Fumigation-extraction Technique Research Articles

Emergence of Diverse Microbes on Application of Biofilmed Biofertilizers to a Maize Growing Soil

Diverse microbial communities in the rhizosphere perform an amazing role in plant growth and productivity. However, conventional agricultural practices such as the use of chemical fertilizer (CF) and tillage have collapsed the diversity of the microbial communities. Direct application of developed fungal-bacterial communities known as biofilmed biofertilizers (BFBFs) to the soil has been introduced recently, and observed to be multi-functional and more effective than conventional biofertilizers. However, the effect of such biofertilizers on soil microbial diversity has not been studied sufficiently world over. Therefore, the current study was carried out to investigate the effects BFBFs on bacterial, cyanobacterial and fungal species in a maize growing soil. A pot experiment was conducted under greenhouse conditions with different fertilizer treatments; 100% chemical fertilizers (CF) recommended for maize, 50% CF, 50% CF + BFBF, and no fertilizer, as the control. Microbial species richness and abundance of bacteria were evaluated after two months of plant growth. Microbial biomass carbon (MBC) was estimated by using chloroform fumigation-extraction technique. Results showed that the species richness of bacteria, fungi and cyanobacteria, and abundance of bacteria were higher in 50% CF + BFBF, compared to 50% and 100% CF. This implies that the action of BFBFs tends to break dormancy of microbial seeds in the soil, resulting in emergence of a diverse microbial community, which may support natural biocontrol of pathogens. An interesting observation was the stimulation of an additional microflora of cyanobacteria by the application of BFBF. MBC levels did not show a significant difference between any of the treatments. We recommend further testing of the BFBFs for increasing microbial diversity and ecosystem functioning and hence sustainability of maize agroecosystems. DOI: http://dx.doi.org/10.4038/cjsbs.v42i2.6612 Ceylon Journal of Science (Bio. Sci.) 42(2): 87-94, 2013

Evaluation of soil biological properties of 9- and 15-year-old stands in the oil palm plantation in Perak, Malaysia

Opening land for oil palm cultivation provokes many debates around the world regarding on the fate of biodiversity. A study was conducted to evaluate and compare soil biological properties of 9-year-old (P1) and 15-year-old (P2) stands of an oil palm plantation in Bikam, Perak, Malaysia. Composite samples were collected at depths of 0-15 cm (topsoil) and 15-30 cm (subsoil) located within six subplots (20 m × 20 m). The microbial population count was estimated using a spread-plate technique, and the Fluorescein diacetate (FDA) hydrolysis assay was used to measure microbial enzymatic activity. A rapid ethanol-free chloroform fumigation extraction technique was used for microbial biomass extraction, and the extracts were respectively analyzed by wet dichromate oxidation and Kjeldahl digestion for biomass carbon (C) and nitrogen (N). At the 0-15 cm depth, the microbial biomass C and N contents in the soils from both plots were significantly different (P<0.05). At the 15-30 cm depth, only microbial enzymatic activity was significantly different between plots. Although the addition of fertilizers to the soil is believed to be a predisposing factor, no significant differences in P1 and P2 plots for the biomass C and N in soils at the 15-30 cm depth were observed. Variations in the MBC/MBN ratio in soils of the P1 and P2 plots indicate that changes occurring in the soil microbial composition are due to the availability of soil organic substrates and N. Thus establishment of an oil palm plantation does contribute to changes in soil biological properties. Key words: Oil palm, different ages, microbial population, enzymatic activity, biomass C and N, rapid chloroform-fumigation extraction technique.

STATUS OF SOIL MICROBIAL POPULATION, ENZYMATIC ACTIVITY AND BIOMASS OF SELECTED NATURAL, SECONDARY AND REHABILITATED FORESTS

Substantial clearance of forests and conversion of forest into various land use types contribute to deterioration of soil fertility and associated nutrients loss. Soils from natural and rehabilitated forest in Chikus Forest Reserve and also enrichment planting forest and secondary forest of Tapah Hill Forest Reserve, Perak, Malaysia were selected in order to assess the influence of land use change on biological properties. This study was carried out to provide fundamental information on soil biological properties and also to compare the differences between natural forest, mono-rehabilitated forest, mixed planting forest and natural regenerated forest (secondary forest). Six subplots (20×20 m) were established at each study plot and soil samples were collected at the depths of 0-15 cm (topsoil) and 15-30 cm (subsoil). Soil microbial population was determined using spread-plate technique. Fluorescein Diacetate (FDA) hydrolysis was used to assess the amount of microbial enzymatic activity for each forest plot. Soil Microbial Biomass C (MBC) and N (MBN) were extracted using chloroform fumigation extraction technique and the amount of MBC was determined by dichromate digestion, while MBN via Kjeldahl digestion technique. Soil acidity was determined by pH meter and moisture content was elucidated using gravimetric method. The levels of microbial population of bacterial and fungal at natural significantly exceeded the corresponding values of rehabilitated and secondary forest. However, microbial population is much higher in rehabilitated forest of Tapah Hill compared to that of secondary forest and also Chikus Forest Reserve planted forest which proves that rehabilitation activities do help increase the level of microbial community in the soils. Longer period of time after planting as in enrichment planting compared to mono planting of S. leprosula plantation showed that restoring and recovery of the planted forest needed time. Deforestation activities decrease soil biological activities; however, proper forest management and rehabilitation activities are able to restore the condition of degraded forest land to its original state.

Posters

In this study I have measured the microbial biomass of soil and stored food material in the mounds of six northeastern Australian termite species using a combination of substrate-induced respiration (SIR) and fumigation extraction techniques. In all but one case mound soil was significantly higher in microbial biomass than nearby surface soils. The microbial biomass of Coptotermes acinaciformis mound material was approximately six times that of the surface soil. The mean microbial biomass content of grass stored in chambers of Nasutitermes magnus, Tumulitermes pastinator and Drepanotermes rubriceps mounds was also very high, up to 2830 μg C g grass−1, which was approximately 20 times greater than the mound soil microbial biomass. It is suggested that microbial decomposition of this food material prior to ingestion by the termites may provide the termite colony with a food source relatively enriched in nitrogen. The microbial decomposition of food material stored in the outer chambers of the mound of an Australian termite has some analogy with the behaviour of the fungus cultivating Macrotermitinae of the African and Asian regions. The association between mound-building termites and microorganisms present in their mounds is discussed in relation to organic matter decomposition.

Modification of the original chloroform fumigation extraction technique to allow measurement of δ13C of soil microbial biomass carbon

In studies of the soil microbial biomass C by the chloroform fumigation extraction (CFE) technique, biomass C is routinely extracted using 0.5 M K 2SO 4 solution. The excessive amounts of salts contained in the extracting solution pause a significant challenge in using 13C isotope techniques to study the nature of C in the soil microbial biomass. This is because the salts can affect the oxidation process and therefore hamper accurate mass spectromic analysis of dried extracts. In spite of this, no standard protocol exists for preparing the K 2SO 4 extracts for 13C isotope analysis. We have modified the original CFE method to allow measurement of the δ 13C of soil microbial biomass C by using 2 M KCl instead of the usual 0.5 M K 2SO 4 solution to extract biomass C. Excess salts were removed by dialysis in 100 molecular weight cut off membranes, after which the extracts were freeze-dried and their δ 13C measured using a mass spectrometer. The soil microbial biomass C and δ 13C of 2 M KCl extracts were compared with those of 0.5 M K 2SO 4 extracts. There was excellent agreement between organic C and δ 13C estimates for dialyzed 2 M KCl and 0.5 M K 2SO 4 extracts, but the speed of dialysis for the latter was very slow, making use of the former more rapid. These results suggest that in procedures where oxidation with potassium dichromate is not critical to analysis of soluble C, 2 M KCl may be used in place of 0.5 M K 2SO 4 to extract soil microbial biomass C for δ 13C measurements. The new procedure is relatively easy and rapid for obtaining indices for both pool sizes and turnover rates of soil microbial biomass C and provides a promising approach to study soil organic C.

Nutrient Input and Carbon and Microbial Dynamics in an Ombrotrophic Bog

Slow rates of plant production and decomposition in ombrotrophic bogs are believed to be partially the result of low nutrient availability. To test the effect of nutrient availability on decomposition, carbon dioxide (CO2) flux dynamics, microbial biomass, and nutrients, we added nitrogen (N) with phosphorus (P) and potassium (K), to prevent limitation of the latter 2 nutrients, over 2 growing seasons to plots at Mer Bleue peatland, Ontario, Canada. After the first growing season, increasing N fertilization (with constant P and K) decreased in vitro CO2 production potential and increased microbial biomass measured with a chloroform fumigation-extraction technique in the upper peat profile, while by the end of the second season, CO2 production potential was increased in response to N plus PK treatment, presumably due to more easily decomposable newly formed plant material. In situ CO2 fluxes measured using chamber-techniques over the second year corroborated this presumption, with greater photosynthetic CO2 uptake and ecosystem respiration (ER) during high N plus PK treatments. The more efficient microbial community, with slower CO2 production potential and larger biomass, after the first year was characterized by larger fungal biomass measured with signature phospholipid fatty acids. The majority of N was likely quickly sequestered by the vegetation and transferred to dissolved organic forms and microbial biomass in the upper parts of the peat profile, while additional P relative to controls was distributed throughout the profile, implying that the vegetation at the site was N limited. However, in situ CO2 flux data suggested the possibility of P or NPK limitation. We hypothesize that nutrient deposition may lead to enhanced C uptake by altering the microbial community and decomposition, however this pattern disappears through subsequent changes in the vegetation and production of more readily decomposable plant tissues.

Relationships of soil respiration to microbial biomass, substrate availability and clay content

The roles of microbial biomass (MBC) and substrate supply as well as their interaction with clay content in determining soil respiration rate were studied using a range of soils with contrasting properties. Total organic C (TOC), water-soluble organic carbon, 0.5 M K 2SO 4-extractable organic C and 33.3 mM KMnO 4-oxidisable organic carbon were determined as C availability indices. For air-dried soils, these indices showed close relationship with flush of CO 2 production following rewetting of the soils. In comparison, MBC determined with the chloroform fumigation–extraction technique had relatively weaker correlation with soil respiration rate. After 7 d pre-incubation, soil respiration was still closely correlated with the C availability indices in the pre-incubated soils, but poorly correlated with MBC determined with three different techniques—chloroform fumigation extraction, substrate-induced respiration, and chloroform fumigation–incubation methods. Results of multiple regression analyses, together with the above observations, suggested that soil respiration under favourable temperature and moisture conditions was principally determined by substrate supply rather than by the pool size of MBC. The specific respiratory activity of microorganisms (CO 2-C/MBC) following rewetting of air-dried soils or after 7 d pre-incubation was positively correlated with substrate availability, but negatively correlated with microbial pool size. Clay content had no significant effect on CO 2 production rate, relative C mineralization rate (CO 2-C/TOC) and specific respiratory activity of MBC during the first week incubation of rewetted dry soils. However, significant protective effect of clay on C mineralization was shown for the pre-incubated soils. These results suggested that the protective effect of clay on soil organic matter decomposition became significant as the substrate supply and microbial demand approached to an equilibrium state. Thereafter, soil respiration would be dependent on the replenishment of the labile substrate from the bulk organic C pool.

Microbial biomass phosphorus turnover in variable-charge soils in China

Turnover of microbial biomass phosphorus (Bp) in three acid red soils (one Arenic aquult and two Plinthic aquults) were investigated by 32P-labeling microorganisms in situ and fumigation–extraction techniques. The turnover rates of Bp in the three red soils were 130, 190, and 217 days, respectively, that is 1.7 to 2.8 times per year. The contents of Bp were 12.2, 17.8, and 31.5 mg kg−1, respectively, for the three soils. Microbial biomass turnover provided a dynamic source of available phosphorus (P) with the size of 2–3 times greater than the Bp. The annual fluxes of P through microbial biomass in these soils could amount to 91 to 146 kg ha−1, which are 3–5 times greater than the amount of P annually removed by the harvested crops. The turnover rate of Bp was greater in the sandy soil than in the other two clayey soils. This was explained by a larger amount of Bp and lower mineralization in the clayey soils, as compared with the sandy soil. Apparently, the turnover of Bp plays a very important role in the supply of plant-available P and ecological cycling of P in the subtropical red soil region.

Biomass development in slow sand filters

Microbial biomass development in the sand and schmutzdecke layer was determined in two full-scale slow sand filters, operated with and without a light excluding cover. A standard chloroform fumigation-extraction technique was adapted to routinely measure microbial biomass concentrations in the sand beds. Sand was sampled to a depth of 10 cm and schmutzdecke was also collected at the same random positions on the uncovered filter. Interstitial microbial biomass in the uncovered sand bed increased with time and decreased with sampling depth. There was a small accumulation of sand biomass with time in the covered filter, but no relationship was apparent between biomass concentration and depth in this filter. Schmutzdecke did not develop on the covered filter and was spatially highly variable in the uncovered condition compared to the consistent patterns observed in interstitial biomass production. It is speculated that microbial biomass in the sand of uncovered filters is largely related to carbon inputs from photosynthetic activity in the schmutzdecke and involves mechanisms that spatially distribute carbon substrate from the schmutzdecke to the sand. However, total organic carbon and dissolved organic carbon removals were similar in both filters suggesting that relatively small biomass populations in covered filters are sufficient to remove residual labile carbon during advanced water treatment and little further advantage to water purification and organic carbon removal is gained by the increased production of biomass in uncovered slow sand filter beds.

Extramatrical ectomycorrhizal mycelium contributes one-third of microbial biomass and produces, together with associated roots, half the dissolved organic carbon in a forest soil.

• A large-scale tree-girdling experiment enabled estimates in the field of the contribution of extramatrical mycelium of ectomycorrhizal (ECM) fungi to soil microbial biomass and by ECM roots and fungi to production of dissolved organic carbon (DOC). • Tree-girdling was made early (EG) or late (LG) during the summer to terminate the flow of photosynthate to roots and ECM fungi. Determination of microbial C (Cmicr ) and microbial N in root-free organic soil was performed by using the fumigation-extraction technique; extractable DOC was determined on unfumigated soil. • Soil Cmicr was 41% lower on LG than on control plots 1month after LG, whereas at the same time (that is, 3months after EG), the Cmicr was 23% lower on EG than on control plots. Extractable DOC was 45% lower on girdled plots than control plots. • Our results, which are of particular interest as they were obtained directly in the field, clearly demonstrate the important contribution by extramatrical ECM mycelium to soil microbial biomass and by ECM roots to the production of DOC, a carbon source for other microbes.

Soil microbial eco-physiology as affected by short-term variations in environmental conditions

Short-term variations in abiotic and biotic environmental conditions modify microbial metabolism. These effects were studied in an arable soil during the oat straw decomposition. Cumulative CO 2 evolution was significantly higher when soil was subjected weekly to drying–rewetting cycles (DR) or had been initially fertilised with nitrogen (+N, as NH 4NO 3) in comparison to samples incubated at constant conditions (CC). The microbial metabolic quotient (respiration rate per unit microbial C) of DR and +N exceeded CC by approximately 100–150%, indicating that variable water availability and improved N availability induced a higher activity level of the microbial biomass. In addition, the metabolic-responsive microbial communities, calculated by the ratio of microbial C estimated by substrate-induced respiration to fumigation–extraction technique, increased from 1.0 to approximately 1.3 after 7 weeks for DR and +N, but remained essentially at 1.0 for CC and control. A high abundance of soil nematodes significantly affected respiration rate, metabolic-responsive biomass and metabolic quotient during the early stages of decomposition. We concluded that short-term variation in environmental conditions promoted decomposition processes by influencing the physiology of soil microorganisms and increasing the metabolic-responsive biomass. Such a stimulated microbial metabolism seems to be unspecific due to stressing environmental conditions. The role of the interacting fauna such as nematodes varied during decomposition.

Bioamelioration of a sodic soil by silvopastoral systems in northwestern India

In sodic soils, excessive amounts of salts have an adverse effect on soil biological activity and stability of soil organic matter. The study analyzes the role of silvopastoral systems to improve soil organic matter and microbial activity with a view for effective management of soil fertility. The silvopastoral systems for the present study (located at Saraswati Reserved Forest, Kurukshetra; 29°4′ to 30°15′ N and 75°15′ to 77°16prime; E) are characterized by tree species of Acacia nilotica, Dalbergia sissoo and Prosopis juliflora along with grass species of Desmostachya bipinnata and Sporobolus marginatus. Soil microbial biomass carbon was measured using the fumigation extraction technique and nitrogen mineralization rates using aerobic incubation method. The microbial biomass carbon in the soils of D. bipinnata and S. marginatus treatments were low. In silvopastoral systems, microbial biomass carbon increased due to increase in the carbon content in the soil – plant system. A significant relationship was found between microbial biomass carbon and plant biomass carbon (r = 0.83) as well as the flux of carbon in net primary productivity (r = 0.92). Nitrogen mineralization rates were found greater in silvopastoral systems compared to 'grass-only' system. Soil organic matter was linearly related to microbial biomass carbon, soil N and nitrogen mineralization rates (r = 0.95 to 0.98, p < 0.01). On the basis of improvement in soil organic matter, enlarged soil microbial biomass pool and greater soil N availability in the tree + grass systems, agroforestry could be adopted for improving the fertility of highly sodic soil.

Soil carbon, microbial activity and nitrogen availability in agroforestry systems on moderately alkaline soils in northern India

The present investigation aimed to analyze the role of agroforestry systems in improving soil organic matter status, microbial activity and nitrogen availability with a view to effective management of the fertility of moderately alkaline soils. The study site was located at Karnal (29°59′N, 76°51′E, 250 m.s.l.) and the systems were characterized by a rice–berseem crop rotation; agrisilvicultural systems of Acacia, Eucalyptus and Populus along with rice–berseem and single species tree plantations. Soil microbial biomass was measured using the fumigation extraction technique and nitrogen mineralization using the aerobic incubation method. Microbial biomass carbon was low in rice–berseem crops (96.14 μg g −1 soil) and increased in soils under tree plantations (109.12–143.40 μg g −1 soil) and agrisilvicultural systems (133.80–153.40 μg g −1 soil). Microbial biomass was higher by 42% (microbial C) and 13% (microbial N) in tree-based systems as compared to monocropping. Microbial biomass immobilized 2.32–2.57% of the soil carbon and 4.08–4.48% of the soil nitrogen in tree-based systems. Soil carbon increased by 11–52% due to integration of trees along with the crops for 6–7 years. Cropland management practices and tree species influenced CO 2-C production, biomass specific respiratory activity, and nitrogen mineralization rates. In tree-based systems, soil inorganic N levels were higher by 8–74% and nitrogen mineralization by 12–37% as compared to monocropping. On the basis of increased soil organic matter content, enlarged soil microbial biomass pool and greater soil N availability, agrisilvicultural systems have been found to be ecologically sustainable land-use systems for utilizing moderately alkaline soils.

14C-labelled glucose turnover in New Zealand soils

The influence of soil mineralogy, as well as texture, on organic-C turnover was determined with 14C-labelled glucose. Samples of 16 soils from major mineralogical classes of New Zealand pastures and providing a range of organic C, clay contents and surface area, were incubated with 14C-labelled glucose for 35 d. The amounts of 12CO 2 and 14CO 2 evolved during incubation were monitored and the residual 14C concentrations determined. Periodically, the samples were removed and microbial biomass 12C and 14C determined using the fumigation-extraction technique. System mean residence times (MRTs) were obtained by three independent methods: (i) a compartmental model using 14C microbial biomass data, (ii) a non-compartmental model using 14C microbial biomass data and (iii) a biexponential equation as an empirical equation from residual 14C data. The effect of soil characteristics on MRTs was compared. The 14CO 2 respired, after 35 d incubation, accounted for 51 to 66% of the glucose 14C input to these soils. The soils differed significantly in their amounts of 14CO 2 evolution and in the proportions of labelled 14C in the biomass. The extent of mineralization of 14C-labelled glucose was influenced by soil clay content and clay surface area. Soils of low clay content (3–12%) had high biophysical quotients (respired: residual 14C); the highest (1.93) was in the soil with least clay (3%) and lowest mineral surface area, suggesting that clay is effective in C stabilization immediately after substrate assimilation. A biexponential model was found to be suitable for describing changes in the residual 14C and microbial biomass 14C during the 35 d glucose decomposition for most of the soils. MRTs for microbial biomass 14C were correlated with clay content ( P<0.001), surface area estimated by para-nitrophenol ( pNP) ( P<0.003) and pH ( P<0.01). Our results also showed that the MRTs of microbially assimilated 14C are similar despite differences in the chemical nature of the applied 14C-labelled substrate. However, the MRT for humus 14C differed with the chemical nature of the applied substrate. Clay and surface area played a major role in controlling the decomposition of added substrate through the stabilization and protection of the microbial biomass.

Microbial activity in the mounds of some Australian termites

In this study I have measured the microbial biomass of soil and stored food material in the mounds of six northeastern Australian termite species using a combination of substrate-induced respiration (SIR) and fumigation extraction techniques. In all but one case mound soil was significantly higher in microbial biomass than nearby surface soils. The microbial biomass of Coptotermes acinaciformis mound material was approximately six times that of the surface soil. The mean microbial biomass content of grass stored in chambers of Nasutitermes magnus, Tumulitermes pastinator and Drepanotermes rubriceps mounds was also very high, up to 2830 μg C g grass −1, which was approximately 20 times greater than the mound soil microbial biomass. It is suggested that microbial decomposition of this food material prior to ingestion by the termites may provide the termite colony with a food source relatively enriched in nitrogen. The microbial decomposition of food material stored in the outer chambers of the mound of an Australian termite has some analogy with the behaviour of the fungus cultivating Macrotermitinae of the African and Asian regions. The association between mound-building termites and microorganisms present in their mounds is discussed in relation to organic matter decomposition.

Biomass carbon measurements and substrate utilization patterns of microbial populations from soils amended with cadmium, copper, or zinc

Samples of a sandy loam soil taken from a long-term liming experiment in southeast England were amended with solutions of metal sulfate salts. Soils with a range of pHs were amended to contain Cu, Cd, or Zn at concentrations around the maximum permissible values for these metals in agricultural land receiving sewage sludge. After a 3-year equilibration period, the microbial biomass was determined by the fumigation-extraction technique. These results were compared with data from substrate utilization patterns of microbial populations extracted by using a weak salt solution. There was no reduction in microbial biomass due to pH or metal treatment in any of the soils except the Cu treatment. Principal-component analysis of the respiration patterns in Biolog plates demonstrated effects of both pH and metal treatment on the extracted microbial population which were independent of gross biomass size. pH and soil amendments with Cu and Zn were found to reduce the metabolic potential of the extracted soil microbial population.

The significance of microbial biomass sulphur in soil

The soil microbial biomass S fraction of total organic S in soil is considered to be relatively labile and the most active S pool for S turnover in soil. Its significance has been demonstrated in studies of S deficiency in agronomic situations and in those of S pollution from high atmospheric inputs. The utility of the CHCl3 fumigation-extraction technique for the measurement of microbial S has been proved for a range of soils and conditions. The various methodologies currently available are discussed, including the need for determination of the conversion (K s) factor. Microbial S values, summarized from the available literature, ranged from 3 to 300 μg S g-1 dry weight soil. They were generally greater in grassland than in arable systems, though the greatest values were obtained in the few examples from forest and peatland soil systems. Microbial S values showed direct relationships with both microbial C and with total soil organic S. Again, there were significant differences between arable and grassland systems. The effect of factors such as organic and inorganic inputs as well as soil physical conditions on microbial S are described. Microbial S turnover rates were estimated from seasonal, 35S-labelling and modelling studies. These rates varied between an approximately annual turnover rate in undisturbed soils up to 80 year-1 following the addition of readily available substrates. Prospective future research areas are also outlined.

Carbon turnover in a range of allophanic soils amended with 14C-labelled glucose

The influence of soil allophane (a short-range-order mineral) content on organic-C turnover was determined with 14C-labelled glucose. Samples from four soils, providing a range of allophane, organic C, clay contents, and some other characteristics, were incubated with 14C-labelled glucose for 28 days. During incubation, microbial biomass 12C and 14C were determined using the fumigation-extraction technique. The amounts of 12CO 2 and 14CO 2 evolved during incubation were also monitored, and residual 14C concentrations determined. Biomass 14C was highest in the soil with the highest allophane content (13%) and least in the soil with least allophane content (<2%). In contrast, 14CO 2 production from the [ 14C]glucose was highest (63%) in the soil with least allophane content and lowest (54%) in the soil with the most allophane. It was concluded, from first-order decay rate constants for residual 14C and exponential decay rate constants for biomass 14C, that allophane retards the turnover rates of 14C derived from added glucose by stabilization of microbial biomass, and also by protection of microbial products. During a 28 day incubation, ca 0.8% more C was diverted from respired CO 2 to new biomass with each 1% increase in allophane content. For allophanic soils, inclusion of mineral surface area rather than clay content should provide a better quantification of the organic matter turnover rate.

Effects of heavy metal contamination on the short-term decomposition of labelled [ 14C]glucose in a pasture soil

The influence of heavy metal contamination on the efficiency of conversion of fresh substrates into new microbial biomass in a pasture soil was examined. Three soils covering a range of chromium, copper and arsenic concentrations, and an uncontaminated control soil, were amended with [U- 14C]glucose and incubated for 28 days. During incubation, microbial biomass 14C was determined using the fumigation-extraction technique. The amounts of 14CO 2 evolved during incubation were monitored, and residual 14C concentrations were determined. Throughout the incubation, the microbial biomass- 14C formed following addition of glucose was consistently lower in the metal-contaminated soils than in the uncontaminated control soil. Soils differed significantly in their rates of 14CO 2 evolution. More glucosederived 14CO 2 was evolved from contaminated soil than from the uncontaminated control. The ratio of both (total respired C): (total biomass-C) and (respired 14CO 2): (biomass- 14C) was greater in the contaminated soils than in the uncontaminated soil. The results suggest that the microbial biomass in soils contaminated with heavy metals are less efficient in the utilization of substrates for biomass synthesis and need to expend more energy for maintenance requirements.

Carbon and nitrogen turnover in two acid forest soils of southeast Australia as affected by phosphorus addition and drying and rewetting cycles

The effects of adding P and of drying and rewetting were studied in two acid forest soils from southeast Australia. The soils were a yellow podzolic with a low soil organic matter content (3.75% C) and a red earth with a high organic matter content (13.5% C). C and N mineralization and microbial C and N contents were investigated in a laboratory incubation for 151 days. Microbial C and N were estimated by a hexanol fumigation-extraction technique. Microbial C was also determined by substrate-induced respiration combined with a selective inhibition technique to separate the fungal and the bacterial biomass. The results obtained by the selective inhibition technique were not conclusive. Adding P to the soil and drying and rewetting the soil reduced microbial N. This effect was more pronounced in rapidly and frequently dried soils. Microbial C was generally less affected by these treatments. Compared with the control, the addition of P caused a reduction in respiration in the red earth (-13%) but an increase in the yellow podzolic soil (+12%). In the red earth net N mineralization was highest following the addition of P. In the yellow podzolic soil highest N mineralization rates were obtained when the soil was subjected to drying and rewetting cycles. In both soils increased N mineralization was associated with a decrease in microbial N, indicating that the mineralized N was of microbial origin. Nitrification decreased with rapid drying and rewetting. The addition of P promoted heterotrophic nitrification in both soils.