Ryegrass root and shoot residues differentially affect short-term priming of soil organic matter and net soil C-balance

Plant root and shoot fractions differ strongly in tissue quality (i.e. concentrations of soluble and recalcitrant components), affecting their decomposition in soil and, in turn, their respective impacts on greenhouse gas emissions and soil C storage. Therefore, root and shoot residues can be used as model substrates to investigate the impact of tissue quality on soil processes. To this end, we used C-13-labelled ryegrass root and shoot residues to quantify their rates of C mineralization, assessed the impacts of residue addition on microbial activity and mineralization of native soil organic matter (SOM) and investigated the influence of root and shoot residues on C partitioning in SOM fractions. This was done using controlled incubation of soil amended with milled root or shoot material and soil only. Addition of shoot residues resulted in higher residue C mineralization rates, accelerated soil microbial activity, and increased SOM priming more than root residues particularly in the first 12 days. Nevertheless, for the first time, we found that at the end of the experiment (36 d) the amount of residue C retained in soil was similar in root versus shoot residue amended soil but net C gain in soil was 44% less in the shoot residue treatment. This is because SOM priming, especially during the initial period of incubation, was greater in the shoot residue treatment. This suggests that low-quality root residues can lower soil CO2 emissions and increase soil C stocks because of not only slow tissue turnover, but also weaker impact on SOM priming.