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

A laboratory incubation was made to compare the amount of inorganic nitroge n (N) released from crop residues incorporated in three soils of different texture and to determine the effect of soil texture on the mineralization o f soil organic N. Residues of N-15-labeled maize (Zea mays L.), soybean (Gl ycine 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 min eralization was measured after 2, 4, and 6 weeks of incubation at 30 degree s C. After 6 weeks, from 6 to 41% of plant residue N was mineralized. Durin g the incubation, alfalfa residue N was mineralized more in the sand soil t han 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 mor e 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 cont ents. The amounts of N mineralized estimated with the isotopic method (IM) and by the difference method (DM) were higher when alfalfa residues were in corporated in the sand soil than in the loam or clay soils. In contrast, so il textural effects were not evident when maize and soybean residue N miner alization were estimated with either method. The isotopic method generally gave higher values than the difference method when used to estimate N miner alization from labeled crop residues. However, both methods were highly cor related (r=0.93; p<0.001), suggesting that the DM is valid for estimating F l mineralization from organic residues. Throughout the entire incubation, a lfalfa residue N stimulated native soil N mineralization in the sand soil ( positive added nitrogen interaction, AMI), whereas soybean and maize residu e N led to negative ANI in all soils. These negative AM values were attribu ted to the immobilization of soil native mineral N as well as to N minerali zed from soil organic matter and crop residues.