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Particulates, Not Plants, Dominate Nitrogen Processing in a Septage-Treating Aerated Pond System

^a Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02143
^b School for Marine Science and Technology, University of Massachusetts, 706 Rodney French Boulevard, New Bedford, MA 02744-1221

View larger version (39K): [in a new window]   Fig. 1. Schematic of the Marion, MA septage-treating pond–wetland system (not to scale). Percentages indicate fate of total nitrogen (TN) flowing into the aerated pond system containing floating plants over a 6-mo period (flow-weighted mass balance; Hamersley et al., 2001). Concentrations indicate composition of N species in pond influent and effluent (flow-weighted average during 6 mo).

View larger version (20K): [in a new window]   Fig. 2. Schematic of experimental tank system with varying root concentrations. Plant root concentrations were manipulated by changing the proportions of the tanks; tank volumes and hydraulic retention times were the same across treatments. One such experimental tank system was inserted into each of the two pond system tank trains. T2 and T5, aerated Tanks 2 and 5; SB, flow splitter box; L3 and L4, low root concentration treatment (2.5 g L-1 septage); H3 and H4, high root concentration treatment (7.1 g L-1 septage).

View larger version (20K): [in a new window]   Fig. 3. Change in NH4–N concentration with time during laboratory batch incubations of septage. Rates of change for particulate organic nitrogen (PON) and NOx–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).

View larger version (21K): [in a new window]   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 (Vmax) and half-saturation (Michaelis) constant (KM). No plant roots treatment (0.0 g L-1): Vmax = 0.608 ± 0.031 mg N L-1 h-1, KM = 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): Vmax = 0.942 ± 0.047 mg N L-1 h-1, KM = 0.62 ± 0.12 mg N L-1, n = 35, R2 = 0.88, P < 0.001. The Vmax 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 KM.

View larger version (19K): [in a new window]   Fig. 5. Substrate limitation of nitrification by NH4–N concentration follows Michaelis–Menten-type kinetics in eight experimental and two standard tanks during the in situ batch experiment (Fig. 2). Each point represents conditions in a single tank over a 2-h period, with measurements repeated every 2 h for 12 h total. Initial NH4–N concentrations varied from tank to tank due to differences in in situ conditions at start of experiment when flow between tanks was stopped. No significant differences were found between the parameters maximum (non-substrate-limited) nitrification rate (Vmax) and half-saturation (Michaelis) constant (KM) of nonlinear Michaelis–Menten fits to high and low (including standard) plant root treatments (P > 0.10). Line shows the Michaelis–Menten fit to the pooled data set. Vmax = 0.811 ± 0.059 mg N L-1 h-1, (P < 0.001), KM = 0.78 ± 0.13 mg N L-1, (P < 0.001), n = 60, R2 = 0.76.