The brigalow lands of central and southern Queensland have largely been cleared of their original Acacia harpophylla (brigalow) and Casuarina lehmanni (belah) vegetation and sown as perennial grass pastures based mainly on buffel grass (Cenchrus ciliaris) and green panic (Panicum maximum). Initially highly productive, these pastures show a decline in productivity to as little as 20% of their initial levels. The deterioration of grass pasture has been attributed to reduced availability of N in the soil despite their being high in total N (0.4% N in 0-20 cm) and despite annual crops not showing this problem.
The aim of this work was to examine the role of native soil microflora in the cycling of N in rundown clay pastures of brigalow region and assess its potential as a source or sink of mineral N in the system. A further aim was to determine the effect of various treatments on microflora, and N cycling under laboratory conditions.
The populations of bacteria, fungi and total microbial biomass N and C, as well as available N levels were evaluated in different seasons in soil under pasture compared with those under annual crop and original brigalow vegetation.
Three laboratory incubation experiments were conducted in which 14C (2 experiments) and 15N (all 3) were used to follow the turnover of N and C in soil cores from rundown brigalow pasture. 14C was added as a cocktail consisting of glucose/sodium acetate and 15N was added in the ammonium form. In addition to the undisturbed soil cores, treatments included a simulated renovation by cultivation (disturbance) and drying-rewetting. The length of these incubations varied from 10 days to 138 days.
The microbial biomass C content in surface clay soils of the native brigalow woodland was on average 3300 µg C.g-1 dry soil, 50% more that an associated rundown pasture and 200% more than an annually cropped legume. Microbial biomass N (500 µg N.g-1 dry soil) in native brigalow woodland soil was on average 41% higher than in pasture soil, which in turn was 164% higher than the legume plot. Soil NO3--N levels increased by nearly 100% in the upper stratum of the woodland soil between winter and spring. This increase was not evident in the pasture or legume sites. Viable counts of bacteria were 69.2% lower in the woodland sites. Regression analysis revealed a highly significant inverse relationship between soil NO3--N (N) and seasonal rainfall prior to sampling (R), which can be represented by the equation
N (µg N.g-1) = 54.11 - 0.67 R (mm.month-1)
A fungal population developed on addition of C/N substrate to brigalow clay soil cores. Simulated cultivation promoted the oxidation of recently synthesized microbial metabolites with a 15% increase in the amount of labile N incorporated into the biomass N fraction. The 14% and 17% increases in the amount of applied 15N label found in the biomass 7 and 14 days after disturbance coincided with 9% and 18% decreases in the non-biomass organic N fraction.
Net mineralization increased 0.17 µg N.g-1.d-1 after soil disturbance with maximum increases in soil NO3--N of 20 µg.g-1 dry soil apparent after 50 days and declining to 16 µq N.g-1 dry soil after 100 days. Forty-three % more was found in the soil NO3--N fraction of the disturbed cores compared to controls after 105 days.
Rapid reductions in mineralization after substrate depletion in the disturbed soils is accompanied by a narrowing of the microbial C/N content from 9.2 to 6.2 between days 3 and 56, suggesting the inability of the developing bacterial community to decompose cellular material from the preceding fungal population.
Between 21 and 63 days after disturbance, and under conditions of reduced microbial activity, 14C transfer rates through the microbial biomass for cultivated and control soils were 0.00486 and 0.00263 days-1 respectively. After 63 days, this trend was reversed with the turnover of C through the biomass in the cultivated situation slowing to 1/10 that of the undisturbed control. Gradual air drying of the surface profile of brigalow clay soils increased soil NO3--N levels by up to 50% or 24 µg N.g-1 dry soil over a 32 day period. Rapid proliferation of soil bacteria resulted on rewetting of the dried soil with a subsequent immobilization of 9 µg N.g-1 dry soil. The majority of the 40% increase in biomass can be attributed to non-biomass organic N.
Net reductions in mineralization of microbial cells after fumigation and incubation has resulted in the kc and kn values of 0.21 and 0.22 respectively being proposed for use in the calculation of microbial biomass C and N in the brigalow environment.
Fungi play a major role in the stabilization of the native brigalow woodland environment. Incubation studies have demonstrated that the addition of readily assimilated C/N substrates (e.g. root exudates) to brigalow pasture soils stimulates fungal growth and the mineralization of soil C. Pasture development has also resulted in the increased presence of prokaryotes in these soils. Under conditions of reduced C inputs, such as a bare fallow, this population may flourish, but is an ineffective decomposer of the cellular material from the preceding fungal population, relying on free and labile forms of N for growth and restricting mineral N accumulation for future plant growth.
The brigalow clay environment contains a very active microbial pool subsisting on a limited N, high energy budget, within the constraints of a large, stable sink of N. Destabilization of both biomass and non-biomass organic pools in the pasture situation is difficult with limited benefit to the plant community, particularly with the continued influx of carbonaceous material from root exudates or litter.