Temporal Response of Soil Denitrification Rates to Rainfall and Irrigation

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~' d~' 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 m~ 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 1 Contribution from the Dep. of Crop & Soil Sciences and of Microbiology & Public Health, Michigan State Univ., East Lansing, MI 48824-1114. Published as Journal Article no. 11212 of the Michigan Agric. Exp. Stn. This work was supported by National Science Foundation Grant DEB-80-12168, USDA Cooperative Agreement no. 58-32U4-1-329 and USDA Regional Research Project NE-39. Received 4 Nov. 1983. Approved 6 Aug. 1984. 2 Former Graduate Student, Research Associate, and Professor of Soil Microbiology, respectively. Present addresses: A.J.S. , Division of Plant and Soil Sciences, West Virginia Univ., Morgantpwn, WV 26506 and T.B. P., Soil Nitrogen & Environmental Chemistry Laboratory, Beltsville Agric. Res. Center, Beltsville, MD 20705. rainfall was not always the case, however, suggesting that NO3 or carbon may also limit nitrogen loss. Additional Index Words: acetylene inhibition method, air-filled porosity, soil cores, soil moisture, nitrogen loss. Sexstone, A. J., T. B. Parkin, and J. M. Tiedje. 1985. Temporal response of soil denitrification rates to rainfall and irrigation. Soil Sci. Soc. Am. J. 49:99-103. T RATE OF NITROGEN lOSS from Soil due tO denitrification is increased by factors that increase the extent of anaerobic sites in soil (Firestone, 1982). Increasing soil moisture acts as one such factor by decreasing the rate of oxygen diffusion through the soil matrix, allowing the development of anaerobic microsites as oxygen consumption exceeds the rate of diffusive supply. Several laboratory studies have demonstrated increased nitrogen loss with increasing moisture content from soils incubated in the presence of an aerobic soil atmosphere (Nommik, 1956; Pilot and Patrick, 1972; Bailey and Beauchamp, 1973). These studies have generally involved the use of disturbed soils and long-term incubations of several weeks. Smith and Tiedje (1979) suggested that it was important to understand the short-term denitrification response to increasing soil moisture, in order to better characterize denitrification N losses from field soils following rainfall or irrigation. They demonstrated that when the soil moisture content of a sandy loam soil was increased from 51% to 94% of saturation the denitrification rate increased rapidly after a lag period of 6 h. McGarity and Rajartnam (1973) recognized that the denitrification rate in field soils would be controlled 100 SOIL SCI. SOC. AM. J., VOL. 49, 1985 by epidsodic factors such as increasing soil moisture, and suggested that techniques were necessary for accurate short-term measurements that preserved the physical control of biological rates imposed by soil structure. Of the techniques subsequently developed for measurement of denitrification in the field, the acetylene inhibition technique (Yoshinari et al., 1976) appears to be the most suitable for short-term measurements. Artificially irrigated soil cores have been used to compare denitrification as affected by tillage practice on several field sites (Rice and Smith, 1982). No-till soils showed significantly greater nitrogen flux than conventionally tilled soils, which was attributed to the higher soil moisture content maintained in notill soils. Aulakh et al., (1982) made once-weekly measurements at various field sites using a soil core technique and observed that the denitrification rate was greatly elevated when rainfall had decreased the soil air-filled porosity (AFP). Ryden et al., (1979), Ryden and Lund (1980), and Rolston et al. (1982) used a soil cover technique to measure denitrification and reported that the N2O flux was greatest immediately following irrigation. The studies all show the expected relationship between moisture and denitrification, but they do not characterize and quantify the temporal response of denitrification rate to rainfall, which can be an important factor in determining the quantity of N lost. In this paper, we report on the short-term response of field denitrification rates to rainfall and irrigation. Peak denitrification rates usually occurred for brief periods after a moisture addition, but the response time and duration were different for light and heavy textured soil. MATERIALS AND METHODS