Nitrogen form, availability, and mycorrhizal colonization affect biomass and nitrogen isotope patterns in Pinus sylvestris

Abstract

Nitrogen (N) isotope patterns are useful for understanding carbon and nitrogen dynamics in mycorrhizal systems but questions remain about how different N forms, fungal symbionts, and N availabilities influence δ15N signatures. Here, we studied how biomass allocation and δ15N patterns in Pinus sylvestris L. cultures were affected by nitrogen supply rate (3% day-1 or 4% day-1 relative to the nitrogen already present), nitrogen form (ammonium versus nitrate), and mycorrhizal colonization by fungi with a greater (Laccaria laccata) or lesser (Suillus bovinus) ability to assimilate nitrate. Mycorrhizal (fungal) biomass was greater with ammonium than with nitrate nutrition for Suillus cultures but similar for Laccaria cultures. Total biomass was less with nitrate nutrition than with ammonium nutrition for nonmycorrhizal cultures and was less in mycorrhizal cultures than in nonmycorrhizal cultures. The sequestration of available N by mycorrhizal fungi limited plant N supply. This limitation and the higher energetic cost of nitrate reduction than ammonium assimilation appeared to control plant biomass accumulation. Colonization decreased foliar δ15N by 0.5 to 2.2‰ (nitrate) or 1.7 to 3.5‰ (ammonium) and increased root tip δ15N by 0 to 1‰ (nitrate) or 0.6 to 2.3‰ (ammonium). Root tip δ15N and fungal biomass on root tips were positively correlated in ammonium treatments (r2 = 0.52) but not in nitrate treatments (r2 = 0.00). Fungal biomass on root tips was enriched in 15N an estimated 6-8‰ relative to plant biomass in ammonium treatments. At high nitrate availability, Suillus colonization did not reduce plant δ15N. We conclude that: (1) transfer of 15N-depleted N from mycorrhizal fungi to plants produces low plant δ15N signatures and high root tip and fungal δ15N signatures; (2) limited nitrate reduction in fungi restricted transfer of 15N-depleted N to plants when nitrate is supplied and may account for many field observations of high plant δ15N under such conditions; (3) plants could transfer assimilated nitrogen to fungi at high nitrate supply but such transfer was without 15N fractionation. These factors probably control plant δ15N patterns across N availability gradients and were here incorporated into analytical equations for interpreting nitrogen isotope patterns in mycorrhizal fungi and plants.