N Mineralization from Residues of Crops Grown with Varying Supply of 15N Concentrations

A laboratory study was conducted to evaluate the effects of varying 15N concentrations on crop residue quality (rice straw=RS and soybean=SY) and N mineralization. Two crops (rice and soybean) were grown in a glass-house under four 15N concentrations, i.e., 0 mM (N0), 0.625 mM (N1), 2.5 mM (N2) , and 10 mM (N3) supplied as CO(15NH2)2, in 30 cm diameter plastic pots containing 5 kg of quartz sand. Eight weeks after planting the above-ground biomass was pruned and oven dried at 60o C for 48 hours and analyzed for polyphenol, lignin, N, C, C/N, organic matter and % 15N-abundance. Each of the eight crop residues produced from this experiment was then incorporated into 10 g of soil in a 50 ml plastic bottle. Mineralization of N from the residues was measured over a 14-week period under controlled non-leaching conditions.The results showed that increasing N concentration in the nutrient solution (0 to 10 mM) increased total N but decreased C, lignin and polyphenol content in the crop residues. The results suggested that N supply may increase the quality of the crop residues, as indicated by decreasing of C/N, lignin/N, polyphenol/N ratios. SY residue released N about 2x faster than RS residue, and the amount N released increased with increasing supply of the N concentration. At the 10 mM N supply, RS and SY residues released respectively 3734,857 mg kg-1 and 4352.34 mg kg-1 cumulative amount of mineral N with the mineralization rate about 7x faster than without 15N in the solution. Differences in the %N mineralized were determined by the quality of the crop residues. Regression analyses showed that the N, lignin and polyphenol contents were the residues quality factors which could be used to predict N mineralization of the crop residues, and the relationship was best described by a linear regression.