CHANGES IN EXTRACTABLE AND MICROBIAL C, N, AND P IN A WESTERN-AUSTRALIAN WHEAT-BELT SOIL FOLLOWING SIMULATED SUMMER RAINFALL

The effects of simulated rainfall events during the summer fallow peri od on extractable and microbial carbon (C), nitrogen (N), and phosphor us (P) in soils under either continuous wheat or the second-year pastu re phase of a 2-pasture-1-wheat rotation in the Western Australian whe atbelt were investigated. A 'single wetting' treatment (45 mm rainfall on Day 0) was compared with a 55 mm 'multiple wetting' treatment (45 mm at Day 0, 5 mm at Day 3, and 5 mm at Day 8). Soil samples from 0-10 cm depth were taken prior to, and at regular intervals up to 14 days following, the inital wetting event. Soil extracts were assayed for to tal soluble N (TSN), total oxidisable C (TOC), Olsen-P, and ninhydrin- positive compounds (NPC). Prior to the simulated rainfall events, extr actable TSN and TOC in the air-dry fellow soils were significantly hig her (P < 0.01), and Olsen-P significantly lower, for the pasture land use compared with the continuous wheat. However, subsequent to wetting there were no significant differences between the 2 land uses, or sin gle and multiple wetting treatments, for extractable TSN, TOC, Olsen P , or NPC. Extractable soluble organic N (SON), calculated by subtracti ng measured inorganic N from TSN, decreased within 48 h of each wettin g event to a minimal value but, after the first 2 wetting events, subs equently increased to at least the prewet value. Microbial C, N, and P were estimated from the difference in TOC, TSN, and Olsen-P of extrac ts from fumigated and unfumigated soils (microbial 'flush') and microb ial C and N were also estimated from the NPC 'flush'. There was genera lly good agreement between the 2 estimates of microbial N (NPC and TSN , R-2 = 0.50), but less so for the 2 estimates of microbial C (NPC and TOC, R-2 = 0.31). There was no significant difference in microbial C, N, or P between the 2 land uses, but there was a highly significant r esponse of the microbial biomass to wetting events and also significan t differences in temporal patterns between the single and multiple wet ting treatments. Microbial C and N increased in the period following i nitial wetting, more rapidly in the wheat than the pasture, reaching a peak at Day 2 for wheat and Day 3 for pasture. Subsequently, for the single wet treatment, there was a steady linear decline in microbial C and N until Day 10, whereas over the same period (Days 4-10) in the m ultiple wet treatment there were 2 highly significant quadratic respon ses to time, manifest as a linear increase in microbial C and N follow ing each re-wetting event, to a peak value 24 h after the event, and a subsequent decline to the pre-wet value after a further 24 h. After D ay 10 until the end of the experiment (Day 14) microbial C and N exhib ited a gradual linear increase in both wetting treatments, resulting i n greater amounts of microbial C and N at the end than at the beginnin g of the experiment. Microbial P increased after the initial wet, more rapidly in the wheat than the pasture, then declined to pre-wet value s by Day 7 and did not appear to respond to the re-wetting events; the re were no signficant differences between wetting treatments and no in teractions of land use and time. Cyclical changes in size and activity of the microbial biomass (i.e. microbial turnover) following sequenti al wetting and subsequent rapid drying of the soil surface during the summer fallow period are accompanied by production of available nutrie nts. These nutrients, in the absence of plants, may be at risk of bein g leached if there are heavy 'opening' rains prior to sowing at the st art of the cropping season. However, this work has demonstrated that t he microbial biomass, using readily available soluble organic C and N as an energy source, will immobilise a proportion of these nutrients t o provide potentially an increased natural resource base for productio n of plant-available nutrients during the next cropping season.