Swine manure as part of the total N source improves red soil resistance to acidification

Objectives Red soil acidification has intensified due to the overuse of chemical N fertilizers; liming and manuring are two main methods of preventing red soil acidification. However, there is a limited understanding of how manure impacts the acid resistance of red soils.

Methods An 11-year field experiment started in 2009 in Qiyang Experimental Station, Hunan Province. Here, we tested red soil in a double rice planting system. The four treatments were 100% chemical fertilizer control (with lime application in 2018 to neutralize severe soil acidity) and an organic fertilizer ratio of 20%, 40%, and 60% of the total N input. The organic fertlizer used in this experiment was swine manure. After harvesting rice in 2018 and 2020, soil samples were collected for pH, exchangeable acidity, cation exchange capacity, organic matter, and acid buffer capacity.

Results CK decreased soil pH by 0.48 units and increased exchangeable acidity and aluminum by 2.74 and 1.06 cmol/kg by 2018 (P<0.05), compared to the initial soil. The liming in 2018 increased soil pH by 0.58 units and decreased the exchangeable acidity and aluminum by 2.62 and 1.45 cmol/kg, respectively. Replacing 40% and 60% of the total N with swine manure controlled soil acidification effectively. By 2020, the 60% manure treatment increased the soil pH by 0.78 units and decreased the exchangeable acidity and Al by 1.10 and 1.25 cmol/kg, respectively, as compared with the initial value in 2009. Soil exchangeable acidity was positively correlated with exchangeable Al and the slopes and negatively correlated with pH and exchangeable Ca (P<0.05); the slopes of soil exchangeable Al to pH had a (P<0.05) negative correlation with soil pH, exchangeable Mg and cation exchange capacity, exchangeable K and organic matter.

Conclusions In conclusion, replacing more than 40% of the total N source with swine manure could effectively prevent red soil acidification. Soil cation exchange capacity and organic matter are two main drivers of soil exchangeable Al response to pH among treatments. This implies that the soil with high cation exchange capacity and organic matter content will decrease by one pH unit, with a slight increase in exchangeable aluminum. However, the mechanism involved needs to be further studied.