Inter-annual changes in the aggregate-size distribution and associated carbon of soil and their effects on the straw-derived carbon incorporation under long-term no-tillage

Abstract

Converting from conventional tillage to no-tillage influences the soil aggregate-size distribution and thus soil organic carbon (SOC) stabilization. However, the dynamics of soil aggregation and the straw-derived carbon (C) incorporation within aggregate fractions are not well understood. An experiment was established in 2004 to test the effects of two treatments, no-tillage with residue (NT) and conventional tillage without residue (CT), on the soil aggregate-size distribution and SOC stabilization in a continuous maize (Zea mays L.) cropping system located in the semiarid region of northern China. Soil samples were collected from the 0–10 cm layer in 2008, 2010 and 2015, and were separated into four aggregate-size classes (>2, 0.25–2, 0.053–0.25, and <0.053 mm) by wet-sieving. In each year, NT soil had a higher proportion of macroaggregates (i.e., >2 and 0.25–2 mm) and associated SOC concentration compared with CT. Additionally, to compare straw-derived C incorporation within NT and CT aggregate fractions, 13C-labeled straw was incubated with intact NT and CT soils. After 90 days, the highest proportion of 13C-labeled straw-derived C was observed in the >2 mm fraction, and this proportion was lower in NT than that in CT soil. Overall, we conclude that long-term continuous NT increased the proportion of macroaggregates and the C concentration within macroaggregates, and the physical protection provided by NT is beneficial for soil C sequestration in the continuous maize cropping system in semiarid regions of northern China.