D.J. Ross

A DIRECT EXTRACTION METHOD TO ESTIMATE SOIL MICROBIAL C: EFFECTS OF EXPERIMENTAL VARIABLES AND SOME DIFFERENT CALIBRATION PROCEDURES

Abstract 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 K2SO4 over 30 min from chloroform-fumigated and unfumigated soil, using a factor (kEC) 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 K2SO4 molarity, and consequently soil moisture content, were not critical. Fumigation flushes of extractable-C and CO2-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 CO2-C flush values. These factors differed appreciably, averaging 0.20 and 0.48 respectively; possible reasons are considered. Preference is currently given to the kEC-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.

Land-use change : effects on soil carbon, nitrogen and phosphorus pools and fluxes in three adjacent ecosystems

Abstract 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.

Soil surface CO2 flux as an index of soil respiration in situ: A comparison of two chamber methods

Predictions of global climate change have recently focused attention on soils as major sources and sinks for atmospheric CO2, and various methodologies exist for measuring soil surface CO2 flux. A static (passive CO2 absorption in an alkali trap over 24 h) and a dynamic (portable infra-red CO2 gas analyzer over 1–2 min) chamber method were compared. Both methods were used for 100 different site × treatment × time combinations in temperate arable, forest and pasture ecosystems. Soil surface CO2 flux estimates covered a wide range from 0 to ca. 300 mg CO2C m−2 h−1 by the static method and from 0 to ca. 2500 mg CO2C m−2 h−1 by the dynamic method. The relationship between results from the two methods was highly non-linear, and was best explained by an exponential equation. When compared to the dynamic method, the static method gave on average 12% higher flux rates below 100 mg CO2C m−2 h−1, but much lower flux rates above 100 mg CO2C m−2 h−1. Spatial variability was large for both methods, necessitating a large number of replicates for reliable field data, with typical coefficients of variation being in the range 10–60%, usually higher with the dynamic than the static method. Diurnal variability in soil surface CO2 flux was partly correlated with soil temperature, whereas day-to-day variability was more unpredictable. However, use of a mechanistic simulation model of CO2 transport in soil, SOILCO2, showed that very large day-to-day changes in soil surface CO2 flux can result from rainfall events causing relatively small changes in soil water content above field capacity (ca. −10 kPa), even if CO2 production rates remained relatively unaffected.

Carbon storage and turnover, and respiratory activity, in the litter and soil of an old-growth southern beech (nothofagus) forest

Abstract Soil and litter carbon pools and turnover, and their relationship to forest floor respiratory activity, were estimated in a lowland old-growth beech (Nothofagus) forest in New Zealand. Although the soils varied spatially over the site, their morphological, chemical and some physical properties were characteristic of spodosols. Two profiles representing the range of soils were analysed by horizon for: C, 14C and a number of chemical and physical properties. C in the fine litter and mineral soil, in the most representative soil at the site, was ca 3.0 and 15.8 kg m−2 respectively. About 0.65 kg CO2-C m−2 was respired annually from the forest floor, based on a simple exponential model relating CO2 efflux measured by a chamber technique, and soil temperature values. Temperature mainly controlled CO2 production by litter and soil because the site was well supplied with rainfall throughout the year. Annual transfers of C from the litter to the atmosphere and the soil, and mean residence times for C in the litter (ca 12 yr) and soils were estimated from the distribution of 14C using a ‘bomb’ radiocarbon model. A major source of soil C was root turnover, based on fine litterfall measurements and modelling of total C input. Root turnover and woody debris together represented 64% of the total C input of ca 0.8kg m−2yr−1 to the soil. Live root respiration, estimated experimentally and from a relationship between total root C allocation and litterfall, was ca 23% of the forest floor respiration. An imbalance of ca 0.35 kg C m−2 was observed in the annual soil C cycle, assuming steady-state conditions. Errors in estimating C fluxes from a combination of direct measurements and models, and underestimation of forest floor respiration, were probably responsible for this imbalance. Modelled mean residence times for soil C ranged from 76 to 207 yr, and resulted from widely different amounts of ‘inert organic matter’ being present in the upper 23 cm of the soils. A long history of tree overturn is the most likely cause of this concentration of recalcitrant C near the soil surface, and may explain the longer C storage time of soils developed under beech in New Zealand compared to those developed under native grassland.

Soil microbial biomass, C and N mineralization and enzyme activities in a hill pasture: Influence of season and slow-release P and S fertilizer

Abstract Hill pastures are widely used in New Zealand for sheep and cattle production. The influence of season and added slow-release fertilizer (rock phosphate and elemental S) on soil microbial pools and biochemical activities in a low-fertility hill pasture was investigated during 2 y. The soil was a Typic Dystrochrept. Specifically determined k ec -and k en -factors were used in the fumigation-extraction procedures for measuring microbial C and N. An appropriate factor for converting substrate-induced respiration values to microbial C was also determined. Fertilizer tended to increase dry matter production and the legume component of the sward in the second year of the trial, but variability was high and differences were generally non-significant. Fertilizer increased extractable soil inorganic P and S, but had no consistent, if any, effect on soil biochemical properties. Soil moisture content was generally high at all sampling times. Amounts of microbial biomass, and the percentages of microbial C and N in total C and N, respectively, were similar to those in more fertile lowland pastures. Temporal fluctuations in microbial C, N and P concentrations were small and mainly non-significant, and were related to variability in total C and N concentrations. Temporal fluctuations were also mainly non-significant for invertase and sulphatase activities, but were very marked for phosphodiesterase activity. CO2C production under standardized conditions was highest in spring samples and significantly correlated with extractable C concentration. Min-N was generally immobilzed initially on incubation at 25°C, but net N mineralization increased over 14–56 d with maxima also in spring; NH4+N predominated throughout. Overall, potential rates of C and N mineralization were more susceptible to seasonal effects than were the microbial biomass pools.

Elevated CO2 and temperature effects on soil carbon and nitrogen cycling in ryegrass/white clover turves of an Endoaquept soil

Effects of elevated CO2 (700 μL L−1) and a control (350 μL L−1 CO2) on the productivity of a 3-year-old ryegrass/white clover pasture, and on soil biochemical properties, were investigated with turves of a Typic Endoaquept soil in growth chambers. Temperature treatments corresponding to average winter, spring, and summer conditions in the field were applied consecutively to all of the turves. An additional treatment, at 700 μL L−1 CO2 and a temperature 6°C higher throughout than in the other treatments, was included.Under the same temperature conditions, overall herbage yields in the ‘700 μL L−1 CO2’ treatment were ca. 7% greater than in the control at the end of the ‘summer’ period. Root mass (to ca 25 cm depth) in the ‘700 μL L−1 CO2’ treatment was then about 50% greater than in the control, but in the ‘700 μL L−1 CO2+6°C’ treatment it was 6% lower than in the control. Based on decomposition results, herbage from the ‘700 μL L−1+6°C’ treatment probably contained the highest proportion of readily decomposable components.Elevated CO2 had no consistent effect on soil total C and N, microbial C and N, or extractable C concentrations in any of the treatments. Under the same temperature conditions, it did, however, enhance soil respiration (CO2-C production) and invertase activity. The effects of elevated CO2 on rates of net N mineralization were less distinct, and the apparent availability of N for the sward was not affected. Under elevated CO2, soil in the higher-temperature treatment had a higher microbial C:N ratio; it also had a greater potential to degrade plant materials.Data interpretation was complicated by soil spatial variability and the moderately high background levels of organic matter and biochemical properties that are typical of New Zealand pasture soils. More rapid cycling of C under CO2 enrichment is, nevertheless, indicated. Futher long-term experiments are required to determine the overall effect of elevated CO2 on the soil C balance.

Temporal variations in some plant and soil P pools in two pasture soils of widely different P fertility status

Temporal variations in plant production, plant P and some soil P (and N) pools were followed over 21 months in two New Zealand pasture soils of widely different P fertility status. Plant growth rates, and herbage composition at the high-fertility site, were closely linked to soil water use, with growth rates falling when soil water deficits exceeded 60 mm. Herbage P concentrations reflected P fertility, and varied with season, being generally higher in winter and lower in summer.A similar temporal pattern was also observed for labile organic P (NaHCO3-extractable P0) in both soils. In the low-fertility soil in spring, net mineralization was especially strong, but from early winter net immobilization occurred. Surprisingly, Olsen P also changed temporally in the high-fertility soil. The microbial biomass remained fairly constant throughout the year, whereas the P content of the biomass varied seasonally. Although microbial biomass was not a useful index of soil fertility, highest microbial P0 contents coincided with periods of maximum labile P0 mineralization, when herbage production was also at a peak.Net N-mineralization in the low-fertility soil, in contrast to the high-fertility soil, was low but varied seasonally, under standardised incubation conditions. Soil P and N dynamics were apparently synchronised in the low-fertility soil through soil microbial processes, with mineral N being negatively correlated with microbial P0 in samples collected two months later.The results of this investigation suggest that the demands of rapid and sustained pasture growth in spring and early summer can best be met by maximising the build-up of organic matter during the preceding autumn and winter. This practice could help to alleviate the common problem of feed shortage in North Island hill country pastures in late winter-early spring.

Microbial C and N, and respiratory activity, in litter and soil of a southern beech (NOTHOFAGUS) forest: distribution and properties

Abstract Microbial biomass C and N, and respiratory activity (CO 2 production), declined with sample depth in the strongly acid litter and soil of a southern beech ( Nothofagus spp) forest, at which annual precipitation is about 2000 mm and mean annual air temperature 9.5°C. Microbial C, and generally N, did not differ significantly between spring and autumn samples, possibly because of spatial variability. As a percentage of total C, microbial C was highest ( c 14.5%) in leaf (L) litter, similar ( c 2.1%) in FH material and the topmost soil depths (0–20 cm), but declined to 0.6% in 50–60 cm depth soil; microbial N, as a percentage of total N, declined similarly with sample depth. Microbial C:N ratios were also highest ( c 13) in L material, but were generally similar (mean 6.7) in all other samples. On an area basis, the mass of microbial C and N to 60cm soil depth was c 4220 and 605 kg ha −1 , respectively, with c 14% occurring in the litter layers. The contribution of litter to total CO 2 production on an area basis with roots excluded was, however, c 42%. Temperature coefficients ( Q 10 values) for CO 2 production, determined with 5 and 13°C data, averaged 3.0. Metabolic quotients ( q CO 2 values) were highest in L and FH materials, and were low and similar in the different depths of soil. Generally, q CO 2 values in this old-growth forest were compatible with other values reported for a late stage of ecosystem succession.

Recolonization of methyl bromide sterilized soils under four different field conditions

SummaryThe course of recovery in biological activity was assessed in the top 5 cm of undisturbed soil cores (29.7 cm diameter, 30 cm deep) that had been fumigated in the laboratory with methyl bromide. The cores were returned to their original pasture and forest sites, two with a moderate and two with a high rainfall, and untreated soils at all sites served as baselines. Sampling took place over 166 days (midsummer to midwinter). Microbial biomass (as measured by fumigation-extraction and substrate-induced respiration procedures) and dehydrogenase activity both recovered rapidly, but remained consistently lower in the fumigated than in untreated samples at both forest sites and at the moister of the two pasture sites. Bacterial numbers also recovered rapidly. Fungal hyphal lengths were, on average over 166 days, 25% lower in the fumigated soils. Levels of mineral N were initially highest in the fumigated soils, but declined with time. Fumigation generally had no detectable effects on the subsequent rates of net N mineralization and little effect on nitrification rates. Fumigation almost totally eliminated protozoa, with one to three species being recovered on day 0; the numbers recovered most rapidly in the moist forest soil and slowly in the dry pasture soil. The recoionization rate of protozoan species was similar in all soils, with species numbers on day 110 being 33 and 34 in the fumigated and untreated soils, respectively. Nematodes were eliminated by fumigation; recolonization was first detected on day 26 but by day 166, nematode numbers were still lower in fumigated than in untreated soils, the abundance being 10 and 62 g-1 soil and diversity 10 and 31 species, respectively. Overall, the results suggest that protozoan and nematode populations and diversities could provide a useful medium-term ecological index of the recovery in comprehensive soil biological activity following major soil pollution or disturbance.

Afforestation of pastures with Pinus radiata influences soil carbon and nitrogen pools and mineralisation and microbial properties

In New Zealand, Pinus radiata D. Don is frequently planted on land under pasture primarily for production forestry, but with the added advantage of potentially offsetting carbon dioxide (CO2) emissions from energy and industrial sources. Conversion of pasture to P. radiata plantations can, however, result in lowered contents of soil carbon (C) at some sites. We here examine the effects of this land-use change on soil C and nitrogen (N) pools, and on microbial properties involved in the cycling of these nutrients, at 5 paired sites, each with an established pasture and P. radiata plantation. Four sites had first-rotation trees aged 12–30 years and the other site second-rotation trees aged 20 years. In mineral soil at 0–10 cm depth, total and microbial C and N, extractable C, CO2-C production, and, generally, net mineral-N production were lower under P. radiata than under pasture; differences were significant (P < 0.05), except for total and extractable C at 2 sites. Differences between these land uses were less distinct in soil at 10–30 cm depth. On an area basis, total C in 0–30 cm depth soil was lower under P. radiata than under pasture at most sites, but significantly lower at only one site. Total N, microbial C and N, and CO2-C and net mineral-N production were, however, again generally significantly lower under P. radiata. These ecosystem differences were less marked, although still present, except for CO2-C production, when forest litter (LFH material) was included in the area calculations. Overall, our study suggests that afforestation with P. radiata leads to a reduction in total N, microbial biomass, and microbial activity, but a less consistent effect on soil C storage after one rotation.