Root-induced changes in nutrient cycling in forests depend on exudation rates

Abstract

a b s t r a c t(1) While it is well-known that trees release carbon (C) to soils as root exudates, the factors that control the magnitude and biogeochemical impacts of this flux are poorly understood.(2) We quantified root exudation and microbially-mediated nutrient fluxes in the rhizosphere for four 80 year-old tree species in a deciduous hardwood forest, Indiana, USA. We hypothesized that trees that exuded the most carbon (C) would induce the strongest rhizosphere effects (i.e., the relative difference in nutrient fluxes between rhizosphere and bulk soil). Further, we hypothesized that tree species that associate with ectomycorrhizal (ECM) fungi would exude more C than tree species that associate with arbuscular mycorrhizal (AM) fungi, resulting in a greater enhancement of nutrient cycling in ECM rhizospheres.(3) Mass-specific exudation rates and rhizosphere effects on C, N and P cycling were nearly two-fold greater for the two ECM tree species compared to the two AM tree species (P < 0.05). Moreover, across all species, exudation rates were positively correlated with multiple indices of nutrient cycling and organic matter decomposition in the rhizosphere (P < 0.05). Annually, we estimate that root exudation represents 2.5% of NPP in this forest, and that the exudate-induced changes in microbial N cycling may contribute~18% of total net N mineralization.(4) Collectively, our results indicate that the effects of roots on nutrient cycling are consequential, particularly in forests where the C cost of mining nutrients from decomposing soil organic matter may be greatest (e.g., ECM-dominated stands). Further, our results suggest that small C fluxes from exudation may have disproportionate impacts on ecosystem N cycling in nutrient-limited forests.

Short-term carbon input increases microbial nitrogen demand, but not microbial nitrogen mining, in a set of boreal forest soils

“…Support for a positive effect of plants on N mining is provided by a range of studies that show higher activities of N-targeting enzymes, gross ammonification rates and soil N losses in rhizosphere than in non-rhizosphere soil (Brzostek et al 2013;Holz et al 2016;Kieloaho et al 2016;Koranda et al 2011;Weintraub et al 2007;Zhu et al 2014), a stimulation of N-targeting enzymes by artificial root exudates (Meier et al 2017), and positive correlations between rates of root exudation, SOC and (gross or net) N mineralization in experiments with intact plants (Bengtson et al 2012;Yin et al 2013Yin et al , 2014. The strongest support for a stimulation of microbial N mining by increased plant C input comes from experiments where plants were exposed to elevated CO 2 concentrations.…”

Biotic and abiotic controls in determining exceedingly variable responses of ecosystem functions to extreme seasonal precipitation in a mesophytic alpine grassland

“…In the late plant growth season, the relatively small vegetation changes could be owed to the lack of treatment effects on soil enzyme activities and extractable nutrients. Labile C released through root exudation has been suggested to trigger positive plant-soil feedback via rhizosphere priming, which is an important mechanism that accounts for sustained forest growth during the experimental warming and CO 2 enrichment (Phillips et al, 2013;Yin et al, 2014). Thus, the low temperature and plant growth activity in the late plant growth season might substantially reduce the rhizodeposition, thereby hindered soil organic matter decomposition and subsequent nutrient cycling for further feedback on growth stimulation.…”

Environmental Factors Influencing Tree Species Regeneration in Different Forest Stands Growing on a Limestone Hill in Phrae Province, Northern Thailand

“…The roots commonly grow deep into the rock fissures of the epikarst substratum (Huang et al 2011). A classification system is required for tree species based on the degree to which their roots alter nutrient availability and soil organic matter decomposition (Yin et al 2014).…”