Soil aggregate stability was an uncertain predictor of ecosystem functioning in a temperate and semiarid grassland

Abstract

We estimate rangeland managers assessing ecosystem health have measured soil stability >800,000 times. Our aim was to use quantitative data from a site in the Northern Great Plains, USA and a semi‐quantitative literature search to demonstrate the robustness of soil stability as an indicator of ecosystem functioning. Empirical data included measurements of plant and soil properties along a local livestock grazing gradient to determine whether soil stability (e.g., % water‐stable aggregates) explained primary productivity and soil water transport for a mixed‐grass prairie site in the Northern Great Plains. We measured: annual net primary productivity (ANPP), elevation, % soil moisture, measures of soil stability, and soil water transport (field‐saturated infiltrability and sorptivity) across points spanning a local gradient in livestock grazing intensity (none vs. light to moderate stocking rates; mean distance separating points = 39.9 m [range = 5.2–71.3 m]). Across the sampled gradient, variation in ANPP was best explained by a model with field‐saturated infiltrability and % soil moisture. Infiltrability explained slightly more of the variation. We then determined that moderate amounts of variation in infiltrability were explained by ANPP, % soil moisture, and % water‐stable aggregates. We determined that most of this variation was explained by ANPP and then soil moisture. Our empirical findings indicate that plant production was correlated with infiltration though we could not determine whether variation in plant production was caused by variation in infiltration or vice versa. We generally failed to show that soil stability (e.g., % water‐stable aggregates) was a useful predictor of primary productivity and soil water transport. Our semi‐quantitative literature review also indicated that soil stability was not a consistent predictor of either plant production or infiltration. The varying evidence reported here on whether soil stability is a predictor of ecosystem function illustrates the difficulty in identifying an indicator of ecosystem health that (1) is a predictor of ecosystem function across grassland types, (2) is sensitive to rangeland management, and (3) can be easily implemented by non‐experts.