The fate of algal nitrogen in a flooded soil system

Algal N labelled with N-15 added to a flooded soil in laboratory columns wi thout plants was studied to determine the changes over time in the fate of N assimilated by algae and to study how its fate is affected by (a) exclusi on of light simulating complete closure of the rice canopy, and (b) additio n of fertilizer-NH4*. In the light but with no added fertilizer-N there was little net mineralization of the added algal N during the first 4 weeks, b ut after 8 weeks 42% had been mineralized, of which 95% was denitrified. Ex clusion of light caused net mineralization to proceed more rapidly in the f irst 4 weeks due to the death of algal cells and lowered reassimilation. Af ter 8 weeks 51% had been mineralized, of which 54% was denitrified, 16% vol atilized and 30% was present as KCl exchangeable NH4+-N. Application of fer tilizer-NH4+ apparently caused mineralization of 25% of the algal N within one week but the results were probably affected by pool substitution in whi ch labelled N mineralized to NH4+-N was diluted with fertilizer - NH4+ and then immobilized leaving more labelled NH4-N in the mineral pool. After 8 w eeks, 42% of algal N had been mineralized, of which 69% was estimated to ha ve been denitrified, 19% lost through NH3 volatilization and 12% remained a s extracted NH4++NO3-. Uptake of N by a rice crop would reduce the gaseous losses. Algal N was mineralized quickly enough to be available during the g rowing season of a rice crop and, depending on field conditions, algae may have a role in assimilating N and protecting it from loss as well as being a major driving force for NH3 volatilization through diurnal increases in p H.