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Plant and microbial N acquisition under elevated atmospheric CO 2 in two mesocosm experiments with annual grasses
Author(s) -
Hu Shuijin,
Wu Jiansheng,
Burkey Kent O.,
Firestone Mary K.
Publication year - 2005
Publication title -
global change biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.146
H-Index - 255
eISSN - 1365-2486
pISSN - 1354-1013
DOI - 10.1111/j.1365-2486.2005.00905.x
Subject(s) - avena fatua , mesocosm , loam , agronomy , growing season , biomass (ecology) , environmental science , ecosystem , environmental chemistry , field experiment , chemistry , ecology , biology , soil water , soil science , weed
The impact of elevated CO 2 on terrestrial ecosystem C balance, both in sign or magnitude, is not clear because the resulting alterations in C input, plant nutrient demand and water use efficiency often have contrasting impacts on microbial decomposition processes. One major source of uncertainty stems from the impact of elevated CO 2 on N availability to plants and microbes. We examined the effects of atmospheric CO 2 enrichment (ambient+370 μmol mol −1 ) on plant and microbial N acquisition in two different mesocosm experiments, using model plant species of annual grasses of Avena barbata and A. fatua , respectively. The A. barbata experiment was conducted in a N‐poor sandy loam and the A. fatua experiment was on a N‐rich clayey loam. Plant–microbial N partitioning was examined through determining the distribution of a 15 N tracer. In the A. barbata experiment, 15 N tracer was introduced to a field labeling experiment in the previous year so that 15 N predominantly existed in nonextractable soil pools. In the A. fatua experiment, 15 N was introduced in a mineral solution [( 15 NH 4 ) 2 SO 4 solution] during the growing season of A. fatua . Results of both N budget and 15 N tracer analyses indicated that elevated CO 2 increased plant N acquisition from the soil. In the A. barbata experiment, elevated CO 2 increased plant biomass N by ca. 10% but there was no corresponding decrease in soil extractable N, suggesting that plants might have obtained N from the nonextractable organic N pool because of enhanced microbial activity. In the A. fatua experiment, however, the CO 2 ‐led increase in plant biomass N was statistically equal to the reduction in soil extractable N. Although atmospheric CO 2 enrichment enhanced microbial biomass C under A. barbata or microbial activity (respiration) under A. fatua , it had no significant effect on microbial biomass N in either experiment. Elevated CO 2 increased the colonization of A. fatua roots by arbuscular mycorrhizal fungi, which coincided with the enhancement of plant competitiveness for soluble soil N. Together, these results suggest that elevated CO 2 may tighten N cycling through facilitating plant N acquisition. However, it is unknown to what degree results from these short‐term microcosm experiments can be extrapolated to field conditions. Long‐term studies in less‐disturbed soils are needed to determine whether CO 2 ‐enhancement of plant N acquisition can significantly relieve N limitation over plant growth in an elevated CO 2 environment.