Rising atmospheric carbon dioxide concentration ([CO 2 ]) has the potential to stimulate ecosystem productivity and sink strength, reducing the effects of carbon (C) emissions on climate. In terrestrial ecosystems, increasing [CO 2 ] can reduce soil nitrogen (N) availability to plants, preventing the stimulation of ecosystem C assimilation; a process known as progressive N limitation. Using ion exchange membranes to assess the availability of dissolved organic N, ammonium and nitrate, we found that CO 2 enrichment in an Australian, temperate, perennial grassland did not increase plant productivity, but did reduce soil N availability, mostly by reducing nitrate availability. Importantly, the addition of 2 1C warming prevented this effect while warming without CO 2 enrichment did not significantly affect N availability. These findings indicate that warming could play an important role in the impact of [CO 2 ] on ecosystem N cycling, potentially overturning CO 2 -induced effects in some ecosystems.

The aims of this study were to determine whether elevated atmospheric CO 2 concentration modifies plant organic matter (OM) fluxes to the soil and whether any change in the fluxes can modify soil OM accumulation. Measurements were made in a grazed temperate grassland after almost 4 years exposure to elevated atmospheric CO 2 (475 ll l -1 ) using a Free Air CO 2 Enrichment (FACE) facility located in the North Island of New Zealand. Aboveground herbage biomass and leaf litter production were not altered by elevated CO 2 but root growth rate, as measured with the ingrowth core method, and root turnover were strongly stimulated by elevated CO 2 particularly at low soil moisture contents during summer. Consequently, significantly more plant material was returned to the soil under elevated CO 2 leading to an accumulation of coarse (>1 mm) particulate organic matter (POM) but not of finer POM fractions. The accumulating POM exhibited a lower C/N ratio, which was attributed to the higher proportion of legumes in the pasture under elevated CO 2 . Only small changes were detected in the size and activity of the soil microbial biomass in response to the POM accumulation, suggesting that higher organic substrate availability did not stimulate microbial growth and activity despite the apparent lower C/N ratio of accumulating POM. As a result, elevated CO 2 may well lead to an accumulation of OM in grazed grassland soil in the long term.

Dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in soils are increasingly recognized as important components of nutrient cycling and biological processes in soil-plant ecosystems. The aims of this study were to: (i) quantify the pools of DON and DOC in a range of New Zealand pastoral soils; (ii) compare the effects of land use changes on these pools; and (iii) examine the seasonal variability associated with these two components of dissolved organic matter. Soil samples (0-7.5 cm depth) from 93 pastoral sites located in Northland, Waikato, Bay of Plenty and Otago/Southland, New Zealand, were collected in autumn. Adjacent sites under long-term arable cropping or native vegetation and forestry land use were also sampled at the same time to estimate the impacts of different land use on DON and DOC in these soils. Twelve dairy and 12 sheep and or beef pastures were sampled in winter, spring, summer and autumn for a 2-year period to study the seasonal fluctuations of DON and DOC. A field incubation study was also carried out in a grazed pasture to examine fluctuations in the concentrations of NO À 3 and NH þ 4 and DON levels in soil. Other soil biological properties, such as microbial biomass-C, biomass-N and mineralizable N, were also measured. Pastoral soils contained the greatest amounts of DON (13-93 mg N kg -1 soil, equivalent to 8-55 kg N ha -1 ) and DOC (73-718 mg C kg -1 soil, equivalent to 44-431 kg C ha -1 ), followed by cropping and native vegetation and forestry soils. The DON concentration in soils was found to be more seasonally variable than DOC. There was approximately 80% fluctuation in the concentration of DON in winter from the annual mean concentration of DON, while DOC fluctuated between 23 and 28% at the dairy and the sheep and beef monitoring sites. Similar fluctuations in the concentrations of DON were also observed in the field incubation studies. These results indicate that DON is a dynamic pool of N in soils. There was a strong and significant positive correlation between DON and DOC in pastoral soils (r ¼ 0.71, P < 0.01). There were also significant positive correlations between DON and total soil C (r ¼ 0.59, P < 0.01), total soil N (r ¼ 0.62, P < 0.01) and mineralizable N (r ¼ 0.47, P < 0.01). The rather poor correlations between total soil C and N with DOC and DON, suggest other biogeochemical processes may be influencing concentrations of DOC and DON in these soils. Given the size of DON and DOC pools in the pastoral soils, we suggest that these pools of C and N should be taken into account when assessing the impact of pastoral land use on soil C and N enrichment of surface and groundwater.

Nitrous oxide emissions from grazed pastures were measured at four sites for a 2 year period. Sites differed in drainage class and N cycle characteristics. At two intensively farmed sites on Kairanga silt loam, which is poorly drained, daily emissions ranged from 0 to 100 g N ha-1 day-1 and annual emission was in the range 3-5 kg N2O-N ha-1. Emissions occurred when the soil was near or above field capacity indicating denitrification is the probable source of N2O. Multiple regression analysis, using soil water content, NO3-, NH4+ and temperature, gave r2 = 0.44 and 0.57 at sites 1 and 2 respectively. Soil water content and NH4+ were significant variables. Emissions at a low fertility hillside site were very low and an annual emission of 0.5 kg N2O-N yr-1, or less, was indicated. The highly fertile hillside site also showed low emission values. It is suggested that grazing animals may have a large impact on emissions through hoof damage on wet soils.

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