Quantifying soil organic carbon in forage-based cow-calf congregation-grazing zone interface

Recent concerns about global warming due to accumulations of atmospheric CO₂ have encouraged the achievement of better understanding of the roles of animal agriculture in mitigating CO₂ emissions. Grazing can accelerate and alter the timing of nutrient transfers, and increase the amount of nutrients cycled from plant to soil. Our reason for conducting this study is to test whether cattle congregation sites (CCS) typical on most Florida ranches, such as mineral feeders (MF), water troughs (WT), and shaded areas (SA) have higher soil organic carbon (SOC) than in other locations of pasture under foraged-based system. Baseline soil samples around the congregations zones (MF, WT, and SA) and grazing zones in established (>10 year), grazed cow-calf pastures were collected in the spring and fall of 2003, 2004, and 2005, respectively. Soil samples were collected from two soil depths (0-20 and 20-40 cm) at different locations around the CCS following a radial (every 90 degrees: N, S, E, and W) sampling pattern at 0.9, 1.7, 3.3, 6.7, 13.3, 26.7, and 53.3 m away from the approximate center of MF, WT, and SA. The levels of SOC varied significantly with CCS (P ≤ 0.001), distance away from the center of the CCS (P ≤ 0.05), sampling depth (P ≤ 0.001), sampling year (P ≤ 0.001) and the interaction of CCS and soil depth (P ≤ 0.001). Sampling orientations did not significantly affect the levels of SOC. The SA sites had the highest level of SOC of 3.58 g kg⁻¹, followed by WT sites (3.47 g kg⁻¹) and MF sites (2.98 g kg⁻¹). Results of our study did not support our hypothesis that cattle congregation sites typical on most ranches, such as MF, WT and SA, may have higher concentrations of SOC. The levels of SOC (averaged across CCS) within the congregation zone (3.42 g kg⁻¹) were not significantly (P ≤ 0.05) different from the concentrations of SOC at the grazing zone (3.16 g kg⁻¹).