Are plot-scale spatial soil moisture patterns universal?

Abstract:

Many factors affecting spatial soil moisture have been explored at hillslope scales and larger, including climate, soil properties, vegetation, seasonality, and topography, but less is known about spatial soil moisture variation at plot scales across these factors. Smaller scale spatial variation is important for understanding moisture controls on processes like nutrient and carbon cycling. Broadly characterizing how variance changes with respect to mean soil moisture across season, topography, or site could also improve nutrient cycling routines in ecohydrological models. The objectives of this study were 1) to test for a universal mean–variance relationship by characterizing them across sites, topographic positions, land uses, and seasons, and 2) to examine the impacts mean–variation relationships have on estimates of nitrification and denitrification at plot and catchment scales. To achieve this, we combined and analyzed over 32,000 point measurements made across lawns, forests, and pastures over more than 15 years. This data was collected as part of the Baltimore Ecosystem Study in Maryland, USA and the Coweeta Long-Term Ecological Research site in North Carolina, USA. We found that plot scale spatial soil moisture was greatest under mesic conditions and varied consistently across land uses, seasons, and sites. The largest differences in spatial soil moisture were explained by riparian versus upland plots. These discrepancies in spatial variance between riparian and upland plots played a critical role in estimates of nitrification and denitrification rates. At the catchment scale, nitrification and denitrification rates computed as a function of soil moisture were correlated with topography. Integrating plot-scale spatial variance patterns into catchment wide nitrification and denitrification estimates revealed large errors, especially in the riparian region. Results from this research suggest that soil moisture sampling strategies should consider capturing riparian and upland areas to adequately characterize spatial variance across the catchment. Further research is needed to explore how plot-scale spatial soil moisture impacts catchment scale denitrification and nitrification in ecohydrological models.