Mycorrhizal strategies have differential effects on soil carbon and nitrogen
Across terrestrial ecosystems, arbuscular mycorrhizal (AM) dominance is associated with a lower soil carbon:nitrogen (C:N) relative to ectomycorrhizal (ECM) dominance, raising the question why mycorrhizal strategies have contrasting biogeochemical effects on soils. ECM soils could achieve elevated soil C:N relative to AM soils by 1) promoting the disproportionate retention of C relative to N in existing soil organic matter or by 2) providing subsidies of autotrophic C, but evidence for the relative importance of these mechanisms is scant. We designed an experiment to test these mechanisms with individual plant-mycorrhizal-soil systems. We grew seedlings of the dominant tree species of southern Appalachian forests [AM (Acer rubrum; Liriodendron tulipifera) and ECM (Betula lenta, Pinus strobus and Quercus rubra)] with pouches of forest soil supplemented with partially decomposed 15N-enriched organic matter that either allowed hyphal but prevented root ingrowth or allowed both hyphal and root ingrowth. After 6 months of plant growth, we harvested plant biomass to determine plant C, N and 15N content and analyzed the soil pouches for C and N. We analyzed data with linear mixed effects models, where species were random effects.
AM plants led to greater reduction of soil C relative to ECM plants. AM plants reduced soil C with increasing soil N recovery, while N recovery by ECM plants had no effect on soil C. AM plants required less biomass C to recover appreciable N from soils, but ECM plants recovered increasingly more N with plant C, suggesting that AM plants rely on mineral N, while ECM plants actively mine organic N. These three findings appear to support the contention that ECM plants promote the retention of organic matter C. However, ECM effects were mostly due to larger C subsidies from root tissue. Both AM and ECM plants reduced soil C with increasing plant C in the absence of roots, but increased soil C in the presence of roots. Our findings demonstrate that 1) both AM and ECM plants stimulate the loss of soil C by promoting microbial activity, 2) and this effect is stronger in the presence of roots than hyphae alone, but 3) root C subsidies in the ECM symbiosis compensate for this loss of soil C. Our results suggest that a dynamic exchange of C within the plant-ECM-soil system is driving differences in soil C:N across ecosystems.