[1] The goal of this study is to understand the interaction between belowground and aboveground ecohydrologic dynamics as facilitated by hydraulic redistribution. We analyze the partitioning of moisture and energy between tall and understory vegetation, and soil evaporation. Both the competitive and facilitative dependencies are examined using a shared resource model where the soil serves as a common reservoir for the interaction between the different vegetation species. The moisture state of the reservoir is altered by the addition and withdrawal by vegetation roots in conjunction with soil-moisture transport. Vertical patterns of soil moisture state and uptake reflect the nonlinear interactions between vegetation species. The study is performed using data from the Blodgett Forest Ameriflux site in the Sierra Nevada Mountains of California. The Mediterranean climate of the region, with wet winters and long dry summers, offers an ideal environment for the study. The results indicate that deep layer uptake of water by the tall vegetation and its release in the shallow layers enhances the productivity of the understory vegetation during the summer. The presence of understory vegetation reduces direct soil-evaporative loss making more moisture available for vegetation which enhances the total ecosystem productivity. The litter layer is also found to play an important role in the partitioning of the water and energy fluxes by damping the radiation reaching the soil and thereby reducing water loss due to soil evaporation.

[2] Surface and subsurface moisture dynamics are strongly influenced by the ability of vegetation to take up and redistribute soil moisture using hydraulic redistribution (HR). These dynamics in turn affect soil biogeochemical cycling through controls on decomposition and mineralization rates and ion transport. The goal of this study is to explore this coupling between HR and biogeochemistry using a numerical model. We examine decomposition and mineralization of organic matter and analyze whether differences in decomposition rates induced by HR influence the long-term storage of carbon in the soil and the movement of nitrate (NO À 3 ) and ammonium (NH þ 4 ) in the rhizosphere. These dynamics are studied in a framework that incorporates the interaction between multiple plant species. The net effect of HR on decomposition is controlled by a trade-off between the resultant moisture and temperature states. This trade-off is conditioned by the availability of fine roots near the surface, and it impacts the long-term storage and vertical distribution of carbon in the soil. HR also impacts the transport and uptake of ions from the soil. It reduces the leaching of nitrate considerably, and, therefore facilitates the uptake of nitrate by vegetation roots. Furthermore, the magnitude and patterns of the feedbacks induced by HR are also influenced by the presence of different plant species that coexist. These results suggest that the alteration of soil moisture by plants through associated processes such as HR can have considerable impact on the below-ground biogeochemical cycling of carbon and nitrogen.

Hydraulic redistribution, a process by which vegetation roots redistribute soil moisture, has been recognized as an important mechanism impacting several processes that regulate plant water uptake, energy and water partitioning, and biogeochemical cycling. We analyze how the magnitude of hydraulic redistribution varies across ecosystems that are exposed to different climates and seasonal patterns of incoming shortwave radiation and precipitation. Numerical simulation studies are performed over 10 Ameriflux sites, which show that hydraulic redistribution predictions are significantly influenced by the specified root hydraulic conductivities. We performed sensitivity analyses by considering expected ranges of root conductivities based on previous experimental studies, and found contrasting patterns in energylimited and water-limited ecosystems. In energy-limited ecosystems, there is a threshold above which high root conductivities enhance hydraulic redistribution with no increase in transpiration, while in water-limited ecosystems increase in root conductivities was always associated with enhancements in both transpiration and hydraulic redistribution. Further we found differences in the magnitude and seasonality of hydraulic redistribution and transpiration across different climates, regulated by interplay between precipitation and transpiration. The annual hydraulic redistribution to transpiration flux ratio (HR/Tr) was significant in Mediterranean climates (HR/Tr 30%), and in the tropical humid climates (HR/Tr 15%). However, in the continental climates hydraulic redistribution occurs only during sporadic precipitation events throughout the summer resulting in lower annual magnitudes (HR/Tr < 5%). These results provide more insights for suitable implementation of numerical models to capture belowground processes in eco-hydrology, and enhance our understanding about the variability of hydraulic redistribution across different climates.

Because of increasing demands for bioenergy, a considerable amount of land in the midwestern United States could be devoted to the cultivation of second-generation bioenergy crops, such as switchgrass and miscanthus. The foliar carbon/nitrogen ratio (C/N) in these bioenergy crops at harvest is significantly higher than the ratios in replaced crops, such as corn or soybean. We show that there is a critical soil organic matter C/N ratio, where microbial biomass can be impaired as microorganisms become dependent upon net immobilization. The simulation results show that there is a threshold effect in the amount of aboveground litter input in the soil after harvest that will reach a critical organic matter C/N ratio in the soil, triggering a reduction of the soil microbial population, with significant consequences in other microbe-related processes, such as decomposition and mineralization. These thresholds are approximately 25 and 15% of aboveground biomass for switchgrass and miscanthus, respectively. These results suggest that values above these thresholds could result in a significant reduction of decomposition and mineralization, which, in turn, would enhance the sequestration of atmospheric carbon dioxide in the topsoil and reduce inorganic nitrogen losses when compared to a corn-corn-soybean rotation.

No abstract