Soil textural effects on mineralization of nitrogen from crop residues and the added nitrogen interaction

Abstract

A laboratory incubation was made to compare the amount of inorganic nitrogen (N) released from crop residues incorporated in three soils of different texture and to determine the effect of soil texture on the mineralization of soil organic N. Residues of 15N‐labeled maize (Zea mays L.), soybean (Glycine max [L.] Merr), or alfalfa (Medicago sativa L.) were mixed with sand, loam, or clay soils at rates of 0 or 220 mg N kg‐1 of soil. Nitrogen mineralization was measured after 2, 4, and 6 weeks of incubation at 30°C. After 6 weeks, from 6 to 41% of plant residue N was mineralized. During the incubation, alfalfa residue N was mineralized more in the sand soil than in the loam or clay soils. In contrast, maize and soybean residue N was mineralized more in the fine‐textured soils. Native soil organic N was more susceptible to decomposition in the clay and loam soils than in the sand soil. In all soils, alfalfa residue released the most inorganic N followed by maize and soybean according to their C:N ratios and insoluble fiber contents. The amounts of N mineralized estimated with the isotopic method (IM) and by the difference method (DM) were higher when alfalfa residues were incorporated in the sand soil than in the loam or clay soils. In contrast, soil textural effects were not evident when maize and soybean residue N mineralization were estimated with either method. The isotopic method generally gave higher values than the difference method when used to estimate N mineralization from labeled crop residues. However, both methods were highly correlated (r=0.93; p<0.001), suggesting that the DM is valid for estimating N mineralization from organic residues. Throughout the entire incubation, alfalfa residue N stimulated native soil N mineralization in the sand soil (positive added nitrogen interaction, ANI), whereas soybean and maize residue N led to negative ANI in all soils. These negative ANI values were attributed to the immobilization of soil native mineral N as well as to N mineralized from soil organic matter and crop residues.