Patterns of root decomposition in response to soil moisture best explain high soil organic carbon heterogeneity within a mesic, restored prairie

• Surface soil organic carbon stocks were greater under wetter soil moisture regimes.
• Fine root biomass, production, and distributions did not vary with soil moisture.
• Fine root decomposition was reduced by approximately 50% in wet versus dry soils.
• Variable root decomposition best explains soil organic carbon spatial heterogeneity.

Spatially heterogeneous patterns of soil organic carbon (SOC) are related to topographically-defined soil moisture levels within Midwestern tallgrass prairies. While roots are regarded as the main contributor to SOC formation, relatively little is known about how fine root dynamics respond to landscape-level changes in soil moisture, and thus the mechanisms promoting spatial heterogeneity of SOC remain uncertain. We evaluated SOC, fine root (≤2 mm) biomass, production, decomposition, and vertical rooting distributions among landscape positions varying in soil moisture within 25+ year old restored tallgrass prairies in Wisconsin, USA. We hypothesized that SOC, root biomass, and root production would increase, while root decomposition would decline with increasing soil moisture. Additionally, we hypothesized that relative root biomass and production distributions would become shallower as soils became wetter. We found no relationship between soil moisture and root biomass, production, or their vertical distributions, but decomposition decreased and SOC increased as expected with increasing soil moisture. However, we also observed a strong relationship between soil moisture and species assemblages, suggesting that community composition changed in response to soil moisture. Our findings indicate that SOC was highest in seasonally wet, lowland landscape positions due to greatly reduced root decomposition, not due to changes in root production or relative distributions. We suggest that species turnover may have reduced the effect of soil moisture on root biomass and production, thereby maintaining similar root production under notably disparate soil moisture conditions. Considering continued interest in monoculture biofuel plantings and their potential to sequester C in roots and soils, additional research is necessary at the landscape scale to elucidate the importance of species spatial heterogeneity on grassland belowground C dynamics.