As part of a broader study, the aim of which is to identify soil factors that might be associated with yield decline of sugar cane, microbial biomass and protease activities were examined in soil samples collected from seven paired old and new land sites in three cane-growing districts of north Queensland. No consistent changes in soil protease activities were observed, although some sites exhibited specific effects, as a result of extensive periods of sugar cane monoculture. Soil microbial biomass, however, was significantly lower in those soils where sugar cane had been grown for extended periods. The implications of a lowering of soil microbial biomass on sugar cane yields and sustainability are discussed.

The critical P concentration in microbial biomass (defined as that required to achieve 80% of the maximum synthesis of microbial biomass C) and minimum amount of available P to obtain the critical P concentration in the microbial biomass of a granitic regosol and an andosol of Japan were examined. Phosphorus was applied as KH2PO4 at rates of 0, 25, 50, 200 and 400 mg P kg-1 soil to a regosol and 0, 25, 100, 400 and 800 mg P kg-1 to an andosol together with 2000 mg C (rice straw) and 200 mg N (ammonium sulphate) kg-1. With increasing P application, the available P in soil markedly increased in the regosol and gradually increased in the andosol. The amount of microbial biomass C and P increased with available P up to 76 and 29 mg P kg-1 in the regosol and andosol, respectively, and either remained constant or was slightly decreased at a higher available P value. The concentration of P in the microbial biomass was higher in the regosol (29 to 89 mg P g-1) than in the andosol (13 to 32 mg P g-1), assuming that 1 g of dry biomass contained 0.5 g C. The microbial biomass C to P ratio was higher in the andosol (16 to 38) than in the regosol (6 to 17). The critical P concentration in microbial biomass was estimated to be 62 mg P g-1 biomass in the regosol and 19 mg P g-1 in the andosol. The corresponding minimum value of available P in soil to increase microbial biomass was estimated as 38 and 6 mg P kg-1 soil in the regosol and andosol, respectively. The specific respiration of microbial biomass was also very high at those P concentrations which were considered optimum in both soils to increase not only the amount of microbial biomass C and P but also microbial activity. These were 38 mg P kg-1 soil in the regosol and 6 mg P kg-1 soil in the andosol.

The horizontal and vertical variability of "total" soil microbial biomass measured by the fumigation-extraction method (FEM), and the extraction efficiency coefficient K(EC), were investigated at the plot scale in an unfertilized soil and in a dairy manure-treated (100 t ha(-1)) soil. Soil cores (0-60 cm) were taken according to a balanced nested sampling design and were sectioned into 20-cm increments. Microbial biomass was measured by the fumigation-extraction method (FEM), and the K(EC) was determined by in situ 14C-labelling: total and soluble C were also measured on all samples. Manure application increased the variability of all measured properties at the 0-20 cm and 20-40 cm depths. The range was generally inferior to 0.1 m in the unfertilized plot and between 0.1 and 1 m in the manure-treated plot. Microbial biomass spatial variability was similar to that of total C, and was highly correlated with total C throughout the soil profile. At the plot scale, sampling distances of greater than 1 m were required to adequately measure biomass. K(EC) values did not correlate with organic carbon, soluble carbon, clay or silt content. K(EC) values for soils sampled from all depths fell within the range of 0.30 to 0.43. close to the commonly used consensus value of about 0.4. However, the K(EC) for subsurface soil samples was significantly lower and more variable than that for surface soils. The results of this study suggest that using K(EC) values for surface soils to calculate subsurface soil biomass using the FEM would result in a significant underestimate of the true subsurface soil biomass value.

The evaluation of pesticide-mineralising microorganisms to clean-up contaminated soils was studied with the widely applied and easily detectable compound atrazine, which is rapidly mineralised by several microorganisms including the Pseudomonas sp. strain Yaya 6. The rate of atrazine removal was proportional to the water content of the soil and the amount of bacteria added to the soil. In soil slurry, 6 mg atrazine kg soil-1 was eliminated within 1 day after application of 0.3 g dry weight inoculant biomass kg soil-1 and within 5 days when 0.003 g kg soil-1 was used. In partially saturated soil (60% of the maximal water-holding capacity) 15 mg atrazine kg soil-1 was eliminated within 2 days by 1 g biomass kg soil-1 and within 25 days when 0.01 g biomass kg soil-1 was used. In unsaturated soil, about 60% [U-ring-14C]atrazine was converted to 14CO2 within 14 days. Atrazine was very efficiently removed by the inoculant biomass, not only in soil that was freshly contaminated but also in soil aged with atrazine for up to 260 days. The bacteria exposed to atrazine in unsaturated sterile soil were still active after a starvation period of 240 days: 15 mg newly added atrazine kg soil-1 was eliminated within 5 days.

Particulate organic matter (POM) is more sensitive than total SOM to changes in management practices and, accordingly, may indicate changes in soil quality. A soil incubation study was conducted to determine the effects of added POM (75 to 250 micrometer size fraction), or macroorganic matter (MOM, 250 to 2000 micrometer size fraction) on C and N mineralization and microbial C and N content. A 1 kg composite made from 16 predominantly silt loam soils was amended with 10 g of POM, MOM or MOM ground to a reduced size of 75 to 250 micrometer (GMOM). The MOM amendment equaled 4.55-fold and POM equaled 1.60-fold of total MOM and POM found in the composite soil. Carbon mineralization of MOM and POM after 8 weeks was approximately 9 and 4%, respectively of the total MOM and POM-C added. Reducing the size of MOM to 75 to 250 micrometer did not affect mineralization. Nitrogen mineralization was slightly greater in the amended soils after 8 weeks and equaled 5 to 6% of the MOM or POM-total N added. Contribution of POM to total mineralized N from soil organic matter (SOM) in the composite soil was proportional to the POM content in SOM or approximately 12%. Amended soils had 25 to 42% more biomass-C than the control soil 2 weeks after amendment application. After 8 weeks, the amended soils contained about 32% more biomass-C. This increase in biomass-C at 8 weeks accounted for approximately 2% of the added C. At 8 weeks, microbial biomass-N in GMOM-, MOM- and POM-amended soils was about 56, 46 and 14% higher, respectively, than in the control soil. These increases were approximately 8% of the MOM-N added and 2% of the POM-N added. Increases in POM resulted in increases in soil respiration and microbial biomass-C and N, which also are suggested indicators of soil quality. Therefore, POM may be a suitable soil quality indicator that provides similar information as soil respiration or microbial biomass determinations.

Microwave irradiation was evaluated as a non-toxic alternate to chloroform fumigation for routine measurement of soil microbial biomass C. Microwave energy was applied to moist soil to disrupt microbial cells. The flush of C released was then measured after extraction or incubation. Microwave irradiation at 800 J g(-1) soil was optimal because this level resulted in an almost instantaneous rise in soil temperature (> or = to 80 degrees C) an abrupt reduction in microbial activity, maximal release of biomass C, and minimal solubilization of humic substances. Both incubation-CO2 titration and extraction-colorimetry methods were used on separate 20-g subsamples to compare the labile C in the microwave-treated and untreated soil samples. The incubation-titration method was also used to measure C in chloroform-fumigated soil samples. Averaged across soils, the chloroform fumigation yielded 123.3 +/ 5.1 mg CO2-C kg(-1). Microwave irradiation yielded 93.6 +/ 3.9 mg CO2-C kg(-1) soil determined by incubation and 52.4 +/- 2.4 mg C kg(-1) soil determined by extraction, accounting for 76% and 42% of the net flush of C measured by the chloroform fumigation. Microwave-stimulated net flushes of C were correlated closely (r2 = 0.974 for incubation or 0.908 for extraction) with microbial biomass C measured by the chloroform fumigation. Little correlation was found with the total soil organic C (r2 = 0.241 for incubation or for 0.166 extraction). Mean efficiency factors for incubation (K(MI)) or extraction (K(ME)) were used to calculate microbial biomass C from net flushes of C between microwaved and unmicrowaved soils. Values of K(MI) and K(ME) were not affected by soil pH, bulk density or clay contents. Extraction of microwaved soil by 0.5M K2SO4 proved to be a simple, fast, precise, reliable, and safe method to measure soil microbial biomass C.

Four soils from various origins, (tropical and temperate regions) were amended with 14C labelled glucose (1mg C.g-1 soil) and incubated at 15ºC and 35ºC to determine the temperature effect on the carbon turnover and on the microbial biomass. The temperature effect on the biomass increased with the glucose addition. The biomass mineralization rates were higher at 35ºC than at 15ºC and higher for Woburn and Pegwell soils (temperate region) than for Capinopolis and Janauba (tropical region). Specific respiration rate (SRR) of new biomass (from glucose) and old biomass showed different behaviors between soils. At 15ºC, the turnover C was 207, 225, 115 and 141 days for Janauba, Capinopolis, Woburn and Pegwell soil, respectively. At 35ºC, it was 92, 69, 69 and 33 days for the same soils. The residual 14C in the soil was higher at 35ºC. The final total biomasses at 15ºC and 35ºC were correlated with the initial soil carbon content. There was an average of 31 and 8 mg of biomass C.g-1 soil organic carbon, respectively at 15ºC and 35ºC. The initial carbon content was an important factor to explain the mineralization rate at 35ºC.

It is well established that distribution of organic C and N within a soil profile is substantially influenced by long-term conversion of soil from plow-tillage to no-tillage management, but little is known about the short-term changes in the character of soil organic matter during transition between these two soil management systems. To characterize the changes in composition of organic C and N pools in soil during tillage transition, we measured total N, organic C, active N, biomass N, and biomass C at depth intervals of 0 to 2.5, 2.5 to 5, 5 to 12.5, and 12.5 to 20 cm within the profile of soil during the first 3 yr in the transition from plow tillage to no tillage. The results obtained showed that transformation of a soil profile from that typical of plow tillage management to one characteristic of no tillage occurred rapidly within a 3-yr period of transition. In this time period, stratification of organic matter in the profile progressed significantly toward that which occurs after 20 yr of no-tillage treatment. For example, substantial increases in total N (30%), organic C (38%), biomass N (87%), and biomass C (33%) were detected in the top layer of notillage soil along with corresponding decreases of 6%, 7%, 35%, and 15% of those respective components in the bottom layer. With transition, the biomass C, biomass N, and active N pools increased more rapidly in the upper soil profile than did the total pools of C and N. Although the characteristic profile of no-tillage soil developed quickly during tillage transition, evidence was equivocal for any significant increase in organic matter content within the first 3 yr after conversion to no-tillage management.

In some Pleistocene soils, forestry practices on sustainability were started 80 years ago. On outwash plains the pine forests were changed into pine-beech forests ("Zweistufenwald"). On moraines, pine-oak-beech forests ("Dreistufenwald") were created.Litter decomposition and carbon mineralization were significantly higher in the more natural, mainly deciduous "Dreistufenwald" than in the pine forest (p≤0.01), while C/N ratio was lower (p≤0.001). Nutrient availability (K⁺, Mg⁺⁺, Ca⁺⁺, P₂O₅) increased, whereas the concentration of Al³⁺ ions was reduced to about one-third of that of the pine forest (p≤0.001). Higher values of microbial biomass were obtained from the soil of the "Dreistufenwald". The increase in microbial biomass was mainly due to higher fungal colonization. Soil respiration (µg CO₂-C/g*h), specific microbial respiration (µg CO₂-C/mg Cₘᵢc*h) and carbon mineralization (µg CO₂-C/mg Cₒᵣg*d) were significantly higher in the mixed forest during the entire investigation (p≤0.01). Furthermore, the abundance of the soil fauna increased in comparison to the coniferous forest. This observation was made concerning the macroinvertebrates (Lumbricidae) as well as the mesofauna (Enchytraeidae, Collembola, Oribatei). With respect to earthworms, the number of species and the biomass was higher for all ecotypes.In the "Zweistufenwald" on outwash plains no improved soil characteristics could be detected. Litter decomposition was even lower than in the coniferous sites (p≤0.001) and the concentration of Al³⁺ was higher (p≤0.001). In the pine-beech forest, microbial biomass and soil respiration were only slightly higher. The biomass of the soil fauna did not differ significantly (p>0.05) in most cases.Changes in decomposition rates were mainly due to the soil type and to interactions between soil and stocking (ANOVA). Both, soil and stocking influenced the microbial characteristics of the soils and the biomass of Collembola and Oribatei (p≤0.001). Differences in the abundance of Enchytraeidae depended on the soil and the interaction of soil and stocking, while the abundance of earthworms could be explained solely by the soil (p≤0.001). Analysis of covariance uncovered the significance of the soil fauna for the increase in soil respiration, specific microbial respiration and carbon mineralization.Moreover, soil fauna and soil microbes biased the C/N ratio, nutrient availability (K⁺, Mg⁺⁺, Ca⁺⁺, P₂O₅) and the concentration of Al³⁺ ions. A remarkable correlation occured between high biotic activity, increase in pH-values and decrease in concentrations of Al³⁺ ions in the more natural, mainly deciduous forest.

The active pool of organic matter plays an essential role in the short-term of nutrients turnover in soil. An approach to characterizing this fraction is through densimetric techniques which isolate soil light fractions. Cropping and tillage systems are determinants of the amount and distribution of soil organic matter, especially in the upper layers of the soil profile. Our objectives were to evaluate the distribution and dynamics of carbon in different density fractions in order to provide a better understanding of soil fertility changes induced by contrasting types of soil management: plow tillage, no-tillage and pasture. The total and active microbial biomass pools and microbial activity were also determined. The experiment was performed on a Typic Argiudoll from the Argentinean Pampa. Organic carbon was highest under pasture, but there were no differences between the others two treatments for the 0-15 cm layer. Under the pasture and no-tillage treatments, organic carbon decreased with depth. The light fraction (density less than or equal to 1.6 g ml-1) was higher under no-tillage than in the plowed soil, indicating that this fraction was more sensitive to management than was total carbon. The carbon mineralized in 160 d of incubation from different density fractions followed the order: light fraction > medium fraction (1.6-2 g ml-1) > heavy fraction (greater than or equal to 2 g ml-1), presumably because of an increase in chemical and physical protection of organic matter in the heavier fractions. Total soil microbial biomass was stratified under the pasture and no-tillage treatments. Basal respiration was significantly associated with the availability of carbon in the light fraction (r2 = 0.98. P < 0.001) and carbon in the soil microbial biomass (r2 = 0.88, P < 0.001). The active microbial biomass differed (P < 0.05) between pasture (29 microgram C g-1), no-tillage (19 microgram C g-1) and plow tillage (9 microgram C g-1). The active microbial biomass, as a fraction of the total soil microbial biomass, increased with depth in all treatments, but especially in plow tillage soils. There was a positive and strong association between the availability of carbon in the light fraction per unit of active soil microbial biomass and the ratio between the respiration in 10 d and the active microbial biomass (r2 = 0.93, P < 0.001). Our results suggest that no-tillage produced the accumulation of carbon in the soil light fraction and increased the potential carbon mineralization. Consequently this tillage treatment can conserve the potential fertility of soil under cultivation.