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

Abstract

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 13C labeled in situ and then traced into whole‐soil samples and three soil organic matter (SOM) fractions (coarse particulate organic matter [CPOM, >250 μm], microaggregates [53–250 μm], and silt and clay [<53 μm]). At the end of the maize (Zea mays L.) growing season, ∼52% of the root‐derived C was still present in the soil, while only ∼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.