Greater carbon allocation to mycorrhizal fungi reduces tree nitrogen uptake in a boreal forest

Abstract

The central role that ectomycorrhizal (EM) symbioses play in the structure and function of boreal forests pivots around the common assumption that carbon (C) and nitrogen (N) are exchanged at rates favorable for plant growth. However, this may not always be the case. It has been hypothesized that the benefits mycorrhizal fungi convey to their host plants strongly depends upon the availability of C and N, both of which are rapidly changing as a result of intensified human land use and climate change. Using large-scale shading and N addition treatments, we assessed the independent and interactive effects of changes in C and N supply on the transfer of N in intact EM associations with ~15 yr. old Scots pine trees. To assess the dynamics of N transfer in EM symbioses, we added trace amounts of highly enriched 15 NO 3 label to the EM-dominated mor-layer and followed the fate of the 15 N label in tree foliage, fungal chitin on EM root tips, and EM sporocarps. Despite no change in leaf biomass, shading resulted in reduced tree C uptake, ca. 40% lower fungal biomass on EM root tips, and greater 15 N label in tree foliage compared to unshaded control plots, where more 15 N label was found in fungal biomass on EM colonized root tips. Short-term addition of N shifted the incorporation of 15 N label from EM fungi to tree foliage, despite no significant changes in below-ground tree C allocation to EM fungi. Contrary to the common assumption that C and N are exchanged at rates favorable for plant growth, our results show for the first time that under N-limited conditions greater C allocation to EM fungi in the field results in reduced, not increased, N transfer to host trees. Moreover, given the ubiquitous nature of mycorrhizal symbioses, our results stress the need to incorporate mycorrhizal dynamics into process-based ecosystem models to better predict forest C and N cycles in light of global climate change.

Effects of nitrogen deposition on soil microbial communities in temperate and subtropical forests in China

“…Our results of the negative effects of N fertilization on fungi, especially arbuscular mycorrhizal fungi, and fungi to bacteria ratios in subtropical forests were in consistent with previous laboratory experiments and studies conducted in boreal, temperate and tropical forests (Högberg et al, 2003;DeForest et al, 2004;Allison et al, 2009;Van Diepen et al, 2011;Wu et al, 2013). Generally, the reduction of fungi resulted from less nutrients allocated to root and arbuscular mycorrhizal fungi due to the enhanced nutrient availability and plant nutrient acquisition by N deposition (Johnson et al, 2003;Treseder 2008;Hasselquist et al, 2016). Although we did not find significant effect of N fertilization on soil C, N and P content, our results highlighted an important direction that changes of fungal taxonomic groups in humid subtropical forests should be considered as well as the shift of the overall microbial community structure.…”

Inorganic nitrogen availability alters Eucalyptus grandis receptivity to the ectomycorrhizal fungus Pisolithus albus but not symbiotic nitrogen transfer

“…However, increased exogenous C availability to P. albus did result in both a decrease in the amount of N transferred to the plant and a trend to fewer mycorrhizal root tips formed. In experiments using plant shading to alter C transfer to ECM fungi, Hasselquist et al (2016) found a similar result with less N delivery to plant tissues when C transfer was high (unshaded). The decrease in N transfer in our experiment was not likely to be associated with a scarcity of N resources for P. albus as colonies with access to high C acquired more N from the substrate and maintained a similar C : N ratio to those colonies with limited access to C. Overall, these results correspond well with previous field-based studies showing that under low-N conditions, although trees allocate more C to their roots (H€ ogberg et al, 2010;Corrêa et al, 2011), this does not appear to correspond to an increase in N gain from the fungus (Corrêa et al, 2011;N€ asholm et al, 2013;Valtanen et al, 2014).…”

Dynamics of soil CO2 efflux under varying atmospheric CO2 concentrations reveal dominance of slow processes

“…Shading experiments can reduce C allocation to belowground less destructively, allowing evaluation of the effect of reduced carbohydrate source on belowground processes. In combination with enhanced N supply, such experiments can also alter carbohydrate sinks both aboveground and belowground, allowing further definition of processes (Hasselquist et al, 2016). However, the approach has only been employed over short vegetation: crops/grasses (Manderscheid, Pacholski, & Weigel, 2010), seedlings (Mao et al, 2016), or young trees (Hasselquist et al, 2016;Walcroft, Whitehead, Kelliher, Arneth, & Silvester, 2002).…”