Effect of canopy and grazing on soil bulk density

Abstract

This study compared soil surface bulk density between: 1) sites not grazed by cattle > 26 years; 2) sites not grazed for 6 years; 3) sites grazed for 15 years to October residual dry matter levels of > 1100 kg ha-1; 4) sites grazed for 15 years to October residual dry matter levels of 670 to 900 kg ha-1; 5) sites grazed for 15 years to October residual dry matter levels of < 450 kg ha-1; and 6) sites subject to concentrated cattle use (trails, corrals, and supplemental feed-water stations). Sites were collected from across the 1,772 ha San Joaquin Experimental Range (SJER) in Madera County, Calif. to represent canopy cover (open grassland, blue oak (Quercus douglasii Hook and Arn.), live oak (Quercus wislizenii A.DC.), foothill pine (Pinus sabiniana Douglas), wedgeleaf ceanothus (Ceanothus cuneatus (Hook) Nutt.), and ceanothus interspace) and topography (swale, uplands) typical of the rocky coarse sandy loam soils of the southern Sierra Nevada foothill oak savannah. Soil surface (0 to 7.62 cm) bulk density (g cm-3) was determined for 1489 soil cores collected across all available combinations of grazing management, canopy cover and topographic position at the SJER. Soil surface bulk density was 0.23 to 0.30 g cm-3 lower under canopy compared to open grasslands. Bulk density was not different (P > 0.05) between sites not grazed > 26 years and sites not grazed for 6 years. Grazing to residual dry matter levels of > 1100, 670 to 900, and < 450 kg ha-1 created bulk densities which were 0.08, 0.18, and 0.21 g cm-3 greater than non-grazed sites, respectively. Cattle concentration sites had bulk densities 0.37 to 0.47 g cm-3 greater than areas not grazed > 6 or 26 years. For the purpose of maintaining soil surface bulk density current residual dry matter recommendations for sites with canopy cover > 50% appear appropriate, but recommendations for open grasslands need additional review. In particular, residual dry matter level must be directly linked to soil surface infiltration capacity.