Root carbon inputs to the rhizosphere stimulate extracellular enzyme activity and increase nitrogen availability in temperate forest soils

The exudation of carbon (C) by tree roots stimulates microbial activity and the production of extracellular enzymes in the rhizosphere. Here, we investigated whether the strength of rhizosphere processes differed between temperate forest trees that vary in soil organic matter (SOM) chemistry and associate with either ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi. We measured rates of root exudation, microbial and extracellular enzyme activity, and nitrogen (N) availability in samples of rhizosphere and bulk soil influenced by four temperate forest tree species (i.e., to estimate a rhizosphere effect). Although not significantly different between species, root exudation ranged from 0.36 to 1.10 g C m−2 day−1, representing a small but important transfer of C to rhizosphere microbes. The magnitude of the rhizosphere effects could not be easily characterized by mycorrhizal associations or SOM chemistry. Ash had the lowest rhizosphere effects and beech had the highest rhizosphere effects, representing one AM and one ECM species, respectively. Hemlock and sugar maple had equivalent rhizosphere effects on enzyme activity. However, the form of N produced in the rhizosphere varied with mycorrhizal association. Enhanced enzyme activity primarily increased amino acid availability in ECM rhizospheres and increased inorganic N availability in AM rhizospheres. These results show that the exudation of C by roots can enhance extracellular enzyme activity and soil-N cycling. This work suggests that global changes that alter belowground C allocation have the potential to impact the form and amount of N to support primary production in ECM and AM stands

Organic carbon transfers in the subtropical Red River system (Viet Nam): insights on CO2 sources and sinks

International audienceThe Red River, draining a 169,000 km2 watershed, is the second largest river in Viet Nam and constitutes the main source of water for a large percentage of the population of North Viet Nam. Here we present the results of an investigation into the spatial distribution and temporal dynamics of particulate and dissolved organic carbon (POC and DOC, respectively) in the Red River Basin. POC concentrations ranged from 0.24 to 5.80 mg C L-1 and DOC concentrations ranged from 0.26 to 5.39 mg C L-1. The application of the Seneque/Riverstrahler model to monthly POC and DOC measurements showed that, in general, the model simulations of the temporal variations and spatial distribution of organic carbon (OC) concentration followed the observed trends. They also show the impact of high population densities (up to 994 in-hab km-2 in the delta area) on OC inputs in surface runoff from the different land use classes and from urban point sources. A budget of the main fluxes of OC in the whole river network, including diffuse inputs from soil leaching and runoff and point sources from urban centers, as well as algal net primary production and heterotrophic respiration was established using the model results. It shows the predominantly heterotrophic character of the river system and provides an estimate of CO 2 emissions from the river of 330 Gg C year-1. This value is in reasonable agreement with the few available direct measurements of CO 2 fluxes in the downstream part of the river network