Effects of altered rainfall on carbon and nitrogen cycling in annual grassland.
Chou, Wendy*,1, Silver, Whendee1, Jackson, Randall2, Allen-Diaz, Barbara1, 1 University of California - Berkeley, Berkeley, CA, USA2 University of Wisconsin - Madison, Madison, WI, USA
ABSTRACT- Given rising concentrations of CO2 and other greenhouse gases, current climate models project contrasting rainfall scenarios for California. Changes in rainfall regime will likely alter rates of carbon (C) loss via soil respiration, as well as fluxes of N2O. Moisture availability can also affect plant productivity in highly seasonal environments. We examined the effects of rainfall change in annual grasslands in the Sierra foothills of northern California by extending the wet season by about 5 weeks and augmenting total annual rainfall by 50 %. Discrete wet-up events took place prior to natural fall rains (early Oct 2003) and early in the drought period (May 2004). Soil respiration, N2O and CH4 effluxes, N mineralization, and above- and belowground plant production were measured in treatment and control plots over a one-year period. Soil CO2 fluxes for the first treatment year, though large, were not statistically different between wet and control plots (1078 ±148 g C m-2 and 1006 ± 138 g C m-2, respectively). The combined wet-up events comprised 17 % of the soil respiration over the 12-month period in treated plots, about twice as much C released by control plots over the same time interval. Aboveground biomass was similar between wet and control plots (415 ± 45 g m-2 y-1 and 374 ±36 g m-2 y-1, respectively) while root biomass increased significantly during the first year of wet treatment (179 ±23 g m-2 y-1 and 111 ±13 g m-2 y-1 for wet and control plots, respectively). The additional biomass C gained in treatment plots (53 g C m-2) partly offset the greater losses from respired C observed in treatment plots (72 g C m-2). Nitrous oxide emissions were negligible during the year except for the period directly after wet-up, when N2O emissions averaged over 78±13 ng N cm-2 h-1. Our first year of water manipulation in annual grasslands suggests that increased water availability via early and late rainfall events releases large pulses of CO2, increases belowground C inputs, and increases N2O emissions.
Key words: global change, precipitation, soil respiration, grassland
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