Long-Term Impacts of Poultry and Swine Manure on Soil Chemical Properties Under Crop Rotation in Subtropical Conditions

The long-term application of manure from animal production can enhance soil quality; however, continuous or excessive use may lead to environmental contamination. This study aimed to evaluate the effects of poultry manure (PM) and liquid swine manure (LSM) applications on soil chemical properties and determine the application rates that significantly alter soil quality after eight years. A random complete block design was used, consisting of five treatments and four replicates. Organic treatments included PM and LSM applied at 67%, 100%, 150%, and 200% of the nitrogen requirement for corn, wheat, and oats, and phosphorus for soybeans, in comparison with a mineral fertilizer control. Soil samples were collected at 0–0.1 m and 0.1–0.2 m depths and analyzed for carbon (C), phosphorus (P), potassium (K+), calcium (Ca2+), magnesium (Mg2+), and cation exchange capacity (CEC). PM and LSM partially or fully replaced mineral fertilization, especially for P, C, Ca2+, and K+, and the application resulted in increased levels of P, C, Ca2+, and K+ at both soil depths. At the same time, LSM enhanced C contents in both layers and increased P only at the highest rates (T4S and T5S). Among the organic sources, PM was more effective in improving soil fertility indicators.