In vivo vs. ex vivo pathways of carbon and nitrogen stabilization – a model analysis

Soil microorganisms contribute to the stabilization of organic carbon and nutrients by breaking down organic matter into relatively small compounds that can be stabilized on mineral surfaces (ex vivo pathway) and by building biomass that eventually turns into necromass, which can also be effectively stabilized (in vivo pathway). Which of these pathways is dominant? We answer this question using a model tracing the fate of plant residues into particulate organic matter (POM) and mineral associated organic matter (MAOM). The model allows partitioning ex vivo and in vivo contributions through a small set of parameters that can be estimated using data from incubation of isotopically labelled plant residues. Leveraging a new database of plant-derived POM and MAOM data from these incubations, we estimated the contributions of the two pathways across nearly 40 soils. We found that the in vivo pathway is in general more important than the ex vivo pathway (especially for stabilization of organic nitrogen). Comparing results across soils, we found that the contribution of the in vivo pathway is particularly high in fine-textured soils with low organic matter content, where a larger area of mineral surfaces is available. We conclude that microbial necromass is a key factor for carbon and nitrogen stabilization, especially in soils that have abundant available mineral surfaces.