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SOIL MICROBES COMPETE EFFECTIVELY WITH PLANTS FOR ORGANIC‐NITROGEN INPUTS TO TEMPERATE GRASSLANDS
Author(s) -
Bardgett Richard D.,
Streeter Tania C.,
Bol Roland
Publication year - 2003
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(2003)084[1277:smcewp]2.0.co;2
Subject(s) - temperate climate , ecology , environmental science , nitrogen cycle , grassland , nitrogen , soil biology , temperate rainforest , ecosystem , biology , soil water , chemistry , organic chemistry
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.