Assessment of Green Manure Crops and their Impacts on Mineralizable Nitrogen and Changes of Nutrient Contents in the Soil

To evaluate the green manure crops those can promote soil properties and the ability of nitrogen (N) mineralization in paddy soil, a pot experiment was conducted at the Department of Agronomy, Yezin Agricultural University, Naypyidaw, Myanmar from April to August 2022. The experimental design was a randomized complete block design with three replications. Ten green manure crops were sown as treatments: soybean (Glycine max (L.) Merr), green gram (Vigna radiata L. Wildzek.), black gram (Vigna mungo L. Hepper.), cowpea (Vigna catjang), cowpea white (Vigna unguiculata (L.) Walp.), lab lab bean (Lablab purpureus (L.) Sweet.), mayflower bean (Phaseolus vulgaris L.), rice bean (Vigna umbellata), sunn hemp (Crotalaria juncea L.), and dhaincha (Sesbania bispinosa (Jacq.)). At the flowering stage, various green manures were incorporated into the soil, and their growth characters were measured along with the mineralizable N, total N, phosphorus (P2O5), and potassium (K2O) contents of the soil. The results indicated that rice bean, sunn hemp, and dhaincha had higher shoot and root length, shoot and root fresh weight, and dry weight compared to other varieties. In addition, they contain a higher percentage of total N content and total P2O5, and consequently, its C:N ratio was lower. After incorporating green manures into the soil, the mineralizable N increased gradually from 0 weeks to 10 weeks and then, the nutrient values decreased. Among green manures, sunn hemp, dhaincha, and rice bean were found to have higher mineralizable N (%) and applicable mineralized N than others. In conclusion, the incorporation of green manures into the soil improves the N, P2O5, and K2O contents of the soil, along with the mineralizable N content. Among green manure crops, dhaincha, rice bean, and sunn hemp were superior to other varieties due to higher biomass yield, mineralizable N (%), and chemical compositions, especially total N content.