Management intensity affects density fractions of soil organic matter from grazed bahiagrass swards

Soil fertility and agricultural systems sustainability depend upon soil organic matter (SOM). The effects of pasture management intensity on SOM are not well understood. The objectives of this study were to determine the effect of management intensity of ‘Pensacola’ bahiagrass (Paspalum notatum Flügge) pastures on the light density fraction of SOM (LD-SOM), the fraction that responds most readily to changes in pasture management practices. Pastures were grazed from 2001-2004 at four management intensities, defined as the combination of stocking method, N fertilization, and stocking rate (SR). Treatments were continuously stocked (CS) Low (40 kg N ha⁻¹ yr⁻¹ and SR of 1.4 animal units ha⁻¹ (AU=500 kg live weight)); CS Moderate (120 kg N and SR of 2.8 AU); CS High (360 kg N and SR of 4.2 AU); and rotationally stocked with a 7-d grazing period and 21-d resting period (360 kg N and SR of 4.2 AU). Composite soil samples (0-8 cm) from each pasture were collected in 2004. Management intensity did not affect C and N concentration in the bulk soil, but it did impact C and N concentrations of size fractions of LD-SOM. In particles from 250 to 2000 μm, both C and N concentration were greater with increasing management intensity. In particles<53 μm, however, the lowest management intensity presented the greatest soil C and N concentrations. Increasing C and N in slow turn over SOM fractions with increased management intensity may result in greater C sequestration and potential soil fertility, but the increased likelihood of negative environmental impact and the questionable sustainability of high N fertilizer rates must also be considered.