Organic Amendments to a Wheat Crop Alter Soil Aggregation and Labile Carbon on the Loess Plateau, China

The Loess Plateau in China is one of the most severely eroded areas in the world. Understanding the characteristics of soil aggregation and the distribution of soil organic carbon (SOC) in aggregates on the plateau is essential for improving regional soil quality. A 2-year study was conducted in a wheat cropping system on the plateau to investigate short-term responses of soil aggregation measured via water stability and total as well as labile SOC (permanganate-oxidizable C [POXC]) fractions to organic amendments (low-, medium-, and high-level maize stalks, stalk composts, and cattle manure) combined with inorganic fertilization (nitrogen/phosphorous/potassium [NPK]). Compared with NPK fertilization, combined organic amendments enhanced soil aggregate stability mostly for 5-to 0.5-mm size classes. Among the treatments, high-level maize stalk plus NPK was associated with the greatest water stability of all aggregates and lowest aggregate deterioration rate (11.3%), followed by cattle manure plus NPK (21.2%). In the 0- to 20-cm soil layer, the SOC content had no significant changes among treatments, whereas the POXC content was significantly higher in the organically amended treatments (especially cattle manure plus NPK, 108.6 %) than in the NPK treatment (P < 0.05). In most aggregate size classes, the SOC content had no significant differences between the organically amended and NPK treatments, whereas the POXC content was significantly higher in the former than in the latter (P < 0.05). There were significant positive relations between the proportions of water-stable aggregates and POXC content (P < 0.05). Application of high-level maize stalk or manure plus NPK was effective for improving soil structure and SOC sequestration of the loess soil.