Microbial regulation of aggregate stability and carbon sequestration under long-term conservation tillage and nitrogen application

The stability of aggregates plays a significant role in soil organic carbon (SOC) sequestration in conservation agriculture soils. However, the regulation of microorganisms within aggregates on aggregate stability and SOC sequestration remains elusive. By dividing the soil into three aggregate size classes [mega-aggregates (>2000 μm), macro-aggregates (250–2000 μm), and micro-aggregates (<250 μm)], we evaluated the response of aggregate stability, SOC and microbial communities within aggregates to long-term conservation tillage, which consisted of two tillage methods (conventional tillage and no-tillage) and three nitrogen application rates (105, 180, and 210 kg N ha−1). Under no-tillage treatment, high nitrogen application rate increased SOC by 2.1–3.7 g·kg−1 within mega- and macro-aggregates but reduced the total amount of phospholipid fatty acids (PLFAs) within all aggregates. Under conventional tillage, high N application rate increased mean weight diameter (MWD) and reduced total PLFAs within all aggregates only in 0–10 cm. With the same nitrogen application rate, no-tillage increased MWD by 8.7 %–42.7 %, SOC content within mega-aggregates by 7.3 %–27.8 % and within macro-aggregates by 13.2 %–28.3 % when compared with conventional tillage. Actinobacteria were recruited by straw under no-tillage and their biomass increased 1.5–7.8 times in all aggregates compared with conventional tillage, where they might participate in aggregate formation via degradation of straw and increasing SOC within mega- and macro-aggregates. Conversely, desulfovibrio biomass within all aggregates was diminished under no-tillage compared with conventional tillage, while desulfovibrio possibly directly inhibited soil aggregate formation and decreased SOC within mega- and macro-aggregates under conventional tillage. Moreover, under no-tillage, arbuscular mycorrhizal fungi biomass increased by 0.4–1.6 nmol g−1 within all aggregates compared with conventional tillage in 0–10 cm, potentially indirectly contributing to soil aggregate formation via co-metabolic processes and increasing SOC within mega- and macro-aggregates. Overall, high nitrogen application under long-term no-tillage protects SOC within mega-aggregates by altering aggregate formation through the microbial communities, providing information that may be useful in developing management strategies to enhance carbon sequestration in agricultural soils.