Temporal Response of Soil Denitrification Rates to Rainfall and Irrigation

Abstract

Abstract The response of soil denitrification to increased soil moisture was compared in a non‐aggregated sandy loam soil and an aggregated clay loam soil using a soil core technique and the acetylene inhibition method. Elevated field denitrification rates were observed on 9 of 11 occasions on three sites following irrigation or rainfall of > 1 cm water. The denitrification rate in the sandy loam soil increased immediately after water addition and reached a maximum rate within 3–5 h and returned to preirrigation levels within 12 h. A similar, but slower denitrification response occurred in the clay loam soil, requiring 8–12 h before a maximum rate was observed and 48 h before the original background rate was restored. Maximum denitrification rates of 209 and 383 ng N g −1 d −1 occurred following water inputs of 7 and 2 cm in the sandy loam and clay loam soils, respectively. These water additions resulted in air‐filled porosities of 0.37 m 3 m −3 in the two soils. Nitrogen losses from the clay loam soil were double that of the sandy loam although the sandy loam received almost twice the water input. This difference was apparently due to the longer duration of the enhanced denitrification rate in the clay loam soil following the increase in the soil moisture. In the two soils 38 and 55% of the total N loss in late spring occurred within 48 h after rainfalls greater than 1 cm. These studies confirm that significant denitrification losses can occur in bursts in response to rainfall, and illustrate that sampling schemes based on integration of denitrification rate measurements must include these episodes to obtain meaningful estimates of N loss. A denitrification response to rainfall was not always the case, however, suggesting that NO ‐ 3 or carbon may also limit nitrogen loss.