Effects of Warming and Oxalic Acid Addition on Plant–microbial Competition in Picea Brachytyla

The importance of oxalic acid for tree seedling growth and the competition for inorganic nitrogen (N) by plants and soil microorganisms under warming was investigated using 15N tracer techniques in Picea brachytyla (Franch.) E. Pritz. Results showed that warming combined with oxalic acid application induced growth enhancements in seedlings primarily through increases in fine root length and fine root surface area. Moreover, soil NH4 +, NO3 –, PO4 3–, N mineralization, microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN) were significantly higher or tended to be higher with oxalic acid application. However, warming with oxalic acid application altered the partitioning of N between plants and soil microorganisms by increasing microbial 15N recovery to a lesser extent than it increased plant 15N recovery. While plants showed no specific preferences between N forms under normal conditions or warming alone, under warming and oxalic acid, plants showed a preference for 15NO3. Microorganisms showed a stronger preference for 15NH4 especially under warming and oxalic acid treatments. These findings suggest that plasticity in resource use could be an important mechanism in alleviating competition for soil N between plants and microbes under warming and oxalic acid addition.