Grassland soils are generally considered a small methane (CH4) sink, based mostly on the findings from chamber experiments. However, micrometeorological studies using the eddy covariance (EC) and flux-gradient (FG) methods have often reported upward CH4 fluxes over pasture, in the absence of grazing animals. Here, we collate available EC and FG data from New Zealand's pasture systems finding a predominance of upwards fluxes, typically approximate to 5-10 mg m(-2) d(-1). We then investigate whether these fluxes constitute real pasture emissions and conclude that they do not. Rather, these small upward fluxes are likely the result of strong CH4 emissions from upwind sources, at distances on the order of 1 km or more, outside the area usually considered the flux footprint. In other words, horizontal advection is often a non-negligible term in the CH4 mass budget and the homogeneity assumption for EC and FG is violated. This conclusion is based on the following points of evidence: 1) Exploratory surveys with chamber measurements, on three farms where EC and FG measurements operated, found small CH4 uptake rates. 2) Simulations with a dispersion model of cow emissions 800-900 m upwind of a measurement point 2 m above ground showed systematically upward EC fluxes of 1-25 mg m(-2) d(-1), and upward FG fluxes of similar magnitude for sufficiently stable or unstable stratification, demonstrating that fluxes of the observed magnitude can be entirely caused by advection. 3) Cospectra of CH4 and vertical wind were compared with their carbon dioxide (CO2) counterparts, because CO2 is known to be taken up or emitted locally and spatially homogeneously, and the CH4 cospectra were substantially different, being dominated at small wavenumbers indicating transport by large, organised structures. Based on our analyses, we urge caution when interpreting micrometeorological CH4 flux data in landscapes where large, transient emissions sources are present.

Environmental DNA (eDNA) is frequently used to infer distributions of microorganisms in Antarctica. Their distributions relative to environmental variables are, in turn, sometimes used to infer their physiological range (and a relationship between the two is generally assumed for conservation purposes). We sought to determine whether ecological inferences based on distributions accurately reflect tolerances of the organisms concerned, using 249 legacy non-marine samples from a latitudinal gradient between 72 and 86°S, Antarctica. A cyanobacterium, a heterotrophic bacterium, two eukaryotic algae, two fungi, and a moss were isolated into culture, and their field distributions inferred using eDNA analysis of the samples above. Tolerances of each organism with respect to environmental predictors were then inferred from the eDNA distribution and metadata using Generalised Additive Models. We then measured growth of the cultured isolates in response to a set of these predictors. Laboratory responses were then compared to inferences from the eDNA/metadata. Predictions from eDNA/metadata agreed with the results of physiological laboratory experiments for strains that were detected at high taxonomic resolution in the field samples. However, errors were never completely eliminated, and direct contradictions occurred when strains were represented at lower taxonomic resolution in the field data. We found that accurate ecological inference from eDNA studies would be best achieved via maximising both taxonomic resolution (through marker choice/read length) and ecological signal (through careful sampling design and rigorous metadata collection).

Methane (CH4) is the second-most important greenhouse gas in terms of its total contribution to global warming, and animal agriculture accounts for a significant share of its emissions. This article discusses the specific challenges of measuring CH4 emissions from systems of animal production, and the solutions that micrometeorological methods can offer. The methods considered include mass-budget methods, eddy covariance, gradient methods, inverse-dispersion modelling (IDM), and tracer-ratio methods that rely on atmospheric transport. The actual CH4 sources of animal agriculture include the animals themselves, manure storage and treatment facilities, and pasture soils. For each of these, the specific measurement challenges are discussed, and the methods employed to overcome them are reviewed. Animals are moving point sources of CH4, and they are managed in grazing or feeding systems, where their density can vary greatly. All the above micrometeorological methods have been used in some configurations to quantify animal CH4 emissions, typically with relative accuracies approaching +/- 10 %. Further improvement on this is possible with carefully designed setups comparing two animal groups in parallel. Such experiments can provide validation of mitigation approaches that rely on management practices not easily applied in controlled chamber or small-plot conditions. For emissions from manure storage and treatment, the main challenge to micrometeorological methods is usually flow disturbance, which can be overcome with both IDM and mass-budget methods, and these methods are well-suited to quantify the effects of mitigation approaches. For soil emissions, the main challenge is resolution; overall, soils contribute little to the total CH4 emissions from animal agriculture. Micrometeorological methods have also been used to integrate emissions over whole farms and regions, with mobile platforms, to corroborate inventory calculations. A challenge for the future is to develop methods that can determine CH4 emissions from a farm's animal herd reliably enough to underpin accounting systems.

Climate change is exposing agricultural systems to more frequent weather extremes, threatening production and challenging the sustainability of current management practices. However, isolating the climatic drivers of CO2 exchange in managed systems can be complicated by the periodic removal of aboveground biomass through grazing or harvesting, abruptly shifting the potential for plant uptake and associated ecosystem biogeochemistry. We used eddy covariance datasets from managed grasslands across New Zealand to determine the relative effects of temperature, vapour pressure deficit, and soil moisture on net CO2 uptake. Using time since grazing or harvest as a proxy for above-ground biomass, we accounted for the shifting potential CO2 exchange associated with these management actions. We then fit light response curves to the outer envelope of data to estimate maximal potential NEE, and subsequently calculate NEE deficit for every measured half hour. Using this analytical framework, we combined data from seven locations making up sixteen individual flux datasets comprising >600 site months and found that high air temperature and vapour pressure deficit were stronger controls over NEE deficit than soil moisture. These results were also consistent with a controlled leaf-level experiment demonstrating increasing photosynthesis deficit as VPD increased under constant temperature. The dominance of atmospheric effects over soil moisture in determining grassland response to dry conditions has important implications because it may be more difficult to mitigate these responses with typical management approaches (e.g., irrigation). We found these patterns were consistent across a diverse range of managed grassland operations, but subtle differences suggest that expanding the approach to a wider array of climatic regions and soil types could provide additional insights into how we might avoid the worst effects of future drying on managed systems.

Mitigation practices for nitrogen leaching losses from livestock agriculture are needed to protect freshwater quality and increase the efficiency of agricultural production. Within New Zealand, the most common pasture type is a two-species mix of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). Ecological theory suggests that increasing species and functional diversity improves ecosystem function, including nitrogen (N) retention. Use of more diverse pasture types, including a mix of pasture grasses, legumes and other forbs, particularly plantain (Plantago lanceolata), with functional traits, including winter activity, deep-rooting, N fixation, and biological inhibition of nitrification in the soil, is a potential mitigation practice that requires further verification with long-term field measurements. Here we utilize a network of large lysimeters to make field-based measurements of N leaching from 5–8 species diverse pasture, including plantain, under a range of soil, climate and management conditions, for comparison with losses from traditional ryegrass-clover pasture. Over 3 years of measurements, leaching from fully established diverse pasture was 2–80 kg N ha−1 y−1. No differences were observed in dry matter production or N leaching of diverse pasture compared to ryegrass-clover lysimeters. Large losses, up to 120 kg N ha−1, were observed during periods when pasture was not fully established, including cultivation and sowing of new pasture, depending on season. Timing of management activities could be optimized to minimize these losses. These data provide critical assessment of diverse pasture as a mitigation approach for reducing N losses. Further work on diverse pastures should include higher diversity mixes as well as consideration of animal mediated effects of diverse pasture diets on N inputs.

We tested an approach to estimate daily canopy net photosynthesis, A, based on estimates of transpiration, E, using measurements of sap flow and water-use efficiency, omega, by measuring delta 13C in CO2 respired from shoots in the canopies of two conifers (Podocarpaceae) native to New Zealand. The trees were planted in adjacent 20-year-old stands with the same soil and environmental conditions. Leaf area index was lower for Dacrycarpus dacrydioides D.Don in Lamb (1.34 m2 m-2) than for Podocarpus totara G.Benn. ex D.Don var. totara (2.01 m2 m-2), but mean (+/- standard error) stem diameters were the same at 152 +/- 21 mm for D. dacrydioides and 154 +/- 25 mm for P. totara. Over a 28-day period, daily A (per unit ground area) ranged almost five-fold but there were no significant differences between species (mean 2.73 +/- 1.02 gC m-2 day-1). This was attributable to higher daily values of E (2.63 +/- 0.83 mm day-1) and lower omega (1.35 +/- 0.53 gC kg H2O-1) for D. dacrydioides compared with lower E (1.82 +/- 0.72 mm day-1) and higher omega (1.90 +/- 0.77 gC kg H2O-1) for P. totara. We attributed this to higher nitrogen availability and nitrogen concentration per unit foliage area, Na, and greater exposure to irradiance in the D. dacrydioides canopy compared with P. totara. Our findings support earlier observations that D. dacrydioides is more adapted to sites with poor drainage. In contrast, the high retention of leaf area and maintaining low rates of transpiration by P. totara, resulting in higher water-use efficiency, is an adaptive response to survival in dry conditions. Our findings show that physiological adjustments for two species adapted to different environments led to similar canopy photosynthesis rates when the trees were grown in the same conditions. We demonstrated consistency between whole-tree and more intensive shoot-scale measurements, confirming that integrated approaches are appropriate for comparative estimates of carbon uptake in stands with different species.

Globally, the soils underlying managed grasslands are an important reservoir of carbon (C), with the management of the land strongly influencing whether the soils gain, lose or maintain C. Further, improved grassland management, such as rotational grazing and increased sward diversity, are widely promoted to increase soil C sequestration despite limited evidence. Using data from New Zealand 's grazed dairy grasslands, we examined the effect of management practices on C stocks through measurement of the net ecosystem C balance (NECB) across 68 site -years. Management practices tested included year-round rotational grazing, differing pasture sward composition, irrigation, periodic supplemental feed cropping and pasture renewal. Excluding years where supplemental feed cropping occurred, the mean (+/- 95% confidence interval) NECB was -12 +/- 30 g C m - 2 y -1 (65 site -years; the negative value represents C loss), indicating the soil C stocks of these ecosystems were likely near steady-state. The processes of pasture renewal and the transition between pasture and crops resulted in reduced C inputs from photosynthesis of between -232 and -94 g C m - 2 . This reduced C input was partially overcome with the application of effluent or manure at the time of cultivation where applied. There was no difference in NECB between irrigated (-26 +/- 114 g C m - 2 y -1 ) and non -irrigated (-19 +/- 198 g C m - 2 y -1 ) pastures. In general, conventional ryegrass/white clover pastures were more beneficial for C stocks than alternative pasture species and mixes. Periodic cropping for supplemental feed resulted in C losses, with the magnitude depending on crop type and management, although data were limited to three site -years.

Abstract Episodic deposition of light absorbing impurities on glaciers reduces albedo and exacerbates snow melt. In 2019/2020 a devastating Australian bushfire and desert dust event combined with favorable meteorological conditions transported an unprecedented mass of impurities across the Tasman Sea turning the Southern Alps of Aotearoa New Zealand red. Here we use time lapse cameras, airmass back trajectories, snow impurity geochemistry, and remote sensing to quantify the timing, provenance, and mass deposition of the event. Deposited in late November 2019, the impurities were dominated by mineral dust with a distinct southeastern Australian geochemical fingerprint. The event deposited ∼4,500 ± 500 tons of red dust to Southern Alps permanent snow and ice with a mean dust mass concentration of 6.5 ± 0.7 g m−2. A southeast Australian desert dust storm generated by the same type of meteorological conditions as the 2020 New Year bushfires was the main driver of the glacier discoloration.

Grazed grasslands are an important source of N2O emissions. Grasslands also store significant amounts of carbon (C), and net changes to these amounts contribute to their net greenhouse gas budgets. We investigated whether net N2O emissions from irrigated pasture grazed by dairy cows can be reduced by planting more diverse species, compared with conventional ryegrass-clover pasture, and whether there are co-benefits for greenhouse gas reduction by net C gains in the ecosystem, or trade-offs through net C losses. Fluxes of CO2 and N2O were measured by eddy covariance, near the boundary of adjacent five-species mixed pasture (MIX) and ryegrass-white clover pasture (RyWC). A split-footprint approach was applied to separate the CO2 and N2O exchange for the two pastures. The CO2 fluxes were gap-filled with marginal distribution sampling (MDS), and the N2O fluxes with k-nearest-neighbour (kNN) regression. To enable calculation of the net ecosystem C balances (NECB) of the two pastures, non-CO2 carbon imports and exports were also determined. Annual N2O emissions from the MIX pasture were 0.14 g N m- 2 yr- 1 (mean of two years), compared with 0.23 g N m- 2 yr- 1 from the RyWC pasture. The N2O emissions accounted for 0.34% and 0.61% of the nitrogen inputs to the MIX and RyWC pasture, respectively. Combining all C gains and losses, both pastures recorded net ecosystem C gains in the first year, of 44 and 282 g C m- 2 for MIX and RyWC, respectively. In the second year, both recorded losses:-207 g C m- 2 for MIX and-86 g C m- 2 for RyWC. Thus, in both years, the net C balance gave the RyWC pasture a considerable advantage over the MIX pasture in the net greenhouse gas balance, and this advantage was one magnitude greater than the difference in N2O emissions, when expressed in CO2-equivalent units.

The effects of four different rates of high nitrogen (N) addition (220, 300, 450, and 750 kg N ha−1) on carbon (C) rhizodeposition, the composition of main soil microbial groups, and microbial processing of rhizodeposited C were investigated for two common grassland species, Lolium perenne L. (perennial ryegrass) and Plantago lanceolata L. (ribwort plantain). We measured net ecosystem carbon dioxide (CO2) exchange and used a 13CO2 pulse-labelling technique to trace recent products of photosynthesis through the plant-soil system. Overall, net C uptake was 10% higher for P. lanceolata than for L. perenne. This was associated with a 62% higher concentration of rhizodeposited C in the soil under P. lanceolata than under L. perenne. Concentrations of rhizodeposited C further increased by 30% per 100 kg N ha−1 added. For both plant species, increasing N addition was associated with compositional differences in soil microbial groups towards a more bacteria-dominated system and increased microbial uptake of rhizodeposited 13C. However, the N-induced changes in rhizodeposited 13C uptake by different microbial groups were much more pronounced for L. perenne than those for P. lanceolata. This suggests that microbial processing of rhizodeposited 13C was more susceptible to the response of L. perenne to high N addition compared to P. lanceolata. The findings highlight the importance of the responses of plant species with contrasting traits to high N inputs and the associated distinct effects on soil C cycling processes through altering the composition of the main soil microbial groups and microbial uptake of rhizodeposited C.

Intensification of grazed grasslands following conversion from dryland to irrigated farming has the potential to alter ecosystem carbon (C) cycling and affect components of carbon dioxide (CO2) exchange that could lead to either net accumulation or loss of soil C. While there are many studies on the effect of water availability on biomass production and soil C stocks, much less is known about the effect of the frequency of water inputs on the components of CO2 exchange. We grew Bermuda grass (Cynodon dactylon L.) in mesocosms under irrigation frequencies of every day (I1 treatment, 30 d), every two days (I2 treatment, 12 d), every three days (I3 treatment, 30 d), and every six days (I6 treatment, 18 d, after I2 treatment). Rates of CO2 exchange for estimating net ecosystem CO2 exchange (FN), ecosystem respiration (RE), and soil respiration (RS) were measured, and gross C uptake by plants (FG) and respiration from leaves (RL) were calculated during two periods, 1–12 and 13–30 d, of the 30-d experiment. During the first 12 d, there were no significant differences in cumulative FN (mean ± standard deviation, 61 ± 30 g C m-2, n=4). During the subsequent 18 d, cumulative FN decreased with decreasing irrigation frequency and increasing cumulative soil water deficit (W ), with values of 70 ± 22, 60 ± 16, and 18 ± 12 g C m-2 for the I1, I3, and I6 treatments, respectively. There were similar decreases in FG, RE, and RL with increasing W , but differences in RS were not significant. Use of the C4 grass growing in a C3-derived soil enabled partitioning of RS into its autotrophic (RA) and heterotrophic (RH) components using a 13C natural abundance isotopic technique at the end of the experiment when differences in cumulative W between the treatments were the greatest. The values of RH and its percentage contributions to RS (43％± 8％, 42％± 8％, and 8％± 5％for the I1, I3, and I6 treatments, respectively) suggested that RH remained unaffected across a wide range of W and then decreased under extreme W . There were no significant differences in aboveground biomass between the treatments. Nitrous oxide (N2O) emission was measured to determine if there was a trade-off effect between irrigation frequency and increasing W on net greenhouse gas emission, but no significant differences were found between the treatments. These findings suggest that over short periods in well-drained soil, irrigation frequency could be managed to manipulate soil water deficit in order to reduce net belowground respiratory C losses, particularly those from the microbial decomposition of soil organic matter, with no significant effect on biomass production and N2O emission.

Intensification of agricultural management practices, including irrigation and addition of nitrogen (N) fertilizers, can lead to enhanced N leaching and loss of soil fertility. In New Zealand, expansion of the dairy industry has rapidly increased irrigated land area, particularly on shallow, stony soils of the Canterbury region that are prone to leaching, leading to degradation of surface- and ground-water quality and losses of soil N and carbon (C). In this study, we measure components of N balance for two adjacent fields of lucerne (Medicago sativa L., alfalfa) harvested for cut-and-carry feed and grazed in situ. One field was non-irrigated and one irrigated with both water and dairy effluent. Inputs from N fixation associated with the legume crop were quantified using a natural abundance isotopic approach. Drainage from the root zone and leaching were measured with 6 large lysimeters in each field. Leaching losses from non-irrigated lucerne were 7-30 kg N ha(-1) y(-1) with the largest losses occurring in a year with primarily grazing management. Losses from irrigated lucerne were 39-102 kg N ha(-1) y(-1) , with the largest losses resulting from summer drainage events exacerbated by irrigation. Fixation of N was the largest input to both systems, contributing 192-257 kg N ha(-1) y(-1) for non-irrigated lucerne. Under irrigation, biomass production increased, but N uptake from effluent and soil stocks contributed to biomass N to a greater extent and fixation was 262-286 kg N ha(-1) y(-1) . Management influenced N balance through inputs from animal excreta and effluent additions and exports through harvest and grazing removals. Management practices which reduce N losses from the soil are needed to minimize environmental impacts and protect soil fertility.

The extent of phosphorus (P) loss from soils under deep rooting crops such as lucerne is currently unknown. This study used large lysimeters (2.0 by 1.5 m) to quantify the amounts and forms of P in drainage under lucerne from two sites, a non-irrigated dairy system and an irrigated dairy system that also received farm dairy effluent (FDE). Results showed despite greater P inputs to the irrigated compared to the non-irrigated site, there was no difference in total P concentrations in drainage. There were also no differences in the forms of P lost between sites, which were dominated by particulate P (48%-52%). More P was lost from the irrigated (0.262 kg ha(-1)) than the non-irrigated site (0.164 kg ha(-1)). The larger P loss was a result of irrigation producing more drainage (42%) than observed at the non-irrigated site. The amount and forms of P lost from lucerne were similar or lower than losses reported for similar soils under grass/legume-based pasture swards amended with P fertiliser and FDE. Because P leaching was regulated by drainage volume rather than P input, management of irrigation water to reduce drainage from the root zone will likely help minimise P leaching.

Previous soil sampling from grazed pastures in New Zealand compared the changes of soil organic carbon (SOC) in adjacent irrigated and unirrigated portions of the same paddocks. It showed that irrigated portions had lower SOC stocks than unirrigated portions, with an average difference of 7.0 tC ha(-1) or 0.6 tC ha(-1) yr(-1). These findings have formed the basis of an assessment for the net effect of conversion of New Zealand's grazed pastures to irrigation. However, since cattle could move freely between irrigated and unirrigated portions of the studied paddocks, there could have been different grazing intensities and/or excreta transfer between the irrigated and unirrigated portions of the same paddocks. Both these factors could have affected SOC stocks. In this study, we used the process-based model, CenW, to simulate the consequences of this possible carbon transfer via animal excreta and different grazing intensities. We found that the observed increase of 0.6 tC ha(-1) yr(-1) in SOC stock in the unirrigated portions could result from a transfer of 20% excreta from the irrigated to unirrigated portions (with an area ratio of 6:1) of a paddock and with the unirrigated portions being grazed only lightly with 2.0 tDM ha(-1) in foliage biomass residuals remaining after grazing. That means that the observed higher SOC stocks in the unirrigated portions could potentially be attributable to the behaviour of grazing animals. We suggest that a realistic extent of carbon transfer and/or differences in grazing intensities could be sufficient to account for the observed differences in SOC stocks even if irrigation per se caused no differences in carbon stocks. It is therefore inappropriate to ascribe the change of SOC to irrigation effects based on experimental findings where SOC changes can be affected by the behaviour of grazing animals. (C) 2021 Elsevier B.V. All rights reserved.

>Dear Editor,Approximately 55% of the agricultural land in New Zealand is grazed grassland to support the expanding dairy and meat industries. Intensified management practices such as irrigation, nitrogen (N) fertiliser application,

Climate warming may be exacerbated if rising temperatures stimulate losses of soil carbon to the atmosphere. The direction and magnitude of this carbon‐climate feedback are uncertain, largely due to lack of knowledge of the thermal adaptation of the physiology and composition of soil microbial communities. Here, we applied the macromolecular rate theory (MMRT) to describe the temperature response of the microbial decomposition of soil organic matter (SOM) in a natural long‐term warming experiment in a geothermally active area in New Zealand. Our objective was to test whether microbial communities adapt to long‐term warming with a shift in their composition and their temperature response that are consistent with evolutionary theory of trade‐offs between enzyme structure and function. We characterized the microbial community composition (using metabarcoding) and the temperature response of microbial decomposition of SOM (using MMRT) of soils sampled along transects of increasing distance from a geothermally active zone comprising two biomes (a shrubland and a grassland) and sampled at two depths (0–50 and 50–100 mm), such that ambient soil temperature and soil carbon concentration varied widely and independently. We found that the different environments were hosting microbial communities with distinct compositions, with thermophile and thermotolerant genera increasing in relative abundance with increasing ambient temperature. However, the ambient temperature had no detectable influence on the MMRT parameters or the relative temperature sensitivity of decomposition ( Q 10 ). MMRT parameters were, however, strongly correlated with soil carbon concentration and carbon:nitrogen ratio. Our findings suggest that, while long‐term warming selects for warm‐adapted taxa, substrate quality and quantity exert a stronger influence than temperature in selecting for distinct thermal traits. The results have major implications for our understanding of the role of soil microbial processes in the long‐term effects of climate warming on soil carbon dynamics and will help increase confidence in carbon‐climate feedback projections.

In New Zealand, increasing areas of dryland farming are being converted to irrigated farming, and there are conflicting findings whether this will lead to gains or losses of soil organic carbon (SOC). In this study, we used 2 years of eddy-covariance data from an irrigated, grazed dairy pasture in Canterbury, New Zealand and compared observed gas exchange fluxes to those from the process-based CenW model. CenW simulations agreed very well with observations for evapotranspiration, gross primary production, and ecosystem respiration rates with Nash-Sutcliffe model efficiencies similar to 0.8-0.9 for daily and weekly mean values. Model efficiency for net ecosystem productivity was similar to 0.7-0.8 for daily and weekly averages. The mean simulated net ecosystem carbon balance (NECB) over the 2 year period was 0.6 t C ha(-1) y(-1) which was comparable to the measured NECB of 1.0 +/- 0.3 t C ha(-1) y(-1). The model also highlighted some processes where current understanding is incomplete. We found that gross primary production (GPP) was substantially reduced after grazing. While some GPP reduction was expected due to reduced leaf area, measured rates were only about half (reduced by a further similar to 30 kg C ha(-1) d(-1)) of that expected due to reduced leaf area alone. This probably resulted from the combined effect of trampling damage by cattle, a minor contribution of foliage coverage by dung pats, and some additional leaf physiological effect when previously shaded parts of the foliage were suddenly exposed to full sun to which they were not fully adapted. GPP then gradually recovered over the subsequent 2 weeks. CenW was then used to compare SOC levels after 50 years of irrigated dairy farming with those under a nonirrigated system. We found that SOC responses to irrigation interacted strongly with imposed grazing patterns. When we used the same grazing regime under irrigated and non-irrigated conditions, the application of irrigation resulted in a small increase of 4 +/- 3 t C ha(-1) (mean +/- standard error) over the range of weather conditions typical for this site. However, with this grazing management, the non-irrigated system had very low productivity (< 0.1 t C ha(-1) y(-1) in animal products). When dryland pasture was combined with a grazing regime typical for rain-fed systems, simulated long-term SOC stocks were increased by 13.9 +/- 1.5 t C ha(-1) under the irrigated system. This highlights the importance of considering the whole management system rather than looking at individual practices in isolation.

Long-term irrigation of temperate pastures has been reported to either increase or decrease soil organic carbon (SOC) stocks when compared with dryland systems. Understanding the short-term effects of irrigation on the fixation and partitioning of carbon (C) to plant and soil components may be important to explaining the observed differences. Continuous (CO2)-C-13 pulse labelling of ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) planted mesocosms was used to quantify the net accumulation and partitioning of new photosynthate C to aboveand below-ground components of the plant-soil system, including soil particle size fractions: > 250 mu m, 53-250 mu m, 20-53 mu m, 5-20 mu m and <5 mu m, under simulated irrigation and dryland conditions. After the (CO2)-C-13 labelling, irrigation increased the quantity of C-13 partitioned into herbage by 16%, while reducing the quantity partitioned into roots in the 15-25 cm soil depth by 35%. However, less new photosynthate C was observed in rhizosphere soil (0-15 cm depth), while more new photosynthate C was partitioned into the 53-250 mu m and <5 mu m soil fractions under irrigation. Despite these differences, the net amount of new photosynthate C in the whole soil (0-25 cm depth) was similar between treatments (2511 kg new C ha(-1) dryland and 2509 kg new C ha(-1) irrigated). Therefore, irrigation did not increase the net amount of new photosynthate C in the soil despite increased above-ground pasture productivity. Based on our results, we hypothesise that the recently reported losses of SOC from irrigated pastures may be driven by faster turnover of root-derived C, which may explain the increase in photosynthate C in the fine POM soil size fraction (53-250 mu m), rather than a reduction in photosynthate C inputs to the soil.

Ecosystem process rates typically increase after plant invasion, but the extent to which this is driven by (i) changes in productivity, (ii) exotic species' traits, or (iii) novel (non-coevolved) biotic interactions has never been quantified. We created communities varying in exotic plant dominance, plant traits, soil biota, and invertebrate herbivores and measured indicators of carbon cycling. Interactions with soil biota and herbivores were the strongest drivers of exotic plant effects, particularly on measures of soil carbon turnover. Moreover, plant traits related to growth and nutrient acquisition explained differences in the ways that exotic plants interacted with novel biota compared with natives. We conclude that novel biological interactions with exotic species are a more important driver of ecosystem transformation than was previously recognized.

Specific surface area can be a strong predictor of organic carbon (SOC) contents in soils. Specific surface area can be estimated reliably and cost-effectively from water adsorption by air-dry soil samples, but SOC itself can also adsorb water. For estimating the mineral component of specific surface area, it is, therefore, necessary to exclude water-adsorption by SOC. Here, we refer to "apparent specific surface area" for measurements that include water adsorption by both mineral soil and SOC. We used a mathematical approach to estimate water adsorption by SOC so that this component can be subtracted from measurements of apparent specific surface area. We used a dataset of apparent specific surface area and soil carbon at seven depths from 50 soil cores collected from a research farm in the Manawatu region in New Zealand. Both apparent specific surface area and SOC content decreased with soil depth with very high correlation (r(2) = 0.98). We estimated the SOC contribution to apparent specific surface area from the slope of the relationship between changes in apparent specific surface area and SOC content. For our soils, the SOC contribution to apparent specific surface area was estimated as 0.43 +/- 0.02 m(2) mgC(-1). This parameter allows apparent specific surface area measurements to be corrected for the water adsorption by SOC to calculate the functionally relevant mineral specific surface area. Highlights Soil surface area can be estimated from the H2O content of air-dry soil but SOC also adsorbs H2O. We developed a mathematical approach to estimate water adsorption by SOC. We estimated the contribution of SOC to apparent specific surface area as 0.43 +/- 0.02 m(2) mgC(-1). Mineral specific surface area can be inferred by subtracting SOC-based H2O adsorption.