Feedbacks between plant N demand and rhizosphere priming depend on type of mycorrhizal association

Abstract

Ecosystem carbon (C) balance is hypothesised to be sensitive to the mycorrhizal strategies that plants use to acquire nutrients. To test this idea, we coupled an optimality-based plant nitrogen (N) acquisition model with a microbe-focused soil organic matter (SOM) model. The model accurately predicted rhizosphere processes and C-N dynamics across a gradient of stands varying in their relative abundance of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) trees. When mycorrhizal dominance was switched - ECM trees dominating plots previously occupied by AM trees, and vice versa - legacy effects were apparent, with consequences for both C and N stocks in soil. Under elevated productivity, ECM trees enhanced decomposition more than AM trees via microbial priming of unprotected SOM. Collectively, our results show that ecosystem responses to global change may hinge on the balance between rhizosphere priming and SOM protection, and highlight the importance of dynamically linking plants and microbes in terrestrial biosphere models.

Root litter decomposition slows with soil depth

“…Recent work incorporating N cycling into the CORPSE model (Sulman et al, 2017) will allow us to investigate the role of N limitation on subsoil SOC decomposition in future analyses.…”

“…We simulated decomposition of isotope-labeled root litter by initializing the model with 0.5 mg of root mass, which was assumed to contain 25% rapidly-decomposing compounds and 75% slowly-decomposing compounds in line with previous CORPSE simulations (e.g., Sulman et al, 2017). We drove the model for the 15, 55, and 95 cm depths by interpolating measured profiles of soil temperature and moisture to the appropriate depths.…”

Vascular plant‐mediated controls on atmospheric carbon assimilation and peat carbon decomposition under climate change

“…The apparent rate of carbon accumulation in the peat layers above the mean water‐table depth approximated c. 90 g C m −2 year −1 for the low elevation site and c. 154 g C m −2 year −1 for the high elevation site over a reference period of 26–30 years for both sites, a result suggesting a lower carbon sink capacity of peatlands at the lower end of their altitudinal distribution (Millar, Cooper, Dwire, Hubbard, & von Fischer, ). The effective annual contribution of the rhizosphere priming to the observed difference in carbon accumulation rate in relation to the dominant vascular plant functional types and their symbiotic microorganisms remains an open question that deserves further attention (Gavazov et al., ; Sulman et al., ; Walker et al., ).…”

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

“…Mycorrhizal types strongly affect soil C cycling via differences in litter decomposition and priming. Based on ecosystem models of enzymatic activities of excised root tips, Sulman et al (2017) suggested that in newly established vegetation, EcM systems retain soil C of previous AM-dominated habitats, but AM vegetation slowly releases soil C originating from previous EcM-dominated forest. These models contradict the earlier suggestions that EcM fungi deplete accumulated soil C in exotic forestry plantations (Chapela et al, 2001), indicating some context dependency or differences in accounting for active and passive losses.…”