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Changes in land use can affect soil organic matter contents and fertility and also atmospheric CO 2 concentrations and global warming through soil respiration. We compared total and microbial C, N and P pools and C and N metabolism in sandy loam soils (Typic Udivitrands) under indigenous broadleaf-podocarp forest, grazed introduced pasture and 19-yr old Pinus radiata D. Don forest (planted on previous pasture) in New Zealand. Total and microbial C and N declined consistently with profile depth (except for total N in L and FH samples), and in comparable depths of mineral soil (to 20 cm) tended to be lower in the pine than in the other systems. Total P, organic P and extractable inorganic P concentrations at comparable depths were, in contrast, lowest in the indigenous forest. Microbial P concentrations did not differ significantly between the different systems. Microbial C-to-microbial N ratios differed little among soil profile depths and ecosystems. In 0–10 cm depth mineral soil, CO 2 -C production, metabolic quotients ( q CO 2 values) and net N mineralization were all highest in the pasture samples. Net nitrification was high in the pine and pasture samples, but much lower in the indigenous forest samples; nitrate-N was, however, consistently present in streamwater from all three ecosystems. Changes in total C and microbial C and N pools on an area basis to 20 cm depth mineral soil were greatest after conversion of the indigenous forest to pasture; total N contents were, however, as high in the pasture as in the forest and net N mineralization was highest in the pasture. On this area basis, changes in total C contents were small after conversion of pasture to pines, although the distribution within the soil profile did differ considerably between the pine and pasture systems.

The influence of soil texture and clay mineral composition on the decomposition of uniformly-labelled ( 14 C) ryegrass ( Lolium hybridum Hausskn) has been investigated. Two clay and two silt loam soils amended with 14 C-labelled ryegrass were allowed to decompose in micro-lysimeters under field conditions for 5 years. Periodically the micro-lysimeters were destructively harvested and the total amount of 14 C remaining in the soil and the fraction of the 14 C incorporated into microbial biomass measured. The influence of the amount and surface area of clay was assessed by calculating the mean and variances of residence times of biomass- 14 C and residual- 14 C. Surface areas were estimated from the mineralogical composition of the soil, and also measured by adsorption of p -nitrophenol. After 5 years' of incubation, 12–25% of the labelled- 14 C was retained by the soils. Decomposition was initially very rapid, between one-third to one-half of the labelled 14 C being lost after 9 weeks. Thereafter, the rate of decomposition was much reduced. During the initial phase of decomposition (9 weeks), a greater portion of the 14 C was retained by the two clay soils than by silt loam soils. In amorphic and smectitic soils the residence times for 14 C were increased because of the protection of microbial metabolites by clay surfaces. The amount of 14 C remaining in the soil did not relate to the amount of clay but correlated with the surface area, as measured by p -nitrophenol adsorption. Apparently, the type of clay, expressed in terms of surface area, controlled carbon turnover rates and residence times. This parameter may provide a reasonable predictor of organic matter decomposition rates and stabilization in different soils.

Comparisons were made of total and microbial C and N pools and C and N metabolism in litter and mineral soil of a mountain beech (Nothofagus solandri var. cliffortioides) forest, ca. 100 m below timberline, and an adjacent tussock grassland (dominated by Chionochloa pallens), ca. 100 m above timberline, in Canterbury, New Zealand. Mean annual precipitation at the sites is ca. 1600 mm and mean annual air temperature ca. 6°C. The silt loam soils are Andic Dystrochrepts. Total C and N and microbial C and N contents in litter and mineral soil (0–50 cm depth) differed appreciably in the two ecosystems and were 1.41, 2.00, 1.71 and 1.98 times, respectively, greater on an area basis at the grassland than at the forest site. Ratios of microbial C-to-total C, microbial N-to-total N and microbial C-to-N generally declined with profile depth. CO2C production, per unit of total C, and metabolic quotients (qCO2 values) tended to be greater in mineral soil from the forest than from the grassland. CO2C production, calculated on an area basis to 50 cm depth of mineral soil, was similar in both ecosystems. Net N mineralization (during 0–56 days at 25°C) was appreciable in the forest litter, but absent in tussock litter; it was similar in both systems at 0–10 cm and 20–50 cm depths of mineral soil, but was lower in the forest than in the grassland at 10–20 cm depth. Nitrification was not detected in the litter samples and was either absent or very low in the samples of mineral soil. Results, overall, show that marked differences in soil and microbial properties can occur in adjacent, indigenous ecosystems in almost the same climatic environment. Although soil microbial biomass levels were lower in the forest than in the grassland, the potential metabolic activity of the component microorganisms tended to be greater in the forest. The relevance of these results to the turnover of organic matter in these ecosystems is briefly discussed.

The influence of soil allophane (a short-range-order mineral) content on organic-C turnover was determined with 14 C-labelled glucose. Samples from four soils, providing a range of allophane, organic C, clay contents, and some other characteristics, were incubated with 14 C-labelled glucose for 28 days. During incubation, microbial biomass 12 C and 14 C were determined using the fumigation-extraction technique. The amounts of 12 CO 2 and 14 CO 2 evolved during incubation were also monitored, and residual 14 C concentrations determined. Biomass 14 C was highest in the soil with the highest allophane content (13%) and least in the soil with least allophane content ( 14 CO 2 production from the [ 14 C]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 14 C and exponential decay rate constants for biomass 14 C, that allophane retards the turnover rates of 14 C 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.

The fumigation-extraction method to estimate microbial C was applied to a range of seven soils with organic-C contents up to 47%. The additional oxidizable organic-C released by chcl3 fumigation at −5 kPa water potential followed by extraction with 0.5 m K2SO4. was compared with the microbial C estimated by a modified substrate-induced-respiration (SIR) method, and by in situ labelling of cells with [14C]glucose. The extractable organic-C flush comprised 37–63% of the microbial C. The overall kecfactors, to convert from the organic-C flush to microbial C were 0.37 for the SIR method and 0.42 by 14C-labelling. The relationship between microbial C and the oxidizable C released by fumigation was similar for both organic and mineral soils. Accumulated data from 168 comparisons of the relationship between the organic-C flush and microbial C were re-examined. The kec-factors varied widely between soils and were strongly influenced by soil water content. It is recommended that soils should be rewetted. when required, to the -5kPa water potential, to ensure that the fumigation stage is fully effective. The reliability of the SIR and fumigation-incubation methods is discussed and a revised calibration for the SIR method: microbial C (/gmg g−1) = 50(/gmlCO2g−1h−1) is proposed, based on 14C-labelling and alternative methods of calcula the CO2-C flush when using the fumigation-incubation method. Taking account of possible underestimation of microbial C by these calibrations, a value of 0.35 is suggested as being representative of an overall kec-factor for New Zealand soils. Variability between soils means there is an error in applying an average factor to all soils, but a standard method should be adequate for most applications of microbial C measurements.

A rapid fumigation-extraction method for measuring soil microbial biomass-C is described. The biomass is calculated from the difference between the C extracted by 0.5 m K 2 SO 4 over 30 min from chloroform-fumigated and unfumigated soil, using a factor ( k EC ) to convert this flush to microbial-C. Sonication experiments with organisms indicated that most of the C extracted after fumigation was probably derived from cytoplasmic material. Standardization of extraction time and shaking conditions was necessary, but K 2 SO 4 molarity, and consequently soil moisture content, were not critical. Fumigation flushes of extractable-C and CO 2 -C produced on incubation were strongly correlated. However, a 1:1 relationship was not apparent in several soils from pasture; possible explanations are considered. Conversion factors for estimating microbial biomass-C from extractable-C values were determined by calibration with cultured organisms (four fungi and two bacteria) and with biomass-C estimated from CO 2 -C flush values. These factors differed appreciably, averaging 0.20 and 0.48 respectively; possible reasons are considered. Preference is currently given to the k EC -factor of 0.33, obtained by Sparling and West (1988). This fumigation-extraction procedure can provide a rapid, reasonable estimate of microbial biomass-C values in grassland soils.