NITROGEN CYCLING IN AN INTEGRATED 'BIOMASS FOR ENERGY' SYSTEM (WATER HYACINTH, SLUDGE, DETRITUS, EFFLUENT, ANAEROBIC DIGESTION)

A series of experiments was conducted to evaluate N cycling in three components of an integrated "biomass for energy" system, i.e. water hyacinth production, anaerobic digestion in hyacinth biomass, and recycling of digester effluent and sludge. Plants assimilated 50 to 90% of added N in hyacinth production systems. Up to 28% of the total plant N was contained in hyacinth detritus. Nitrogen loading as plant detritus into hyacinth ponds was 92 to 148 kg N ha('-1) yr('-1).; Net mineralization of plant organic ('15)N during anaerobic digestion was 35 and 70% for water hyacinth plants with low (10 g N kg('-1) dry tissue) and high (35 g N kg('-1)) N content, respectively. Approximately 20% of the ('15)N was recovered in the digested sludge while the remaining ('15)N was recovered in the effluent.; Water hyacinth growth in digester effluents was affected by electrical conductivity (0.7 to 6.7 dS m('-1)) and ('15)NH(,4)('+)-N concentration (23 to 289 mg N L('-1)). Biomass yields were maximum at electrical conductivities of < 2.5 dS m('-1) and ('15)NH(,4)('+)-N concentrations of < 100 mg N L('-1).; Addition of water hyacinth biomass to soil resulted in decomposition of 39 to 50% of added C for fresh plant biomass and 19 to 23% of added C for digested biomass sludge. Only 8% of added ('15)N in digested sludges was mineralized to ('15)NO(,3)('-)-N despite differences in initial N content (27 and 39 g N kg('-1) dry sludge). In contrast, 3 and 33% of added ('15)N in fresh biomass with low and high N content, respectively, was recovered as ('15)NO(,3)('-)-N.; Total ('15)N recovery after anaerobic digestion ranged from 70 to 100% of the initial plant biomass ('15)N. Land application of digester sludge resulted in the mineralization of 2% of initial biomass ('15)N into plant available form. Use of water hyacinth for digester effluent treatment resulted in recycling of 21 to 38% of the initial biomass ('15)N. Total N recovery by sludge and effluent recycling in the integrated "biomass for energy" system was 48 to 60% of the initial plant biomass ('15)N. The remaining ('15)N was lost from the system during anaerobic digestion and effluent recycling.