Fig. 3. Change in NH₄–N concentration with time during laboratory batch incubations of septage. Rates of change for particulate organic nitrogen (PON) and NO_x–N are reported in the text. Values are means ± SE (n = 5); error bars are often smaller than points. (a) Increase in NH₄–N concentration due to N mineralization in treatments where nitrification was inhibited by addition of N-Serve. Initially, NH₄–N concentration declined due to sorption onto septage particulates. In treatments containing plant roots, N-Serve inhibition of nitrification decreased after 8 h. Nitrogen mineralization rates were calculated from the slopes (±SE) of linear regressions (lines shown) of unaveraged NH₄–N concentrations during the period of linear NH₄–N increase. Nitrogen mineralization was threefold higher with plant roots present (0.190 ± 0.031 mg N L^-1 h^-1, n = 20, P < 0.001), versus absent (0.0605 ± 0.0089 mg N L^-1 h^-1, n = 25, P < 0.001). (b) Decrease in NH₄–N concentration, due to nitrification in excess of N mineralization in treatments containing no plants or 7.1 g plant roots L^-1. Earlier work showed that plant uptake was <4% of denitrification losses (Hamersley et al., 2001).