Vermicompost Addition Improved Soil Aggregate Stability, Enzyme Activity, and Soil Available Nutrients

Vermicompost is an environment-friendly and effective organic fertilizer, and has been widely used to improve soil quality with different addition doses. However, the potential effects of vermicompost size that may affect soil porosity and nutrient release are rarely studied. Two sizes (0–2 mm and 3–5 mm) and four volume proportions (5%, 10%, 20%, and 30%) of vermicompost were added in the poor soil to improve soil fertility with the originally unimproved soil as the control (CT). Almost all physical parameters, nutrients, aggregates, and soil enzyme activity measured in this study were positively affected by vermicompost addition, and significantly related to addition proportion and vermicompost size. Overall, vermicompost addition decreased soil pH and bulk density, while increased electrical conductivity and porosity. The mean weight diameter (MWD) and geometric mean diameter (GMD) in the S8 (30% proportion, 3–5 mm size) were 15.2% and 9.1% higher, and the water stable aggregate (WSA_0.25) in the S4 (30% proportion, 0–2 mm size) was 27.7% higher than the CT. On average, the MWD and GMD in the bigger size group were 19.7% and 19.4% higher than those in the smaller size group. The activities of urease, saccharase, dehydrogenase, and glutamate decarboxylase in the S4 and S8 were 3.3 and 2.1 times, 2.9 and 4.9 times, 50.7% and 69.5%, 72.1% and 72.7% higher than the CT. Vermicompost addition increased the content of organic matter, available nitrogen, phosphorus, and potassium with the biggest enhancement in the S4 and S8 (ranging from 1.2 to 4.9 times), which was related to the increase in aggregates and enzyme activity. Vermicompost application had positive effects on soil improvement with addition proportion being more significant than vermicompost size.