Consistent with past studies conducted in this site (Midgley et al. 2015; Midgley and Phillips 2016), we found that the effect of stand mycorrhizal type on soil properties was stronger and more consistent than the effect of N addition on soil properties (Table S1). Specifically, ECM stands were characterized by significantly lower NO3- concentrations (F1, 12 = 169.61, P < 0.0001; Table S1) and pH (F1, 13.77 = 31.18, P < 0.0001; Table S1) but significantly higher microbial biomass C (F1, 186.06 = 73.18, P < 0.0001), microbial biomass N (F1, 186.16 = 29.77, P < 0.0001), microbial biomass C:N ratios (F1, 183.29 = 7.44, P = 0.007) and C mineralization (F1, 12.03 = 48.96, P < 0.0001) rates compared to AM stands (Table S1). In contrast to most soil properties measured, NH4+ concentrations were similar between stand types (Table S1). We found that N addition increased soil NH4+ and NO3- concentrations (F1,93 = 17.45, P < 0.0001; F1,93 = 44.09, P < 0.0001), and decreased soil pH (F1,94.90 = 7.05, P = 0.009) with average pH values of 4.57 and 4.71 for N addition and control plots, respectively (Table S1). Although nitrogen addition had no effect on microbial biomass C or microbial biomass N, microbial biomass C:N ratios were significantly greater in control compared to N addition plots in ECM-dominated stands (P = 0.0031), but not AM-dominated stands. The effect of N addition on C mineralization rates was marginally significant with control plots trending towards greater C mineralization rates relative to N addition plots (F1, 204.08 = 3.07, P = 0.08; Table S1).
We found that gross N mineralization and specific gross N mineralization differed significantly between ECM and AM forest stands, but the effect of stand mycorrhizal type was opposite for the two metrics of mineralization (Fig. 1a,b). Gross N mineralization was greater in ECM stands compared to AM stands (F1,102.03 =8.95, P = 0.003; Fig. 1a) whereas specific gross N mineralization was greater in AM stands compared to ECM stands (F1,99.06=14.44, P = 0.0003; Fig. 1b). Opposite to patterns in gross N mineralization, net N mineralization rates were significantly greater in AM stands compared to ECM stands (F1, 12 = 49.41, P < 0.0001; Fig. 2a). In contrast, gross NH4+ assimilation aligned with patterns of gross N mineralization, with greater rates in ECM relative to AM stands (F1,65.23 = 21.29, P < 0.0001; Fig. 2b). Net N mineralization (F1,93 = 16.60, P < 0.0001) and NH4+ assimilation (F1,65.09 = 8.35, P = 0.005) increased with N addition, but there was no effect of N addition on gross N mineralization or specific gross N mineralization (Fig. 1ab, 2ab).
The effect of stand mycorrhizal type on nitrification rates was consistent across both nitrification assays and control versus N addition treatments, with lower rates in ECM stands. Potential and net rates were lower in ECM compared to AM-dominated stands, though this effect was only marginally significant for potential nitrification rates (Potential, F1, 12.25 = 3.68, P = 0.08; Fig. 3a; net, F1,12 = 253.89, P < 0.0001; Fig. 2c), with similar patterns across N addition and control treatments. Nitrogen addition resulted in significantly greater net nitrification rates (F1,93 = 21.96, P < 0.0001), but had no effect on potential nitrification rates (Fig. 2c, 3a). In contrast to NO3- production processes, NO3- assimilation rates did not differ between ECM and AM stands and increased with N addition (F1,74.02 = 23.95, P < 0.0001; Fig. 2d).
We found that total (nitrification- plus denitrification-derived), nitrification-derived, and denitrification-derived net N2O fluxes exhibited similar patterns to nitrification rates with lower fluxes in ECM compared to AM stands (total, F1,12.02 = 32.56, P < 0.0001; nitrification-derived, F1,11.93 = 7.46, P = 0.02; denitrification-derived, F1,79 = 10.34, P < 0.0001; Fig. 4a-c). However, for nitrification-derived N2O fluxes, this effect was only significant in N addition plots (Fig. 4a). Potential total denitrification and potential incomplete denitrification were also significantly lower in ECM compared to AM stands (F1,12.10 = 27.35, P < 0.0002; F1,12.05 = 41.63, P < 0.0001; Fig. 3bc). Total net N2O fluxes were significantly greater in N addition compared to control plots (F1,204.07 = 21.87, P < 0.0001) (Fig. 4c). The effect of N addition on denitrification-derived N2O fluxes was marginally significant with greater fluxes in N addition compared to control plots (F1,79 = 3.08, P = 0.08; Fig. 4b). However, for nitrification-derived net N2O fluxes and for potential total and potential incomplete denitrification, there was no effect of N addition (Fig. 3bc, 4a).