Transformations of nitrogen-15-labelled urea in a flooded soil as affectedby floodwater algae and green manure in a growth chamber

The effects of floodwater algae and green manure on transformations of N-15 -urea were studied in columns of a sandy loam soil in a growth chamber. The columns were flooded and either kept in the light, to allow algal growth, or in the dark (control) for 17 days before adding the labelled urea. Chang es in urea-, NO3- and NH4+-N levels and the pH of the floodwater were measu red over the subsequent 41-day period, during which the control column rema ined in the dark and those containing algae were maintained either in the d ark to cause the death of the algae or in the light. Volatilized NH3 was mo nitored, and on termination of the experiment the distribution of N-15 betw een NO3-, NH4+ and organic forms was measured in the soil. Urea hy drolysis was most rapid in the presence of both living algae and green manure, foll owed by dead algae, and was slowest in the control. The concentration of NH 4--N in the floodwater was, however, reduced in the presence of algae due t o assimilation and NH3 volatilization owing to the raised day-time pH in th e floodwater. NH3 volatilization for the first 10 days was rather high in t he columns kept in the light compared to those in the dark. Total volatiliz ation plus denitrification losses were greatest where dead algae were prese nt, owing to the absence of live algae which assimilated more than half of the applied N. Algal growth in floodwater increased the depth of the aerobi c soil layer present at the soil-water interface. Subsequently, under dark conditions, stimulated algal growth reduced the depth of the aerobic layer causing less nitrification, which resulted in lower losses of N due to deni trification, i.e. 17% of the applied urea-N as compared to 39% in the light treatments. Although the presence of green manure caused a marked increase in the rate of hydrolysis, algal assimilation prevented excessive N losses via volatilization, indicating that the retention of higher quantities of NH4+-N may have increased fertilizer-N use efficiency.