Spatial Distribution of Soil Carbon in Pastures with Cow-Calf Operation: Effects of Slope Aspect and Slope Position

Authors: Sigua, Gilbert; Coleman, Samuel

Submitted to: Journal of Soils and Sediments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2009
Publication Date: 2/11/2010
Citation: Sigua, G.C., Coleman, S.W. 2010. Spatial distribution of soil carbon in pastures with cow-calf operation: Effects of slope aspect and slope position. Journal of Soils and Sediments. 10(2):240-247.

Interpretive Summary: Widespread concerns about increasing atmospheric CO2 and global changes have increased the need for data and information on the global carbon (C) cycle. 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 and position on slope 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. We hypothesize that slope aspect and position on slope could be of relative important factors affecting spatial variability of soil organic carbon (SOC). This study addressed the effects of slope aspects and position on slope on the spatial distribution of soil organic carbon in a forage-based cow-calf operation in subtropical region of southeastern U.S.A. 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 cause differences in the biotic soil component and SOC trend. Slope aspect 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 biomass when compared with the south-facing slope. There was also a decreasing trend in the average forage biomass with decreasing slope. . Between the top slope and the bottom slope, forage biomass declined from of 2,484 to 1,448 kg ha-1, probably due to more grazing activity of cattle at the bottom of the slope. When livestock graze some pastured areas more than others, plant communities can degrade and soil physical and chemical attributes can change with time.

Technical Abstract: The forests of the world have been the focus of most of the research on terrestrial C sequestration while other parts of the agro-ecological systems like pastures or grasslands have received less research attention. Broad knowledge of cattle movement in pasture situations is critical to understanding their impact on agro-ecosystems. 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). There was a slope aspect (SA) x positions on slope (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 quadratic equation. (SOC = 0.05*clay2 – 0.29*clay +4.4; p=0.001) related with total clay content. Concentrations of SOC have declined by about 49%, 40% and 32% for 2004, 2005 and 2006, respectively as soil depth changed from 0-20 cm to 20-40 cm. Results of our study are suggesting that slope aspect and position on slope could be of relative important factors affecting spatial variability of SOC. Beef cattle pasture as a major part of the agro-ecological system may represent a potential sink for reducing atmospheric carbon dioxide concentrations. Based on the average concentration of SOC, the south-facing slope may have sequestered about 6,460 kg ha-1 of SOC.