Quantification of the dynamics of soil organic carbon (SOC) pools under the influence of different cropping systems is essential for predicting carbon (C) sequestration. We combined soil fractionation with C isotope analyses to investigate the SOC dynamics of the various soil fractions in a black soil of Northeast China after 25 years of tillage. Soil samples from the initial condition (CK) and 2 cropping treatments including continuous maize cropping (MM), maize–soybean rotation (MS) were separated into 4 aggregate sizes (< 53 µm, 250–53 µm, 2000–250 µm, and > 2000 µm) and 3 density fractions: free light fraction (LF), intra-aggregate particulate organic matter (iPOM), and mineral-associated organic matter (mSOM). All fractions were analyzed for their C content and δ13C values.
Results/Conclusions
The 25 years of cropping with manure application significantly increased the SOC storage, mainly by enhancing the soil C in the macroaggregates (2000-250 μm), with most of the C stored in the iPOM (62.01-90.32%) in the black soils of northeastern China. The enhanced organic C was largely controlled by the amount of each aggregate size and was ascribed to the increased SOC in all density fractions (LF, iPOM, and mSOM) of each aggregate in both cropping systems. Additionally, the long-term maize–soybean rotation was more favorable for SOC accumulation in macroaggregates (> 250 μm), mainly due to a higher belowground humification rate for soybean than for maize roots. However, continuous maize cropping may be a more effective measure for future soil C sequestration because of more stable C stored in the small size fraction (< 53 μm). A faster C turnover rate of soil fractions occurred in the MS than in the MM system, possibly related to the lower C:N ratio of soybean residues as well as the nitrogen-fixation ability of soybean. Our results suggest that long-term crop management could alter soil aggregate weight distribution and/or C storage, consequently affecting soil C pool dynamics in agricultural ecosystems.