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Science Results (Summer 2005)
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Tracing sediment dynamics and sources in eroding rills with rare earth elements


T.W. Lei                      China Agricultural University

Q. W. Zhang                China Agricultural University

J. Zhao                         China Agricultural University

M. A. Nearing              SouthwestWatershedResearchCenter


Rates of soil erosion are quite variable from place to place in the field.  In order to most effectively develop plans for conserving soil we need to know where on the field the erosion is most critical. Some of our models of soil erosion give conservation planners estimates of this variability (of where the greatest erosion occurs in the field), but data to verify such models is severely limited.  The purpose of this study was to develop a method whereby sediment particles could be tracked so that we can identify where the particles originate, how far they move, and which ones leave the field.  Thus we developed a method using special chemicals called Rare Earth Elements, which bind strongly to soil and sediment particles and can be measured in sediment and soil samples.  This kind of data is unique in the world and an important advance for the science of soil erosion and conservation.  The Rare Earth Element technique has been applied previously on plots and small watersheds.  In this study we applied the method to individual rills in order to help understand the basic processes of soil erosion.  This will help us to develop better technologies for both predicting soil erosion and for developing measures to control erosion in order to protect this valuable resource for use by future generations to produce food and fiber.


Watershed monitoring and research at the Walnut Gulch Experimental Watershed, Arizona, USA.


M.H. Nichols               SouthwestWatershedResearchCenter

M.A. Nearing               SouthwestWatershedResearchCenter


Understanding of hydrology and sediment movement is important to all areas of the world, but none more so than in arid and semi-arid regions where water is a scarce resource.  In 1953 the USDA initiated an outdoor laboratory in southeastern Arizona near Tombstone with  the stated purposes of understanding the effects of conservation projects on water yields and sediment movement, and to better understand hydrologic processes in semi-arid regions.  This 50 square mile watershed is called Walnut Gulch, and it has been heavily instrumented over the years to help address these and other questions.  Much of the understanding that we have of semi-arid hydrologic systems come from the research that has been collected over the years in this facility.  Concepts such as A) the dominance of infiltration of runoff into channels that is the major contributor to groundwater recharge, B) the percentage of rainfall that is available to recharge groundwater aquifers, and C) the effects of vegetation changes on runoff and erosion were all developed largely at this facility.  The implications of this research are enormous.  The knowledge that has been gained here is used by water resource planners in government and business on a daily basis to help us live and manage our water resources in the semi-arid western rangeland environment.


Amino acid enrichment following soybean in an Iowa corn-soybean rotation.


D.A. Martens               SouthwestWatershedResearchCenter

D.B. Jaynes                  National Soil Tilth Laboratory

T.S. Colvin                   National Soil Tilth Laboratory

T.C. Kaspar                 National Soil Tilth Laboratory

D.L. Karlen                  National Soil Tilth Laboratory


Soil nitrogen (N) is composed of an organic fraction and an inorganic or mineral N fraction. The mineral fraction can account for up to 5% of the total soil N with the remaining 95%, an organic fraction composed of amino acids, amino sugars and unidentified N compounds. Understanding the amount of organic N that cycles to mineral N would provide scientists a way to predict the amount of N the soil could provide, potentially limiting the over application of N fertilizers. Ten Iowa soils from a privately managed 16 ha field were sampled each spring and fall from 1997 - 1999 and the surface 15 cm soil was prepared and analyzed for organic N composition. The amount of organic N following corn production decreased 255 kg N ha-1 while the soil organic N content increased following soybean production by 247 kg N ha-1. The spring total amino acid N content and certain microbial amino acids were significantly correlated with fall corn yields suggesting that understanding the soil organic N content and production rotation may provide an important tool the producer may use to reduce N fertilizer additions. The benefits may include reduced N costs to producers and reduced N nutrient pollution in the environment while not impacting economic yields.


Comparison of responses from multiple soil moisture sensors installed in a semi-arid shrub dominated rangeland site.


G.P. Paige                    University of Wyoming

T.O. Keefer                 SouthwestWatershedResearchCenter


Soil water is an important feature for agriculture management, water resource engineering and the science of hydrology. Soil moisture, the amount of water held in the soil as a percentage of the volume of the soil, can be difficult to accurately measure, especially in rangeland soils such as those covering much of the semi-arid western US. There are several distinct types of soil moisture sensors which are in general use and which measure soil moisture adequately in specific types of soils, especially those conducive to agricultural crops. However, adapting those sensors to be used in rangeland soils can be challenging because of high rock content and low moisture contents. Three different types of soil moisture sensors were compared to each other and to the results of two different types of computer models in semi-arid rangeland conditions in southeastern Arizona.  The sensors were installed at shallow depths to monitor soil moisture conditions for an eighteen month period, including many sequences of wetting due to precipitation, infiltration into the soil and subsequent drying of the soil. The sensors all measured responses to precipitation events, but were very different in the percent change in moisture and the timing of the response. The model results indicate that no one of the sensors was significantly better than the others on an event basis, nor were the sensors better than the models in determining the amount of moisture in the soil during individual events or over seasons.


Ecosystem water use efficiency in a rangeland shrub and grass plant


W.E. Emmerich            SouthwestWatershedResearchCenter


Plant uptake of carbon dioxide from the atmosphere to reduce the atmospheric concentration is tied to the ecosystem water use efficiency (EWUE) of different plant communities.  EWUE is a measure of the amount of carbon dioxide taken up to the amount of water lost through evapotranspiration (ET).  Knowing the EWUE of plant communities is important to the understanding of how they take up carbon and interact.  The uptake of carbon dioxide and the loss of water was measured at a shrub and grass site in Arizona.  The grass community was 1.4 to 3.6 times more EWUE in taking up carbon dioxide into the plant biomass.  Rangeland managers can use this information to help reduce the carbon dioxide concentration in the atmosphere to reduce the tremendous cost to society caused by global warming.


Inferences of all-sky irradiance using terra and AGUA  MODIS and GOES-10 satellite data


R. Houborg                  University of Copenhagen

W. E. Emmerich           SouthwestWatershedResearchCenter

M.S. Moran                 SouthwestWatershedResearchCenter


Solar radiation is the driving force for the world energy and carbon dioxide budgets.  Being able to estimate solar radiation continuously at all point on the earth surface will greatly increase our ability to understand climate and associated climate change.  Presently we have estimates from individual measurement points.  This work was able to take satellite remote sensing data that is available for entire earth surface and through modeling make close estimates to the actual measurements made for solar radiation at many locations in southern Arizona.  This result will be of great benefit to scientist studying climate to allow for much better climatic models and to make better predictions of climate change.  Society will benefit from knowing how climate is going to change and being able to make decisions on adjustments to coming climate change.


Empirical analysis and prediction of nitrate loading and crop yield for a corn-soybean rotation


R. Malone                    Agricultural Land and Watershed Mgmt. Research

L. Ma                           Great Plains Systems Research

D.L. Karlen                  Soil and Water Quality Research       

T.G. Meade                 Agricultural Land and Watershed Mgmt. Research

P. Heilman                    SouthwestWatershedResearchCenter

R.S. Kanwar                IowaStateUniversity

J.L. Hatfield                  National Soil Tilth Research Laboratory


Crop yield and nitrate nitrogen losses through subsurface drainage are determined by multiple climatic and management variables; however the interactive affect of these variables is not well understood.  Simple equations that predict nitrate loading and crop yield as a function of important variables may improve our understanding of agricultural systems.  Therefore, we developed regression equations to predict crop yield, nitrate concentration, drainage volume, and nitrate loading from a corn and soybean rotation in response to rainfall amount, N source, N rate, and timing of N application in northeastern Iowa.  The regression equations were then used to analyze nitrate leaching and crop yield under a variety of N management and climate scenarios.  The regression equations improve our understanding of variable interactions on nitrate leaching, offer a simple method to quantify potential N losses from Midwestern corn-soybean rotations, and are a step toward development of easy to use N management tools.  This work will initially help scientists develop easy to use N management tools.  Also, this work will eventually help decision-makers and farmers design farming practices that reduce nitrate leaching to shallow groundwater and tile drains while maintaining crop production goals.


AGWA design documentation:  Migrating to ARCGIS and the internet


A. Cate                        University of Arizona

D. Semmens                 U.S. EPA

I. Burns                        University of Arizona

D.C. Goodrich             SouthwestWatershedResearchCenter

W.G. Kepner               U.S. EPA


Planning and assessment in land and water resource management are evolving from simple, local-scale problems toward complex, spatially explicit regional ones.  Such problems have to be addressed with spatially distrubuted models that can compute runoff and erosion over a range of watershed sizes and lengths of time.  The extensive data requirements and the difficult task of building model input files, however, have long represented an obstacle to the timely and cost-effective use of such complex models by resource managers.  In addition, to evaluate mangment practices and their impacts on water quality, land and resource managers need to describe and simulate the impacts of land use and best management conservation practices on watershed response.  The USDA-ARSSouthwestWatershedResearchCenter, in cooperation with the U.S. EPA-Office of Research and Development and the University of Arizona, have developed the Automated Geospatial Watershed Assessment (AGWA) Geographic Information System (GIS) tool to facilitate the distributed hydrological modeling process.  This document serves as a design to move the capability of the AGWA modeling tool to an internet based environment.  the new tool is deemed DotAGWA.


Sensitivity of riparian ecosystems to moisture pulses in semiarid environments


D.G. Williams               University of Wyoming

R.L.Scott                     SouthwestWatershedResearchCenter

T.E. Huxman                University of Arizona

D.C. Goodrich             SouthwestWatershedResearchCenter

G. Lin                           ChineseAcademy of Sciences


How the structure and functioning of riparian vegetation in arid and semiarid basins depends on different hydrological processes is not well understood.  In order to begin to better understand these dependencies, this paper brings together the findings of various studies conducted along the SanPedroRiver in southeastern Arizona to show that rooting depth and access to groundwater are key factors that control the vegetations water use and carbon dioxide exchange.  Depth to groundwater, which varies substantially across the riparian landscape, is a key factor controlling the sensitivity of cottonwood (Populus fremontii) transpiration, leaf photosynthetic metabolism, and plant water sources to pulsed inputs of growing season moisture. Because mesquite (Prosopis velutina) accesses groundwater in these habitats, ET and gross ecosystem production are decoupled from precipitation. But ecosystem respiration remains highly coupled to rainfall due to the dominant contribution of litter and bulk soil organic matter decomposition. Responses of net ecosystem exchange of carbon dioxide to rainfall variability in riparian floodplain is therefore not simple, but depends on vegetation structure and the connection of dominant plants to the water table. The complex vegetation patterns, hydrologic setting and disturbance dynamics in the riparian landscape offer unparalleled opportunities to investigate fundamental processes linking water and carbon exchange.


Organic forms of nitrogen


D.C. Olk                                  Soil and Water Quality Research

D.A. Martens                           SouthwestWatershedResearchCenter


Nitrogen (N) is essential to crop growth.  Most nitrogen in the soil is bound in stable organic molecules.  Some of this organic nitrogen is slowly released from the organic molecules as inorganic forms that are immediately available to the crop.  This inorganic pool is a vital supply of nitrogen to a growing crop.  Identification of the chemical structures of the organic nitrogen molecules would allow better understanding of the gradual release of organic N and contribute to improved management of this large nutrient pool.  However, current laboratory analyses allow identification of only half of all organic nitrogen molecules in soil.  A newly developed laboratory analysis identifies more than 80 percent of all organic nitrogen molecules in soil.  This report explains the steps and analytical instruments that are involved in the new procedure.  Use of this procedure will enable scientists to better understand the factors of nitrogen cycling in the soil, possibly leading to more precise fertilizer rates and less environmental contamination by excess nitrogen.


Speciation of selenium (IV) and selenium (VI) using coupled ION Chomatography - Hydride generation atomic absorption spectrometry


S.R. Goldberg                          Soil and Water Chemistry Research

D.A. Martens                           SouthwestWatershedResearchCenter

H.S. Forster                             Soil and Water Chemistry Research

M.J. Herbel                              UC Riverside


Selenium is an essential trace element that can be toxic to animals at elevated concentrations.  It occurs in two oxidation states, selenium(IV) and selenium(VI).  The selenium(IV) oxidation state is considered to be more toxic than the selenium(VI) state.  For this reason, analytical methods must differentiate between the two states.  A rapid continuous flow-through method has been developed to quantitate selenium according to oxidation state at the part per billion level of concentration.  Since selenium may be present in elevated amounts in soil solution and drainage waters, its accurate analysis is important.  This method can be used by researchers, action and regulatory agencies to identify waters high in selenium, as well as to identify the dominant selenium oxidation state.