Fig. 4. Substrate limitation of nitrification in laboratory batch experiment differs between treatments containing plants versus no plants. Values are means ± SE (n = 5). Lines are nonlinear Michaelis–Menten fits to unaveraged data points for calculation of maximum (non-substrate-limited) nitrification rate (V_max) and half-saturation (Michaelis) constant (K_M). No plant roots treatment (0.0 g L^-1): V_max = 0.608 ± 0.031 mg N L^-1 h^-1, K_M = 0.85 ± 0.14 mg N L^-1, n = 70, R² = 0.85, P < 0.001. Plant-containing treatment (7.4 g roots L^-1): V_max = 0.942 ± 0.047 mg N L^-1 h^-1, K_M = 0.62 ± 0.12 mg N L^-1, n = 35, R² = 0.88, P < 0.001. The V_max of nitrification in treatments containing plants was significantly higher than that of treatments containing no plants (P < 0.001), but no significant difference was found for K_M.