B52C-08
Pulse driven productivity in semi-arid lands

Friday, 18 December 2015: 12:05
2006 (Moscone West)
Amy C Bennett1, Scott L Collins1, Gregory E Maurer2, Albert Ruhi3 and Marcy E Litvak1, (1)University of New Mexico Main Campus, Albuquerque, NM, United States, (2)University of New Mexico Main Campus, Department of Biology, Albuquerque, NM, United States, (3)Arizona State University, Julie Ann Wrigley Global Institute of Sustainability, Tempe, AZ, United States
Abstract:
Drylands cover 41% of global land surface, and the carbon balance of these ecosystems is water limited. In the Southwestern United States precipitation events are infrequent and unpredictable with great spatial and temporal variation. “Pulses” of water availability are often the major driver of nutrient cycling and vegetation productivity. Rain induces biological activity and the ecosystm carbon balance depends on carbon fixed by plants in photosynthesis and carbon lost via heterotrophic and autotrophic respiration during and after precipitation. The current paradigm is that soil microbes respond rapidly to small rainfall events whilst plants respond to relatively larger pulses. Therefore arid ecosystems are predicted to act as a carbon source immediately following rainfall followed by a period of carbon accumulation while soil moisture is available.

At three flux-tower sites, two desert grasslands and a creosote shrubland, all in New Mexico, continuous measurements of carbon, water and energy fluxes and micrometeorology can be used to determine if respiration pulses occur following precipitation events. We identified all rain pulses during the growing season each year that were followed by five dry days to assess the structure of pulses and their abiotic controls.

Regardless of conditions before rain only 39% (68/175) of precipitation pulses show a classic carbon-out, carbon-in response, with variation in timing (on day of rain to 5 days post-rain) and size (0.1–1 g C m-2 day-1) of respiratory peak. Rain often led to no identifiable gross primary production (carbon uptake) by plants. Rather, pulses are remarkably variable in size and duration. Analyses using multivariate autoregressive state-space (MARSS) models indicated that the shape of different subsets of pulses varied as a function of season, available soil moisture before a pulse, size of a rain pulse, and whether the system is a carbon source or sink before rain.

The complexity of precipitation pulses and carbon fluxes is in the size and sensitivity of both photosynthetic and respiratory responses. Understanding how these natural systems respond to rain is important for estimating future carbon storage capacity under altered precipitation regimes and assessing the potential contribution of arid and semi-arid ecosystems to the global carbon budget.

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