Testing rangeland health theory in the Northern Great Plains.
Correctly assessing whether rangeland ecosystem services are stable, improving, or degrading is of global importance. Soil aggregate stability (SAS) is widely used to infer rangeland health, partly because high SAS is thought to reduce run-off by increasing infiltration. We studied the sensitivity of SAS to grazing and other disturbances, the effects of SAS on infiltration, and the utility of alternative indicators of infiltration in the Northern Great Plains. To test grazing effects on SAS, we compared SAS between paired areas that were lightly to moderately grazed or excluded from grazing for 6 years. Additionally, we compared SAS between grazed and not-grazed plots of a 2-year controlled grazing experiment with moderate and severe grazing. We also applied herbicide, mowing, and fungicide treatments to test SAS responses to disturbances more generally, as well as effects of SAS and other factors on infiltration. To more generally test for a SAS-infiltration relationship, we performed a meta-analysis of our data combined with other data from the region. Grazing often reduced stability of small macroaggregates (0.25-1 mm) in the controlled grazing experiment but not the paired grazing area experiment. Grazing had no detectible effect on SAS of larger macroaggregates (1-2 mm). Herbicide tended to reduce SAS, and mowing sometimes increased SAS. Infiltration exhibited high plot-to-plot variation and was not significantly affected by treatments. Variation in infiltration was best explained by plant community composition variables and was not explained by either SAS or other soil properties. Our meta-analysis revealed no general SAS-infiltration relationship. Synthesis and applications. Our findings counter prevailing expectations that soil aggregate stability (SAS) is consistently sensitive to rangeland disturbance(s) and a leading indicator of soil water transport. Plant community composition properties were better predictors of infiltration. Our findings support the theory that excessive grazing increases the prevalence of a grazing tolerant species, which was associated with low levels of infiltration irrespective of SAS.