This study was a stratified random survey of soil surface bulk density across all available combinations of three factors at San Joaquin Experimental Range in California. The three factors were: 1) grazing management, 2) canopy cover, and 3) topographic position. Soil surface bulk density was 16 to 22% lower under canopy compared to open grasslands or wedgeleaf ceanothus (Ceanothus cuneatus) interspace. There was no significant difference in soil surface bulk density between sites not grazed > 26 years and sites not grazed for 6 years, indicating that reversal of soil compaction from grazing occurs rapidly in this landscape. Sites with a 15-year history of grazing to residual dry matter levels of >1100, 670-900, and <450 kg/ha-1 created bulk densities which were 6, 15, and 17% greater than non-grazed sites, respectively. Cattle concentration sites had bulk densities 30 to 40% greater than areas not grazed > 26 year, and 18 to 22% greater than surrounding grazed areas, and clearly represent the largest risk of increased runoff and contaminant transport on these rangelands. Current residual dry matter recommendations for sites with canopy cover > 50% appear appropriate for maintaining soil surface bulk densities. The authors note that residual dry matter recommendations for open grassland sites need further critical review to determine if the observed increases in soil surface bulk density translate to sufficient reductions in soil infiltration capacity to increase surface runoff and subsequent contaminant transport.
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