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ABSTRACT Labile carbon (C) inputs in soils are expected to increase in the future due to global change drivers such as elevated atmospheric CO2concentrations or warming and potential increases in plant primary productivity. However, the role of mycorrhizal association in modulating microbial activity and soil organic matter (SOM) biogeochemistry responses to increasing below‐ground C inputs remains unclear. We employed18O–H2O quantitative stable isotope probing to investigate the effects of synthetic root exudate addition (0, 250, 500, and 1000 μg C g soil−1) on bacterial growth traits and SOM biogeochemistry in rhizosphere soils of trees associated with arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi. Soil respiration increased proportionally to the amount of exudate addition in both AM and ECM soils. However, microbial biomass C (MBC) responses differed, increasing in AM and decreasing in ECM soils. In AM soils, exudate addition increased taxon‐specific and community‐wide relative growth rates of bacteria, leading to enhanced biomass production. Conversely, in ECM soils, relative growth rates were less responsive to exudate addition, and estimates of MBC mortality increased with increasing exudate addition. In the AM soils, aggregated bacterial growth traits were predictive of soil respiration, but this relationship was not observed in ECM soils, perhaps due to substantial MBC mortality. These findings highlight the distinct responses of bacterial communities in AM and ECM rhizosphere soils to exudate addition. Considering that microbial products contribute to the formation of stable soil organic carbon (SOC) pools, future increases in labile exudate release in response to global change may consequently lead to greater SOC gains in AM soils compared to ECM soils.
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Shifts in bacterial traits under chronic nitrogen deposition align with soil processes in arbuscular, but not ectomycorrhizal‐associated trees
Abstract Nitrogen (N) deposition increases soil carbon (C) storage by reducing microbial activity. These effects vary in soil beneath trees that associate with arbuscular (AM) and ectomycorrhizal (ECM) fungi. Variation in carbon C and N uptake traits among microbes may explain differences in soil nutrient cycling between mycorrhizal associations in response to high N loads, a mechanism not previously examined due to methodological limitations. Here, we used quantitative Stable Isotope Probing (qSIP) to measure bacterial C and N assimilation rates from an added organic compound, which we conceptualize as functional traits. As such, we applied a trait‐based approach to explore whether variation in assimilation rates of bacterial taxa can inform shifts in soil function under chronic N deposition. We show taxon‐specific and community‐wide declines of bacterial C and N uptake under chronic N deposition in both AM and ECM soils. N deposition‐induced reductions in microbial activity were mirrored by declines in soil organic matter mineralization rates in AM but not ECM soils. Our findings suggest C and N uptake traits of bacterial communities can predict C cycling feedbacks to N deposition in AM soils, but additional data, for instance on the traits of fungi, may be needed to connect microbial traits with soil C and N cycling in ECM systems. Our study also highlights the potential of employing qSIP in conjunction with trait‐based analytical approaches to inform how ecological processes of microbial communities influence soil functioning.
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- Award ID(s):
- 2114570
- PAR ID:
- 10477418
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Global Change Biology
- Volume:
- 30
- Issue:
- 1
- ISSN:
- 1354-1013
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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