Plot‐ and watershed‐scale soil moisture variability in a humid Piedmont watershed

Abstract

We investigate space‐time dynamics of soil water through extreme drought and wet periods for a Mid‐Atlantic Piedmont catchment. Soil moisture is a nonlinear control of soil nitrogen cycling processes, providing “hot spot” and “hot moment” dynamics that are not well modeled using time‐ or space‐averaged conditions. We document spatial variation in near‐surface soil moisture and examine relationships between mean and variance in soil moisture at plot and watershed scales. Riparian plot soil moisture spatial variance was significantly higher than the variance in uplands and was high relative to soil moisture variance reported by other studies. Plot soil moisture means closely followed a linear trend with the topographic wetness index. However, high within‐plot variance obscured this relationship for experiments where smaller subsets of samples were used, emphasizing the importance of replicate plot measurements to account for fine‐scale heterogeneity in soil moisture. We demonstrate the importance of these spatial‐temporal patterns of soil moisture for estimates of hydrologic controls on biogeochemical cycling by showing that undersampling of fine‐scale soil moisture variation is likely to lead to overestimation and underestimation of nitrification and denitrification, respectively.