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Title: MYCORRHIZAL MEDIATION OF PLANT N ACQUISITION AND RESIDUE DECOMPOSITION: IMPACT OF MINERAL N INPUTS

Author
item TU, CONG - NORTH CAROLINA STATE UNIV
item Booker, Fitzgerald
item WATSON, DOROTHY - NORTH CAROLINA STATE UNIV
item CHEN, XIN - NORTH CAROLINA STATE UNIV
item RUFTY, THOMAS - NORTH CAROLINA STATE UNIV
item SHI, WEI - NORTH CAROLINA STATE UNIV
item HU, SHUIJIN - NORTH CAROLINA STATE UNIV

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2005
Publication Date: 5/10/2006
Citation: Tu, C., Booker, F.L., Watson, D.M., Chen, X., Rufty, T.W., Shi, W., Hu, S. 2006. Mycorrhizal mediation of plant n acquisition and residue decomposition: impact of mineral n inputs. Global Change Biology. 12:793-803.

Interpretive Summary: Mycorrhizal fungi that grow in association with plant roots have important roles in mineral nutrient uptake by plants. In this study, we examined how nitrogen (N) fertilizer additions affected mycorrhizally-mediated plant N uptake in a microcosm system. The microcosms consisted of Plexiglas boxes that were each subdivided into six cells. Each pair of cells was separated into HOST and TEST compartments by a replaceable screen that allowed fungal hyphae but not plant roots to grow into the TEST compartments. Wild oat (Avena fatua L.) was planted in the HOST compartment soils. The HOST compartment soil was inoculated with either a single fungus, Glomus etunicatum, or a mixture of fungi species. Uptake of N by the fungus-plant association was directly assessed by introducing 15N tracer to the adjacent TEST compartment. Results showed that fungal hyphae grew into the TEST compartment and directly transported N from the TEST soil to the host plant, leading to a 125% increase in 15N in the HOST plant and a 20% reduction in N in the TEST soil. In a second experiment, N fertilizer was added to the HOST compartments containing the oat plants. The fertilizer increased biomass and root length colonized by mycorrhizae. Mineral N additions to the HOST compartment also reduced N in the adjacent TEST soil by 58% as a result of increased fungal colonization of host roots and hyphal penetration of the TEST compartment soil. Also, decomposition of dead plant matter in the TEST compartment was enhanced by mineral N additions to the HOST compartment through stimulated fungal production. The mycorrhizal species mixture was more effective in facilitating N transport and residue decomposition than the single species. These findings suggest that low-level additions of N fertilizer may significantly enhance nutrient cycling and plant resource capture in terrestrial ecosystems by stimulating root growth, mycorrhizal production, and residue decomposition rates.

Technical Abstract: Arbuscular mycorrhizal (AM) fungi are ubiquitous plant-fungus mutualists in terrestrial ecosystems and play important roles in plant resource capture and nutrient cycling. In this study, we examined how mineral N additions affected mycorrhizal mediation of plant N acquisition and residue decomposition in a microcosm system. Each microcosm unit was separated into HOST and TEST compartments by a replaceable screen that prevented or allowed AM hyphae but not plant roots to grow into the TEST compartments. Wild oat (Avena fatua L.) was planted in the HOST compartments that were inoculated with either a single species of AM fungus, Glomus etunicatum, or a mixture of AM fungi. Mycorrhizal contributions to plant N acquisition and residue decomposition were directly assessed by introducing 15N tracer and 13C-rich residues of a C4 plant to the TEST compartments. Results from 15N tracer measurements showed that AM fungal hyphae directly transported N from the TEST soil to the host plant, leading to a 125% increase in 15N in the HOST plant and a 20% reduction in total extractable N in the TEST soil. Mineral N additions to the HOST compartments increased both oat biomass and total root length colonized by mycorrhizae. Mineral N additions to the HOST compartment also reduced extractable N in the adjacent TEST soil by 58% as a result of increased AM fungal colonization of host roots. Decomposition of 13C-rich plant residues in the TEST compartment was enhanced by mineral N additions to the HOST compartment through stimulated AM fungi production. The mycorrhizal species mixture was more effective in facilitating N transport and residue decomposition than the single species. These findings suggest that low-level N inputs may significantly enhance nutrient cycling and plant resource capture in terrestrial ecosystems by stimulating root growth, mycorrhizal production, and residue decomposition rates.