Tracing Root vs. Residue Carbon into Soils from Conventional and Alternative Cropping Systems

We investigated the fate and rate of stabilization of root vs. residue C and the role of soil aggregates in root- vs. residue-derived C accumulation within long-term conventional (mineral fertilizer), low-input (mineral fertilizer and cover crop), and organic (manure and cover crop) cropping systems. Both hairy vetch (Vicia dasycarpa Ten.) roots and residue were C-13 labeled in situ and then traced into whole-soil samples and three soil organic matter (SOM) fractions (coarse particulate organic matter [CPOM, >250 mu m], microaggregates [53-250 mu m], and silt and clay [53 mu m]). At the end of the maize (Zea mays L.) growing season, similar to 52% of the root-derived C was still present in the soil, while only similar to 4% of residue-derived C remained. These results suggest that root C contributes more to overall C stabilization than residue C, which supports a nascent body of research demonstrating greater retention of root-derived than residue-derived C in SOM. The ratio of root- to residue-derived C (an indicator of relative root contribution) was higher in the microaggregates and silt-and-clay fractions than the CPOM of low-input and conventional systems. In contrast, relative root contribution was greater in the whole soil of the organic (6.76) than the conventional (1.43) and low-input cropping systems (3.24), and particularly greater in the CPOM of the organic system (7.53). This trend mirrored long-term soil C stocks across the cropping systems, i.e., organic > low input = conventional, and suggests that the CPOM fraction is pivotal to short-term accumulation of root-derived C and, ultimately, to long-term C sequestration under organic crop management.