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Nitrogen isotope fractionation during nitrogen uptake by ectomycorrhizal and non‐mycorrhizal Pinus sylvestris
Author(s) -
HÖGBERG PETER,
HÖGBERG MO.,
QUIST MAUD E.,
EKBLAD ALF,
NÄSHOLM TORGNY
Publication year - 1999
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1046/j.1469-8137.1999.00404.x
Subject(s) - paxillus involutus , ectomycorrhiza , fractionation , mycorrhiza , botany , nitrogen , biology , nutrient , symbiosis , chemistry , ecology , bacteria , genetics , organic chemistry
An experiment was performed to find out whether ectomycorrhizal (ECM) fungi alter the nitrogen (N) isotope composition, δ 15 N, of N during the transport of N from the soil through the fungus into the plant. Non‐mycorrhizal seedlings of Pinus sylvestris were compared with seedlings inoculated with either of three ECM fungi, Paxillus involutus , Suillus bovinus and S. variegatus . Plants were raised in sand in pots supplied with a nutrient solution with N given as either NH 4 + or NO 3 − . Fractionation against 15 N was observed with both N sources; it decreased with increasing plant N uptake, and was larger when NH 4 + was the source. At high ratios of N uptake /N supplied there was no (NO 3 − ), or little (NH 4 + ), fractionation. There seemed to be no difference in fractionation between ECM and non‐mycorrhizal plants, but fungal rhizomorphs were sometimes enriched in 15 N (up to 5‰ at most) relative to plant material; they were also enriched relative to the N source. However, this enrichment of the fungal material was calculated to cause only a marginal decrease (−0.1‰ in P. involutus ) in δ 15 N of the N passing from the substrate through the fungus to the host, which is explained by the small size of the fungal N pool relative to the total N of the plant, i.e. the high efficiency of transfer. We conclude that the relatively high 15 N abundance observed in ECM fungal species should be a function of fungal physiology in the ECM symbiosis, rather than a reflection of the isotopic signature of the N source(s) used. This experiment also shows that the δ 15 N of plant N is a good approximation of δ 15 N of the available N source(s), provided that N is limiting growth.