Dynamics of physically-separated soil organic carbon pools assessed from δ13C changes under 25 years of cropping systems

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 mu m, 250-53 mu m, 2000-250 mu m, and > 2000 pm) and 3 density fractions: free light fraction (LF), intra-aggregate particulate organic matter (iPOM), and mineral-associated organic matter (mSOM). The 25 years of cropping with manure application significantly increased the SOC storage, mainly by enhancing the soil C of the macroaggregates (2000-250 pm), with most of the C stored in the iPOM (62.01-90.32%). The MS system was more beneficial for the SOC accumulation in macroaggregates (> 250 pm) than the MM system because of enhanced SOC in heavier fractions (iPOM and mSOM); this was probably induced by the differentiation of the below ground humification rate between soybean and maize roots, while the MM system may be a more effective measure for future soil C sequestration because most of the stable C is stored in the small size fraction (< 53 mu m). The delta C-13 values indicated that, among aggregate sizes, the fastest soil C turnover occurred in microaggregates (250-53 mu m). Moreover, C in the MS soils had a faster turnover rate than in the MM soils.