Nitrogen cycle and microorganisms in a hydroponic system as influenced by the amount of nitrogen applied

To avoid potential water pollution from intensive glasshouse horticulture, closed systems with recirculated drain water are required. Nevertheless, nitrogen losses up to 30 % of the N applied have been observed also in closed growing systems. Therefore, the aims of the present study were to determine the nitrogen in- and output in a closed hydroponic system and to monitor the microbial population dynamics. These investigations focused on organisms involved in ammonification, nitrification, nitrate reduction and denitrification in the rhizoplane of tomato plants as well as in rain water. The experiment was carried out in 1994 under commercial conditions during a 6-month growth period with tomatoes (Lycopersicon lycopersicum cv. Counter) at the Institute in Grossbeeren. In a greenhouse of 1,500 m(2) three N concentrations between 11, 38 and 57 mmol(.)l(-1) were investigated controlled by means of three electrical conductivity levels (EC 1.6, 5.3 and 7.1 dS m(-1)). The plants absorbed different amounts of N depending on the EC level. The N content in all plant organs increased with increasing EC. Total uptake per growing area showed its maximum at EC 5.3 dS m(-1). With higher EC levels the uptake decreased due to the lower yield and, consequently, biomass production. Total bacterial cell numbers were in the order of 10(6) to 10(7) cells per independent of their fresh roots. Generally, bacterial populations declined with increasing EC > 1.6 dSm(-1) function in the N cycle, except for denitrifying bacteria. During the growing period the denitrifying bacteria proliferated while all other populations decreased, especially in the rhizoplane of plants grown with high EC level. Bacterial groups involved in N turnover were found in the rainwater used for mixing the nutrient solution and in extremely high concentrations on the tomato roots during the whole growing period.