Heterogeneity of soil properties and associated structure exert major controls on the storage and fluxes of water through the surface soils and provide habitat and resources for life in the critical zone - that part of the terrestrial biosphere extending from the bottom of the root zone to the top of the canopy. Also, soil structure and properties are not fixed, but are altered dynamically, in part by the organisms that inhabit it. The importance of soil properties in controlling resource availability and primary production formed the basis of the ecological optimality hypothesis (Eagleson, 1978) that biological processes alter soil properties over time in such a way that increases ecosystem productivity, e.g., by increasing the permeability of fine soils, and/or increasing the retention capacity of sandy soils. This project aims to understand the role of biologic alterations of soil hydrologic properties on soil moisture fluxes and the feedback with associated vegetation dynamics in water limited ecosystems. The objective is to test the overarching hypothesis that alteration of the soil environment by inputs of soil organic matter and the development of macropores increases the access of plants to water and nutrients and thus reduces the need for plants to expend carbon to access those resources. We will critically examine this hypothesis using a holistic ecohydrologic modeling framework, and will test model predictions of soil properties with observed field measurements in New Mexico and Arizona. The collection of chosen field sites, especially in Arizona, pairs granitic and schist-underlain catchments at three elevations representing desert shrub, oak woodland and ponderosa pine forest. Specifically, we will 1) develop understanding of biologically mediated alteration of soil properties in a small New Mexico watershed to establish a foundation for the interaction of ecohydrologic processes, 2) then incorporate biotic effects of soil properties into the CRS canopy-root-system model being developed as part of another NSF project (ATM 06-28687, PI Praveen Kumar), which has been validated in a number of humid sites, and which we now will validate using data from arid sites, including the New Mexico site, 3) generate predictions of hydrologic fluxes, vegetation dynamics and soil properties based on appropriate climate forcing and the new understanding gained, 4) validate these predictions using soil structure data and hydrologic data taken from sites located along several climo-topo-sequences, including one in the Catalina Mountains in Arizona, and 5) develop an interpretive framework using the concept of hydrologic regimes and available global data sets. The work will be conducted in partnership with the new NSF-funded Critical Zone Observatory at the University of Arizona (especially arid-land ecology specialists Travis Huxman, Scott Saleska and David Breshears).
Responses to Comments of Reviewer and Panel Summaries 1. Sampling strategy: We appreciate the comments of the reviewers about the lack of details on the field sampling. The co-PIs Peter Troch and Kathleen Lohse have a history of collaborating with pedologist Dr Craig Rassmussen of the University of Arizona, who has considerable experience in soil sampling and analysis in these arid systems. We will collaborate with Dr Rasmussen to design a comprehensive field sampling program as one of the first steps in the project. 2. Adequacy of the 1-D CRS model: We accept that 3-dimensional spatial patterns are very prominent and very important in arid soil and ecosystems. Hydrologically, lateral redistribution in these environments occurs mainly on the surface, driven by spatially variable soil properties and topography. However, once the soil is wetted, unsaturated zone fluxes are generally vertical, with little lateral redistribution, except through lateral roots. By separately modeling under-canopy and between-canopy patches on the landscape, while accounting for the surface redistribution effects on infiltration, we believe it will be possible to accurately capture the relevant hydrologic fluxes using the 1-D model. 3. Overlap with funded project ATM 06-28687 (PI Praveen Kumar): There is indeed an overlap with this ongoing project. The only overlap is however the use of the 1-D CRS model that has been developed as part of that project. However, due to the crucial role of this comprehensive 1-D coupled model of water, energy, carbon and nutrients, this overlap is inevitable and highly useful. As part of the previously funded project the model has been validated using flux data from Illinois, Duke Forest, and the Amazon. What remains is to implement and validate this model, and any needed refinements, using data from arid sites, including in New Mexico and Arizona. This will be done as part of the new project. 4. On separating the effects of differing microclimates on the north and south-facing hillslopes at the New Mexico site from species effects: Our aim is not to consider these hillslopes as different 'treatments' whose interaction with the soils can be compared. If we were, then clearly the different radiation inputs would be a confounding factor. However, we see an opportunity here: we consider that the vegetation and soils on the two hillslopes are different as a result of the different microclimates. In this broader perspective, we can therefore compare how their microclimates may have led to the dominance of contrasting vegetation types and strategies and their combined effect on consequent soil development. 5. Vivoni letter of support: we could not obtain this letter of support at the time of application because Dr Vivoni was in the middle of a move from New Mexico Tech to Arizona State University. We will endeavor to obtain this letter from him, which we will be forwarding to you in the next few days.
