02986nas a2200157   4500008004100000245014500041210006900186300000800255490000700263520235900270100001702629700002102646700002002667700001702687856012402704       2005                        eng d00aSimulated carbon sink response of shortgrass steppe, tallgrass prairie and forest ecosystems to rising [CO2], temperature and nitrogen input0 aSimulated carbon sink response of shortgrass steppe tallgrass pr  a1 -0 v193 a[1] The response of plant ecosystems to environmental change will determine whether the terrestrial biosphere will remain a substantial carbon sink or become a source during the next century. We use two ecosystem models, the Generic Decomposition And Yield model (G'DAY) and the daily time step version of the Century model (DAYCENT), to simulate net ecosystem productivity (NEP) for three contrasting ecosystems (shortgrass steppe in Colorado, tallgrass prairie in Kansas, and Norway spruce in Sweden) with varying degrees of water, temperature, and nutrient limitation, to determine responses to gradual increases in atmospheric CO2 concentration ([CO2]), temperature, and nitrogen input over 100 years. Using G'DAY, under rising [CO2], there is evidence of C sink “saturation,” defined here as positive NEP reaching an upper limit and then declining toward zero, at all three sites (due largely to increased N immobilization in soil organic matter) but a positive C sink is sustained throughout the 100 years. DAYCENT also predicts a sustained C sink at all three sites under rising [CO2], with evidence of C sink saturation for the Colorado grassland and the C sink levels off after 80 years for the Kansas grassland. Warming reduces soil C and the C sink in both grassland ecosystems but increases the C sink in the forest. Warming increases decomposition and soil N mineralization, which stimulates net primary productivity (NPP) at all sites except when inducing water limitation. At the forest site some of the enhanced N release is allocated to a woody biomass pool with a low N:C ratio so that warming enhances NEP without increased N input at the forest site, but not at the grassland sites. Responses to combinations of treatments are generally additive for DAYCENT but more interactive for G'DAY, especially under combined rising [CO2] and warming at the strongly water- and N-limited shortgrass steppe. Increasing N input alleviates C sink saturation and enhances NEP, NPP, and soil C at all sites. At the water-limited grassland sites the effect of rising [CO2] on growth is greatest during the drier seasons. Key sensitivities in the simulations of NEP are identified and include NPP sensitivity to gradual increase in [CO2], N immobilization as a long-term feedback, and the presence or not of plant biomass pools with low N:C ratio.1 aPepper, D.A.1 aDel Grosso, S.J.1 aMcMurtrie, R.E.1 aParton, W.J.  uhttp://knz.lternet.edu/content/simulated-carbon-sink-response-shortgrass-steppe-tallgrass-prairie-and-forest-ecosystems03031nas a2200349   4500008004100000245009100041210006900132300001100201490000800212520198400220653001202204653003002216653002602246653001502272653001202287100001702299700001702316700001502333700001402348700002102362700002002383700002002403700001702423700001902440700001802459700002302477700001602500700001702516700001702533700001502550856011602565       2004                        eng d00aWater relations in grassland and desert ecosystems exposed to elevated atmospheric CO20 aWater relations in grassland and desert ecosystems exposed to el  a11 -250 v1403 a<p>Atmospheric CO2 enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO2 enrichment, with an emphasis on water relations. Increasing CO2 led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO2 enrichment were relatively greater in dry years. In contrast, CO2-induced aboveground biomass increases in the Texas C3/C4 grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO2 in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO2-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO2 were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO2 on plant and soil water relations may contribute substantially to experimentally induced CO2-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO2 effects on photosynthesis and those indirect CO2 effects via soil moisture as documented here.</p>10abiomass10aCarbon dioxide enrichment10aLandscape predictions10asoil water10aStomata1 aMorgan, J.A.1 aPataki, D.E.1 aKorner, C.1 aClark, H.1 aDel Grosso, S.J.1 aGrunzweig, J.M.1 aKnapp, Alan, K.1 aMosier, A.R.1 aNewton, P.C.D.1 aNiklaus, P.A.1 aNippert, Jesse, B.1 aNowak, R.S.1 aParton, W.J.1 aPolley, H.W.1 aShaw, M.R.  uhttp://knz.lternet.edu/content/water-relations-grassland-and-desert-ecosystems-exposed-elevated-atmospheric-co2