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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- 13 C- 15 N) we measured preferential uptake of amino-acid N vs. inorganic N ((( 15 NH 4 ) 2 SO 4 )) 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 15 N 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 15 N from both N sources, but especially in the low-productivity grassland where the bulk of 15 N added was sequestered by the microbial biomass. A significantly greater amount of added 15 N was captured by the microbial biomass in the unimproved than in the improved grassland, and substantially more 15 N 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.
Ecology – Ecological Society of America
Published: May 1, 2003
Keywords: agricultural grassland ; amino acid ; glycine ; microbial biomass ; nitrogen ; nitrogen mineralization ; nitrogen retention ; organic-nitrogen uptake ; plant––microbial competition
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