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Soil organic carbon (C) and aggregates are the important components of soil fertility and the foundation of sustainable agriculture. The storage and protection of SOC in aggregates is widely regarded as the material basis of soil organic C accumulation. However, current understanding of soil aggregate and its associated organic C is insufficient to elucidate the regulation mechanism of soil organic C. A nine-year field experiment including chemical fertilizer (FR) and organic manure (OM) treatments was set up in the eastern plain of Funiu Mountain, central China. Using chemical analysis, physical sieving as well as nuclear magnetic resonance (NMR) methods, we mainly probed into the response of soil organic C concentration and composition, and C functional groups, water-stable aggregates to different treatments. Furthermore, scanning electronic microscopy (SEM) and partial least square structural equation modelling (PLS-SEM) was conducted to characterise the different size aggregates and to analyse the mechanism of soil organic C accumulation and stabilisation at aggregate scales. After nine years of farming, OM treatment substantially increased soil organic C content (by 3.77 g kg−1) and significantly enhanced the formation of macro-aggregates (> 250 μm), while FR had no significant influence on soil organic C. At the aggregate scale, the amounts of soil organic C, C physical fractions (particulate and mineral-associated organic C), total nitrogen and microbial biomass carbon associated in macro-aggregates (> 250 μm) were obviously higher than that in micro-aggregates and silt + clay fraction, and OM treatment greatly increased the accumulation of soil organic C and its components in macro-aggregates. Moreover, microbial biomass carbon (MBC) amounts in aggregates were remarkably increased (27–116%) by the application of OM. And MBC had a positively effect on the physical fractions of SOC but not on the C chemical structure within aggregates. The present study indicated that soil organic C accumulation mainly rely on macro-aggregates (> 250 μm). Intra–particulate organic carbon (POC) and mineral-associated organic carbon (MOC) within macro-aggregates played an important role in soil organic C accumulation. Meanwhile, soil microbes were a driving force for the accumulation of soil organic C physical fractions (POC and MOC). We concluded that OM treatment accelerated the synergistic process between organic C sequestration and soil aggregation, and showed great potential to increase soil organic C accumulation. [ABSTRACT FROM AUTHOR]

Agricultural soil organic carbon (SOC) plays a critical role in soil ecosystems, and SOC sequestration is a key ecosystem service. This study investigated the SOC fractions, including mineral‐incorporated organic carbon (MSC), intra‐particulate organic carbon (iPOC) and microbial biomass carbon (MBC), in bulk soil and water‐stable aggregates, as influenced by soil fertilization managements, using three treatments: no fertilizer (CK), chemical fertilizer (FR) and organic manure (OM). Fertilizer application significantly increased SOC content and its fractions (MSC, iPOC and MBC) compared to CK, and the effect of organic manure amendment had more impact on the SOC accumulation than chemical fertilizer. Macroaggregate (>250 μm)‐associated SOC, MSC and iPOC (including fine and coarse iPOC) increased by 22%–283%, 18%–84% and 45%–232%, respectively, in FR and OM treatments compared with CK. In addition, the iPOC:MSC ratio values (%) within all size aggregates were all increased in the FR (about 84%–183%) and OM (about 103%–230%) treatments compared to CK (about 65%–146%). Meanwhile, MBC:SOC ratio values were the highest (about 2.16%–3.07%) in small macroaggregates (250–2000 μm), but decreased sharply (about 2%–2.35%) in large macroaggregates (>2000 μm) in different treatments. Our findings suggested that macroaggregates were the main sites of carbon sequestration and that carbon became more unstable with the increase of SOC in aggregates. Furthermore, the microbial biomass showed obvious differentiation characteristics in macroaggregates (>250 μm). Specifically, small macroaggregates (250–2000 μm) contained more microbial biomass, and large macroaggregates (>2000 μm) were inclined to inhibit microbial activity. [ABSTRACT FROM AUTHOR]

• The application of reduced tillage plus organic manure promoted soil aggregation. • SOC and its active fractions were preferentially fixed in macroaggregates. • Reduced tillage plus organic manure improved the quantity and quality of SOC. Studying the impact of tillage and fertilization on soil organic carbon (SOC) content and aggregate is essential to define better strategies for improving soil structure and SOC sequestration. A field experiment was conducted for 9 years to investigate effects of different tillage and fertilization management practices on SOC and its active fractions [labile organic carbon (LOC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC)] within the bulk soil and in soil aggregates under rice (Oryza sativa L.)–wheat (Triticum aestivum L.)-based cropping systems. Five treatments were included: conventional tillage with no fertilizer (CT), conventional tillage with chemical fertilizer (CTFR), reduced tillage with chemical fertilizer (RTFR), conventional tillage with organic manure (CTOM), and reduced tillage with organic manure (RTOM). The results showed that the organic manured plots (CTOM and RTOM) significantly increased the concentrations of SOC and active carbon fractions (LOC, DOC and MBC) within the bulk soil and soil macroaggregates, and increased the mass proportion of macroaggregates (>250 μm) compared with chemical fertilizer treatments (CTFR and RTFR) or CT. The highest SOC concentration and macroaggregates (>250 μm) were observed in CTOM. The increased SOC, especially active carbon fractions, was mainly concentrated in soil macroaggregates (>250 μm). Soil aggregates were dominated by macroaggregates (>250 μm), which had higher MBC and lower PAOC (the percentage of slow plus passive carbon fractions) after 9 years of RTOM treatment than after CT or CRFR treatment. The results indicated that RTOM played a positive role in promoting the formation of the soil macroaggregate structure. By studying the responses of SOC and water-stable aggregate contents to different tillage and fertilization managements, the continual application of organic manure may be feasible and the periodic organic manure addition may be combined with reduced tillage practice to produce an integrated soil management strategy for the paddy–upland rotation region in Middle China. [ABSTRACT FROM AUTHOR]