Influence of Nitrogen Limitation and Long-Term Use of Rockwool on Nitrous Oxide Emissions in Hydroponic Systems

To mitigate Nitrous Oxide (N2O) emissions derived from Nitrogen (N) fertilizer of agroecosystems, establishment of best management protocols for cultivation is necessary. Hydroponic systems using rockwool have the potential to reduce N2O emissions; however, the effects of nutrient condition and retained N compounds in rockwool on N2O emissions remain unclear. The primary objective of our study was to understand the crucial factors behind emissions of N2O. Tomato cultivation with low levels of nutrient showed reduced growth and yield, but increased N2O emission. In contrast, growth and N2O emissions were increased by cultivation with normal levels of nutrient and used (1-y-old) rockwool containing excess N compounds from the previous year's cultivation. Though the long-term use of rockwool significantly enhanced seasonal N2O emission, the availability of N2O precursors NO3- and NH4+ did not clearly explain the variation in N2O fluxes during cultivation. Rather, environmental factors, such as relative water content of rockwool in the rhizosphere, were significantly correlated to N2O emissions during cultivation under various conditions. We conclude that environmental factors most strongly influence the fate of available environmental substrates remaining in rockwool, and thereby control N2O emissions