Abstract. Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15 N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15 N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15 N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (,1 week after 15 N tracer application), total ecosystem 15 N recovery was negatively correlated with fine-root and soil 15 N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15 N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg NÁha À1 Áyr À1 above which most ecosystems showed net losses of applied 15 N tracer in response to N fertilizer addition.

Groundwater discharge to the Cockburn River, southeast Australia, has been estimated from comparison of natural 222Rn activities in groundwater and river water, interpreted using a numerical flow model that simulates longitudinal radon activities as a function of groundwater inflow, hyporheic exchange, evaporation, gas exchange with the atmosphere, and radioactive decay. An injection of SF6 into the river to estimate the gas transfer velocity assisted in constraining the model. Previous estimates of groundwater inflow using 222Rn activities have not considered possible input of radon due to exchange between river water and water in the hyporheic zone beneath the streambed. In this paper, radon input due to hyporheic exchange is estimated from measurements of radon production by hyporheic zone sediments and rates of water exchange between the river and the hyporheic zone. Total groundwater inflow to the Cockburn River is estimated to be 18500 m3/d, although failure to consider hyporheic exchange would cause overestimation of the volume of groundwater inflow by approximately 70%.

Water quality was monitored in Boreal Shield lakes for 3 years following their simultaneous impact by clearcut logging or wildfire. Seventeen similar undisturbed lakes served as references. Dissolved organic carbon (DOC) and the light attenuation coefficient (εPAR) were up to threefold higher in cut lakes than in reference and burnt lakes. Compared with median values for reference lakes, cut and burnt lakes had higher concentrations of total phosphorus (TP) (two- to three-fold), total organic nitrogen (TON) (twofold), and K+, Cl-, and Ca2+ (up to sixfold). NO3- and SO42- concentrations were up to 60- and 6-fold higher, respectively, in burnt lakes than in reference and cut lakes. In most cases, impacts were directly proportional to the area harvested or burnt divided by the lake's volume or area. These simple models correctly predicted the changes observed in three lakes harvested during the study. Some of the ob served effects occur on different time scales. Mobile ions released by fire (K+, Cl-, SO42-, NO3-) or harvesting (K+, Cl-, some DOC) are rapidly flushed out of the watershed (50% decrease in 3 years). Other constituents or properties (TP, TON, DOC, εPAR, Ca2+, Mg2+) show little change or are still increasing after 3 years and will take a longer time to reach normal levels.

Soil water matric potentials (J m ) and the deuterium (d 2 H) composition at natural abundance levels of xylem water, soil water, river water and groundwater were used to evaluate whether trees use groundwater during the dry season in the riparian zone of the Daly River (Northern Territory, Australia). Groundwater was a significant source of water for plant transpiration, probably accounting for more than 50% of the water transpired during the dry season. Groundwater use occurred either when trees used water from the capillary fringe or when low J m induced by soil water uptake lifted groundwater in the vadose zone. Several water use strategies were inferred within the riparian plant community. Melaleuca argentea W. Fitzg and Barringtonia acutangula (L.) Gaertn. appeared to be obligate phreatophytes as they used groundwater almost exclusively and were associated with riverbanks and lower terraces with shallow (!5 m) water tables. Several species appeared to be facultative phreatophytes (including Cathorium umbellatum (Vahl.) Kosterm. and Acacia auriculiformis A. Cunn. ex Benth.) and tended to rely more heavily on soil water with increased elevation in the riparian zone. The levee-bound Corymbia bella K.D. Hill and L.A.S. Johnson mostly used soil water and is either a facultative phreatophyte or a non-phreatophyte. The temporal variability in groundwater utilisation by the trees is unclear because the study focused on the end of the dry season only. A decline in the regional water table as a result of groundwater pumping may affect the health of riparian zone vegetation in the Daly River because groundwater use is significant during the dry season. q

Element export rates from the drainage area of nine harvested, nine burnt, and 16 reference Boreal Shield lake basins in Haute-Mauricie, Québec, were estimated for the 3 years following harvesting or fires. Export rates from the drainage area of the basins were evaluated using lake sampling surveys, estimated hydrological budgets, and estimated nutrient retention during lake transit. Increases in K + , total N, and total P export rates were similar between harvested and burnt drainage areas. However, harvested drainage areas exported more dissolved organic C, while burnt drainage areas exported more Mg 2+ , NO 3 -, and SO 4 2-. Potassium cumulative losses in runoff during the 3 years of the study were of a similar magnitude as volatilization losses during fires and corresponded to -33% of the losses by biomass removal during harvesting. While the increased export rates for N and P following fires or harvesting represented negligible losses of nutrients for the forest, they were important supplementary inputs to lakes. The differences in element export rates observed between harvested and burnt drainage areas indicate that these disturbances have different impacts on biogeochemical cycles in the Boreal Shield forest.

Within Australia and globally there is considerable concern about the potential impacts of groundwater extraction on ecosystems dependent on groundwater. In this study we have combined heat pulse and isotopic techniques to assess groundwater use by riparian vegetation along the Daly River in the Northern Territory. The riparian forests of the Daly River exhibited considerable structural and floristic complexity. More than 40 tree species were recorded during vegetation surveys and these exhibited a range of leaf phonologies, implying complex patterns of water resource partitioning within the riparian forests. Water use was a function of species and season, and stand water use varied between 1.8 and 4.1 mm day–1. In general, however, water use tended to be higher in the wet season than during the dry season, reflecting the contribution to stand water use by dry-season deciduous tree species. There was a strong relationship between stand basal area and stand water use in the wet season, but the strength of this relationship was lower in the dry season. The amount of groundwater use, as determined by analysis of deuterium concentrations in xylem sap, was principally a function of position in the landscape. Trees at lower elevations, closer to the river, used more groundwater than trees higher on the levees. By using a combination of techniques we showed that riparian vegetation along the Daly River was highly groundwater dependent and that these water-use requirements need to be considered in regional management plans for groundwater.

Radon-222 is a naturally occurring radioactive gas (half-life = 3.8 d) that is emitted by virtually all geologic materials. Stream sediment porewater tends to approach an equilibrium 222 Rn activity determined by the 222 Rn production rate of the sediments and radon half-life. However, this equilibrium may not be reached when porewaters are diluted with low Rn disequilibrium technique is only applicable to estimate t h in the range of hours to days, but this is the range of interest in many hyporheic studies.

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