Changes in extractable and microbial C, N, and P in a Western Australian wheatbelt soil following simulated summer rainfall

The effects of simulated rainfall events during the summer fallow period on extractable and microbial carbon (C), nitrogen (N), and phosphorus (P) in soils under either continuous wheat or the second-year pasture phase of a 2-pasture-1-wheat rotation in the Western Australian wheatbelt 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 total soluble N (TSN), total oxidisable C (TOC), Olsen-P, and ninhydrin-positive compounds (NPC). Prior to the simulated rainfall events, extractable TSN and TOC in the air-dry fellow soils were significantly higher (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 single and multiple wetting treatments, for extractable TSN, TOC, Olsen P, or NPC. Extractable soluble organic N (SON), calculated by subtracting measured inorganic N from TSN, decreased within 48 h of each wetting event to a minimal value but, after the first 2 wetting events, subsequently increased to at least the prewet value. Microbial C, N, and P were estimated from the difference in TOC, TSN, and Olsen-P of extracts from fumigated and unfumigated soils (microbial 'flush') and microbial C and N were also estimated from the NPC 'flush'. There was generally 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 response of the microbial biomass to wetting events and also significant differences in temporal patterns between the single and multiple wetting treatments. Microbial C and N increased in the period following initial 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 multiple wet treatment there were 2 highly significant quadratic responses to time, manifest as a linear increase in microbial C and N following 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 Day 10 until the end of the experiment (Day 14) microbial C and N exhibited a gradual linear increase in both wetting treatments, resulting in greater amounts of microbial C and N at the end than at the beginning of the experiment. Microbial P increased after the initial wet, more rapidly in the wheat than the pasture, then declined to pre-wet values by Day 7 and did not appear to respond to the re-wetting events; there were no signficant differences between wetting treatments and no interactions of land use and time. Cyclical changes in size and activity of the microbial biomass (i.e. microbial turnover) following sequential wetting and subsequent rapid drying of the soil surface during the summer fallow period are accompanied by production of available nutrients. These nutrients, in the absence of plants, may be at risk of being leached if there are heavy 'opening' rains prior to sowing at the start of the cropping season. However, this work has demonstrated that the microbial biomass, using readily available soluble organic C and N as an energy source, will immobilise a proportion of these nutrients to provide potentially an increased natural resource base for production of plant-available nutrients during the next cropping season.