Roots contribute more to refractory soil organic matter than above-ground crop residues, as revealed by a long-term field experiment

Abstract

We revisited the well documented and ongoing long-term ‘Ultuna continuous soil organic matter field experiment’ which started in 1956 at the Swedish University of Agricultural Sciences. The objective of the experiment is to quantify effects of six organic amendments and mineral N fertilizers on the crop and soil. We used the ‘equivalent soil mass’ concept for estimating changes in the topsoil carbon stocks in all 15 treatments. C inputs from amendments were measured and those from crops were calculated using allometric functions and crop yields. Clustering C inputs into seven categories by quality allowed us to calculate a ‘humification’ coefficient for each category. Here, these coefficients were simply based on the fraction of total C input that still remains in the topsoil after about 50 years. As indicated by previous studies, this coefficient was highest for peat, followed by sewage sludge, manure, sawdust and above-ground crop residues. The most interesting result from the current investigation is that the optimized coefficient for root-derived carbon was about 2.3 times higher than that for above-ground plant residues. The calculated results were found to be robust in a sensitivity analysis. Our findings strongly support the hypothesis that root-derived carbon contributes more to relatively stable soil C pools than the same amount of above-crop residue-derived C.