The changes in microbial biomass C, soil respiration, microbial activity (respiration/microbial C) and the content of oxidizable organic C extracted by 0-5 M K2SO4, were measured in four soils of contrasting characteristics (a sand, two silt loam soils and a peat) which were air-dried at 22�C at three different rates in the laboratory. Respiration was also measured on samples of the drying soils rewetted with water. The rates of drying were: <10 h (fast), <33 h (medium) and <62 h (slow); drying was carried out for 6 h on consecutive days, with overnight storage. Measurements were also made on soils stored at field-moisture content over the 15 day duration of the experiment. Respiration and activity declined continuously and in a generally linear manner as the volumetric water content (W,) decreased. The decline in respiration in relation to water content W, was similar for all four soils and for the three rates of drying. Microbial biomass C also declined but generally only after a considerable initial period of drying (after the soils had reached Wv of 0-1-0.3). Extractable C values increased, but only after an initial drying period (Wv below 0.06-0.12). The increases in extractable C were approximately coincident with the decreases in microbial C, but only part of the increase in extractable C could be accounted for by the decrease in microbial C. Rewetting of dried soils caused a marked increase in respiration, particularly when the rewetted soils had reached Wv values where extractable C had begun to increase. The relationship between microbial activity and extractable C was similar for all four soils and was not affected by the rate of drying. The similarity of the microbial responses in these contrasting soils, and the absence of any detectable differences between rates of drying suggest that the microbial communities had similar survival strategies to resist desiccation, and occupied comparable physical niches in the soils, despite these soils having widely differing textures, organic matter content, and soil moisture characteristics.
A clay loam, a silt loam and a sand soil were gradually dried from field moisture content to air-dryness at 25�C in the laboratory. Microbial C measured by substrate-induced respiration (SIR), fumigation-incubation (FI) and fumigation-extraction (FE), microbial N-flush measured by FI and FE, microbial ATP content, and soil phosphatase and sulfatase activities were monitored throughout a drying period of approx. 60 h achieved over 16 days. All the microbial and enzyme variables declined as the gravimetric soil water content ( W) decreased to air-dryness. In general, the relationship between microbial C or N-flush and W was linear, but was exponential between ATP or phosphatase and W. Soil texture appeared to affect the rates of decline and also the amounts of the microbial and enzyme variables remaining in air-dry soil; e.g., the lowest rate of microbial C decline and the largest amount remaining at air-dryness occurred in the clay loam soil. Sulfatase activity was not significantly affected by soil drying. Agreement between the SIR and FE estimates of microbial C was good (r = 0.92***). These two methods were applicable over a wide range of water contents. Microbial N-flush, estimated by the FE method, also showed a consistent trend and correlated highly with microbial C estimated by SIR or FE. In contrast, microbial C and N-flush estimated by the FI method were not significantly correlated with W or any of the other variables. ATP and phosphatase activity appeared to relate more closely to microbial activity (CO2 respiration/microbial C) than microbial mass. The reliability of the methods to measure the biomass and the influence of soil texture, water and carbon contents on microbial survival are discussed.
The influence of the soil moisture regime on the tolerance of the soil micro-organisms to increased osmotic stress was examined by laboratory tests with a range of New Zealand soils. Soils from various climatic regions (moist, intermediate and dry) were amended with glucose-NaCl solutions, incubated for 0.5 h, and the respiration rate over the following 2 h was used as a measure of the response of the microbial biomass to the changed osmotic potential. Osmotic potentials were varied between -4 and -80 bar by altering the concentration of NaCl. Air-drying the soils at 25�C decreased the respiration response of the microbial biomass by 3-60% but had little effect on the tolerance of the surviving populations to decreased osmotic potentials. In general, the soils showed the same patterns: an osmotic potential of -23 bar decreased the respiration response by 28-45% (18-44% after air-drying) and a -80 bar potential decreased it by 64-86% (52-84% after air-drying). For the majority of soils, a consistent relationship was obtained between the respiration rate of the moist soils and the osmotic potential applied. A reasonable prediction of the respiration response after air-drying could be obtained from the respiration response of moist soils at -25 bar osmotic potential.
The influence of age of Fusarium oxysporum mycelia on mineralization in soil of their C, N and P contents following chloroform fumigation was determined with two soils from pasture of different productivity. The mineralization factors kc and kn could vary with mycelial age and, in prior-incubated soils, were lowest in the oldest (44-day) culture with the lowest cytoplasm and total N contents. Changes in kp with mycelial age showed no consistent trends. The use of k factors derived from physiologically young cells may lead to under-estimation of C and N in the indigenous soil biota.
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