4.2 Trade-offs of nutrient uptake and soil exploration types inAbies faxoniana
As the different impacts of soil exploration types on root and foliar N and P nutrients, we confirmed the hypothesis about the differences in nutrient uptake of soil explorations types. We found that the concentrations of root and foliar N and P in A. faxoniana were positively associated with the frequency of contact exploration type (Figure 3; Figure 4a, b; Figure 5a), and negatively with that of the short-distance and the medium-distance exploration types (Figure 4a, b). Especially, root N and P decreased with the improvement of the frequency of short-distance exploration type, while concentration of root P varied with the frequency of the medium-distance exploration type in 9%-30% with a quadratic relationship in this study (Figure 5b, c, d). This suggests that the extension of emanates across natural environmental gradients may simply be a passive response of ECM fungi to N and P deficiency rather than for facilitating N and P uptakes of host trees. This is different with these findings which often demonstrate positive relationships between the nutrient status of host plants and the length of emanates of ECM roots (Agerer, 2001; Brandes et al ., 1998; Hobbie & Agerer, 2010; Lilleskov et al. , 2011). In this study, the soil N and P were mostly deficient across the study sites (alkaline N: 40.78±18.83 mg g-1, total P: 0.87±0.26 mg g-1). It is likely that the contact exploration type and the short-distance exploration type facilitated the uptake of alkaline N and available P, whereas the medium-distance exploration type helped forage the organic N and P far from the root distal (Agerer, 2001; Hobbie & Agerer, 2010). Our study showed the facilitations of the contact exploration type in nutrient uptake (Figure 4; Figure 5). Interestingly, we found that root P limitation could be relieved when colonization ratio of medium-distance exploration type up toc. 15%, whilst root N limitation was strengthening when the frequency of medium-distance exploration type over 20% (Figure 5e). It suggested the converse or negative ecological function on plant nutrient uptake among the soil exploration types which need considerable C requirement despite the host tree under both heavy nutrient limitations. Ostonen et al. (2007, 2011) proposed that host tree always relied on the high efficiency of resource capture of the root-mycorrhiza continuum whilst investing little C to ECM root systems. Besides, plants would cut down the investment when C allocation outweigh the benefit obtained from ECM fungi (Johnson et al. 2003; Treseder, 2004), or ECM fungi sometimes hold the nutrients for itself in priority resulting in host tree remaining nutrient deficient under extreme nutrient limitations (Treseder & Allen, 2002). The improved root and foliar N and P by the occurrence of the contact exploration type and the negative relationships with the frequency of the short-distance and the medium-distance exploration types may partially attribute to trade-offs between the C allocation to ECM emanates and nutrient uptake in host plants (Johnson et al. , 2013; Magyar et al. , 2007).