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Title: Non-Linear Responses to Precipitation and Shrub Encroachment in Semi-Arid Grassland: Isotopes and CO2 Fluxes Reveal Soil Microsite Alteration as Explanation 1875

item CABLE, J.
item SUN, W.
item OGLE, K.
item POTTS, D.
item Scott, Russell - Russ
item HUXMAN, T.

Submitted to: Trans American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 10/1/2006
Publication Date: 12/11/2006
Citation: Cable, J.M., Sun, W., Ogle, K., Williams, D.G., Potts, D.L., Scott, R.L., Huxman, T.E. 2006. Non-Linear Responses to Precipitation and Shrub Encroachment in Semi-Arid Grassland: Isotopes and CO2 Fluxes Reveal Soil Microsite Alteration as Explanation. Eos. Trans. AGU, 87(52), Fall Meet. Suppl., Abstract B33C-1201.

Interpretive Summary:

Technical Abstract: Responses of net ecosystem production (NEP) to growing season rainfall amount is non-linear over a gradient of woody-plant encroachment in semi-arid riparian grassland. NEP is positively correlated with growing season precipitation amount in the grassland, but is negatively correlated with precipitation amount in a former C4 grassland now occupied by large mesquite (Prosopis) individuals. NEP at sites with intermediate stages of mesquite encroachment have a complex, threshold response to precipitation amount. Mesquite encroachment creates patchy soil microsites and spatial variation in rooting depth and activity. We hypothesized that variation in soil microsite properties (e.g., temperature, labile carbon) and root activity affect soil CO2 efflux in such a way that explains the non-linearity in response of NEP to precipitation. We measured soil CO2 efflux during the dry pre-monsoon (early summer) and wet monsoon (mid summer) periods on old floodplain terraces along the San Pedro River in southeastern Arizona. We made intensive spatial and temporal measurements of soil CO2 flux in four microsites associated with woody-plant encroachment: inter-canopy space and beneath the canopies of grasses, medium mesquite, and large mesquite. We also measured the '13C of soil-respired CO2, which provided insight into the contribution of different sources (e.g., roots vs. microbes) to soil CO2 efflux. Soil respiration was highest beneath large mesquite near the canopy center, and lowest beneath medium mesquite and in inter-canopy spaces. The '13C data revealed that soil respiration was dominated by a C4 signal during the pre-monsoon, but it switched to being dominated by the C3 mesquite signal during the wet monsoon period. Respiration was most sensitive to precipitation inputs beneath the large mesquite, where labile carbon in the form of mesquite litter is readily available. Conversely, soil respiration was least sensitive to precipitation in the open, inter- canopy space, where microbial respiration is expected to exceed root respiration. Spatial and temporal variation in root and microbial responses to precipitation suggest that root activity patterns and heterogeneous soil microsites contribute to the non-linearity between ecosystem production and seasonal precipitation. To develop a more process-based understanding of this non-linear phenomenon, we are developing a Bayesian inverse model that couples the respiration and isotope data, spatial variability in soil microsite properties, and models of root and microbial respiration. The inverse model is being used to explore the importance of small- scale processes (e.g., plant and microsite fluxes) to emergent properties at larger-scales (e.g., ecosystem dynamics along the encroachment gradient).