Article

Ectomycorrhizal responses to organic and inorganic nitrogen sources when associating with two host species.

Louis Calder Center and Department of Biological Sciences, Fordham University, 53 Whippoorwill Road, Armonk, NY 10504, USA.
Mycological Research (Impact Factor: 2.81). 06/2009; 113(Pt 8):897-907. DOI: 10.1016/j.mycres.2009.05.001
Source: PubMed

ABSTRACT While it is established that increasing atmospheric inorganic nitrogen (N) deposition reduces ectomycorrhizal fungal biomass and shifts the relative abundances of fungal species, little is known about effects of organic N deposition. The effects of organic and inorganic N deposition on ectomycorrhizal fungi may differ because responses to inorganic N deposition may reflect C-limitation. To compare the effects of organic and inorganic N additions on ectomycorrhizal fungi, and to assess whether host species may influence the response of ectomycorrhizal fungi to N additions, we conducted an N addition experiment at a field site in the New Jersey pine barrens. Seedlings of two host species, Quercus velutina (black oak) and Pinus rigida (pitch pine), were planted at the base of randomly-selected mature pitch pine trees. Nitrogen was added as glutamic acid, ammonium, or nitrate at a rate equivalent to 227.5 kg ha(-1) y(-1) for eight weeks, to achieve a total application of 35 kg ha(-1) during the 10-week study period. Organic and inorganic N additions differed in their effects on total ectomycorrhizal root tip abundance across hosts, and these effects differed for individual morphotypes between oak and pine seedlings. Mycorrhizal root tip abundance across hosts was 90 % higher on seedlings receiving organic N compared to seedlings in the control treatment, while abundances were similar among seedlings receiving the inorganic N treatments and seedlings in the control. On oak, 33-83 % of the most-common morphotypes exhibited increased root tip abundances in response to the three forms of N, relative to the control. On pine, 33-66 % of the most-common morphotypes exhibited decreased root tip abundance in response to inorganic N, while responses to organic N were mixed. Plant chemistry and regression analyses suggested that, on oak seedlings, mycorrhizal colonization increased in response to N limitation. In contrast, pine root and shoot N and C contents did not vary in response to any form of N added, and mycorrhizal root tip abundance was not associated with seedling N or C status, indicating that pine received sufficient N. These results suggest that in situ organic and inorganic N additions differentially affect ectomycorrhizal root tip abundance and that ectomycorrhizal fungal responses to N addition may be mediated by host tree species.

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    ABSTRACT: Observed modifications of ectomycorrhizal (ECM) communities have been connected to the increased N depositions of the 20th century. Because of their narrow niche width, small disturbances of soil conditions can produce greater effects on the fungal species than on their host trees. This study investigated the ECM community in a black spruce (Picea mariana (Mill.) BSP) stand subjected to long-term additions of 9 and 30 kg N·ha–1·year–1 of ammonium nitrate, representing 3 and 10 times the atmospheric N deposition at the site, respectively. Root tip vitality and ECM presence were detected on samples collected from the organic horizon and ECM were classified into morphotypes according to their morphological and anatomical characters. In the control, 80.6% of the root tips were vital, 76.5% of them showing ECM colonization. Higher root tip vitality and mycorrhization were observed in the treated plots. Forty-one morphotypes were identified, most of them detected at the higher N inputs. Results diverging from the expectations of a reduction in ECM presence and diversity could be related to a higher growth rate of the trees following fertilization. The repeated application of small N doses could have been a better imitation of natural inputs from atmospheric deposition and could have provided more reliable responses of ECM to treatment.
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