Nitrogen and phosphorus co-limit mineralization of labile carbon in deep subsoil

The growth of deep-rooted crops has the potential to increase carbon deposition within deep subsoil layers, thereby contributing to climate change mitigation. Yet, the mechanisms of labile carbon stabilization in deep subsoils remain uncertain. Here we studied the effect of nitrogen, phosphorus and sulphur limitations on the microbial mineralization of added glucose and an artificial root exudate mixture (both spiked with 14C-glucose) in intact soil samples from a deep subsoil (5–6 m) over a 10-week incubation period, where the temporal respiration of CO2 and 14CO2 were quantified. We found that mineralization was co-limited by nitrogen and phosphorus, but the cumulative amount of CO2 emitted was significantly larger from artificial root exudates than glucose, suggesting that the nitrogen carried in amino acids in root exudates (here, L-arginine) was sufficient to overcome the nitrogen limitation in this subsoil. Highlights: Labile C substrates and nutrients from deep-rooted crops may affect the potential for C sequestration in subsoil. Deep subsoil was incubated with glucose and artificial root exudates amended with N, P, and S. C mineralization was co-limited by N and P, but less so in soil amended with glucose. Amino acid derived-N in exudates from deep-rooted crops may alleviate the N limitation in deep subsoils.