Plant litter effects on soil carbon stabilization and nitrogen availability: A trade‐off and some versatile species

Globally, the constrained stoichiometry of living organisms is responsible for the coupling of carbon (C) and nitrogen (N) elements in living organic matter, with afterlife effects in dead organic matter. This coupling has long been thought to foster trade‐offs among several key functions of ecosystems, such as soil fertility and C sequestration. However, while there is evidence for a general coupling of C and N cycling in ecosystems, there are many ways in which these cycles also diverge, both temporally and spatially, under the influence of multiple biotic and abiotic drivers. Here, focusing on the role of plant residues in feeding and steering the C and N cycles in soil, we examine how 24 leaf and root litters with contrasting chemistry differentially influence the temporal release of compounds with varying C:N stoichiometry to soil, and how C and N elements end up as stabilized (mineral‐associated organic matter, MAOM) or more bioavailable organic (particulate organic matter, POM) or mineral forms (e.g. CO 2 , NO 3 − , NH 4 + ). There were major differences in the C:N stoichiometry of compounds released during decomposition, from low C:N early on to very high C:N at later stages. We observed a trade‐off in the role of litters towards increasing soil N availability (i.e. N in the soil solution) versus soil C stabilization (i.e. C in MAOM). Slow‐decomposing litters (with high lignin and low N concentrations, C‐poor leachates), particularly roots, favoured soil C stabilization over N availability. For each gram of litter decomposed, roots contributed 33% more C to the MAOM fraction of the soil, whereas leaves contributed 87% more N to the soil solution. This pattern was strongly driven (44% of variance explained) by the contrasting biochemistry of leaf versus root litters. Synthesis . These results suggest that leaf and root litters are highly complementary in the way they contribute to soil C stabilization and N availability. As such, global changes that influence the production and turnover of above and below‐ground litter inputs will likely have cascading effects on the balance between these functions. Our results also reveal substantial variation around the trade‐off between soil C stabilization and N availability, suggesting a continuum from ‘underachieving species’ to ‘versatile species’ contributing more to both functions. This opens perspectives for selecting versatile species capable of influencing positively several agro‐ecosystem functions.