Soil Microbes Compete Effectively With Plants for Organic-Nitrogen Inputs to Temperate Grasslands

Although agricultural grassland soils have inherently high rates of net nitrogen (N) mineralization, they often have soil concentrations of soluble organic N that are comparable to inorganic N. We set out to examine in situ the significance of organic N for plant nutrition in grasslands of differing management intensity and soil fertility. Using in situ dual-labeling techniques (glycine-2-13C-15N) we measured preferential uptake of amino-acid N vs. inorganic N (15NH4)2SO4 in early and late season in low-productivity Agrostis capillaris–Festuca ovina grassland and in agriculturally improved, high-productivity Lolium perenne-dominated grassland. The dominant soluble-N form differed greatly between grasslands. Inorganic N (especially nitrate N) dominated the soluble N pool of the highly productive improved grassland whereas amino acid N was the dominant soluble N form in the low-productivity unimproved grassland. There was no difference in the amount of 15N taken up by plants from the two N forms in either grassland. However, our data indicate that amino-acid N was taken up directly by plants of both grasslands and that more N was captured in this way by plants of low-productivity grassland where amino acids were the dominant soluble N form in soil. Our data from both grasslands also indicate significant microbial competition for added 15N from both N sources, but especially in the low-productivity grassland where the bulk of 15N added was sequestered by the microbial biomass. A significantly greater amount of added 15N was captured by the microbial biomass in the unimproved than in the improved grassland, and substantially more 15N was detected in the microbial biomass than in plant tissue in the unimproved grassland. On the basis of our findings, we predict that subsequent microbial turnover and release of this N into the plant–soil system is the major pathway for plant N capture in these temperate grasslands. Microbial sequestration of added N might be an important mechanism of N retention in these grasslands, especially in the low-productivity systems where microbial N sink strength is greater and organic matter slowly accumulates. Corresponding Editor: J. B. Yavitt.