Abstract
Plants spend a high proportion of their photosynthetically fixed carbon (C) belowground to support mycorrhizal associations in return for nutrients, but this C expenditure may decrease with increased soil nutrient availability. In this study, we assessed how the effects of nitrogen (N) fertiliser on specific root respiration (SRR) varied among mycorrhizal type (Myco type). We conducted a multi-level meta-analysis across 1,600 observations from 32 publications. SRR increased in ectomycorrhizal (ECM) plants with more than 100 kg N ha−1 applied, did not change in arbuscular mycorrhizal (AM) and non-mycorrhizal (NM) plants, but increased in plants with a dual mycorrhizal association in response to N fertilisation. Our results suggest that high N availability (>100 kg N ha−1) could disadvantage the growth of ECM plants because of increased C costs associated with maintaining higher root N concentrations, while the insensitivity in SRR by AM plants to N fertilisation may be because AM fungi are more important for phosphorus (P) uptake.
Highlights
Plants allocate up to 50% of photosynthetically fixed carbon (C) to root biomass and rhizodeposition (Pausch and Kuzyakov, 2018)
Under high N fertilisation, specific root respiration (SRR) increased in ECM plants (Figure 3B), while it showed a non-significant increase in plants associated with arbuscular mycorrhizal (AM)+ECM
N fertilisation level was the second most important moderator of SRR effect sizes. When both factors were examined together, SRR increased in ECM plants at high N fertilisation rates (>100 kg N ha−1) and for AM+ECM plants at low N fertilisation rates (≤100 kg N ha−1)
Summary
Plants allocate up to 50% of photosynthetically fixed carbon (C) to root biomass and rhizodeposition (Pausch and Kuzyakov, 2018). The C cost associated with forming and maintaining different types of mycorrhizas remains highly uncertain, with estimates as low as 4% and as high as 25% of the net primary production of a plant allocated to the fungi (Hobbie, 2006; Johnson and Gehring, 2007; Stuart and Plett, 2020). Plants associated with AM and ECM fungi differ in their nutrient economies and related biogeochemical transformations of C and nutrients (Phillips et al, 2013). It can, be expected that variation in nutrient availability will affect root respiratory C costs of AM and ECM plants, with consequences for their productivity and abundance in terrestrial ecosystems
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