N, P and straw return influence the accrual of organic carbon fractions and microbial traits in a Mollisol

Abstract Soil organic carbon (SOC) is the key to soil fertility and quality. However, SOC has severely declined in the past several decades due in part to inappropriate nitrogen (N) and phosphorus (P) input levels. Thus far, the influences of N and P input levels on the accrual of different SOC fractions and relevant microbial traits at aggregate scales remain uncertain. Here, a 3-year field experiment was used to reveal the effects of N and P input levels on SOC fractions in a Mollisol. Treatments included straw return (SR) and combinations of high nitrogen (N) and high phosphorus (P) inputs (N + P + SR), high N and low P inputs (N + P1/2 + SR), and low N and high P inputs (N1/2 + P + SR). Macroaggregates (>0.25 mm), microaggregates (0.25–0.053 mm), and clay and silt (