and macroaggregates. PLS-PM analysis indicates that while the aromatic-C/aliphatic ratio directly enhances aggregated formation, Feo and Ca2+ promote MWD indirectly by facilitating higher aromatic-C and polysaccharide-C compounds. In different sites, although Phaeozem had highest baseline SOC storage, NT led to the greatest increases in MWD and SOC sequestration in Calcaric Cambisol compared to CT.
(2) In Phaeozem, RT and NT significantly increased SOC content by 13.6% and 17.9% due to an increase in the mineral associated organic carbon (MAOC) compared to CT. The contents of amorphous iron oxides (Feo) and complex iron oxides (Fep) increased under NT and RT by 10.6–14.4% and 12.7–41.1%, respectively, in bulk soil and silt + clay fractions within macroaggregates (> 0.25 mm). The contents of Feo and Fep were strongly positively correlated with aggregate stability, and promoted the physical protection of SOC. In addition, both NT and RT enhanced the aromatic-C content and aromatic-C/aliphatic-C ratio in macroaggregates compared with CT, which showed positive relationship with Feo plus Fep contents. The SEM analysis indicated that conservation tillage augmented the accessibility of Fe for binding C by forming organo-Fe complexes, which subsequently improved soil aggregation, and thus promoted the chemical stability and long-term sequestration of SOC.
(3) In Calcaric Cambisol, we found that ST and NT increased SOC storage by 9.9–14.3 % by promoting organo-Ca associations compared to CT and PT. Moreover, NT and ST reduced total SOC mineralization due to increased macroaggregates formation. Compared to CT and PT, 22-years NT and ST not only promoted the transformation of CaCO3 to exchangeable Ca, but also increased microbial biomass, especially the proportion of Gram-negative and arbuscular mycorrhizal fungi. Exchangeable Ca was positively correlated with the particle- and mineral-associated SOC and negatively correlated with SOC mineralization. The exchangeable Ca and bacteria biomass primary explained the final SOC sequestration (R2 = 0.482, p < 0.01) and contributed to higher SOC storage under NT and ST (R2 = 0.560, p < 0.05). Mechanistically, higher Ca could shift bacterial communities and encourage the microbial by-product adhering to the mineral surface to form stable organo-Ca complexes, supported by the presence of common bacterial groups, and significant positive relationships between them.
(4) The results showed that (NT, RT, ST) promotes SOC accumulation by reducing priming effect (PE) and increasing microbial carbon use efficiency (CUE). Conservation tillage significantly reduced PE by 27.3–45.4% but increased microbial CUE by 20.1–77.5% compared to CT. Furthermore, conservation tillage promoted the glucrose derived MAOC content and cPOC, whith MAOC (53.2–76.7%) contribute more to the final SOC content. The lower priming effect observed under conservation tillage was positively related to higher nitrogen availability, increased glucose-derived MAOC, and lower BG/PER and RC:N – TERC:N values. Compared to Calcaric Cambisol, Phaeozem exhibited a lower priming effect due to their higher nitrogen availability, more clay and MAOC contents. Based on the stoichiometric regulation, conservation tillage reduces the enzymes required for energy consumption and substance secretion to improve microbial CUE, and limits microbial contact with carbon through co-metabolism mechanisms, thereby reducing PE and promoting carbon stability.
Taken together, in different sites, the selective binding of Feo and Ca2+ to small-molecule SOC functional groups, positively enhanced MAOC formation and soil aggregation. Furthermore, conservation tillage promoted SOC sequestration mainly via MAOC formation after exogenous carbon input. Our results also emphasized that conservation tillage through mineral-mediated microbial communities and further enzyme stoichiometric, reduced SOC mineralization and priming effect, thus protected SOC. From this, we conclude that conservation tillage is an effective strategy to improve SOC sequestration by regulating the interaction effect of physo-chemial protection and microbial properties.
