A PRELIMINARY-STUDY TO PREDICT NET NITROGEN MINERALIZATION IN A FLOODED RICE SOIL USING ANAEROBIC INCUBATION

Complementary field and laboratory studies were conducted to determine whether laboratory measurements of net nitrogen (N) mineralisation un der anaerobic conditions could be used to predict field rates in a flo oded soil and N uptake by a rice crop. The laboratory experiment consi sted of measurements of ammonium accumulation at 10, 20, 30, and 40-de grees-C for 7, 14, and 28 days of anaerobic incubation. There was no a ccumulation of ammonium at 10-degrees-C, but increasing ammonification rate at temperatures of 20-degrees-C was observed, except for a slowe r rate at 40-degrees-C after 14 days. Two models were tested on the da ta: a zero-order reaction in which rate of mineralisation was a linear function of temperature; a first-order reaction in which net N minera lisation rate was a proportion of a depleting pool of labile organic N . In the second model, the rate was also linearly related to temperatu re. Both models fitted the laboratory data well (R2 = 0.94 and 0.97, r espectively), but the second model accounted better for mineralisation at 40-degrees-C for the 28-day incubation. These models were then run , using daily mean temperatures over a rice-growing season, to predict net mineralisation in the field. The predictions were compared with m easured net N mineralisation in a flooded soil and N uptake by the cro p measured throughout the season in the field from which the incubated soil was sampled. Net N mineralisation and crop uptake increased thro ughout the season, reaching maximum values of 115 and 111 kg N/ha at m aturity. The zero-order and first-order models both predicted net N mi neralisation accurately until the middle of the season, after which th e zero-order model overestimated net N mineralisation but the first-or der model predicted the reduction in the rate of net N mineralisation with reasonable accuracy. The close agreement between the laboratory i ncubations and field measurements of net mineralisation and crop N upt ake suggest that incubation tests may provide useful information for i ncluding in a model to assist ricegrowers' decisions about N fertilise r.