Checking the pulse of rainfall pulse dynamics
In semi-arid environments such as the Southwestern United States, infrequent and unpredictable precipitation events (“pulses” of water availability) induce biological activity and ultimately ecosystem structure and function. North American monsoon precipitation is highly variable across space and time but is often the major driver of nutrient cycling and vegetation productivity. The carbon balance of aridland ecosystems depends on carbon fixed by plants in photosynthesis and the carbon lost via heterotrophic and autotrophic respiration during and after rainfall pulses. The current paradigm is that soil microbes respond rapidly to small rainfall events whilst plants respond more 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.
Data from flux towers, which have been directly measuring carbon, water and energy fluxes, and micrometeorology continuously since 2007, can be used to determine if respiration pulses occur following precipitation events. To do so, we identified all precipitation pulses during the summer monsoon each year that were followed by five dry days to assess the structure of pulses and their abiotic controls in two Chihuahuan desert grasslands and a creosote shrubland, all in New Mexico.
Preliminary analysis suggests that regardless of conditions before rain only 16% (5/31) of precipitation pulses show a classic carbon-out, carbon-in response, with variation in timing (on day of rain through 5 days post-rain) and size (0.1 – 1 g C m-2 day-1) of respiratory peak. Rain often lead to no identifiable gross primary production (carbon uptake) by plants. Rather, pulses are remarkably variable in size and duration. Understanding the nuances of precipitation pulses and carbon fluxes involves disentangling the effects of rainfall event size, antecedent soil water content (SWC), and change in SWC on the size and sensitivity of both photosynthetic and respiratory responses.
Drylands make up 41% of the global land surface, and the carbon balance of these ecosystems is both water limited and driven by precipitation pulses. Understanding how these natural systems respond to rainfall 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.