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

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, C-13-labeled straw was incubated with intact NT and CT soils. After 90 days, the highest proportion of C-13-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.