Soil Carbon Stocks in Agricultural Areas Under Swine Manure Application in Pará de Minas, MG

Swine farming is one of the leading animal protein production activities in Brazil. Due to its large-scale production, a significant volume of swine manure (SM) is generated, posing a challenge for proper disposal. The agricultural use of SM is a common practice among farmers, as it contains high nutrient levels that reduce the need for industrial fertilizers. In addition to supplying essential nutrients to crops, SM application contributes to organic carbon accumulation in the soil. This study hypothesized that the long-term application of swine manure (SM) increases soil carbon stocks compared to a native forest. To test this, we determined the carbon stock in an agricultural area where SM has been applied for 25 years at a rate of 1 million liters per week (equivalent to 7 liters/m²/week), using a nearby native forest as a reference due to its stable long-term carbon reservoir. This comparison helps assess whether SM application can enhance soil carbon sequestration relative to natural conditions. Soil samples were collected from a production unit in the rural zone of Pará de Minas, Brazil, from both a native forest area and an SM-treated area to compare carbon stocks. Samples were taken at six depths: 0-10, 10-20, 20-30, 30-50, 50-70, and 70-100 cm. Organic carbon content (SOC) was determined using the Yeomans & Bremner (1988) method, and carbon stock was calculated using the equation ∑(SOC×D×E), where SOC represents soil organic carbon content, D is soil bulk density, and E is layer thickness. The results were analyzed using ANOVA, and means were compared using the Scott-Knott test at a 5% significance level. Across all evaluated depths, carbon stock values were higher in the SM application area, except for the 30-50 cm layer, where the forest area exhibited a greater carbon reserve, with a difference of 2.76 Mg C/ha. Higher carbon values in the 0-10 cm and 10-20 cm layers result from manure concentration in the upper soil, consistent with studies showing limited vertical organic matter movement. Longer-term applications or different methods (e.g., incorporation) may enhance carbon storage at deeper layers. These findings indicate that swine manure application enhances surface soil carbon but does not significantly affect deeper carbon stocks over short timescales. Long-term or alternative application strategies may further improve carbon sequestration in deeper layers.