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

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. 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. 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 also varied significantly among SA (p ≤ 0.0001), SP (p ≤ 0.001), and SD (p ≤ 0.0001). Averaged across years and SP, soils on the south-facing slope contained the greatest amount of SOC, while soils on the north-facing slope had the least amount of SOC. Average concentrations of SOC in top slope and middle slope were comparable. These values were significantly (p ≤ 0.05) higher when compared with soils from bottom slope. About 73% of SOC spatial variability could be explained by total clay content. Concentrations of SOC were quadratically (SOC = 0.05 × clay2 − 0.29 × clay + 4.4; p ≤ 0.001) related with total clay content. No other significant correlations between SOC and other soil properties were found. Our results have shown that soils on the south-facing slope had greatest concentration of SOC, while soils on the north-facing slope had the lowest concentration of SOC. The differences may be attributed to topographic aspect-induced microclimatic differences, which are causing differences in the biotic soil component and SOC trend. SA may be acting as an important topographic factor influencing local site microclimate mainly because it determines the amount of solar radiation received. Differences in microclimate are often linked to varying soil moisture and erosion potential and in turn could be used to explain distribution of plant communities. The north-facing slope had the lower forage availability when compared with the south-facing slope. There was a decreasing trend in the average forage availability with decreasing slope. Between the top slope and the bottom slope, forage availability declined from 2,484 to 1,448 kg ha−1, which can be attributed to more grazing activities of cattle at bottom slope. Differences in SOC among different SA and SP could also be explained by varying amount of total clay. Concentrations of SOC were linearly related with increasing total clay content. The greatest amount of SOC was observed from soils located at the top slope of south-facing slope. Of the entire SA, south-facing slope had the greatest concentration of total clay, while the greatest clay content among SP was observed from the top slope. Results further revealed that 73% of SOC spatial variability could be explained by total clay content. The relationship between SOC and total clay content was best described by a quadratic equation: SOC = 0.05 × clay2 − 0.29 × clay + 4.4; R 2 = 0.73; p ≤ 0.001. Results of our study are suggesting that SA and SP could be of relative importance in controlling spatial variability of SOC. Averaged across years, soils on the south-facing slope contained the greatest amount of SOC, while soils on the north-facing slope had the least amount of SOC. Based on the average concentration of SOC, the south-facing slope may have sequestered about 6,460 kg ha−1 of SOC. Results have shown that landscape attributes (e.g., SA and SP) associated with beef cattle pastures as a part of the agro-ecological system could be potential sink for C sequestration, thus reducing atmospheric carbon dioxide concentrations. It is still critical to understand how the interactions of pasture management and landscape are affecting soil C dynamics. More studies are needed to assess the rate at which soil C is accumulating as well as the mechanisms responsible for the current and future C sink in forage-based pastures with cow-calf operations.