Importance of Phytobiomass and Ungulates to Hydrologic Function in a Temperate Grassland

Abstract

Hydrologic dysfunction associated with desertification of grazing lands likely depends on two opposing biophysical drivers, compaction by livestock and bioturbation by plant roots and soil biota. To understand which is most important, we conducted a long-term field experiment. We tested the effects of phytobiomass (control vs. herbicide treated) and three ungulate disturbance treatments (none, once, and twice disturbed) on soil water transport in northern mixed-grass prairie. Field-saturated hydraulic conductivity (i.e., infiltration) was primarily reduced by the most severe (twice disturbed) ungulate disturbance treatment. To a lesser degree, ungulate disturbance and phytobiomass interacted, and similar levels of hydrologic dysfunction were detected for plots annually treated with herbicide and disturbed by a single livestock disturbance event (and while soils were dry and resistant to compaction) and naturally vegetated plots that had been twice disturbed (including while soils were wet and susceptible to compaction). Our findings suggest hydrologic dysfunction in grazing lands is mainly due to compaction by ungulates, especially when grazing at high stocking rates and while areas are vulnerable to compaction (e.g., fine soils, wet soils, not well vegetated).