Spatial distribution of soil carbon in pastures with cow-calf operation: effects of slope aspect and slope position

Background, aim, and scope The rate at which soil carbon (C) accumulates in terrestrial beef agro-ecosystem is uncertain, as are the mechanisms responsible for the current C sink. Broad knowledge of cattle movement in pasture situations is critical to understanding their impact on agro-ecosystems. Movement of free-ranging cattle varies due to spatial arrangement of forage resources within pastures and the proximity of water, mineral feeders, and shades to grazing sites. The effects of slope aspect (SA) and slope position (SP) on nutrient dynamics in pastures are not well understood. Few studies have been made of soil-vegetation and soil-landscape relationships along an elevation gradient in tropical and subtropical regions. Current literature suggests no clear general relationships between grazing management and nutrient cycling. Early study reported no effect of grazing on soils nutrients, while other studies determined increases in soil nutrients due to grazing. We hypothesize that SA and SP could be of relative importance in controlling spatial variability of soil organic carbon (SOC). This study addressed the effects of SA and SP on the spatial distribution of SOC in forage-based pastures with cow-calf operation in subtropical region of southeastern USA. Materials and methods Soil samples were collected at 0-20 and 20-40 cm on contiguous south-, north-, east-, and west-facing slopes across different landscape positions (top slope, middle slope, and bottom slope) of 100 ha pastures during three summer seasons (2004-2006). Soil samples were air-dried, passed through a 2-mm mesh sieve, and visible roots were removed prior to analyses of SOC and other soil properties likely to affect spatial distribution of SOC. Analyses of soils were conducted at the Subtropical Agricultural Research Station in Brooksville, FL, following the dry-ash or the ‘loss-on-ignition' method. Concentrations of organic carbon in soils from four different SA, three SP, and two soil depths (SD) in 2004, 2005, and 2006 were analyzed statistically following a four-way analysis of variance using the SAS PROC general linear model. Results There was an SA × SP interaction (p ≤ 0.0001) effect on the concentration of SOC. The two highest concentrations of SOC were observed from top slope (8.4 g kg⁻¹) and middle slope (7.8 g kg⁻¹) in south-facing slope, and the two lowest levels of SOC were in top slope (2.6 g kg⁻¹) and middle slope (3.0 g kg⁻¹) of north-facing slope, respectively. Soil C als