Soil carbon and nitrogen responses to global change are informed by soil organic matter fractions

As the largest terrestrial carbon (C) pool, the feedbacks of soil C to global environmental change have significant implications for our future climate. It is increasingly recognized that studying solely bulk soil C and nitrogen (N) responses to global change is not sufficient. Because different fractions of soil organic matter (SOM) have distinct controls, they likely respond differently to global changes. To investigate the responses of SOM fractions to global change we must combine data synthesis with mechanistic experiments. We investigated the responses of particulate and mineral-associated organic matter (POM and MAOM) to global change through a global meta-analysis and an increased precipitation experiment. In our meta-analysis we found that POM was more strongly influenced by global change than MAOM and that these fractions responded uniquely to global changes. In particular, increased precipitation caused opposing, but non-significant, responses of POM and MAOM C (decrease and increase, respectively) when investigated with meta-analysis. In investigating POM and MAOM responses to an increased precipitation experiment, we find greater support for changes in plant biomass and diversity driving changes in SOM fractions than changes in microbial necromass. Unique, statistically stronger, and plant- and microbially-informed responses of SOM fractions as compared to bulk SOM suggest these fractions are useful for understanding SOM responses to global change. Altogether, our work provides strong evidence that fractionating SOM into POM and MAOM will help determine whether soil C will feed back positively or negatively to climate change.