Mineralisation of soil organic nitrogen and microbial respiration after simulated summer rainfall events in an agricultural soil

Abstract

Simulated rainfall events were applied during the summer fallow period to a Western Australian agricultural soil, a loamy sand, under continuous wheat and after the second pasture phase of a 2 pasture : 1 wheat (2P : 1W) rotation. The aims of this study were to determine the change in microbial activity, amount of net and gross nitrogen (N) mineralised, and inorganic N produced after dry soil was re-wet during summer rainfall. Three irrigation treatments were applied: (i) control soils received no water, (ii) the single wet treatment received 45 mm of water on day 0 and was allowed to dry, and (iii) the multiple wet treatment received 45 mm of water on day 0 plus further applications of 5 mm on days 3 and 8. The CO₂ production and N mineralisation were measured in the surface 10 cm of soil for a period of 14 days after initial irrigation. Net N mineralisation was measured from in situ incubation of soil cores and gross N mineralisation by ¹⁵N isotopic dilution. The CO₂ production was measured by infrared gas analysis of air samples taken from a closed headspace above the soil. A large flush in CO₂ production and gross N mineralisation occurred immediately after rewetting the dry soil. This response was short-lived and rates of CO₂ production and gross N mineralisation declined rapidly after 2 days. After irrigation on day 0, CO₂ production was twice as large and gross N mineralisation was slightly larger in the 2P : 1W soil than continuous wheat. Gross N mineralisation and CO₂ production were not significantly different in the multiple and single wet treatments after the re-wet on day 3, but an additional flush in activity occurred in the multiple wet treatment after the re-wet on day 8. The patterns of gross N mineralisation and CO₂ production corresponded to changes in the soil water content, especially in the surface 2·5 cm of soil. Net and gross N mineralised and gross N immobilised over the 14-day period after the initial re-wet were greater in the multiple wet compared with the single wet treatments under both wheat and 2P : 1W. However, gross N mineralisation was approximately 4-fold greater than net N mineralisation under wheat and 15-fold greater under 2P : 1W. The majority of gross N mineralised after simulated summer rainfall was immobilised which resulted in an increase in inorganic N within the soil profile (0-50 cm) during opening winter rains.