Mineralization of nitrogen from crop residues and N recovery by maize inoculated with vesicular-arbuscular mycorrhizal fungi

Legume crop residues serve as a source of nitrogen (N) for succeeding crops in low-input production systems, and characterizing the release of this N supports efforts to develop sound economic and environmental management pra ctices. Nitrogen mineralization of N-15-labelled field crop residues was mo nitored in a Greenville sandy loam during a 140-day laboratory incubation a t 25 degrees C. Residue type strongly influenced the rate of N mineralizati on; decomposition rate constants were 0.283, 0.083, 0.00047 and 0.0014 day( -1) for alfalfa (Medicago sativa L.) stover (above-ground plant parts), alf alfa roots, maize (Zea mays L.) stover (above-ground plant parts excluding cob and kernels) and maize roots, respectively. At the end of the incubatio n, 50% of alfalfa stover and 25% of alfalfa root residues N were mineralize d, whereas these proportions were 8% for maize stover and 12% for root resi dues. Mineralization of N-15 from alfalfa stover residues was also monitore d in a greenhouse experiment to determine N availability during the growth of maize inoculated or not with vesicular-arbuscular mycorrhizal (VAM) fung i, and to evaluate the effects of the presence of plants on the decompositi on of the residues. Stover and root dry matter yields were greatest for mai ze inoculated withVAM fungi and grown in residue-amended soil. At the final harvest, maize grown in residue-amended soils had accumulated 44% more dry matter and 40% more N than maize grown in unamended soils. Enhanced VAM co lonization of roots inoculated with a mixture of three Glomus spp. increase d the residue N accumulation in maize roots at 5 weeks after silking and at the final harvest. Alfalfa stover decomposed rapidly both in the presence and absence of maize plants, but the amount of N-15 mineralized at the end of the experiment was influenced by the presence of living roots; 23% of th e N-15 in alfalfa stover residues was mineralized in soil without plants co mpared to about 38% when maize plants were present. These results suggest t hat N mineralization is enhanced by the presence of living roots.