Long-Term Minimum Tillage and Straw Retention Promote Macroaggregate Formation and Carbon and Nitrogen Sequestration in a Semi-Arid Area of Northern China

Abstract

Conservation tillage is widely believed to promote soil aggregate stability and carbon (C) and nitrogen (N) sequestration, but the underlying mechanisms remain unclear. Here, we collected soil samples from an 18-year experiment in northern China that included conventional tillage with straw removal (CT), deep scarification with straw mulching (DS), and no-tillage with straw mulching (NT) models. The results showed that NT and DS increased total soil organic carbon (SOC) and N by 9.3–16.4% and 10.8–25.8%, respectively, in addition to increasing the weight proportion of macroaggregates and subfractions, and the contribution of macroaggregate-associated C and N to total SOC and N. Although the C and N concentrations of all aggregates and subfractions increased, the C and N contributions of microaggregates and silt + clay fractions decreased under long-term conservation tillage. The C change in the total POM (sum of M_POM, m_POM, mM_POM) accounted for 77.4% and 79.9% of the total SOC increase by NT and DS, while the MAOM (sum of s+c, m_s+c, M_s+c, and mM_s+c) only accounted for 29.2% and 25.2%. Meanwhile, microaggregates-within-macroaggregates accounted for 96.9% and 90.5% of the SOC increase by NT and DS. The total SOC and N were significantly positively correlated with the C and N of the macroaggregates and subfractions. In conclusion, conservation tillage enhanced soil C and N sequestration by promoting the formation of macroaggregates and the protection thereof, and POM and mM were important functional C pools for soil C sequestration and turnover under conservation tillage.

Keywords: Carbon sequestration mechanism, Conservation tillage, Straw mulching, Aggregate-associated C and N, Mineral-associated organic matter