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Methane (CH4) emissions from natural landscapes constitute roughly half of global CH4 contributions to the atmosphere, yet large uncertainties remain in the absolute magnitude and the seasonality of emission quantities and drivers. Eddy covariance (EC) measurements of CH4 flux are ideal for constraining ecosystem-scale CH4 emissions due to quasi-continuous and high-temporal-resolution CH4 flux measurements, coincident carbon dioxide, water, and energy flux measurements, lack of ecosystem disturbance, and increased availability of datasets over the last decade. Here, we (1) describe the newly published dataset, FLUXNET-CH4 Version 1.0, the first open-source global dataset of CH4 EC measurements (available at https://fluxnet.org/data/fluxnet-ch4-community-product/, last access: 7 April 2021). FLUXNET-CH4 includes half-hourly and daily gap-filled and non-gap-filled aggregated CH4 fluxes and meteorological data from 79 sites globally: 42 freshwater wetlands, 6 brackish and saline wetlands, 7 formerly drained ecosystems, 7 rice paddy sites, 2 lakes, and 15 uplands. Then, we (2) evaluate FLUXNET-CH4 representativeness for freshwater wetland coverage globally because the majority of sites in FLUXNET-CH4 Version 1.0 are freshwater wetlands which are a substantial source of total atmospheric CH4 emissions; and (3) we provide the first global estimates of the seasonal variability and seasonality predictors of freshwater wetland CH4 fluxes. Our representativeness analysis suggests that the freshwater wetland sites in the dataset cover global wetland bioclimatic attributes (encompassing energy, moisture, and vegetation-related parameters) in arctic, boreal, and temperate regions but only sparsely cover humid tropical regions. Seasonality metrics of wetland CH4 emissions vary considerably across latitudinal bands. In freshwater wetlands (except those between 20∘ S to 20∘ N) the spring onset of elevated CH4 emissions starts 3 d earlier, and the CH4 emission season lasts 4 d longer, for each degree Celsius increase in mean annual air temperature. On average, the spring onset of increasing CH4 emissions lags behind soil warming by 1 month, with very few sites experiencing increased CH4 emissions prior to the onset of soil warming. In contrast, roughly half of these sites experience the spring onset of rising CH4 emissions prior to the spring increase in gross primary productivity (GPP). The timing of peak summer CH4 emissions does not correlate with the timing for either peak summer temperature or peak GPP. Our results provide seasonality parameters for CH4 modeling and highlight seasonality metrics that cannot be predicted by temperature or GPP (i.e., seasonality of CH4 peak). FLUXNET-CH4 is a powerful new resource for diagnosing and understanding the role of terrestrial ecosystems and climate drivers in the global CH4 cycle, and future additions of sites in tropical ecosystems and site years of data collection will provide added value to this database. All seasonality parameters are available at https://doi.org/10.5281/zenodo.4672601 (Delwiche et al., 2021). Additionally, raw FLUXNET-CH4 data used to extract seasonality parameters can be downloaded from https://fluxnet.org/data/fluxnet-ch4-community-product/ (last access: 7 April 2021), and a complete list of the 79 individual site data DOIs is provided in Table 2 of this paper.

While wetlands are the largest natural source of methane (CH4 ) to the atmosphere, they represent a large source of uncertainty in the global CH4 budget due to the complex biogeochemical controls on CH4 dynamics. Here we present, to our knowledge, the first multi-site synthesis of how predictors of CH4 fluxes (FCH4) in freshwater wetlands vary across wetland types at diel, multiday (synoptic), and seasonal time scales. We used several statistical approaches (correlation analysis, generalized additive modeling, mutual information, and random forests) in a wavelet-based multi-resolution framework to assess the importance of environmental predictors, nonlinearities and lags on FCH4 across 23 eddy covariance sites. Seasonally, soil and air temperature were dominant predictors of FCH4 at sites with smaller seasonal variation in water table depth (WTD). In contrast, WTD was the dominant predictor for wetlands with smaller variations in temperature (e.g., seasonal tropical/subtropical wetlands). Changes in seasonal FCH4 lagged fluctuations in WTD by ~17±11days, and lagged air and soil temperature by median values of 8±16 and 5±15days, respectively. Temperature and WTD were also dominant predictors at the multiday scale. Atmospheric pressure (PA) was another important multiday scale predictor for peat-dominated sites, with drops in PA coinciding with synchronous releases of CH4 . At the diel scale, synchronous relationships with latent heat flux and vapor pressure deficit suggest that physical processes controlling evaporation and boundary layer mixing exert similar controls on CH4 volatilization, and suggest the influence of pressurized ventilation in aerenchymatous vegetation. In addition, 1- to 4-h lagged relationships with ecosystem photosynthesis indicate recent carbon substrates, such as root exudates, may also control FCH4. By addressing issues of scale, asynchrony, and nonlinearity, this work improves understanding of the predictors and timing of wetland FCH4 that can inform future studies and models, and help constrain wetland CH4 emissions.

High water tables (WT) stabilise peatland carbon (C) through regulation of biogeochemical processes. The impact of peatland WT on ecosystem function, including C exchange, alters over time, and the factors that cause some peatlands to display resilience and others to undergo degradation are poorly understood. Here we use CO2 flux measurements, measured by eddy covariance, to compare ecosystem function between two raised bogs; one drainage-affected, with a deep and fluctuating water table and the other near-natural, with a shallow and stable water table. The drainage-affected bog was found to be a moderate sink for CO2 (69 g C m−2 yr−1), which was 134 g C m−2 yr−1 less than the near-natural bog (203 g C m−2 yr−1). Greater ecosystem productivity has allowed the drainage-impacted bog to act as a CO2 sink despite higher ecosystem respiration; most likely due to an increase in photosynthetic capacity caused by expansion of ericaceous shrub cover. The tolerance of the vegetation community, particularly the main peat former Empodisma robustum (Restionaceae), to low and fluctuating WT appears to have been key in allowing the site to remain a sink. Despite the current resilience of the ecosystem CO2 sink, we found gross primary production to be limited under both high and low water tables, even in a year with typical rainfall. This is best explained by the limited physiological ability of ericaceous shrubs to tolerate a fluctuating WT. As such we hypothesise that if the WT continues to drop and become even more unstable, then without further vegetation change, a reduction in gross primary production is likely which may in turn cause the site to become a source for CO2.

Agricultural greenhouse gas (GHG) emissions account for almost half of New Zealand’s total emissions, and therefore considerable attention has been given to identifying and testing mitigation options. At plot scale, plantain (Plantago lanceolate L.) in the pasture sward has been demonstrated to reduce nitrous oxide (N2O) emissions but has not been tested at paddock scale on an operating farm. Our aim was to test the efficacy of a pasture sward containing >30% plantain as a GHG mitigation option at paddock scale (2.5-3 ha) on a year-round rotationally grazed commercial dairy farm in the Waikato region of New Zealand. Utilising eddy covariance measurements of CO2, N2O and CH4 coupled to farm management records, N2O, carbon (C) and GHG balances (sign convention: positive value = emission to the atmosphere) were calculated for two adjacent paddocks – a control paddock containing an existing ryegrass/clover sward (RC), and a paddock that underwent renovation with the establishment of a ryegrass/clover/plantain sward (RCP). Establishment of RCP was via spraying and direct drilling and occurred in March 2018 (autumn). For the establishment period between initial herbicide application and the first grazing of the new RCP sward 66 days later, N2O emissions were 2.58 kg N ha-1 compared with 1.69 kg N ha-1 for the RC paddock. During the same period, C losses from the RCP paddock were greater than from the RC paddock (2.40 t C ha-1 for RCP and 1.29 t C ha-1 for RC) primarily due to reduced photosynthetic inputs associated with the herbicide application. The GHG budget (including enteric methane emissions from feed grown and eaten in the paddock) during the 66 day establishment period was an emission of 6.56 t CO2-eq ha-1 for RC and 9.85 t CO2-eq ha-1 for RCP. Unfortunately, the RCP sward establishment was poor, and after one year, total pasture production was unexpectedly lower than RC. Additionally, plantain accounted for 2O, C and GHG balances for RCP in the first year following (and including) establishment were 6.61 kg N ha-1 y-1, 3.25 t C ha-1 y-1 and 21.40 t CO2-eq ha-1 y-1 respectively, while for RC they were 7.21 kg N ha-1 y-1, 0.95 t C ha-1 y-1 and 13.29 t CO2-eq ha-1 y-1. Due to the poor establishment of plantain, any N2O and GHG benefits of this species were unable to be initially concluded, but additional plantain was sown and measurements are ongoing. However, we did identify several relevant findings: any N2O/GHG benefits of plantain must firstly offset emissions (including C losses) associated with the establishment of the sward (>3 t CO2-eq ha-1 in this study), and furthermore, there is a risk that should the establishment be poor, GHG emissions can be considerably greater (and pasture production lower) than an existing pasture.

Peatland biological, physical and chemical properties change over time in response to the long-term water table position. Such changes complicate predicting the response of peatland carbon stocks to sustained drying. Here we use Eddy Covariance measurements of CO2 exchange to study the effect of sustained water table lowering on peatland carbon dynamics. We compare measurements from a near-pristine peatland with those of a drying remnant, both raised bogs dominated by Empodisma robustum (Restionaceae), across two different time periods separated by a 16-year interval. We found that the remnant bog was initially a source of CO2 following water table lowering. However, the CO2 sink recovered and strengthened after the 16-year interval between measurements. The increase in CO2 sink strength in the remnant bog was primarily due to increased photosynthetic uptake of CO2, which exceeded that of the near-pristine site in both time periods. Additionally we found the loss of CO2 via ecosystem respiration to have declined with time, however, ecosystem respiration remained elevated compared to the near-pristine site. These trends of increasing photosynthesis and declining ecosystem respiration resulted in the CO2 sink in the dry bog reaching half the sink strength of the near-pristine bog. We consider two factors to have been key for the recovery of the CO2 sink in the remnant bog. These were 1) resilience of the peat-forming plant community to water-table change and 2) the expansion of ericoid shrubs. Our results demonstrate that the peatland carbon sink can recover from drying over a multi-decadal timescale, but questions remain as to the long-term trajectory of dry bogs and the stability of carbon fixed after water table lowering.

Projections indicate that climate change may exacerbate existing challenges to the productivity of New Zealand dairy farming systems. To assess the importance of these projections and understand adaptation challenges at farm level, detailed farm-scale model simulations of climate change impacts were undertaken for six representative pasture-based dairy farms located in the major dairying regions of New Zealand. The analysis suggested that without adaptation, climate change is likely to have a negative impact in most of the study locations. However, the level and type of impact depends to a large degree on regional climate variability as well as on the management practices of each farm. Under current management, responses to projected climate changes ranged from no change to an 18% decrease in average annual pasture production. A number of modelled adaptations demonstrated the potential to reduce climate change impacts under current management. The modelling work, together with farmers' responses, showed the adaptations' potential to provide both benefits and management challenges across different regions and climate conditions. In particular, it highlighted the need for the results of farm systems modelling under climate change scenarios to be considered in the context of their specific and localised climatic and management challenges.

Pasture renewal (or pasture renovation) is a common management practice in grass-based production systems aimed at restoring pasture production and forage quality. It is important to understand the impact of pasture renewal on soil organic carbon stocks. Here we report the CO2 and C balance of three blocks of an intensively managed temperate grassland in New Zealand. Two blocks underwent pasture renewal using the spray and direct drill approach (no ploughing), while the third block served as an undisturbed Control. Net ecosystem production (NEP) was measured using eddy covariance, and additional in- and outputs of C (e.g. C in pasture removed by grazers and returned in dung) either measured or estimated. NEPs of the renewed blocks were between 149 and 212 g C m−2 y−1 lower than the NEP of the Control block during the year of pasture renewal. While CO2 sink strength was obviously diminished as a result of pasture renewal, neither renewed block was a source for CO2, presumably due the relatively warm climate which enabled year-round growing conditions. Despite the non-negative NEPs during the year of pasture renewal, both renewed sites were found to lose soil C as indicated by the negative net ecosystem carbon balances (NECBs). NECBs of the renewed blocks were −156 and −222 g C m−2 y−1, compared to a near-neutral NECB of the Control block. Comparison of the findings from the current study on pasture renewal without ploughing to those from an earlier study on pasture renewal including ploughing led to the following conclusions: i) our data did not suggest lower rates of respiratory CO2 losses when spray-and-direct-drill approach was used instead of ploughing; ii) as both the direct CO2 losses due to microbial degradation and the decrease in CO2 uptake through photosynthesis contribute to the total impact of pasture renewal on NEP, the duration of fallow period was found to be important in determining the short-term CO2 losses due to pasture renewal, with shorter fallow period leading to lower net CO2 losses; iii) addition of C in the form of manure, effluent, or extra supplemental feed for cattle may help to reduce the negative impact of pasture renewal on soil C loss at the renewed site.

Appropriate management of grasslands may aid the sequestration of carbon (C) in soil organic matter, thereby increasing soil quality and offsetting other CO2 emissions to the atmosphere. In ungrazed grassland trials, higher species diversity has been found to enhance soil C sequestration due to increases in plant production and C inputs to the soil via roots. Little is known about the impact of increased species diversity on the change in soil C stocks in intensively grazed pastoral systems common in temperate regions. Here, we report the CO2 and C balances of three blocks of an intensively managed dairy farm in temperate New Zealand. Two blocks underwent pasture renewal (PR): one block was renewed back to ryegrass-clover (NewRye), while the second was sown in a mix of higher species diversity including ryegrass, clover, timothy, prairie grass, lucerne, chicory and plantain (NewMix). A third block served as an undisturbed ryegrass-clover control (Control). We hypothesised that the block renewed to a more diverse pasture would have higher pasture production and C sink strength than either the unmodified pasture or the pasture renewed to ryegrass-clover. Net ecosystem production (NEP) was measured using eddy covariance, and other inputs and outputs of C (e.g. C in pasture removed by grazers and C deposited in dung) were calculated. NEP and the net ecosystem carbon balance (NECB) were determined for four years: one year before PR, and three years after. In the year before PR, we measured important differences in annual NEP (and thus NECB) which suggested unanticipated inherent site differences between blocks which affected C cycling: the NEP of the NewRye block was 116 to 160 g C m−2 y−1 higher than that of the other blocks. These differences between blocks were accounted for when considering differences in NECBs as a result of the treatments in years following renewal. During the three years following PR, dry matter production was similar for the NewMix and NewRye blocks (15,033 and 14,708 kg DM ha−1 y−1 respectively) and greater than the Control block (13,116 kg DM ha−1 y−1). For 11 out of the 12 block-years the soil-pasture systems lost C as indicated by negative NECBs. Taking into consideration the pre-treatment differences between blocks, the NewMix pasture had a higher NECB (by 254 g C m−2 over 3 years) than the NewRye pasture, indicating smaller C losses at the NewMix block. In contrast, there was no difference in NECB between NewMix and Control blocks. When PR is undertaken to increase pasture performance, renewal to a more diverse sward may decrease C losses compared to renewal to a ryegrass-clover sward. In contrast, there was no evidence that PR to a higher diversity sward decreased C losses compared to an unmodified ryegrass-clover sward. Many C balance studies based on eddy covariance techniques have not accounted for pre-existing differences between treatment blocks and we have demonstrated that this may be critically important for drawing conclusions about the true effect of imposed treatments on C balances.

In New Zealand, pasture renewal is a routine management method for maintaining pasture productivity. However, knowledge of the renewal effects on soil organic carbon (SOC) stocks is still limited. Here we use a process-based model, CenW, to comprehensively assess the effects of pasture renewal on the carbon balance of a temperate pasture in the Waikato region of New Zealand. We investigated the effects of renewal frequency, length of fallow period, renewal timing, and the importance and quantification of age-related reductions in productivity. Our results suggest that SOC change depends on the combined effects of renewal on gross primary productivity (GPP), autotrophic and heterotrophic respiration, carbon removal by grazing and carbon allocation to roots. Pasture renewal reduces grazing removal proportionately more than GPP because newly established plants need to allocate more carbon to re-build their root system following renewal which limits foliage production. That lengthens the time before above-ground biomass has grown sufficiently to be grazed again. New plants have a lower ratio of autotrophic respiration to GPP, however, which partly compensates for the GPP loss during renewal. Our simulations suggested an average SOC loss of 0.16 tC ha−1 yr−1 if pastures were renewed every 25 years, but could gain an average of 0.3 tC ha−1 yr−1 if pastures were renewed every year. For maximizing pasture production, the optimal renewal frequency depends on the rate of pasture deterioration with more rapid deterioration rates favouring more frequent renewal. Additionally, the length of the fallow period, renewal timing, and associated environmental conditions are important factors that can affect SOC temporally, but the importance of those effects diminishes at the annual or longer time scales. A major uncertainty for a full understanding of the renewal effect on SOC lies in the rate of pasture deterioration with time since previous renewal.

Ecosystem scale measurements of evaporation (E) from intensively managed pasture systems are important for informing water resource decision making and validation of hydrologic models and remote sensing methods. We measured E from a year round intensively grazed temperate pasture system in New Zealand using the eddy covariance method for three years (2012⿿2014). Evaporation varied by less than 3% both spatially (770⿿783 mm) and temporally (759⿿776 mm) at an annual scale. The low spatial and temporal variation largely occurred because E was strongly controlled by net radiation (r2 = 0.81, p

Contemporary measurements of carbon (C) accumulation rates in peatlands around the world often show the C sink to be stronger on average than at times in the past. Alteration of global nutrient cycles could be contributing to elevated carbon accumulation in the present day. Here we examine the effect of volcanic inputs of nutrients on peatland C accumulation in Moanatuatua Bog, New Zealand, by examining a high-resolution Late Holocene C accumulation record during which powerful volcanic eruptions occurred, depositing two visible rhyolitic tephra layers (Taupo, 232 ± 10 CE; Kaharoa, 1314 ± 12 CE). Carbon accumulation rates since c. 50 CE, well before any human presence, increased from a background rate of 23 g C m−2 yr−1 up to 110 g C m−2 yr−1 following the deposition of the Taupo Tephra, and 84 g C m−2 yr−1 following the deposition of the Kaharoa Tephra. Smaller but nevertheless marked increases in C accumulation additionally occurred in association with the deposition of three andesitic-dacitic cryptotephras (each ≤ ∼1 mm thick) of the Tufa Trig Formation between the Taupo and Kaharoa events. These five periods of elevated C uptake, especially those associated with the relatively thick Taupo and Kaharoa tephras, were accompanied by shifts in nutrient stoichiometry, indicating that there was greater availability of phosphorus (P) relative to nitrogen (N) and C during the period of high C uptake. Such P was almost certainly derived from volcanic sources, with P being present in the volcanic glass at Moanatuatua, and many of the eruptions described being associated with the local deposition of the P rich mineral apatite. We found peatland C accumulation to be tightly coupled to N and P accumulation, suggesting nutrient inputs exert a strong control on rates of peat accumulation. Nutrient stoichiometry indicated a strong ability to recover P within the ecosystem, with C:P ratios being higher than most other peatlands in the literature. We conclude that nutrient inputs, deriving from volcanic eruptions, have been very important for C accumulation rates in the past. Therefore, the elevated nutrient inputs occurring in the present day could offer a more plausible explanation, as opposed to a climatic component, for observed high contemporary C accumulation in New Zealand peatlands.

Peatland biological, physical and chemical properties change over time in response to alterations in long-term water table position. Such changes complicate our ability to predict the response of peatland carbon stocks to sustained drying. In order to better understand the effect of sustained lowering of the water table on peatland carbon dynamics, we re-visited a drainage-affected bog, repeating eddy covariance measurements of CO2 flux after a 16-year interval. We found the ecosystem CO2 sink to have strengthened across the intervening period, despite a deep and fluctuating water table. This was mostly due to an increase in CO2 uptake through photosynthesis associated with increased shrub growth. We also observed a decline in CO2 loss through ecosystem respiration. These changes could not be attributed to environmental conditions. Air temperature was the only significant contemporaneous driver of monthly anomalies in CO2 fluxes, with higher temperatures decreasing the net CO2 sink via increased ecosystem respiration. However, the effect of air temperature was weak in comparison to the underlying differences between time periods. Therefore, we demonstrate that for drying peatlands, long-term changes within the ecosystem can be of primary importance as drivers of CO2 exchange. In this peatland, the ecosystem carbon sink has shown resilience to water table drawdown, with internal feedbacks leading to a recovery of the CO2 sink after a 16-year interval.

Estimates of regional and national nitrous oxide (N2O) emissions rely on emission factors (EFs) commonly derived from measurements using static chambers. These measurements can include high uncertainties and might obscure the quantification of N2O fluxes. Advances in micrometeorological eddy covariance technique (EC) now allow direct measurements of N2O fluxes at the field scale. Here, we compared N2O emissions calculated from site-specific EFs with N2O flux data derived from year-round EC measurements on an intensively grazed dairy pasture in the Waikato region, NZ. Annual N2O emissions of 7.30 kg N2O-N ha−1 yr−1 determined using gap-filled EC flux data were greater than N2O estimates of 3.82 kg N2O-N ha−1 yr−1 based on site-specific EFs for cattle urine (1.53%), cattle dung (0.24%) and urea fertiliser (0.16%). Likely reasons for this difference were that the EF approach did not take into account the seasonal variability of EFs, the effect of supplementary feed on cattle nitrogen (N) excretion and background N2O emissions (BNE). Including calculated emissions from supplementary feed N (0.92 kg N2O-N ha−1 yr−1) and BNE (1.09 kg N2O-N ha−1 yr−1) increased annual EF-based emissions to 5.83 kg N2O-N ha−1 yr−1. The site-specific EFs were established in spring 2017 and may not have adequately represented summer, winter and particularly autumn N2O emissions. The EF approach, therefore, did not fully account for the seasonal variability of N2O fluxes as measured by EC but, if quantified, could have led to further agreement between measurements. Using EC measurements to complement static chambers and EF approaches altered annual N2O emissions estimates from intensively grazed pastoral land. Hence, we conclude that N2O budgets derived from EFs need to better capture the effect of seasonal variability, supplementary feed and BNE.

We estimated the net ecosystem carbon (C) balance (NECB) of a temperate pasture in the North Island of New Zealand for four years (2008–2011). The pasture was intensively managed with addition of fertiliser and year-round rotational grazing by dairy cows. Climatic conditions and management practices had a large impact on CO2 exchange, with a severe drought in one year and cultivation in another both causing large short-term (∼3 months) net losses of CO2–C (100–200 g C m−2). However, CO2 was regained later in both of these years so that on annual timescales, the site was a CO2 sink or CO2 neutral. Management practices such as effluent application and harvesting silage also influenced non-CO2–C fluxes, and had a large impact on annual NECB. Despite these major environmental or management perturbations, both NEP and NECB were relatively constant on annual timescales. It is likely that this apparent resilience of the CO2 and C balance to perturbations was at least partly attributable to the relatively warm temperatures, also in winter, providing good growing conditions year-round (in the absence of major perturbations such as moisture stress). In several instances, the farmer’s decisions aimed at maintaining a constant milk yield between years also appeared to contribute to a relatively stable C balance. Averaged over the full four-year study period, the site was a net sink for both CO2 (NEP = 165 ± 51 g C m−2 y−1), and total C (NECB = 61 ± 53 g C m−2 y−1) after non-CO2–C fluxes were accounted for. Annual NEP and NECB values were similar to results collated from other managed temperate grasslands on mineral soils globally, for which average NEP and NECB were 188 ± 44 g C m−2 y−1 and 44 ± 33 g C m−2 y−1, respectively. In the global dataset, we noted a general trend for increased C sequestration with increasing NEP, suggesting that it may be possible to meet the dual goal of increased pasture production (thus milk, meat and fiber production) and increasing soil C storage in managed temperate grasslands. Identification of management practices that increase C storage while maintaining or enhancing pasture production requires more standardised reporting between NECB studies, and experiments involving side-by-side comparison of treatment and control plots.

There are still large uncertainties in peatland methane flux dynamics and insufficient understanding of how biogeochemical processes scale to ecosystems. New Zealand bogs differ from Northern Hemisphere ombrotrophic systems in climatic setting, hydrology, and dominant vegetation, offering an opportunity to evaluate our knowledge of peatland methane biogeochemistry gained primarily from northern bogs and fens. We report eddy covariance methane fluxes from a raised bog in New Zealand over 2.5 years. Annual total methane flux in 2012 was 29.1 g CH4 m−2 yr−1, whereas during a year with a severe drought (2013) it was 20.6 g CH4 m−2 yr−1, both high compared to Northern Hemisphere bogs and fens. Drier conditions led to a decrease in fluxes from ~100 mg CH4 m−2 d−1 to ~20 mg CH4 m−2 d−1, and subsequent slow recovery of flux after postdrought water table rise. Water table depth regulated the temperature sensitivity of methane fluxes, and this sensitivity was greatest when the water table was within 100 mm of the surface, corresponding to the shallow rooting zone of the dominant vegetation. A correlation between daytime CO2 uptake and methane fluxes emerged during times with shallow water tables, suggesting that controls on methane production were critical in determining fluxes, more so than oxidation. Water table recession through this shallow zone led to increasing methane fluxes, whereas changes in temperature during these periods were not correlated. Models of methane fluxes should consider drought-induced lags in seasonal flux recovery that depend on drought characteristics and location of the critical zone for methane production.

It is commonly assumed that agricultural peatlands are net sources of CO2 to the atmosphere because of lowered water tables and intensive land management altering the balance of plant productivity and respiration. Yet actual farm-scale fluxes of CO2 have been infrequently quantified. We measured net ecosystem exchange of CO2 (NEE) using a permanent and a mobile eddy covariance tower installed over dairy farms with year-round rotational grazing on deep peats in New Zealand. The permanent tower was in place for one year and the mobile tower was deployed for periods of 3–4 weeks at three other farms on peat between spring and autumn. At all sites, grazing cycles caused large variations in pasture biomass and consequent daytime NEE and we accounted for these variations using an index of photosynthesising biomass (phytomass index, Lohila et al., 2004 ) automatically derived from daily CO2 flux measurements. We estimated annual CO2 loss of 190 gC m−2 yr−1 for the permanent site, which is in broad agreement with other agricultural peatland studies. Including other farm-scale exports of C, overall net ecosystem carbon loss estimated for the permanent site was 294 gC m−2 yr−1. Accounting for changes in phytomass index, daytime NEE was similar for permanent-mobile site farm pairings, except when there were very large differences in water table depths between farms in autumn. In contrast, night-time respiration losses were almost identical between farms even when water tables were markedly different, suggesting that spatial differences in NEE in these agricultural peatlands are caused by reduced photosynthesis in dry periods, due to plant water stress, rather than increased respiration. Comparisons between permanent and mobile towers appeared a useful approach for determining spatial variability of CO2 fluxes from peat soils. Taken together, our results suggested that the CO2 losses measured at the permanent site were representative of CO2 losses for farmed peats in the Waikato region when the water table was within ∼0.5 m of the surface. Where water tables were deeper net CO2 losses would be expected to be greater due to reduced pasture photosynthesis and production. Maintaining higher water tables might achieve dual benefits of increasing pasture productivity and reducing CO2 losses.

We used eddy covariance measurements of net ecosystem CO2 exchange (NEE) in an undisturbed New Zealand bog to determine the major controls on gross primary production (GPP). In particular, we were interested in the relative importance of air temperature, saturation vapor pressure deficit (VPD), and diffuse fraction of incoming radiation (fdiff) in driving increases and restrictions to ecosystem photosynthesis. We found that NEE light response parameters for this peatland were remarkably similar year-round. We used four different light response functions to explore the influence of driving meteorological variables on daytime CO2 uptake. This revealed that including VPD as a model input reduced the bias associated with estimates of NEE relative to the standard rectangular hyperbola. An apparent temperature restriction of light-saturated photosynthetic capacity was relieved when high VPD data were excluded from light response curve fits. We thus, utilized the model that incorporated VPD to fill gaps in daytime NEE data and to estimate GPP. Increases in mid-day GPP were associated with increasing total photosynthetic radiation and air temperature, especially during winter and spring/autumn seasons. However, in summer, mid-day GPP was capped by high VPD, likely due to a stomatal response triggered to reduce transpiration water losses. Although GPP did not directly respond to fdiff, both temperature and VPD were coupled to sky conditions. Furthermore, the highest mid-day mean GPP for high light levels always occurred during the periods of highest fdiff. These results have important implications for the future C sink strength of New Zealand peatlands given a trend toward drier summers with clearer skies.

We used two years of eddy covariance (EC) measurements collected over an intensively grazed dairy pasture to better understand the key drivers of changes in soil organic carbon stocks. Analysing grazing systems with EC measurements poses significant challenges as the respiration from grazing animals can result in large short-term CO2 fluxes. As paddocks are grazed only periodically, EC observations derive from a mosaic of paddocks with very different exchange rates. This violates the assumptions implicit in the use of EC methodology. To test whether these challenges could be overcome, and to develop a tool for wider scenario testing, we compared EC measurements with simulation runs with the detailed ecosystem model CenW 4.1. Simulations were run separately for 26 paddocks around the EC tower and coupled to a footprint analysis to estimate net fluxes at the EC tower.Overall, we obtained good agreement between modelled and measured fluxes, especially for the comparison of evapotranspiration rates, with model efficiency of 0.96 for weekly averaged values of the validation data. For net ecosystem productivity (NEP) comparisons, observations were omitted when cattle grazed the paddocks immediately around the tower. With those points omitted, model efficiencies for weekly averaged values of the validation data were 0.78, 0.67 and 0.54 for daytime, night-time and 24-hour NEP, respectively. While not included for model parameterisation, simulated gross primary production also agreed closely with values inferred from eddy covariance measurements (model efficiency of 0.84 for weekly averages). The study confirmed that CenW simulations could adequately model carbon and water exchange in grazed pastures. It highlighted the critical role of animal respiration for net CO2 fluxes, and showed that EC studies of grazed pastures need to consider the best approach of accounting for this important flux to avoid unbalanced accounting.

Peatland ecosystems have been important global carbon sinks throughout the Holocene. Most of the research on peatland carbon budgets and effects of variable weather conditions has been done in Northern Hemisphere Sphagnum-dominated systems. Given their importance in other geographic and climatic regions, a better understanding of peatland carbon dynamics is needed across the spectrum of global peatland types. In New Zealand, much of the historic peatland area has been drained for agriculture but little is known about rates of carbon exchange and storage in unaltered peatland remnants that are dominated by the jointed wire rush, Empodisma robustum. We used eddy covariance to measure ecosystem-scale CO2 and CH4 fluxes and a water balance approach to estimate the sub-surface flux of dissolved organic carbon from the largest remaining raised peat bog in New Zealand, Kopuatai bog. The net ecosystem carbon balance (NECB) was estimated over four years, which included two drought summers, a relatively wet summer, and a meteorologically average summer. In all measurement years, the bog was a substantial sink for carbon, ranging from 134.7 to 216.9 gC m−2 yr−1, owing to the large annual net ecosystem production (161.8 to 244.9 gCO2–C m−2 yr−1). Annual methane fluxes were large relative to most Northern Hemisphere peatlands (14.2 to 21.9 gCH4–C m−2 yr−1), although summer and autumn emissions were highly sensitive to dry conditions, leading to very predictable seasonality according to water table position. The annual flux of dissolved organic carbon was similar in magnitude to methane emissions but less variable, ranging from 11.7 to 12.8 gC m−2 yr−1. Dry conditions experienced during late summer droughts led to significant reductions in annual carbon storage, which resulted nearly equally from enhanced ecosystem respiration due to lowered water tables and increased temperatures, and from reduced gross primary production due to vapor pressure deficit-related stresses to the vegetation. However, the net C uptake of Kopuatai bog during drought years was large relative to even the maximum reported NECB from Northern Hemisphere bogs. Furthermore, global warming potential fluxes indicated the bog was a strong sink for greenhouse gases in all years despite the relatively large annual methane emissions. Our results suggest that adaptations of E. robustum to dry conditions lead to a resilient peatland drought response of the NECB.

We measured net ecosystem CO2 exchange (NEE) for two years in a remnant New Zealand raised peat bog using the eddy covariance technique and estimated the component CO2 fluxes gross primary production (GPP) and ecosystem respiration (ER). Vegetation was dominated by vascular plants belonging to the southern hemisphere family Restionaceae, comprising an overstory of the erect Sporadanthus ferrugineus and a dense understory of Empodisma robustum. Annual NEE (GPP, ER) totals for the two years were −218.2 (1124.2, 906.0) gC m−2 yr−1 and −250.3 (1148.3, 897.8) gC m−2 yr−1, indicating substantially larger CO2 sink strength than has been reported for peatland ecosystems in the northern hemisphere. The bog was a net sink for CO2 in nine months each year, with small differences in autumn and wintertime NEE accounting for most of the difference in annual NEE totals. Summer daytime average NEE were similar to published values for northern hemisphere peatlands, despite differences in vegetation type and a comparatively deep water table. Fitted ecosystem light response and respiration model parameters were also within the ranges of values reported for peatlands elsewhere. The major distinction compared to northern peatlands was the year-round productivity of the evergreen vegetation cover in New Zealand, which largely offset respiration losses during the short, mild winter season.

It is well known that frequent cultivation of cropped soils leads to increased soil respiration and loss of soil carbon (C). In contrast, little is known about the impact of occasional cultivation of permanent grasslands on soil C and CO2 dynamics. Occasional cultivation of pastures is common if a pasture is part of an arable-ley rotation, or as part of pasture renewal. Here we report on the CO2 balance following three cultivation events of temperate permanent pasture in New Zealand. For two experiments, one during a drought during late summer/autumn 2008 and one under moist soil conditions in spring 2008, CO2 losses following cultivation were measured using the closed chamber technique. During the spring 2008 experiment, two soils with different clay mineralogy and drainage were studied. During a third cultivation event in autumn 2010 CO2 exchange was measured using eddy covariance. Measured short-term respiratory losses following cultivation across the three experiments ranged from 151 to 329 g C m−2 over 39 to 43 days. Rates of CO2 loss measured during non-drought conditions were generally higher than those previously reported from studies in Europe and North America, presumably because of generally high soil temperatures, non-limiting moisture conditions and high organic carbon availability at our study site. The ‘net impact of cultivation’ (taking into account both direct respiratory losses of CO2 and the lack of photosynthetic carbon input following cultivation) across the three experiments ranged between 77 and 406 g C m−2 over 39–43 days. Both direct CO2 respiratory losses and the net impact of cultivation appeared highly dependent on soil moisture status, with lowest losses measured during a severe drought and highest losses measured in spring when ample moisture was present. Rates of respiratory CO2 losses did not decrease over the duration of our experiments (39–43 days). Our results suggest that when aiming to reduce C losses resulting from cultivation of permanent grassland, it is preferable to cultivate when conditions for soil microbial activity and photosynthesis are sub-optimal; for our study site this meant in autumn instead of spring because of lower soil moisture availability. We also recommend minimising the duration of the period between spraying the old sward and establishment of the new sward or crop.

Flooding and land use changes can be a significant source of nutrients to wetlands and contribute to ecosystem changes at a community level. We studied changes in vegetation composition and peat properties in a New Zealand wetland incorporated into a flood control scheme. Lack of hydrological records pre-development of the flood control scheme allowed the opportunity to use nutrient and physical indicators developed for restiad vegetation groups, to identify if increased flooding was the primary cause for ecological succession. Vegetation changes along a 2.3-km transect were related to hydrological processes, peat characteristics and atmospheric ammonia inputs. Measurements for peat and vegetation composition were taken from 27 sites, whereas hydrological analysis incorporated seven automated water level sites and a historical investigation of 46 years of river flood records. Ammonia deposition was higher on the farmland/wetland fringe (4.0 kg N ha−1 year−1) but decreased to background levels (2–2.5 kg N ha−1 year−1) 500 m into the wetland. Classification and ordination of vegetation data with environmental variables identified six main groups along a gradient from low nutrients and stable water tables (farthest from the river) to high nutrients and a dynamic water table near the river. Invasion over 50 years of the native scrub tree Leptospermum scoparium into restiad bog was linked to enhanced nutrient inputs and physical degradation from flooding, portrayed through levels higher than restiad indicator target ranges. This study shows that target ranges for vegetation indicators can be used as a tool to identify ecological succession and invasion. Copyright © 2012 John Wiley & Sons, Ltd.

Summary Field studies have demonstrated that ground surface rainfall accumulation can be detected at depth by synchronous increases in static confined groundwater pore pressures. This opens the way for “geological weighing lysimeters” providing disturbance-free water storage monitoring of the surface environment, in effect by weighing a significant land area in real time. Such systems require specific hydrogeological conditions, which are not easily verified by field observations and replicated observations from multiple geological formations are a prerequisite for quality control. Given replication over two monitored formations, we introduce an expression which utilises the respective formation piezometric water levels to give an improved combined estimate of the ground surface water budget. The expression utilises raw piezometric levels and has the advantage of direct correction for Earth tide noise, which may sometimes be influenced by local effects in addition to the pure solar/lunar tidal potential. The expression is particularly simple, if the two formations have similar (but possibly unknown) undrained Poisson ratios and porosities. Surface water budgets can then be estimated using only the respective formation barometric coefficients and piezometric levels. An example application to two vertically separated confined aquifers at a New Zealand site indicate an improved accuracy over single-formation observations. The two-formation expression for surface storage could find use as an accurate water budget tool with particular application to monitoring diffuse hydrological systems such as wetlands, arid regions, and heavily forested localities.

Seabee Hook is a low lying gravel spit adjacent to Cape Hallett, northern Victoria Land, in the Ross Sea region of Antarctica and hosts an Adélie penguin (Pygoscelis adeliae) rookery. Dipwells were inserted to monitor changes in depth to, and volume of, groundwater and tracer tests were conducted to estimate aquifer hydraulic conductivity and groundwater velocity. During summer (November–February), meltwater forms a shallow, unconfined, aquifer perched on impermeable ice cemented soil. Groundwater extent and volume depends on the amount of snowfall as meltwater is primarily sourced from melting snow drifts. Groundwater velocity through the permeable gravel and sand was up to 7.8 m day−1, and hydraulic conductivities of 4.7 × 10−4 m s−1 to 3.7 × 10−5 m s−1 were measured. The presence of the penguin rookery, and the proximity of the sea, affects groundwater chemistry with elevated concentrations of salts (1205 mg L−1 sodium, 332 mg L−1 potassium) and nutrients (193 mg L−1 nitrate, 833 mg L−1 ammonia, 10 mg L−1 total phosphorus) compared with groundwater sourced away from the rookery, and with other terrestrial waters in Antarctica.

Land-use changes have contributed to increased atmospheric CO2 concentrations. Conversion from natural peatlands to agricultural land has led to widespread subsidence of the peat surface caused by soil compaction and mineralization. To study the net ecosystem exchange of carbon (C) and the contribution of respiration to peat subsidence, eddy covariance measurements were made over pasture on a welldeveloped, drained peat soil from 22 May 2002 to 21 May 2003. The depth to the water table fluctuated between 0.02m in winter 2002 to 0.75m during late summer and early autumn 2003. Peat soil moisture content varied between 0.6 and 0.7m 3 m � 3 until the water table dropped below 0.5m, when moisture content reached 0.38m 3 m � 3 . Neither depth to water table nor soil moisture was found to have an effect on the rate of nighttime respiration (ranging from 0.4‐8.0lmolCO2m � 2 s � 1 in winter and summer, respectively). Most of the variance in night-time respiration was explained by changes in the 0.1m soil temperature (r 2 50.93). The highest values for daytime net ecosystem

Increased accuracy in measuring temporal variations in the Earth's gravity field allow inprinciple the use of gravity observations to deduce subsurface water-mass changes. This canbe with respect to a small area, or as a larger spatial average of water mass change usinggravity observations from low-altitude satellites, such as the forthcoming GRACE mission.At both scales, there is a need to validate gravity-based estimates against field recordings ofactual subsurface water-mass variations. In practice, this could prove difficult because thespatial integral of all water-storage change components can be subject to considerable fieldmeasurement error. An alternative approach to the validation process is proposed by whichsuitable geological formations are utilized as giant weighing devices to directly measure area-integratedwater-mass changes. The existence of such “natural geological weighing lysimeters”is demonstrated using observations from a replicated experimental site in New Zealand. Sitesof this type could be used to verify water-storage change estimates derived from sensitiveground surface gravity instrumentation. In addition, geological lysimeters could be used tomake local checks on the accuracy of any estimated regional water-mass time series, whichis proposed for satellite calibration. The land area “weighed” by a geological lysimeter increaseswith formation depth and it is speculated that recordings made at oil well depth may allowdirect monitoring of subsurface water mass changes at the regional scale.

The evaporation regimes of two northern New Zealand raised peat bogs with vegetation dominated by the restionaceous rush Sporadanthus traversii were investigated. One of the bogs is unmodified while the other is affected by the drainage practices of surrounding agricultural land. Measurements of the latent heat flux obtained concurrently with Bowen ratio and eddy covariance techniques were in close agreement, and good energy balance closure was demonstrated when there was adequate fetch. Evaporation rates and energy partitioning behaviour were similar for the two bogs despite differences in water table elevation and peat moisture content. Evaporation rates averaged 2.9 mm day−1, equivalent to 55% of the equilibrium evaporation rate. Energy balance partitioning favoured the sensible heat flux, and Bowen ratios of around 2 were common. The importance of the latent heat flux in the energy balance depended on the state of canopy wetness following frequent rainfall events. When the canopy was completely wet, evaporation rates approximated, or exceeded, the equilibrium evaporation rate. Evaporation rates were evidently constrained by a combination of plant physiological and canopy structural factors which combined to prevent high rates of evaporation from the moist peat surface. These findings provide evidence of strong canopy-hydrological feedbacks within the raised peat bogs of northern New Zealand which result in conditions favourable for peat formation.

Preliminary results from a groundwater experiment set up near Matamata (New Zealand) reveal that confined aquifers can act as giant weighing lysimeters, with pore water pressures giving real-time measures of changes in amounts of surface and near-surface water. This opens the possibility of water balance studies at the scale of hectares using measurements from a single site. Given suitable confined aquifers, the technique has application in various environments for hydrological measurements as diverse as quantification of evapotranspiration loss, areal precipitation measurement, monitoring the water content of an accumulating snowpack, and net lateral groundwater transfers in unconfined aquifers.

A soil climate monitoring network, consisting of seven automated weather stations, was established between 1999 and 2003, ranging from Minna Bluff to Granite Harbour and from near sea level to about 1700 m on the edge of the polar plateau. Active layer depth was calculated for each site for eight successive summers from 1999/2000 to 2006/2007. The active layer depth varied from year to year and was deepest in the warm summer of 2001-02 at all recording sites. No trends of overall increase or decrease in active layer depth were evident across the up-to-eight years of data investigated. Average active layer depth decreased with increasing latitude from Granite Harbour (778S, active layer depth of . 90 cm) to Minna Bluff (78.58S, active layer depth of 22 ± 0.4 cm), and decreased with increasing altitude from Marble Point (50 m altitude, active layer depth of 49 ± 9 cm) through to Mount Fleming (1700 m altitude, active layer depth of 6 ± 2 cm). When all data from the sites were grouped together and used to predict active layer depth the mean summer air temperature, mean winter air temperature, total summer solar radiation and mean summer wind speed explained 73% of the variation (R 2 5 0.73). Received 9 December 2008, accepted 8 June 2009

Variations in the isotopic composition of precipitation are determined by fractionation processes which occur during temperature- and humidity-dependent phase changes associated with evaporation and condensation. Oxygen stable isotope ratios have therefore been frequently used as a source of palaeoclimate data from a variety of proxy archives, which integrate this signal over time. Applications from ombrotrophic peatlands, where the source water used in cellulose synthesis is derived solely from precipitation, have been mostly limited to Northern Hemisphere Sphagnum-dominated bogs, with few in the Southern Hemisphere or in peatlands dominated by vascular plants. New Zealand (NZ) provides an ideal location to undertake empirical research into oxygen isotope fractionation in vascular peatlands because single taxon analysis can be easily carried out, in particular using the preserved root matrix of the restionaceous wire rush (Empodisma spp.) that forms deep Holocene peat deposits throughout the country. Furthermore, large gradients are observed in the mean isotopic composition of precipitation across NZ, caused primarily by the relative influence of different climate modes. Here, we test whether δ18O of Empodisma α-cellulose from ombrotrophic restiad peatlands in NZ can provide a methodology for developing palaeoclimate records of past precipitation δ18O. Surface plant, water and precipitation samples were taken over spatial (six sites spanning >10° latitude) and temporal (monthly measurements over one year) gradients. A link between the isotopic composition of root-associated water, the most likely source water for plant growth, and precipitation in both datasets was found. Back-trajectory modelling of precipitation moisture source for rain days prior to sampling showed clear seasonality in the temporal data that was reflected in root-associated water. The link between source water and plant cellulose was less clear, although mechanistic modelling predicted mean cellulose values within published error margins for both datasets. Improved physiological understanding and modelling of δ18O in restiad peatlands should enable use of this approach as a new source of palaeoclimate data to reconstruct changes in past atmospheric circulation.