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Previous studies have demonstrated the effective utility of volcanic pumice soil to mitigate both high and low levels of methane (CH4) emissions through the activity of both γ-proteobacterial (type I) and α-proteobacterial (type II) aerobic methanotrophs. However, the limited availability of volcanic pumice soil necessitates the assessment of other farm soils and potentially suitable, economical and widely available biofilter materials. The potential biofilter materials, viz. farm soil (isolated from a dairy farm effluent pond bank area), pine biochar, garden waste compost and weathered pine bark mulch, were inoculated with a small amount of volcanic pumice soil. Simultaneously, a similar set-up of potential biofilter materials without inoculum was studied to understand the effect of the inoculum on the ability of these materials to oxidise CH4 and their effect on methanotroph growth and activity. These materials were incubated at 25 °C with periodic feeding of CH4, and flasks were aerated with air (O2) to support methanotroph growth and activity by maintaining aerobic conditions. The efficiency of CH4 removal was monitored over 6 months. All materials supported the growth and activity of methanotrophs. However, the efficiency of CH4 removal by all the materials tested fluctuated between no or low removal (0–40 %) and high removal phases (>90 %), indicating biological disturbances rather than physico-chemical changes. Among all the treatments, CH4 removal was consistently high (>80 %) in the inoculated farm soil and inoculated biochar, and these were more resilient to changes in the methanotroph community. The CH4 removal from inoculated farm soil and inoculated biochar was further enhanced (up to 99 %) by the addition of a nutrient solution. Our results showed that (i) farm soil and biochar can be used as a biofilter material by inoculating with an active methanotroph community, (ii) an abundant population of α-proteobacterial methanotrophs is essential for effective and stable CH4 removal and (iii) addition of nutrients enhances the growth and activity of methanotrophs in the biofilter materials. Further studies are underway to assess the feasibility of these materials at small plot and field scales.

In developing countries, the simple biogas digesters installed underground without heating or stirring are seen as a 'green' technology to convert animal waste into biogas, a source of bio-energy. However, quantitative estimates of biogas production of manures from steers fed local feed diets at actual incubation temperatures have yet to be carried out. The aim of this study was to determine the methane (CH4) production potential of manures from steers in Vietnam offered traditional feed rations or supplemental diets. Biochemical CH4 production (BMP) was measured in batch tests at 30°C using manures collected from two different experiments of steers fed diets containing feed supplements. BMP was 110.1 (NLkg-1 VS) for manure from steers receiving a control diet, significantly lower 79.0 (NL kg-1 VS) for manure from steers fed a diet containing 0.3% tannin (%DM), but then showed an increasing trend to 90.9 and 91.2 (NL kg-1 VS) for manures from steers receiving 0.4 and 0.5% tannin (%DM) supplements, respectively. Similarly, the CH4 production (NL kg-1 VS) of manure from steers was 174 for control, 142 for control supplemented concentrate (C), 143 for control added rice straw treated with urea (R), and 127 for control supplemented C and R. Our results show there was a decrease in CH4 emissions from steer manures through using supplemented rations.

We have followed the responses of the properties of a sandy soil (0–50 mm), including carbon (C) and nitrogen (N) concentrations, to 10 years of ambient and elevated (475 μl l −1 ) CO 2 in a FACE (Free Air CO 2 Enrichment) experiment in a grazed pasture in North Island, New Zealand. We have previously reported results for the first 5 years where, at this relatively low level of CO 2 enrichment, most soil properties changed gradually and non-significantly. Here we add data from a further 5 years of enrichment and find that most soil properties had changed significantly. Soil moisture was greater under elevated CO 2 but we could find no evidence that this was the cause of changes in other properties, suggesting that increased inputs of C and N in the high CO 2 regime were driving soil differences. When soil total C and N were adjusted to account for differences in starting concentrations there was evidence of increased pool sizes and increased C/N under elevated CO 2 . Microbial C and N pools also increased with CO 2 as did heterotrophic respiration and labile C and N fractions. There were few changes in N mineralization but no evidence for reduced rates under elevated CO 2 . There was little evidence for a direct effect of changes in soil properties on plant biomass responses to CO 2 but the changes we observed relate to pools and fluxes that are known to be important in both production and environmental outcomes in grassland systems.

Dairy farm effluent ponds are a source of methane (CH 4 ), a potent greenhouse gas. Biofiltration, whereby CH 4 is oxidised by methanotrophic bacteria, is a potentially cost-effective CH 4 mitigation technology. We report on the performance of a field-scale biofilter treating dairy farm effluent pond CH 4 emissions for 16 months. This study is the first to report on the feasibility of using biofiltration to mitigate dairy waste CH 4 emissions. The 70-L filter comprised a 1:1 volumetric mixture of volcanic soil from a landfill and perlite. Biogas collected in a floating cover on a 4-m 2 section of the pond was directed through the biofilter's base. Air was pumped through the filter to supply oxygen to the methanotrophs. The filter's maximum CH 4 removal rate was 16 g m −3 h −1 (or 53 μg g −1 h −1 ), which is high compared with literature landfill soil oxidation rates (typically −1 h −1 ). At the trial's conclusion, the filter experienced acid accumulation, due to oxidation of H 2 S in the inlet biogas (evidenced by low pH [3.9] and high sulphate-S [1079 mg −1 kg −1 ] at the base of the filter compared with the top [pH = 4.6, sulphate-S = 369 mg −1 kg −1 ]). Nonetheless, the filter's oxidation rate peaked at the end of the experiment indicating negligible H 2 S impact on overall performance over the 16-month period. The results showed that a 50-m 3 filter would be needed to offset CH 4 emissions (approximately 720 g h −1 ) from a typical 1000-m 2 dairy effluent pond. As this calculation is based on the efficiency of a single experimental filter, field testing of replicate biofilters is needed to accurately establish full-scale filter sizing. Nonetheless, this study has shown that biofilter technology is feasible to mitigate dairy effluent pond CH 4 emissions. Current research is underway to make the filter more economically viable through design optimisation.

The two non-CO2 greenhouse gases (GHGs) nitrous oxide (N2O) and methane (CH4) comprise 54.8% of total New Zealand emissions. Nitrous oxide is mainly generated from mineral N originating from animal dung and urine, applied fertiliser N, biologically fixed N2, and mineralisation of soil organic N. Even though about 96% of the anthropogenic CH4 emitted in New Zealand is from ruminant animals (methanogenesis), methane uptake by aerobic soils (methanotrophy) can significantly contribute to the removal of CH4 from the atmpsphere, as the global estimates confirm. Both the net uptake of CH4 by soils and N2O emissions from soils are strongly influenced by changes in land use and land management. Quantitative information on the fluxes of these two non-CO2 GHGs is required for a range of land-use and land-management ecosystems to determine their contribution to the national emissions inventory, and for assessing the potential of mitigation options. Here we report soil N2O fluxes and CH4 uptake for a range of land-use and land-management systems collated from published and unpublished New Zealand studies. Nitrous oxide emissions are highest in dairy-grazed pastures (10–12 kg N2O–N ha−1 year−1), intermediate in sheep-grazed pastures, (4–6 kg N2O–N ha−1 year−1), and lowest in forest, shrubland and ungrazed pasture soils (1–2 kg N2O–N ha−1 year−1). N deposited in the form of animal urine and dung, and N applied as fertiliser, are the principal sources of N2O production. Generally, N2O emissions from grazed pasture soils are high when the soil water-filled pore-space is above field capacity, and net CH4 uptake is low or absent. Although nitrification inhibitors have shown some promise in reducing N2O emissions from grazed pasture systems, their efficacy as an integral part of farm management has yet to be tested. Methane uptake was highest for a New Zealand Beech forest soil (10–11 kg CH4 ha−1 year−1), intermediate in some pine forest soils (4–6 kg CH4 ha−1 year−1), and lowest in most pasture (

Afforestation and reforestation of pastures are key land-use changes in New Zealand that help sequester carbon (C) to offset its carbon dioxide (CO2) emissions under the Kyoto Protocol. However, relatively little attention has been given so far to associated changes in trace gas fluxes. Here, we measure methane (CH4) fluxes and CO2 production, as well as microbial C, nitrogen (N) and mineral-N, in intact, gradually dried (ca. 2 months at 20 °C) cores of a volcanic soil and a heavier textured, non-volcanic soil collected within plantations of Pinus radiata D. Don (pine) and adjacent permanent pastures. CH4 fluxes and CO2 production were also measured in cores of another volcanic soil under reverting shrubland (mainly Kunzea var. ericoides (A. Rich) J. Thompson) and an adjacent pasture. CH4 uptake in the pine and shrubland cores of the volcanic soils at field capacity averaged about 35 and 14 μg CH4–C m−2 h−1, respectively, and was significantly higher than in the pasture cores (about 21 and 6 μg CH4–C m−2 h−1, respectively). In the non-volcanic soil, however, CH4–C uptake was similar in most cores of the pine and pasture soils, averaging about 7–9 μg m−2 h−1, except in very wet samples. In contrast, rates of CO2 production and microbial C and N concentrations were significantly lower under pine than under pasture. In the air-dry cores, microbial C and N had declined in the volcanic soil, but not in the non-volcanic soil; ammonium–N, and especially nitrate–N, had increased significantly in all samples. CH4 uptake was, with few exceptions, not significantly influenced by initial concentrations of ammonium–N or nitrate–N, nor by their changes on air-drying. A combination of phospholipid fatty acid (PLFA) and stable isotope probing (SIP) analyses of only the pine and pasture soils showed that different methanotrophic communities were probably active in soils under the different vegetations. The C18 PLFAs (type II methanotrophs) predominated under pine and C16 PLFAs (type I methanotrophs) predominated under pasture. Overall, vegetation, soil texture, and water-filled pore space influenced CH4–C uptake more than did soil mineral-N concentrations.

In situ farm-scale measurements are a prerequisite for improving the accuracy of greenhouse gas inventories, and for developing and modifying the modelling approaches to quantify emissions. We describe here the changes made to the process-based model NZ-DNDC to allow us to simulate emissions of nitrous oxide (N2O) from typical New Zealand grazed grassland soils, and subsequent improvements to account for periodic changes in pasture growth, N input from animals, and water balance/soil moisture status. In grazed pastures, N2O fluxes varied widely both spatially due to uneven excretal N inputs, and seasonally due to climatic conditions. The model was validated against field measurements from two dairy pastures with contrasting soils and from a sheep pasture. The model simulated effectively most of the soil water-filled pore-space (WFPS) and the general pulses and trends in N2O emission from both the sheep- and dairy-grazed pastures, and also captured the observed effects of excretal and fertiliser N inputs. It also fairly reproduced the real variability in underlying processes regulating N2O emissions. A series of sensitivity tests conducted on NZ-DNDC showed the model predicted changes in pasture production and N2O emissions with changes in climate, soil properties, fertiliser management and grazing regimes. Pasture production was more sensitive to rainfall, temperature and initial soil carbon levels than to fertiliser additions, and stocking rate while N2O emissions were strongly sensitive to rainfall, quantity and frequency of N inputs through both fertiliser and increased stocking rate, and initial soil carbon levels.

Plant and Soil, 282 (1-2), ISSN:0032-079X, ISSN:1573-5036

Soil N2O emissions were measured during four seasons from two highly productive grass-clover dairy pastures to assess the influences of soil moisture, temperature, availability of N (NH 4 + and NO 3 – ) and soluble C on N2O emissions, and to use the emission data to validate and refine a simulation model (DNDC). The soils at these pasture sites (Karapoti fine sandy loam, and Tokomaru silt loam) differed in texture and drainage characteristics. Emission peaks for N2O coincided with rainfall events and high soil moisture content. Large inherent variations in N2O fluxes were observed throughout the year in both the ungrazed (control) and grazed pastures. Fluxes averaged 4.3 and 5.0 g N2O/ha/day for the two ungrazed sites. The N2O fluxes from the grazed sites were much higher than for the ungrazed sites, averaging 26.4 g N2O/ha/day for the fine sandy loam soil, and 32.0 g N2O/ha/day for the silt loam soil. Our results showed that excretal and fertiliser-N input, and water-filled pore space (WFPS) were the variables that most strongly regulated N2O fluxes. The DNDC model was modified to include the effects of day length on pasture growth, and of excretal-N inputs from grazing animals; the value of the WFPS threshold was also modified. The modified model ‘NZ-DNDC’ simulated effectively most of the WFPS and N2O emission pulses and trends from both the ungrazed and grazed pastures. The modified model fairly reproduced the real variability in underlying processes regulating N2O emissions and could be suitable for simulating N2O emissions from a range of New Zealand grazed pastures. The NZ-DNDC estimates of total yearly emissions of N2O from the grazed and ungrazed sites of both farms were within the uncertainty range of the measured emissions. The measured emissions changed with changes in soil moisture resulting from rainfall and were about 20% higher in the poorly drained silt loam soil than in the well-drained sandy loam soil. The model accounts for these climatic variations in rainfall, and was also able to pick up differences in emissions resulting from differences in soil texture.

Globally, land-use change is occurring rapidly, and impacts on biogeochemical cycling may be influenced by previous land uses. We examined differences in soil C and N cycling during long-term laboratory incubations for the following land-use sequence: indigenous forest (soil age = 1800 yr); 70- year-old pasture planted after forest clearance; 22-year-old pine (Pinus radiata) planted into pasture. No N fertilizer had been applied but the pasture contained N-fixing legumes. The sites were adjacent and received 3-6 kg ha1 yr1 "volcanic" N in rain; NO3 -N leaching losses to streamwater were 5-21 kg ha 1 yr 1 , and followed the order forest pine = forest, and total N: pasture > pine > forest. Nitrogen mineral- ization followed the order: pasture > pine > forest for mineral soil, and was weakly related to C min- eralization. Based on radiocarbon data, the indigenous forest 0-10 cm soil contained more pre-bomb C than the other soils, partly as a result of microbial processing of recent C in the surface litter layer. Heterotrophic activity appeared to be somewhat N limited in the indigenous forest soil, and gross nitri- fication was delayed. In contrast, the pasture soil was rich in labile N arising from N fixation by clover, and net nitrification occurred readily. Gross N cycling rates in the pine mineral soil (per unit N) were similar to those under pasture, reflecting the legacy of N inputs by the previous pasture. Change in land use from indigenous forest to pasture and pine resulted in increased gross nitrification, net nitrification and thence leaching of NO3 -N.

Summary The growth and chemical composition of most plants are influenced by elevated CO2, but accompanying effects on soil organic matter pools and mineralization are less clearly defined, partly because of the short-term nature of most studies. Herein we describe soil properties from a naturally occurring cold CO2 spring (Hakanoa) in Northland, New Zealand, at which the surrounding vegetation has been exposed to elevated CO2 for at least several decades. The mean annual temperature at this site is ≈ 15.5 °C and rainfall ≈ 1550 mm. The site was unfertilized and ungrazed, with a vegetation of mainly C3 and C4 grasses, and had moderate levels of ‘available’ P. Two soils were present − a gley soil and an organic soil – but only the gley soil is examined here. Average atmospheric CO2 concentrations at 17 sampling locations in the gley soil area ranged from 372 to 670 ppmv. In samples at 0–5 cm depth, pH averaged 5.4; average values for organic C were 150 g, total N 11 g, microbial C 3.50 g, and microbial N 0.65 g kg−1, respectively. Under standardized moisture conditions at 25 °C, average rates of CO2-C production (7–14 days) were 5.4 mg kg−1 h−1 and of net mineral-N production (14 −42 days) 0.40 mg kg−1 h−1. These properties were all correlated positively and significantly (P

The coniferous forests of Siberia contain a significant fraction of the world's terrestrial C. We have assessed organic matter storage and potential metabolic activities in one of these forest stands by determining C and N pools, microbial properties and C and N mineralization at different temperatures in litter and a sandy mineral soil (a Pergelic Cryochrept) from a 215-y old forest of Pinus sylvestris L. Relationships between CO2–C production in the laboratory and field were also assessed. The site was near Zotino in central Siberia at 61° N, 89° E and has an average air temperature of ca. −4°C and an annual precipitation of ca. 620 mm; it is subject to water deficits during summer and to recurrent fires. Results were evaluated by comparisons with data from other coniferous forests, including those with sandy or sandy loam soils at three temperate sites under Pinus radiata D. Don in New Zealand. The very acid (pH 4.1) litter (ca. 3 cm depth) from the Zotino site contained (to 50 cm depth of mineral soil) ca. 41% of the total C (2.8 kg m−2), 34% of the total N (86 g m−2) and about 50% of the microbial biomass and respiratory and net N-mineralizing activity. The low values in the mineral soil were, except for total N which was lowest at the Zotino site, generally similar to those in two young coastal sands under P. radiata in New Zealand. Rates of C and net N mineralization in the Zotino mineral soil declined proportionately more between 13.5°C and 5.5°C than between 25 and 13.5°C. Use of temperature relationships suggested that less than half of the CO2–C produced from the soil profile in the field under well-watered conditions during summer would have originated from microbial respiration. In litter and in mineral soil, ratios of microbial C and N-to-total C and N, microbial C-to-microbial N, rates of CO2–C and net mineral-N production on a total C and N basis, and metabolic quotients (qCO2 values) were all similar to at least some of the values found for the temperate pine-forest soils. Our results strongly suggest that soil organic matter content is low at this Zotino site, not because of distinctive metabolic potentials of the microbial populations, but because of environmental constraints on ecosystem processes.

The effects of elevated CO2 on rhizosphere processes, including the response of soil faunal populations and community structure, have so far received little attention. We report on significant responses in the soil fauna of ryegrass/white clover swards to both increasing CO2 from 350 to 750 μl · l–1 and, to a period of 60 days when some of the turves were subject to drought, in a controlled climate growth room experiment. The nematodes which increased were predominantly Enoplia, including dorylaimids, alaimids and trichodorids. This accords with both the doubling of Alaimus under elevated CO2 conditions reported in a similar experiment and with the common association of Enoplia with less disturbed habitats. The most marked decrease was in the bacterial-feeding Rhabditis (Secernentea). The increase in omnivorous and predacious nematodes may have been responsible for the decrease in populations of bacterial-feeding nematodes. However, in contrast to their standing crops, the turnover rate of bacterial-feeding nematodes and soil microbial biomass probably increased as a result of increased grazing by these omnivorous and predacious nematodes. Increases in earthworm and enchytraeid populations were related to increased below-ground productivity reported for the same trial.

In grassland ecosystems, most of the carbon (C) occurs below-ground. Understanding changes in soil fluxes induced by elevated atmospheric CO2 is critical for balancing the global C budget and for managing grassland ecosystems sustainably. In this review, we use the results of short-term (1–2 years) studies of below-ground processes in grassland communities under elevated CO2 to assess future prospects for longer-term increases in soil C storage. Results are broadly consistent with those from other plant communities and include: increases in below-ground net primary productivity and an increase in soil C cycling rate, changes in soil faunal community, and generally no increase in soil C storage. Based on other experimental data, future C storage could be favoured in soils of moderate nutrient status, moderate-to-high clay content, and low (or moderateIy high) soil moisture status. Some support for these suggestions is provided by preliminary results from direct measurements of soil C concentrations near a New Zealand natural CO2-venting spring, and by simulations of future changes in grassland soils under the combined effects of CO2 fertilization and regional climate change. Early detection of any increase in soil C storage appears unlikely in complex grassland communities because of (a) the difficulty of separating an elevated CO2 effect from the effects of soil factors including moisture status, (b) the high spatial variability of soil C and (c) the effects of global warming. Several research imperatives are identified for reducing the uncertainties in the effects of elevated atmospheric CO2 on soil C.

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.

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.

The 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.

The reliability of three methods (microbial C and mineral-N flush by fumigation-incubation, and ATP) for measuring soil microbial biomass was assessed on two silt-loam soils of different P fertility status under grazed perennial pastures. The mineral-N flush and ATP methods provided a reasonably reliable index of microbial biomass, but the fumigation-incubation procedure for CO2-C flush, using preincubated samples and an unfumigated 0–10 day control, was inappropriate for these soils. The numbers of bacteria (direct microscopy) and the percentage metabolically active were also measured.

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.

We investigated the effect of afforestation and reforestation of pastures on methane oxidation and the methanotrophic communities in soils from three different New Zealand sites. Methane oxidation was measured in soils from two pine ( Pinus radiata ) forests and one shrubland (mainly Kunzea ericoides var. ericoides ) and three adjacent permanent pastures. The methane oxidation rate was consistently higher in the pine forest or shrubland soils than in the adjacent pasture soils. A combination of phospholipid fatty acid (PLFA) and stable isotope probing (SIP) analyses of these soils revealed that different methanotrophic communities were active in soils under the different vegetations. The C 18 PLFAs (signature of type II methanotrophs) predominated under pine and shrublands, and C 16 PLFAs (type I methanotrophs) predominated under pastures. Analysis of the methanotrophs by molecular methods revealed further differences in methanotrophic community structure under the different vegetation types. Cloning and sequencing and terminal-restriction fragment length polymorphism analysis of the particulate methane oxygenase gene ( pmoA ) from different samples confirmed the PLFA-SIP results that methanotrophic bacteria related to type II methanotrophs were dominant in pine forest and shrubland, and type I methanotrophs (related to Methylococcus capsulatus ) were dominant in all pasture soils. We report that afforestation and reforestation of pastures caused changes in methane oxidation by altering the community structure of methanotrophic bacteria in these soils.

Since 1992, afforestation with Pinus radiata D. Don in New Zealand has led to the establishment of over 600 000 ha of new plantation forests, about 85% of which are on fertile pastures used previously for grazing sheep and cattle. While this leads to rapid accumulation of carbon (C) in vegetation, the effects of afforestation on soil C are poorly understood. We examined key soil C cycling processes at the (former) Tikitere agroforestry experimental site near Rotorua, New Zealand. In 1973, replicated stands of P. radiata (100 and 400 stems/ha) were established on pastures, while replicated pasture plots were maintained throughout the first 26-year rotation. In 1996, soil C and microbial biomass C in 0–0.10 m depth soil, in situ soil respiration and net N mineralisation, and soil temperature were lower in the forest than in the pasture, and tended to decline with increasing tree-stocking density. In the 400 stems/ha stands, mineral soil C (0–0.50 m depth) was lower than in the pasture (104 and 126 Mg C/ha, respectively; P < 0.01). Carbon accumulation in the forest floor during the first rotation of these forest stands was 12 Mg C/ha. Using the Rothamsted soil C model (Roth-C), we examined how changes in plant C inputs following afforestation might lead to changes in soil C content to 0.30 m depth. Steady-state pasture inputs of 9.0 Mg C/ha.year were estimated using Roth-C; these C inputs were assumed to decrease linearly during the first 12 years following tree establishment (until canopy closure). Below-ground C inputs in the forest were estimated using steady-state relationships between litterfall and soil respiration; these inputs were assumed to increase linearly between years 1 and 12, after which they remained constant at 1.53 Mg C/ha.year until harvest. Measured changes in soil C (0−0.30 m) during the first rotation, in conjunction with the below-ground inputs, were used to estimate above-ground inputs (as a proportion of total litterfall [3.81 Mg C/ha.year]) to the soil. Our results suggest 10% of litterfall C over one rotation actually entered the mineral soil. Using these results and estimates of additional C inputs to the soil from harvest slash and weeds following harvest, we found mineral-soil C stocks would continue to decline during second and third rotations of P. radiata; the magnitude of this decline depended in part on how much slash enters the mineral soil matrix. We confirmed our modelling approach by simulating soil C changes to within 8% over 19 years following afforestation of pasture at another previously studied site, Purukohukohu. Whether afforestation leads to an increase or decrease in mineral-soil C may depend on previous pasture management; in highly productive pastures, high C inputs to the soil may maintain soil C at levels that cannot be sustained when trees are planted onto these grasslands.

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.

Although the contribution of roots to soil carbon (C) fluxes and biochemical processes is recognised, it is difficult to quantify. One approach to assess their importance is the use of trenched plots, in which C inputs to the soil and respiration by living roots has ceased. We give here an account of C and nitrogen (N) pools and mineralisation in samples taken 27 months after trenching in a 26-year-old Pinus radiata D. Don plantation on a coastal sand (an Aquic Udipsamment); above-ground litter inputs continued throughout the 27-month period.Moisture contents were higher in FH material and mineral soil from the trenched than from the control plots. Trenching had no effect on total organic C and N concentrations, but led to decreases in extractable C, microbial C and N, and CO2-C production values at some depths in the soil profile. Mineral-N concentrations and gross nitrification rates were, in contrast, initially higher in the trenched-plot samples, but were similar in both treatments after incubation of the samples at 25°C for 57 days. Mineral-N concentrations were also higher in the trenched than control mineral soil after in situ incubation. On an area basis (to 20 cm depth of mineral soil), inputs from roots were estimated to account for about 40% of the extractable C pool, 28% of microbial C, 26% of microbial N, and 23% of heterotrophic CO2-C production (0–7 days at 25°C) in the control soil. Overall, our results suggest a tight connection between N cycling rates and the labile C pools derived from below-ground inputs, with nitrification in particular increasing as C availability declined as a result of trenching.

Afforestation may lead to an accumulation of carbon (C) in vegetation, but little is known about changes in soil C storage with establishment of plantation forests. Plantation forest carbon budget models often omit mineral soil C changes from stand-level C budget calculations, while including forest floor C accumulation, or predict continuous soil C increases over several rotations. We used national soil C databases to quantify differences in soil C content between pasture and exotic pine forest plantations dominated by P. radiata (D. Don), and paired site studies to quantify changes in soil C with conversion of pasture to plantation forest in New Zealand. Overall, mineral soil C to 0.10 m was 20–40% lower under pine for all soil types (p

Grazing and fertilizer management practices are of prime importance for maintaining summer-moist hill pastures of introduced grasses and clovers in New Zealand for sheep and cattle production. The influence of withholding grazing (a pastoral fallow) from spring to late summer on microbial biomass, C and N mineralization, and enzyme activities was investigated in a Typic Dystrochrept soil from unfertilized and fertilized (rock phosphate and elemental S) low-fertility pastures at a temperate hill site. The fallow increased pasture but not legume growth in the following year in the unfertilized treatment, but had no effect on pasture or legume growth in fertilized plots. High background levels of the biochemical propel-ties examined, and very variable rates of N mineralization, complicated data interpretation. Extractable-C concentration and CO2-C production were enhanced at the completion of the fallow. Increases in net N mineralization (14-56 days incubation), following initial immobilization, after the fallow were clearly indicated in the unfertilized treatment, but were less distinct, in the fertilized treatment. The fallow had no detectable influence on the concentrations of total C and N or microbial C and P, or on invertase, phosphodiesterase and sulfatase activities. Some small changes in microbial N and an increased proportion of bacteria in the microbial population were, however, suggested. Results are consistent with the concept of fallowing giving a short-term increase in pools of readily decomposable soil organic matter. Generally, the changes that did occur in these soil biochemical properties are, with the partial exception of increased N availability, unlikely to have had any pronounced impact on subsequent pasture performance.

The restoration of soil biochemical activities after topsoil removal to depths 0 (S0), 10 (S10) or 20 (S20)cm, and re-establishment of a grass-clover pasture, was studied with Judgeford silt loam, a Typic Dystrochrept, in the Wellington area. Organic matter contents of the S10 and S20 samples were initially 58 and 40%, and biochemical activities, on average, 32 and 20%, respectively of S0 values. Organic matter content increased only slowly in the stripped plots. Yields of pasture herbage, soil CO2 and mineral-N production, three indices of microbial biomass, and invertase, amylase, cellulase, xylanase, urease, phosphatase, and sulphatase activities increased more rapidly, but had not reached S0 levels after 3 yr. The ratios of biomass indices varied with depth of topsoil removed; over the trial, biomass-C to ATP ratios averaged 236, 205, and 168 in the S0, S10, and S20 samples respectively. Biochemical activities were generally correlated significantly with herbage yields in the S10 or S20, but not in the S0 plots; organic C or total N contents were not generally correlated significantly with yields over the first 2 yr. Overall, invertase, and then sulphatase, activity appeared to be the best indicators of soil fertility status in this stripped soil.

Biochemical indices of microbial biomass and other biochemical properties of a Typic Haplaquoll, sampled under pasture over four seasons, were compared in intact cores and soil sieved through a 6 mm and a 2 mm mesh. Sieving had an inconsistent influence on biomass C estimates, which tended, however, to be higher in