Precipitation pulses and carbon fluxes in semiarid and arid ecosystems

Abstract

In the arid and semiarid regions of North America, discrete precipitation pulses are important triggers for biological activity. The timing and magnitude of these pulses may differentially affect the activity of plants and microbes, combining to influence the C balance of desert ecosystems. Here, we evaluate how a "pulse" of water influences physiological activity in plants, soils and ecosystems, and how characteristics, such as precipitation pulse size and frequency are important controllers of biological and physical processes in arid land ecosystems. We show that pulse size regulates C balance by determining the temporal duration of activity for different components of the biota. Microbial respiration responds to very small events, but the relationship between pulse size and duration of activity likely saturates at moderate event sizes. Photosynthetic activity of vascular plants generally increases following relatively larger pulses or a series of small pulses. In this case, the duration of physiological activity is an increasing function of pulse size up to events that are infrequent in these hydroclimatological regions. This differential responsiveness of photosynthesis and respiration results in arid ecosystems acting as immediate C sources to the atmosphere following rainfall, with subsequent periods of C accumulation should pulse size be sufficient to initiate vascular plant activity. Using the average pulse size distributions in the North American deserts, a simple modeling exercise shows that net ecosystem exchange of CO2 is sensitive to changes in the event size distribution representative of wet and dry years. An important regulator of the pulse response is initial soil and canopy conditions and the physical structuring of bare soil and beneath canopy patches on the landscape. Initial condition influences responses to pulses of varying magnitude, while bare soil/beneath canopy patches interact to introduce nonlinearity in the relationship between pulse size and soil water response. Building on this conceptual framework and developing a greater understanding of the complexities of these eco-hydrologic systems may enhance our ability to describe the ecology of desert ecosystems and their sensitivity to global change.

Rainfall-induced carbon dioxide pulses result from sequential resuscitation of phylogenetically clustered microbial groups

“…The source of the CO 2 pulse is a hotly debated topic, with proposed contributions from displacement of soil porespace CO 2 (2) or microbial metabolism of soil organic carbon originating from photodegradation of plant material over the summer (3)(4)(5), dead microbial biomass (6), microbial intracellular osmolytes (7,8), or previously inaccessible soil carbon made available for mineralization by soil wet-up (9). No matter what the origin of the organic carbon, the substrate must be decomposed to CO 2 by a microbial community emerging from stasis.…”

Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a Chihuahuan Desert shrubland

“…Patterns of soil water availability exert overwhelming control over biogeochemistry in these regions, not only by filtering potential vegetation type but also by determining specific temporal windows during which biologically-mediated chemical transformations can occur (Noy-Meir, 1973;Austin et al, 2004;Huxman et al, 2004a). In turn, vegetation type and geomorphology interact to affect how water is partitioned following precipitation in different hydroclimatic zones.…”

“…In these regions, the response of the biological component of the system to climatic forcing such as rising CO 2 or temperature are mediated through variation in soil water availability (Hamerlynck et al, 2000;Smith et al, 2000). As such, there has been an important push to understand how ecological and hydrological cycles are coupled in both time and space for these regions (Huxman et al, 2004a;Scott et al, 2006).…”

“…GEP is the photosynthetic activity of plants in the ecosystem, and thus should be at least proportional to water loss to the atmosphere through transpiration (Enquist, 2002;Huxman et al, 2004a). Ecosystem respiration quantifies plant (autotrophic), microbial (heterotropic), and possibly, inorganic carbonate dynamics.…”

“…The relationship of respiration to evaporation and transpiration is not well known, but it is likely to be correlated with near-surface soil water status. This is especially true for systems dominated by wet-dry cycles where a substantial contribution of soil micro-organisms to the CO 2 efflux is expected (Austin et al, 2004;Huxman et al, 2004a). However, some fraction of efflux should be related to the respiratory activity of plants and correlated with deeper, rooting zone soil moisture and transpiration (Franzluebbers et al, 2002;Chimner and Welker, 2005), and another fraction could be related to the release of CO 2 from the precipitation of carbonates in the soil profile during drying cycles (Emmerich, 2003).…”