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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.

A biofilter made using volcanic pumice soil from a landfill in Taupo, New Zealand has been found to mitigate CH4 emissions from New Zealand dairy effluent ponds. However, the biofilter after drying out following almost 5 years of use removed little or no CH4. Furthermore, H2S present in the biogas (from the dairy effluent ponds) had increased the acidity (pH) in the soil biofilter from 5.2 to 3.72 during this 5-year period. In this study, we adjusted the soil moisture to 60 % water-holding capacity (WHC) and investigated the CH4-oxidising capacity of a reconstituted acidic soil biofilter operating at low pH (3.72) and characterised the abundance and diversity of methane-oxidising bacteria (MOB) using quantitative polymerase chain reaction (qPCR) and terminal-restriction fragment length polymorphism (T-RFLP). The acidic soil biofilter achieved a maximum CH4 removal rate of 30.3 g m–3 h–1. Both types I and II MOB communities, along with some uncultured novel MOB strains or species in the biofilter column, were present. Among these, Methylocapsa-like type II methanotrophs were significantly more prominent than the other MOB. Other MOB, Methylococcus (type I), Methylobacter/Methylomonas/Methylosarcina (type I) genera, Methylosinus and Methylocystis (type II), were least abundant. During the 90-day study, the population of Methylocapsa-like MOB increased 4-fold, demonstrating the ability of these soil microorganisms to grow under acidic pH conditions in the biofilter, whereas the populations of type I MOB remained stable, and the populations of type II MOB (except Methylocapsa) decreased. Our results indicated that (i) a soil biofilter can effectively regain efficiency if sufficient moisture levels are maintained, regardless of the soil acidity; (ii) changes in the MOB population did not compromise the capacity of the volcanic pumice soil to oxidise CH4; (iii) the more acidic environment (pH 3.72) tends to favour the growth and activity of acid loving Methylocapsa-like MOB while being detrimental to the growth of Methylobacter/Methylocystis/Methylococcus group of MOB; and (iv) novel species or strains of uncultured Methylomicrobium / Methylosarcina/Methylobacter (type I MOB) could be present in the soil biofilter. This study has revealed the MOB population changes in the biofilter with acidification did not compromise its capacity to oxidise CH4 demonstrating that soil biofilter can operate effectively under acidic conditions.

Methane (CH4) is the second greatest contributor to anthropogenic climate change. Emissions have tripled since preindustrial times and continue to rise rapidly, given the fact that the key sources of food production, energy generation and waste management, are inexorably tied to population growth. Until recently, the pursuit of CH4 mitigation approaches has tended to align with opportunities for easy energy recovery through gas capture and flaring. Consequently, effective abatement has been largely restricted to confined high-concentration sources such as landfills and anaerobic digesters, which do not represent a major share of CH4’s emission profile. However, in more recent years we have witnessed a quantum leap in the sophistication, diversity and affordability of CH4 mitigation technologies on the back of rapid advances in molecular analytical techniques, developments in material sciences and increasingly efficient engineering processes. Here, we present some of the latest concepts, designs and applic...

Global atmospheric methane (CH4) concentrations are now approaching 1800 ppbv as a result of the growing imbalance between the net CH4 emissions from natural and anthropogenic sources of this potent greenhouse gas, and its consumption by physical and biological processes. The main focus of this review is on how land-use change and soil management can be used to correct this imbalance. Currently, the main terrestrial source for CH4 is from natural wetlands and irrigated rice cultivation, although improvements in water management during rice production have resulted in major reductions of CH4 emissions from this source. Afforestation and reforestation can also enhance soil CH4 oxidation by influencing the composition and activity of the soil methanotroph (aerobic proteobacteria) community. The effects of these and other land-use changes on soil CH4 oxidation are not generally well understood, but are known to influence this process through their effects on a range of soil properties such as soil moisture, nitrogen status, and pH that also affects methanotroph community structure and function. Recent advances in molecular techniques have confirmed the central role of methanotroph communities in regulating soil CH4 consumption by revealing how they respond to land-use change. Community-level molecular analyses of methanotroph populations under different conditions now provide new insights into the distinct traits of the different subgroups and their ecology. These advances in understanding the abiotic and biological processes regulating soil CH4 oxidation now offers the possibility of being able to predict which land-use and management practices, especially for afforestation and reforestation, will achieve high soil CH4 oxidation rates They also improve the prospects for integrated assessment of the atmospheric impacts on the global greenhouse gas budget from net soil emissions of CH4, N2O, and CO2 with land use and management change.

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.

Mitigating methane (CH) emissions from New Zealand dairy effluent ponds using volcanic pumice soil biofilters has been found to be a promising technology. Because the soil column biofilter prototype previously used was cumbersome, here we assess the effectiveness of volcanic pumice soil-perlite biofilter media in a floating system to remove high concentrations of CH emitted from a dairy effluent pond and simultaneously in a laboratory setting. We measured the CH removal over a period of 11 mo and determined methanotroph population dynamics using molecular techniques to understand the role of methanotroph population abundance and diversity in CH removal. Irrespective of the season, the pond-floating biofilters removed 66.7 ± 5.7% CH throughout the study period and removed up to 101.5 g CH m h. By contrast, the laboratory-based floating biofilters experienced more biological disturbances, with both low (∼34%) and high (∼99%) CH removal phases during the study period and an average of 58% of the CH oxidized. These disturbances could be attributed to the measured lower abundance of type II methanotroph population compared with the pond biofilters. Despite the acidity of the pond biofilters increasing significantly by the end of the study period, the biofilter encouraged the growth of both type I ( and ) and type II ( and ) methanotrophs. This study demonstrated the potential of the floating biofilters to mitigate dairy effluent ponds emissions efficiently and indicated methanotroph abundance as a key factor controlling CH oxidation in the biofilter.

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.

We describe experiments to better understand how CH 4 oxidation rates by different methanotroph communities respond to changing CH 4 concentrations. We used a novel system of automatically monitored chambers to investigate the response of CH 4 oxidation rates in a New Zealand pasture and adjacent pine forest soil exposed to varying atmospheric CH 4 concentrations. Type II methanotrophs that dominate CH 4 oxidation in the forest soil became progressively saturated as CH 4 concentrations rose from ambient (1.8 ppmv) to 570 ppmv, as shown by a decrease in uptake efficiency from 20% to 2% removal. By contrast, CH 4 oxidation in the pasture soil where Type I methanotrophs dominate increased in proportion to the increase in CH 4 inlet concentration, oxidising about 2% of the inlet CH 4 flux throughout. Modelling based on Michaelis-Menten kinetics revealed that low-affinity (Type I) methanotrophs were solely responsible for CH 4 oxidation in pasture soils, whereas high affinity (Type II) methanotrophs only contributed about 10% of the CH 4 oxidation in the forest soil. Increased aeration status using a soil–perlite (1:1) mixture doubled CH 4 oxidation rates at both ambient (1.8 ppmv) and 40 ppmv atmospheric CH 4 . A similar volcanic soil previously exposed for 8 y to high CH 4 fluxes from a landfill had removal efficiencies consistently above 95% for atmospheric CH 4 concentrations up to 7500 ppmv when the CH 4 oxidation rate was7000 μg CH 4 kg −1 soil h −1 .

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 transition of land between forest and grassland has important implications for greenhouse gas emissions and removals. In this paper, we use New Zealand as a case study to comprehensively assess, compare and quantify the net climate change impact of shifting land use between temperate forest and grassland. Forests store large amounts of carbon in their biomass, whereas grasslands contain relatively little biomass carbon. These biomass changes tend to dominate the carbon balance under land-use change. Soil carbon stocks usually do not change much after deforestation unless subsequent erosion occurs, but some soil carbon is often lost when grasslands are reforested with exotic plantations. Forest soils usually release little nitrous oxide or methane and can even oxidise small amounts of methane. Grasslands, on the other hand, can release a large amount of nitrous oxide, which may be further increased with fertilisation, and is higher for cattle- than sheep-grazed systems. Grazing animals increase emissions because the concentrated forms of nitrogen in their excreta allow it to escape from the system. Ruminant animals can also emit large amounts of methane. Land cover change in addition has direct radiative effects through the amount of solar radiation that is either absorbed by vegetated surfaces or reflected back into space. As forests typically absorb more radiation than grasslands, this slightly negates the greenhouse consequences of changes in carbon storage, and methane and nitrous oxide emissions under land-use change.

Soil methane (CH(4)) biofilters, containing CH(4)-oxidizing bacteria (methanotrophs), are a promising technology for mitigating greenhouse gas emissions. However, little is known about long-term biofilter performance. In this study, volcanic pumice topsoils (0-10 cm) and subsoils (10-50 cm) were tested for their ability to oxidize a range of CH(4) fluxes over 1 yr. The soils were sampled from an 8-yr-old and a 2-yr-old grassed landfill cover and from a nearby undisturbed pasture away from the influence of CH(4) generated by the decomposing refuse. Methane was passed through the soils in laboratory chambers with fluxes ranging from 0.5 g to 24 g CH(4) m(-3) h(-1). All topsoils efficiently oxidized CH(4). The undisturbed pasture topsoil exhibited the highest removal efficiency (24 g CH(4) m(-3) h(-1)), indicating rapid activation of the methanotroph population to the high CH(4) fluxes. The subsoils were less efficient at oxidizing CH(4) than the topsoils, achieving a maximum rate oxidation rate of 7 g CH(4) m(-3) h(-1). The topsoils exhibited higher porosities; moisture contents; surface areas; and total C, N, and available-P concentrations than the subsoils, suggesting that these characteristics strongly influence growth and activity of the CH(4)-oxidizing bacteria. Soil pH values and available-P levels gradually declined during the trial, indicating a need to monitor chemical parameters closely so that adjustments can be made when necessary. However, other key soil physicochemical parameters (moisture, total C, total N) increased over the course of the trial. This study showed that the selected topsoils were capable of continually sustaining high CH(4) removal rates over 1 yr, which is encouraging for the development of biofilters as a low-maintenance greenhouse gas mitigation technology.

Denitrification and nitrogen (N) fixation are two major processes which drive N transformation in ecosystems. We investigated the effect of afforestation of pasture in New Zealand on denitrifying and N-fixing communities using molecular techniques. We optimised and applied multiplex-terminal restriction fragment length polymorphism for simultaneous analysis of denitrifying and N-fixing communities. Soil samples from four pine (Pinus radiata) forests of various age and adjacent pastures were analysed for microbial communities, denitrification rate and some environmental variables. Analysis of variance revealed that the denitrifying community was mainly influenced by site while the N-fixing community was influenced by both site and land-use types. Among environmental variables, moisture and porosity had the strongest impact on denitrifying communities while C:N ratio explained most of the variability in the N-fixing community. Denitrification rate as measured by nitrous oxide (N2O) flux, was significantly lower in older forest soils compared to adjacent pasture soils. Simple and step-wise regression analyses revealed that the N2O flux rate was strongly linked to the denitrifying community composition, providing evidence for the influence of community composition on denitrification. Moreover, we explored the relationship between denitrifying and N-fixing communities using various statistical approaches. Our results for the first time showed that denitrifying and N-fixing communities are related in situ, and suggest that changes in land-use may have a significant impact on N-transformations and ecosystem processes.

Methanotrophs use methane (CH(4)) as a carbon source. They are particularly active in temperate forest soils. However, the rate of change of CH(4) oxidation in soil with afforestation or reforestation is poorly understood. Here, soil CH(4) oxidation was examined in New Zealand volcanic soils under regenerating native forests following burning, and in a mature native forest. Results were compared with data for pasture to pine land-use change at nearby sites. We show that following soil disturbance, as little as 47 years may be needed for development of a stable methanotrophic community similar to that in the undisturbed native forest soil. Corresponding soil CH(4)-oxidation rates in the regenerating forest soil have the potential to reach those of the mature forest, but climo-edaphic fators appear limiting. The observed changes in CH(4)-oxidation rate were directly linked to a prior shift in methanotrophic communities, which suggests microbial control of the terrestrial CH(4) flux and identifies the need to account for this response to afforestation and reforestation in global prediction of CH(4) emission.

In forest ecosystems, few data are available on transfer rates of litterfall carbon (C) to mineral soil. Such data are, nevertheless, needed for predicting the effects of forest land-use change and management on soil C stocks. We here report the transfer of C from Pinus radiata litter to mineral soil over 117 weeks, using 14 C-labelled needles mixed with unlabelled needles in lysimeters placed on the forest floor of a 5-year-old P. radiata stand. Mean annual temperature was about 12.9°C and mean annual rainfall about 995 mm; the soil was a well-drained silt loam. Measurements were made at 39, 78 and 117 weeks of 14 C in residual litter and in 10-mm increments of mineral soil to 50 mm depth and 25-mm increments from 50 to 100 mm depth. Measurements were also made of 14 C-CO 2 production, at frequent intervals over the first 5 weeks and then less frequently. Recovery of 14 C in litter and mineral soil ranged from 51 to 66% at the different exposure times. Cumulative respired 14 C-CO 2 , expressed as a percentage of the initially added 14 C, increased from 16 to 19% after exposure times of 39 and 117 weeks, respectively. Total recovery of the added 14 C did not differ significantly (P > 0.12) with time over the three measurement periods, with means ranging from 68 to 84%. Apparent losses of 14 C were associated with large replicate variability, with coefficients of variation ranging from 20 to 31 %, and with some disturbance of litter in the lysimeters by birds. We estimate that 1.0-2.2% of the litter 14 C was transferred annually to 10-100-mm depth of mineral soil. On the basis of microbial biomass- 14 C measurements, this transferred C contained proportionately more microbial (labile) C than did the older total C in mineral soil. Results therefore are consistent with other studies on forests with well-developed organic horizons indicating that C in humified organic matter, rather than in newly fallen litter, is the main above-ground source for transfer of C to the mineral soil.

The rivers that drain active, collisional margins of the southwest Pacific deliver up to 35% of particulate organic carbon (POC) to the world ocean, and are a key component of the global organic carbon flux. However, knowledge of the fate of terrestrial POC in the ocean is both limited and necessary for quantifying terrestrial and coastal ocean carbon budgets. Here, the fate of terrestrial POC is determined off the high discharge, Waipaoa River (sediment yield 15 Mt y − 1 ) based on a transect of seven cores from the river floodplain to the adjacent continental shelf and slope. Total organic carbon (%TOC), δ 13 C, 14 C, C/N ratios and lipid biomarker compounds were used to determine biogeochemical characteristics of surface sediments from terrestrial source to marine sink, and how these characteristics vary with river discharge. Complementary to surface sediments, down-core characteristics of three multi-cores covering the shelf and slope regions were used to identify perturbations in sediment supply via major floods. The presence of flood deposits allows us to compare their OC characteristics with non-flood sediment, thereby helping address the question of how flood events in the river catchment affect the transfer and fate of terrestrial OC through the marine environment. Results from this study show that as surface sediments are physically and biologically processed across the continental margin, they gain a marine signature. Biomarker analyses of surface samples show decreases in terrigenous vascular plant sources with increasing distance offshore. Biomarkers also demonstrate that terrestrial OC is being transferred across the continental margin, with plant sterols, long-chain alcohols and long-chain fatty acids (biomarkers indicative of vascular plants) persisting as far offshore as the mid-continental slope. In contrast to ambient conditions represented by surface sediments, rapid delivery by floods allows for more complete transfer of terrestrial carbon to the marine environment. A 1–10 cm thick flood layer preserved from Cyclone Bola (March 1988) contains a significant amount of terrestrially-sourced OC which subsequently was rapidly buried by sediments delivered during less extreme conditions.

Afforestation and deforestation are key land-use changes across the world, and are considered to be dominant factors controlling ecosystem functioning and biodiversity. However, the responses of soil microbial communities to these land-use changes are not well understood. Because changes in soil microbial abundance and community structure have consequences for nutrient cycling, C-sequestration and long-term sustainability, we investigated impacts of land-use change, age of stand and soil physico-chemical properties on fungal and bacterial communities and their metabolic activities. This study was carried out at four sites in two geographical locations that were afforested on long-established pastures with Pinus radiata D. Don (pine). Two of the sites were on volcanic soils and two on non-volcanic soils and stand age ranged from 5 to 20 y. Microbial communities were analysed by biochemical (phospho-lipid fatty acids; PLFA) and molecular (multiplex-terminal restriction fragment length polymorphism; M-TRFLP) approaches. Both site and stand age influenced microbial properties, with changes being least detectable in the 5-y-old stand. Land use was a key factor influencing soil metabolic activities as measured by physiological profiling using MicroResp. Pasture soils had higher microbial biomass (P

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.

Soil has been identified as a possible carbon (C) sink to mitigate increasing atmospheric CO2 concentration. However, several recent studies have suggested that the potential of soil to sequester C is limited and that soil may become saturated with C under increasing CO2 levels. To test this concept of soil C saturation, we studied a gley and organic soil at a grassland site near a natural CO2 spring. Total and aggregate-associated soil organic C (SOC) concentration showed a significant increase with atmospheric CO2 concentration. An asymptotic function showed a better fit of SOC and aggregation with CO2 level than a linear model. There was a shift in allocation of total C from smaller size fractions to the largest aggregate fraction with increasing CO2 concentration. Litter inputs appeared to be positively related to CO2 concentration. Based on modeled function parameters and the observed shift in the allocation of the soil C from small to large aggregate-size classes, we postulate that there is a hierarchy in C saturation across different SOC pools. We conclude that the asymptotic response of SOC concentration at higher CO2 levels indicates saturation of soil C pools, likely because of a limit to physical protection of SOC.

Forest harvest results in the removal of a large reservoir of terrestrial carbon with potential significant effects on net CO 2 emissions, but concomitant effects on other atmospheric trace gas fluxes are poorly understood. We measured CO 2 , CH 4 and N 2 O fluxes between soils and the atmosphere over 3 years at a recently harvested site under Pinus radiata on a volcanic soil of high-fertility status using replicated, large, in situ chambers to enclose three harvest residue treatments. Temporal changes in CO 2 and N 2 O emissions were also measured over a wider harvested area using small chambers. The residue (slash) treatments were a control (‘no-slash’), ‘normal-slash’ typical of the site and ‘high-slash’ (three times normal-slash). Mass loss was inversely related to all size categories of slash, and averaged 98% for litter and 34% for large wood (76–120 mm diameter). C:N ratios generally declined as a result of increased N concentrations. Overall, CO 2 -C production was significantly higher ( P = 0.02) in the ‘normal-slash’ (by 27%) and ‘high-slash’ (by 72%) than in the ‘no-slash’ treatments. An interaction between treatment and time explained ( P = 0.05) the CO 2 -C flux data better than did the interaction between treatment and soil moisture ( P = 0.07). Evidence from small-chamber CO 2 -C flux data collected over a wider area before and after harvest suggested little apparent effect of soil disturbance during harvest. Averaged over 3 years, the annual CO 2 -C efflux from the large chambers was 8.3 ± 1.1 Mg ha −1 . Methane uptake was apparently depressed by surface soil disturbance during harvest, because it increased after harvest by 70% in the ‘normal-slash’ and ‘high-slash’ treatments to average 12 ± 1 kg CH 4 ha −1 year −1 . Neither NH 4 + nor NO 3 − -N concentrations had any measurable effect on CH 4 uptake. Despite the high N fertility of the harvest site, N 2 O emissions were low overall (0.56 ± 0.17 kg N 2 O-N ha −1 year −1 ) and differed little between treatments, apart from a spike shortly after harvest. Overall, during the time frame of the first commitment period under the Kyoto Protocol, the three greenhouse gases (CO 2 , CH 4 and N 2 O) contributed 87, −1 and 14%, respectively, to combined emissions on a CO 2 -equivalent basis from this fertile harvest site.

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

La Nouvelle-Zelande va utiliser les provisions de boisement/reboisement (A/R) de l'article 3.3 du protocole de Kyoto pour compenser les emissions de gaz a effets de serre, pendant la premiere periode d'engagement 2008-2012. Nous evaluons ici le potentiel initial de puits de C rendus disponible par l'A/R des pâturages marginaux par les especes arbustives les plus communes de Nouvelle-Zelande: le mânuka (Leptospermum scoparium) et le kânuka (Kunzea ericiodes). Les mesures realisees sur des parcelles d'etude de comparaison par paire montrent que la moyenne du taux net d'accumulation de carbone pour le mānuka/kānuka, sur une periode d'environ 40 ans de la phase de croissance active, est de l'ordre de 1,9 a 2,5 t C ha -1 par an. Des etudes sur les changements en C mineral du sol sur une A/R arbustive de prairies suggerent des pertes mineures, qui seraient apparemment largement compensees par une accumulation dans la litiere et dans la couche d'humus. Des etudes sur les changements en C mineral du sol sur une A/R arbustive de prairies suggerent des pertes mineures, qui seraient apparemment largement compensees par une accumulation dans la litiere. Des analyses montrent qu'au niveau national environ 1,45 Mha de pâturages marginaux seraient appropries pour une A/R par arbuste ou foret indigene. Ces zones pourraient accumuler environ 2.9 ± 0.5 Mt de C par an, une compensation significative aux emissions annuelles de combustible fossile en CO 2 de la Nouvelle-Zelande, estimees a 8,84 Mt de C-CO 2 par an. Des analyses economiques preliminaires d'activite d'elevage sur une region dont les prairies comprennent d'importantes zones marginales pour une agriculture pastorale durable suggerent que la « culture de carbone » pourrait s'averer une alternative interessante d'utilisation du sol, si les prix internationaux de carbone de biomasse atteignent environ 10 » par tonne de CO 2 .

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.

Rising concentrations of atmospheric [CO2] in a multi-species ecosystem can influence species composition and increase plant productivity, but have a less predictable effect on soil C storage and nutrient availability. Using a free-air [CO2]-enriched (FACE) system and seasonal sampling over a 5-year period, we examined the influence of elevated atmospheric [CO2] (475 μL L−1) on soil C and N pools and mineralization in a fertilized (P, K, S), sheep-grazed pasture of mixed grass, clover, and forb species on a seasonally dry sand (Mollic Psammaquent). Annual yields of herbage dry matter ranged from about 300 to 1600 g m−2. Total yields did not increase significantly under elevated [CO2], but the proportions of clovers and forbs increased markedly. Most properties in 0–50 mm-depth soil differed significantly (P 0.10) for moisture, pH, total C and N, extractable C and organic N, microbial C, and mineral-N. However, microbial N, CO2-C production (0–14 days) in field-moist soil, and net mineral-N production (14–56 days) in soil at 60% of water-holding capacity were significantly higher (per unit weight of soil) in the elevated-[CO2] treatment (P=0.071, 0.063, 0.003, respectively); the degree of these treatment differences was roughly similar when values were also expressed on a total C or N basis. Relationships with soil moisture were mainly non-significant for microbial C and N, but mainly significant (P

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.

Both CO 2 –C production and the decomposability of grass leaf litter in a gley soil from a naturally occurring CO 2 spring were previously shown to be influenced by the atmospheric CO 2 concentrations under which the soil and litter were sampled. Here we investigate C mineralization in an organic soil from very high CO 2 environments (range 1220–3900 μl l −1 ) at the same spring, and the effect of added leaf litter on CO 2 –C production. Carbon mineralization in the organic soil was unusual in two respects: (1) the proportion of labile components was very high, with more than 11% of the initial soil C being metabolized to CO 2 –C after 56 d at 25 °C; (2) rates of CO 2 –C production in autumn samples increased on incubation, after an initial decline. Decomposition was initially more rapid in C3 Holcus lanatus (Yorkshire fog) than in C4 Pennisetum clandestinum (kikuyu) litter, but differed little in samples from different atmospheric CO 2 concentrations. Overall, the effects of environmental variables on estimates of litter decomposability in the organic soil were similar to, although much less marked than, those in the gley soil. Results suggest that organic components in the organic soil were metabolized at least as readily as those of added litter during the later stages of the 56-d incubation.

In this study, we simulated pasture to Pinus radiata land-use change with the Generic Decomposition And Yield (G'DAY) ecosystem model to examine mechanisms responsible for the change in soil carbon (C) under pine. We parameterized the model for paired sites in New Zealand. Our simulations successfully reproduced empirical trends in ecosystem productivity and soil inorganic nitrogen (N), and modeled an increase in soil C and a small decline in soil N after 30 years under pine. We determined the mechanisms contributing to soil C change based on an established hypothesis that attributes increases in soil C storage to three main factors: increased ecosystem N inputs relative to outputs, increased C/N ratios in plant and soil, or a shift of N from plant to soil. The mechanisms we attributed to the simulated increase in soil C under pine were increased soil C inputs through tree litterfall, and an increase in the soil C/N ratio. In the first 7 years following pine establishment, a decline in soil C was simulated; this was matched by a decline in soil N. The simulated longer-term increase in soil C with afforestation by pine contrasts with results from published field studies, which show either a decline or no change in soil C under pine. The discrepancy between measured and simulated changes in soil C was attributed to the G'DAY model overestimating the transfer of litter C into the mineral soil.

We designed a soil carbon monitoring system for New Zealand using country-specific land use and soil carbon information. The system pre-stratifies the country by soil type, climate, and land use. Soils were placed in six IPCC soil categories; Podzols were added as they are widespread throughout New Zealand. Temperature was stratified into two categories, each spanning 7 degrees C. Moisture categories were based on water balance, and included five categories. Temperature and moisture stratification was based on the USDA Soil Classification system. Land use (10 categories) was based on 1980s survey data. Overall, 39 combinations of these three factors (cells) described 93% of the New Zealand landscape. Geo-referenced soil carbon data (carbon concentration and bulk density) were used to quantify average soil carbon for each of the 39 cells. Aggregating the polygons gave an estimated 1990 soil carbon baseline of 1152+/-44, 1439+/-73, and 1602+/-167 Mt C (mean+/-S.D.) for the 0-0.1, 0.1-0.3, and 0.3-1.0 m depth increments (not including forest floor carbon). The system described could also be used to quantify equilibrium changes in soil C associated with land-use change if land use is updated periodically.

Elevated concentrations of atmospheric CO2 can influence the relative proportions, biomass and chemical composition of plant species in an ecosystem and, thereby, the input of litter nutrients to soil. Plant growth under elevated CO2 appears to have no consistent effect on rates of litter decomposition; decomposition can, however, differ in C3 and C4 plant material from the same CO2 environment. We here describe the decomposability of leaf litter of two grass species – the C3 Holcus lanatus L. (Yorkshire fog) and C4 Pennisetum clandestinum Hochst. (kikuyu) - from an unfertilized, ungrazed grassland at a cold CO2 spring in Northland, New Zealand. Decomposability was measured by net CO2–C production from litter incubated for 56 days at 25 °C in a gley soil from the site; net mineral-N production from litter was also determined. Both litter and soils were sampled under `low' and `high' concentrations of atmospheric CO2. Decomposition of H. lanatus litter was greater than that of P. clandestinum litter throughout the 56-day incubation. Decomposition tended to be greater in `high-CO2' than in `low-CO2' H. lanatus litter, but lower in `high-CO2' than `low-CO2' P. clandestinum litter; differences were, however, non-significant after 28 days. Overall, litter decomposition was greater in the `low-CO2' than `high-CO2' soil. Differences in decomposition rates were related negatively to litter N concentrations and positively to C:N ratios, but were not predictable from lignin:total N ratios. Net mineral-N production from litter decomposition did not differ significantly in `high-CO2' and `low-CO2' samples incubated in `low-CO2' soil; in `high-CO2' soil some net immobilization was observed. Overall, results indicate the likely complexity of litter decomposition in the field but, nevertheless, strongly suggest that rates of decomposition will not necessarily decline in a `high-CO2' environment.

Interest in national carbon (C) budgets has increased following the signing of the Kyoto Protocol as countries begin to develop source/sink C inventories. In this study, specific-site measurements, regional databases, satellite observations, and models were used to test the hypothesis that New Zealand’s terrestrial ecosystems are C neutral because C uptake by planted forests and scrub is roughly balanced by C losses from indigenous forests and soils. Net ecosystem C balance was estimated from the difference between net primary production (NPP) and heterotrophic soil respiration. The productivity portion of the CASA model and NOAA–AVHRR imagery were used to estimate national NPP ( 128±14 Mt C per year). Main sources of uncertainty were the coarse spatial scale ( 1×1 km 2 grid cells), and the general lack of information on photosynthetically active radiation, light-use efficiency, and below-ground C allocation for the major vegetation types: indigenous and exotic forests, scrub, and grasslands (improved, unimproved and tussock). Total soil CO2-C production predicted from an Arrhenius-type function coupled to climate and land-cover data was 380±30 Mt C per year, suggesting that New Zealand’s terrestrial ecosystems may be either (a) a net source of atmospheric CO2 or (b) roughly in C balance if ca. 252 Mt CO2-C per year (66%) can be attributed to roots. Soil moisture limitations on respiration were small, reducing the national value to 365±28 Mt C per year. Differences between NPP and heterotrophic soil respiration were −29 Mt C per year for improved pastures, −8 Mt C per year for indigenous forests, and +4 Mt C per year for planted forests; the large negative value for improved grasslands may be due to under-estimation of NPP and root respiration. Soil C losses to coastal waters, as estimated from a consideration of all the major erosion processes, were ca. 3–11 Mt C per year. These national-scale estimates of ecosystem C balance were in general agreement with those based on plot-scale data for some major ecosystems including planted forests (4 Mt C per year vs 3.7 Mt C per year, respectively) and indigenous forest (−8 Mt C per year vs ca. −2.8 Mt C per year, respectively). Poor agreement for forest regenerating after land abandonment (−17 Mt C per year vs +3 Mt C per year) was probably due to an underestimate of NPP at the national scale. Overall, the results suggest that New Zealand is a net C source, despite the fact that some ecosystems are accumulating C. For some land-use types, using the balance between NPP and soil respiration at the national scale to estimate the net ecosystem C balance may be too coarse, and studies of land-use changes at finer spatial scales are needed to reduce uncertainties in national-scale C balance estimates.

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

Summary Methane emissions from ruminant livestock are responsible for 45 % of New Zealand’s combined CO2-equivalent greenhouse gas inventory, and arise principally from sheep. Using a flock of 6-month old sheep (20 ha–1) grazing abundant pasture, we compare micrometeorological measurements of net methane emission rates with measurements from individual sheep based on a sulphur-hexafluoride tracer technique. Individual sheep emission rates were highly variable and averaged 19.5 ± 4.8 (SD) g CH4 sheep–1 d–1, or 39 ± 9.6 mg CH4 m–2 d–1 on an areal basis. Emission rates were poorly correlated with animal live weight or dry matter intake but represented an average dietary energy loss of 3.6%. Methane fluxes from the surface were determined as half hourly averages by a flux-gradient technique using temperature and methane gradients. Soil methane consumption was measured using chambers and found to be negligible (

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.

All current mathematical models of the soil system are underpinned by a wealth of research into soil biology and new research continues to improve the description of the real world by mathematical models. In this review we examine the various approaches for describing soil biology in mathematical models and discuss the use of each type of model in global change research. The approaches represented among models participating in the Global Change and Terrestrial Ecosystems (GCTE) Soil Organic Matter Network (SOMNET) are described. We examine the relative advantages and constraints of each modelling approach and, using these, suggest appropriate uses of each. We show that for predictive purposes at ecosystem scale and higher, process-orientated models (which have only an implicit description of soil organisms) are most commonly used. As a research tool at the ecosystem level, both process-orientated and organism-orientated models (in which functional or taxonomic groups of soil organisms are explicitly described) are commonly used. Because of uncertainties introduced in internal model parameter estimation and system feedbacks, the predictive use of organism-orientated models at the ecosystem scale and larger is currently less feasible than is the use of process-orientated models. In some specific circumstances, however, an explicit description of some functional groups of soil organisms within models may be required to adequately describe the effects of global change. No existing models can adequately predict the feedback between global change, a change in soil community function, and the response of the changed system to future global change. To find out if these feedbacks exist and to what extent they affect future global change, more research is urgently required into the response of soil community function to global change and its potential ecosystem-level effects.

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.

The organic carbon in New Zealand's vegetation and soils has been estimated for 1992 from updated national databases of vegetative cover and soil carbon. These databases were augmented by inclusion of vegetation and soils information for Stewart Island, and addition of carbon estimates for upland and high‐country soils of South Island. Plant biomass estimates from literature were combined with the Vegetative Cover Map of New Zealand to give an estimate of 2420 Mt carbon for vegetation carbon above and below ground, including litter and humus. More than 80% of this carbon occurs in indigenous forested ecosystems on less than 26% of the land area, with only about 5% in planted forests. Soil organic carbon estimates for two depth ranges (0–0.25 m, 0–1 m) were derived from the N.Z. Land Resource Inventory, the Soil Map of Stewart Island, and the National Soils Database. Totals were 2500 ± 77 Mt and 4260 ± 190 Mt C respectively. Yellow‐brown earths made the largest contribution, with 727 Mt C in North Island a...

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.

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.

Effects of elevated CO2 (525 and 700 μL L−1), and a control (350 μL L−1 CO2), on biochemical properties of a Mollic Psammaquent soil in a well-established pasture of C3 and C4 grasses and clover were investigated with continuously moist turves in growth chambers over four consecutive seasonal temperature regimes from spring to winter inclusive. After a further ‘spring’ period, half of the turves under 350 and 700 μL L−1 were subjected to ‘summer’ drying and were then re-wetted before a further ‘autumn’ period; the remaining turves were kept continuously moist throughout these additional three consecutive ‘seasons’. The continuously moist turves were then pulse-labelled with 14C-CO2 to follow C pathways in the plant/soil system during 35 days. Growth rates of herbage during the first four ‘seasons’ averaged 4.6 g m−2 day−1 under 700 μL L−1 CO2 and were about 10% higher than under the other two treatments. Below-ground net productivity at the end of these ‘seasons’ averaged 465, 800 and 824 g m−2 in the control, 525 and 700 μL L−1 treatments, respectively. in continuously moist soil, elevated CO2 had no overall effects on total, extractable or microbial C and N, or invertase activity, but resulted in increased CO2-C production from soil, and from added herbage during the initial stages of decomposition over 21 days; rates of root decomposition were unaffected. CO2 produced h−1 mg−1 microbial C was about 10% higher in the 700 μL L−1 CO2 treatment than in the other two treatments. Elevated CO2 had no clearly defined effects on N availability, or on the net N mineralization of added herbage. In the labelling experiment, relatively more 14C in the plant/soil system occurred below ground under elevated CO2, with enhanced turnover of 14C also being suggested. Drying increased levels of extractable C and organic-N, but decreased mineral-N concentrations; it had no effect on microbial C, but resulted in lowered microbial N in the control only. In soil that had been previously ‘summer’-dried, CO2 production was again higher, but net N mineralization was lower, under elevated CO2 than in the control after ‘autumn’ pasture growth. Over the trial period of 422 days, elevated CO2 generally appears to have had a greater effect on soil C turnover than on soil C pools in this pasture ecosystem.

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.

Biofiltration, whereby CH(4) is oxidized by methanotrophic bacteria, is a potentially effective strategy for mitigating CH(4) emissions from anaerobic dairy effluent lagoons/ponds, which typically produce insufficient biogas for energy recovery. This study reports on the effectiveness of a biofilter cover design at oxidizing CH(4) produced by dairy effluent ponds. Three substrates, a volcanic pumice soil, a garden-waste compost, and a mixture of the two, were tested as media for the biofilters. All substrates were suspended as 5 cm covers overlying simulated dairy effluent ponds. Methane fluxes supplied to the filters were commensurate with emission rates from typical dairy effluent ponds. All substrates oxidized more than 95% of the CH(4) influx (13.9 g CH(4) m(-3) h(-1)) after two months and continued to display high oxidation rates for the remaining one month of the trial. The volcanic soil biofilters exhibited the highest oxidation rates (99% removal). When the influx CH(4) dose was doubled for a month, CH(4) removal rates remained90% for all substrates (maximum = 98%, for the volcanic soil), suggesting that biofilters have a high capacity to respond to increases in CH(4) loads. Nitrous oxide emissions from the biofilters were negligible (maximum = 19.9 mg N(2)O m(-3) h(-1)) compared with CH(4) oxidation rates, particularly from the volcanic soil that had a much lower microbial-N (75 mg kg(-1)) content than the compost-based filters (240 mg kg(-1)). The high and sustained CH(4) oxidation rates observed in this laboratory study indicate that a biofilter cover design is a potentially efficient method to mitigate CH(4) emissions from dairy effluent ponds. The design should now be tested under field conditions.

Land-use change between forestry and agriculture can cause large net emissions of carbon dioxide (CO2), and the respective land uses associated with forest and pasture lead to different on-going emission rates of methane (CH4) and nitrous oxide (N2O) and different surface albedo. Here, we quantify the overall net radiative forcing and consequent temperature change from specified land-use changes. These different radiative agents cause radiative forcing of different magnitudes and with different time profiles. Carbon emission can be very high when forests are cleared. Upon reforestation, the former carbon stocks can be regained, but the rate of carbon sequestration is much slower than the rate of carbon loss from deforestation. A production forest may undergo repeated harvest and regrowth cycles, each involving periods of C emission and release. Agricultural land, especially grazed pastures, have much higher N2O emissions than forests because of their generally higher nitrogen status that can be further enhanced through intensification of the nitrogen cycle by animal excreta. Because of its longevity in the atmosphere, N2O concentrations build up nearly linearly over many decades. CH4 emissions can be very high from ruminant animals grazing on pastures. Because of its short atmospheric longevity, the CH4 concentration from a converted pasture accumulates for only a few decades before reaching a new equilibrium when emission of newly produced CH4 is balanced by the oxidation of previously emitted CH4. Albedo changes generally have the opposite radiative forcing from those of the GHGs and partly negate their radiative forcing. Overall and averaged over 100 years, CO2 is typically responsible for 50% of radiative forcing and CH4 and N2O for 25% each. Albedo changes can negate the radiative forcing by the three greenhouse gases by 20-25%.

Landfills that generate too little biogas for economic energy recovery can potentially offset methane (CH(4)) emissions through biological oxidation by methanotrophic bacteria in cover soils. This study reports on the CH(4) oxidation efficiency of a 10-year old landfill cap comprising a volcanic pumice soil. Surface CH(4) and CO(2) fluxes were measured using field chambers during three sampling intervals over winter and summer. Methane fluxes were temporally and spatially variable (-0.36 to 3044 mgCH(4)m(-2)h(-1)); but were at least 15 times lower than typical literature CH(4) fluxes reported for older landfills in 45 of the 46 chambers tested. Exposure of soil from this landfill cover to variable CH(4) fluxes in laboratory microcosms revealed a very strong correlation between CH(4) oxidation efficiency and CH(4)/CO(2) ratios, confirming the utility of this relationship for approximating CH(4) oxidation efficiency. CH(4)/CO(2) ratios were applied to gas concentrations from the surface flux chambers and indicated a mean CH(4) oxidation efficiency of 72%. To examine CH(4) oxidation with soil depth, we collected 10 soil depth profiles at random locations across the landfill. Seven profiles exhibited CH(4) removal rates of 70-100% at depths