Nutrient Cycling in Agroecosystems 55: 89–94, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 89 The fate of algal nitrogen in a flooded soil system H.S. Thind & D.L. Rowell ∗ Department of Soil Science, The University of Reading, Whiteknights, PO Box 233, Reading RG6 6DW, Berkshire, UK ( ∗ corresponding author) Received 8 July 1997; accepted in revised form 8 January 1999 Key words: algae, flooded soils, N cycling, nitrogen, 15 N, rice Abstract Algal N labelled with 15 N added to a flooded soil in laboratory columns without 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) exclusion of light simulating complete closure of the rice canopy, and (b) addition of fertilizer-NH 4 ∗ . In the light but with no added fertilizer-N there was little net mineralization of the added algal N during the first 4 weeks, but after 8 weeks 42% had been mineralized, of which 95% was denitrified. Exclusion of light caused net mineralization to proceed more rapidly in the first 4 weeks due to the death of algal cells and lowered reassimilation. After 8 weeks 51% had been mineralized, of which 54% was denitrified, 16% volatilized and 30% was present as KCl exchangeable NH 4 + -N. Application of fertilizer-NH 4 + apparently caused mineralization of 25% of the algal N within one week but the results were probably affected by pool substitution in which labelled N mineralized to NH 4 + -N was diluted with fertilizer – NH + 4 and then immobilized leaving more labelled NH 4 –N in the mineral pool. After 8 weeks, 42% of algal N had been mineralized, of which 69% was estimated to have been denitrified, 19% lost through NH 3 volatilization and 12% remained as extracted NH 4 + +NO − 3 . Uptake of N by a rice crop would reduce the gaseous losses. Algal N was mineralized quickly enough to be available during the growing 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 NH 3 volatilization through diurnal increases in pH. Introduction Rice accounts for 21% of the total energy content of the world’s food. It ranks second to wheat in terms of area harvested but in terms of importance as a food crop, rice provides more calories per hectare than any other cereal crop. About 87% of the 145 million hec- tares of rice land is in lowlands where rice grows in flooded fields during part or all of the cropping period (IRRI, 1989). Early in the growth of the crop, urea is the main source of N. Algae flourish in the flood- water and affect nutrient uptake and floodwater pH. The latter is largely controlled by algal photosynthesis and fluctuates diurnally with the maximum value of 9.5 or above (Bowmer & Muirhead, 1987; Fillery et al., 1984, 1986; Mikkelsen et al., 1978; Simpson et al., 1988). Ammonia volatilization from fertilizer -N is encouraged by high pH and warm conditions. Stud- ies with 15 N show that applied N is also assimilated by algae (Craswell et al., 1985; Meyer et al., 1989; Vlek & Craswell 1979; Vlek et al., 1980) and, with subsequent mineralization may be an important part of the N cycle in rice fields. The availability to rice of the N in blue-green algae and Azolla has been quantified in 15 N experiments. The N recovery from Azolla ranged from 12 to 34% (Roger, 1988). Wilson et al. (1980) found that 14–50% was recovered, the greatest recovery being when fresh algal material was incorporated into the soil. Residual effects of algal N have been observed in which 4–7% of algal N was recovered by a second rice crop (Grant & Seegers, 1985; Tirol et al., 1982). In all these stud- ies, 15 N recovery was measured in mature rice crops. The algae and Azolla had been grown separately and then applied to the soil, in some cases to a dry soil surface before flooding. This is a poor simulation of