MINERALIZATION OF SOIL ORGANIC NITROGEN AND MICROBIAL RESPIRATION AFTER SIMULATED SUMMER RAINFALL EVENTS IN AN AGRICULTURAL SOIL

Simulated rainfall events were applied during the summer fallow period to a Western Australian agricultural soil, a loamy sand, under contin uous 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) mineralise d, and inorganic N produced after dry soil was re-wet during summer ra infall. Three irrigation treatments were applied: (i) control soils re ceived 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 i n the surface 10 cm of soil for a period of 14 days after initial irri gation. Net N mineralisation was measured from in situ incubation of s oil cores and gross N mineralisation by N-15 isotopic dilution. The CO 2 production was measured by infrared gas analysis of air samples take n from a closed headspace above the soil. A large flush in CO2 product ion and gross N mineralisation occurred immediately after rewetting th e dry soil. This response was short-lived and rates of CO2 production and gross N mineralisation declined rapidly after 2 days. After irriga tion on day 0: CO2 production was twice as large and gross N mineralis ation was slightly larger in the 2P:1W soil than continuous wheat. Gro ss N mineralisation and CO2 production were not significantly differen t 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 trea tment after the re-wet on day 8. The patterns of gross N mineralisatio n and CO2 production corresponded to changes in the soil water content , especially in the surface 2.5 cm of soil. Net and gross N mineralise d 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 trea tments under both wheat and 2P:1W. However, gross N mineralisation was approximately 4-fold greater than net N mineralisation under wheat an d 15-fold greater under 2P:1W. The majority of gross N mineralised aft er simulated summer rainfall was immobilised which resulted in an incr ease in inorganic N within the soil profile (0-50 cm) during opening w inter rains.