Organic amendments affect soil organic carbon sequestration and fractions in fields with long-term contrasting nitrogen applications

Organic amendments are widely used to both manage soil organic C (SOC) and improve soil fertility in agroecosystems. However, crop production and the mechanisms of SOC sequestration following organic amendment may be affected by long-term differential N fertilizations, hence driving the dynamics in SOC fractions in cultivated fields. Our 3-yr (2016-2019) field experiment evaluated the responses of SOC sequestration and SOC fractions to organic amendments in soils with long-term contrasting N applications in a winter wheat monoculture system. We compared wheat straw return (St) and farmyard manure application (Fm) in soils with either long-term (2002-2019) N absence (-N) or normal N application (+N). Both St and Fm increased SOC stocks in the 0-20-cm layer, but Fm increased it more than St did. However, St exhibited a greater SOC sequestration rate in +N soils than that in -N soils, whereas Fm exhibited both a lower SOC sequestration rate and efficiency in +N soils than in -N soils. SOC increased across all fractions in all soils after both St and Fm. However, compared with -N soils, +N soils exhibited a lower SOC increase, specifically in the very labile fraction after St, but in the nonlabile fraction after Fm. St, Fm, and +N all increased the soil C management index (CMI), but only +N increased wheat yield and its sustainable yield index (SYI). Those relationships indicated that both straw and manure amendments were responsible for both SOC sequestration and increasing the CMI, and N input was fundamentally responsible for an increased SYI and associated plant-C input. Consequently, straw return was expected to be more effective at sequestering SOC when applied to soil having had adequate long-term N application, whereas farmyard manure application was likely more effective when applied in N-deficient soil. Our field study findings provide empirical evidence that soil N availability affects SOC sequestration capacities following organic amendments, and that impacts strategies aimed at further increasing soil fertility improvements via organic amendment, especially considering that different agro-ecosystems are subject to different fertilization regimes.