Isotopic evaluation of the role of arbuscular mycorrhizae in the nitrogen preference in Chinese fir seedlings

Chinese fir seedlings grow well in shrubland (including deciduous forest) soils without or less fertilizer application, but they sometimes harbor disease and show symptoms of nitrogen deficiency in ploughed (including several rotation of Chinese fir plantation) soils, where agricultural practice and clear-felling reduce the abundance and diversity of mycorrhizal fungi, and lead to destruction of mycorrhizae. Based on measurements of foliar δ15N or foliar δ15Nfol-soil in seedlings collected from 33 nurseries, we compared the effect of an AM-mediated process on nitrogen resource use between shrubland and ploughed soils. In mycorrhizal seedlings growing in shrubland soils, both foliar δ15N and foliar δ15N (fol-soil) were significantly higher than those in ploughed soils, likely because of enhanced high δ15N/NO3− absorption through AM-mediated pathways. Those results showed that foliar δ15N typically reflected the isotopic signature of the source pools of N. We suggest that the dominant N form taken up by fir seedlings growing in ploughed soils was NH4+-N rather than NO3−-N, where colonized root epidermis play an important role in exploiting soil N resource. However, the N form taken up by fir seedling growing in shrubland soils was primarily NO3−-N compared to NH4+-N, which is attributed to the high efficiency in an AM-mediated process rather than the dominance of N species in the different habitats. It is conceivable that combined colonized root epidermis with AM-mediated process may be more important than root epidermis alone in exploiting different forms of N in nursery soils. Therefore, in low N and acidic ecosystems, species other than the dominant N-NH4+, should be considered to satisfy the N demand for Chinese fir survival and growth, while the efficiency of an AM-mediated process should be determined by soil abiotic conditions.