Heterogeneous spatial and temporal distributions of soil resources important to plant growth have been documented in the sagebrush steppe ecosystem. There can exist as much variability in soil resources within the root zone of individual plants as exists across an entire field. The objective of this dissertation research was to evaluate how plants respond to, utilize and influence the spatial heterogeneity of soil resources. The three specific sets of questions addressed are outlined in the three main chapters of this dissertation. My first study addressed how the number and concentration of phosphorus (P) patches in the root zone of an individual Artemisiaplant influenced the ability of the plant to increase root P uptake capacity from the enriched patches as compared to roots from unenriched soil. I found that root uptake kinetics in the most enriched patches in general was not limited by the number or concentration of phosphorus patches experienced by the plant. However, the plants could modulate the quantity of P acquired from a target patch as the number of patches experienced increased. My second study addressed how six species common to the Great Basin, which represent three different growth forms, utilized nitrogen (N) from patches or a uniform distribution. The two species within the two perennial growth forms, shrub and tussock grass, revealed different capacities for acquiring N from concentrated patches immediately adjacent to a plant and from N applications at a distance from plants. This suggests the potential for different root foraging behavior. The two annual species used concentrated N patches more effectively than uniform applications. My third study described decimeter scale variability of soil water potential (Ψs) in the interspace of two perennial plants at different time scales and at different soil moisture conditions. The mean Ψs was more spatially consistent in the interspace between plants during a midsummer dry period compared to an early summer period. Diel Ψs fluctuations during an early summer dry period was more spatially consistent than a midsummer dry period. When soil moisture was recharged by precipitation there were no spatial diel patterns and the mean Ψs was autocorrelated across the area evaluated.

Natural resource conservation is important for human well-being, especially in fragile environments of developing countries. This study occurred in 2006 among 6,500 smallhold farmers residing along a 25-km segment of a heavily utilized river. Research objectives were to determine use and adoption constraints for 14 soil and water conservation practices (SWCPs). Farms were reportedly contributing to a decline in river water quality via soil erosion. Recent occupation of the upper watershed by immigrants magnified concerns that resource degradation could escalate. A multi-method approach incorporating quantitative surveys, qualitative interviews, and participant observation was used to interpret constraining factors within the biophysical and historical context of the watershed. Adoption rates for SWCPs were expected to be low (less than 20 percent). Increased formal education, income, access to information, and security of land tenure and soil characteristics, were expected to positively influence adoption. Data analysis included descriptive statistics and use of classification and regression trees. Results indicated that all sampled farms had adopted at least two SWCPs, with an average of six per farm. Favored practices were those that were easier to implement and more effective for resource protection and food production. Years in residence (tenure security) and income emerged as primary explanatory variables for adoption of SWCPs, while soil quality and formal education were secondary. Only 27 percent of surveyed farmers held title deeds, but the others perceived that land occupation conferred "ownership" and hence implemented SWCPs. A follow-up visit in 2009, after the region had endured a year of highly publicized ethnic conflict, immigration and farm expansion continued with SWCPs being adopted. Njoro communities mostly remained intact and appeared resilient. While small farms likely contribute to watershed-scale problems and declines in quality and quantity of water in the River Njoro, farmers have made remarkable strides--largely on their own--to conserve natural resources. Future research should examine how a general lack of infrastructure off-farm and study-site context contributes to reduced watershed-resource quality. Further protection of soil and water is best served by a more aggressive policy and extension education framework that links food security, household well-being, and natural resource management.

Agricultural yields in Northwest Yucatán are constrained by climate, thin soils, and low fertility. Despite this, the ancient Maya city of Chunchucmil Yucatán, Mexico, boasted an immense, dense, and wealthy population during the Middle Classic period (ca A.D. 400-700). Soil physical and chemical properties were explored to determine how the ancient Maya of Chunchucmil fed themselves. Soil profiles were collected from various locations within ancient Chunchucmil's suspected sustaining area. The physical and chemical properties, carbon isotopes, black carbon, and coprostanols of soil profiles sampled were compared to ancient rural settlement and remotely sensed images, such as AIRSAR (airborne synthetic aperture radar). Our objectives were to geographically determine the areas of agricultural importance and determine whether evidence of ancient agricultural intensification could be observed in the surrounding soil resources of Chunchucmil. Indigenous Maya of the area identify three major soil classes, boxlu'um, saklu'um, and kancab. The ancient Maya likely preferred kancab because it provided some security with higher soil moisture, greater soil depth, and improved nutrient availability. The land use capability is severely limited in the swamp/estuary and tzekel. The lack of rural settlement within these zones suggests that they were not used for cultivation in ancient times; however, the wood resources likely provided Chunchucmil with vital raw materials. The carbon isotopic signature of ancient C4 crops was not detected suggesting that either maize was not extensively produced or that the mix of native C3 and C4 plants in the savanna hid the signature. There were no soil chemical or biomarker evidences of ancient agricultural intensification, suggesting that ancient agriculture was mainly based on shifting cultivation at Chunchucmil. Concentrations of black carbon, calcium, phosphorus, potassium (Olsen Method), magnesium, and organic carbon within urban and rural settlements were enhanced by incidental human activities. We determined that the land requirement would have been extensive to sustain the population of Chunchucmil during the Middle Classic based on traditional agricultural methods. The ancient Maya of Chunchucmil likely traded marine and estuary products from the Gulf coast and other high value trade items for agricultural products from the nearby Puuc Hills.