Thirty years of increased precipitation modifies soil organic matter fractions but not bulk soil carbon and nitrogen in a mesic grassland

Grassland ecosystems, which are known to be sensitive to climate change, have shown minimal responses of soil carbon (C) and nitrogen (N) pools to increased moisture availability, despite moisture-induced changes in plants and soil microbes (e.g., expected drivers of soil C and N). However, it is not clear if this apparent limited response is due to an unresponsive belowground system or because alterations in multiple soil organic matter (SOM) component pools and fluxes offset each other. To investigate potential responses of C and N in SOM to decadal increases in precipitation, we sampled soils from a 30-year precipitation augmentation experiment in an annually burned mesic grassland. We measured C and N in three SOM fractions which vary in their plant and microbial controls 1) free particulate OM (fPOM), 2) occluded POM plus heavy, coarse OM (oPOM + hcOM), and 3) mineral-associated OM (MAOM), as well as amino sugars, pyrogenic C and N, and root quality metrics. We found no changes in bulk C or N under increased precipitation, but SOM fractions were modified. Altered plant inputs and soil N availability appeared to drive the responses of fPOM N and oPOM + hcOM C:N, which were increased and decreased, respectively, by increased precipitation. In contrast, the observed increase in MAOM C and N under increased precipitation could not be connected to a specific driver, suggesting additional plant, microbial, or mineral measurements may be required. Regardless, our results indicate that investigating SOM fractions may more directly connect soil C and N pools and their drivers, compared to measuring only bulk SOM. Further, our finding of greater stable OM (MAOM) under increased precipitation suggests soil C storage could provide a negative feedback to climate change with increased moisture availability, but the lack of bulk SOM response suggests that this feedback is not strong in mesic grasslands.