Location: Soil, Water & Air Resources Research2020 Annual Report
Objective 1: Quantify the water and light use efficiency of corn-soybean and other cropping systems using a range of management practices (cover crops, tillage, N fertilizer, shelter) relative to carbon and water dynamics throughout the year. Objective 2: Evaluate the effectiveness of microclimates modified by agroforestry practices on production efficiency of row crop and silvopasture systems.
To fulfill the objectives of this project there are four major research projects: 1) comparison of energy and C exchanges between cover crop and reduced tillage corn-soybean systems compared to conventional systems, 2) comparison of the effect of increased air temperature and soil water availability on wheat growth and productivity, 3) evaluation of the effect of tree windbreaks on crop performance and energy exchanges compared to rainfed and irrigated cropping systems, and 4) comparison of the water and light use efficiency in pasture systems grown under silvopasture and conventional pasture. The research approach utilizes surface energy balance methods to quantify differences among management practices or microclimate modifications. These data are then used to estimate the water use and gross and net ecosystem productivity using daily values across the growing season with a direct contrast of cumulative water and carbon fluxes over a year and over portions of the year to represent different aspects of management systems. Studies on spring wheat will be conducted in the NLAE rhizotron to quantify the effect of increasing minimum air temperatures on phenological development, biomass, and grain yield components under a range of soil water conditions. The windbreak experiment involves a direct comparison of energy balance, biophysical properties, and productivity of rainfed and irrigated crops with rainfed crops protected by a windbreak at the Eastern Nebraska Research and Extension Center. A silvopasture research site in Fayetteville, Arkansas consists of rows of five tree species with orchardgrass in the alleys that is used for grazing and hay. Eddy covariance fluxes will be compared with Bowen ratio and surface renewal estimates in both agroforestry studies. Forage height, biomass, and leaf area index will be measured before each grazing event. Biomass produced and cumulative crop water use from the onset of growth or since the last grazing event will be used to calculate water use efficiency. These objectives focus on components of agricultural systems, provide a suite of observations on a common set of measurements to quantify carbon and energy exchanges, and lead to the direct comparison of water use efficiency and radiation use efficiency of these different systems. One critical aspect in this integration is the collaboration with crop modeling programs to evaluate how crop simulation models can be improved for these management alternatives.
Objective 1 Hypothesis 1.1. Conventional management of corn and soybean production in Iowa typically includes tillage prior to planting, fertilizer and pesticide application(s), and additional tillage after corn harvest to incorporate the substantial amounts of crop residues into the soil to promote mineralization. Due to the large extent of corn–soybean rotations in Iowa, changes in crop management may substantially influence water and light use efficiency of agricultural land at the regional scale. Water and light use efficiency are defined as the amount of water used (i.e. evapotranspiration) and radiation absorbed per unit of Gross Primary Production (GPP). Since most of the agricultural land in Iowa grows corn and soybean, a small but widespread change in management practices may substantially increase or decrease carbon and water cycle components, or impact radiation dynamics (absorption or reflection of sunlight). Studies were conducted in central Iowa on two sites, each with two adjacent farmer-managed fields with each phase of the corn–soybean rotation present. The first site is under conventional crop and soil management, while the second site is part of the Upper Mississippi River Basin Long-Term Agroecosystem Research Network (LTAR) of USDA-ARS, where “aspirational” crop management strategies are investigated under field conditions compared with conventional management. All four fields were equipped with eddy-covariance stations and recorded carbon and water fluxes, and incoming radiation. Water use efficiency in the aspirational crop management system was not improved as evapotranspiration was similar in both cropping systems, while GPP was lower in the aspirational system. This was due to reduced respiration in the aspirational system. Long-term study of the conventional site from 2006-2015 showed that there also differences in water- and light use efficiency among crops that depend on the weather conditions and season. Data collected within the eddy flux stations of conventional and “aspirational” managed fields will be incorporated into the biogeochemical model, DayCent. The combination of weather station data and sensors will be used to improve mechanistic capabilities to simulate production and water and nutrient fluxes in response to different cropping systems and climatic scenarios. Objective 1 Hypothesis 1b. Climate trends indicate that increasing average temperatures are driven by rising daily low temperatures rather than rising daily high temperatures. Additionally, precipitation trends suggest that rainfall events will be less consistent but greater in intensity. Controlled environment chambers (rhizotrons) were used to compare the phenological development of maize, soil respiration, and soil water interactions under two temperature regimes: 30-year average Iowa temperatures for each 24 hour cycle and 30-year average Iowa temperatures during daytime with nighttime temperatures increased 3 degrees C. Soil monoliths were maintained at a moisture content to exert water stress on the maize by providing either too little or too much water. Maize development was monitored weekly until harvest, while soil respiration, soil water content, and temperature were measured continuously throughout the growing cycle. One growing cycle has been completed of this study, with one additional growing cycle planned to provide study replication. Upon completion of data collection after the second cycle, the data will be incorporated into the Agricultural Production Systems Simulation (APSIM) program to improve performance in estimating the impacts of changing climate and water regimes on crop development and productivity. A second rhizotron experiment compared phenological development of maize under imposed water stress conditions (same as in experiment 1) but with stress conditions applied at an early vegetative stage. Maize development was monitored weekly, while soil water content and temperature were measured continuously throughout the experiment. After imposing water stress at the early vegetative stage, plant samples were collected, and analyzed using RNA-sequencing to explore differences in gene expression. Three cycles using maize plants have been completed for this study, with additional runs planned using soybeans. Data collected in these runs can be used to identify water stress-related genes that could be used for selection of water-stress traits. The data from these experiments will be incorporated into the biogeochemical model, DayCent, to improve performance in simulating water stress response on plant production. Objective 2 Hypothesis 2. Enhancements of micromet measurement systems at both Mead, Nebraska (tree windbreak) and Fayetteville, Arkansas (silvopasture) field sites have continued. A flux station and supporting microclimate sensors were installed in the open pasture at Fayetteville to allow direct comparison with sensors deployed in the silvopasture. Data analysis includes an analysis of the livestock heat stress benefits of the shade in the silvopasture. Collaborators on the Fayetteville project have been instrumental in assisting with site management and sensor maintenance at this location that is over 350 miles from Ames. Over 100 sycamore and cottonwood trees were cut in a planned thinning operation to enhance tree growth and improve aerodynamics for the eddy covariance flux station. Aboveground biomass and carbon were measured on the trees removed to enable estimates of carbon sequestration by the remaining trees as they grow. Yield of pecan nuts was measured on one tree in each plot in the fall of 2019 to determine if fertilizer treatments were affecting nut yield and to provide additional information on the economic viability of silvopasture systems in the Ozarks. The full complement of micromet sensors and supporting equipment were installed at Mead for the 2019 growing season. Preliminary analyses indicate relatively small differences in air temperature, relative humidity and other microclimate parameters with distance from the tree windbreak. Crop yield and water use comparisons between the sheltered and open site operated by University of Nebraska-Lincoln collaborators is underway. Wind direction and speed have a large impact on all parameters necessitating careful data screening to identify periods of similar wind conditions for assessment of the windbreak effect. Eddy covariance data collection and analysis at both sites continues and has focused on footprint analysis, energy balance closure, and comparison of sensible heat flux measured by eddy covariance and estimated using the surface renewal technique. As the trees at both sites represent air flow obstructions for the sonic anemometer of the eddy covariance system, a careful analysis of air flow is underway to determine when conditions occur that result in flow distortion and errors with the eddy covariance technique.
1. Carbon sequestration in silvopasture systems. Agroforestry systems (AFS) have the potential to foster long-term carbon sequestration and nutrient uptake. The rate and amount of carbon and nitrogen uptake in a 17-year-old northern red oak (Quercus rubra)–pecan (Carya illinoinensis) silvopastoral planting in Fayetteville, Arkansas, was measured. Seven oak and pecan trees were felled to develop AFS-specific allometric equations to enable estimation of above-ground biomass, carbon, and nitrogen content. Tree-stand woody biomass, carbon and nitrogen and leaf biomass, carbon, and nitrogen were calculated with these equations. Diameter at 1.37 m above ground (diameter at breast height, DBH) was measured annually, and a non-linear mixed-effect model was used to estimate absolute (AGR) and relative growth rates. Woody biomass and its carbon content were 7.1 and 3.4 Mg ha-1 for pecan and 26.6 and 12.7 Mg ha-1 for oak, which corresponds to carbon sequestration rates of 0.20 and 0.75 Mg C ha-1 yr-1, for pecan and oak, respectively. Total N uptake was approximately 66 and 71 g N tree-1 yr-1 for oak and pecan. The mixed effect model with individual-tree-level random effects for all parameters provided the best representation of DBH growth of oak and pecan, likely due to the high heterogeneity of site characteristics. The AGR explained the non-linear plant growth and reached its maximum of 0.017 and 0.0179 m yr-1 for oak and pecan, respectively, 11 years after planting. This study reveals the dynamics of tree growth rates and carbon accumulation in an agroforestry setting. Such information is important to scientists interested in accurate estimates of tree growth in situations with wide tree spacing and for landowners interested in combining timber and animal production on the same land.
2. Changes in profile soil properties in tree windbreaks of the Great Plains. Agroforestry systems such as tree windbreaks became a common practice in the U.S. Great Plains following a large tree planting program during the Dust Bowl of the 1930s. Tree windbreaks combine the potential to increase biomass and soil carbon (C) storage while maintaining agricultural production. However, our understanding of the effect of trees on soil organic carbon (SOC) is largely limited to the upper 30 cm of the soil. Research was conducted to examine the impact of tree plantings ranging in age from 15 to 115 years on SOC storage and relevant soil properties. ARS researchers in Ames, Iowa, and university collaborators quantified SOC stocks to 1.25 m depth within eight tree plantings and in the adjacent farmed fields within the same soil map unit. Soil samples were also analyzed for inorganic carbon, total nitrogen, pH, bulk density, and water stable aggregates. Averaged across 8 sites in North Dakota, South Dakota, Nebraska, and Kansas, SOC stocks in the 1.25 m soil profile were 16% higher beneath trees than the adjacent farmed fields. Differences ranged from 10.54 to a – 5.05 kg m-2 depending on the site, climate, and tree species and age. The subsurface soils (30-125 cm depth) beneath trees had 7% greater SOC stocks than the surface 30 cm (9.54 vs. 8.84 kg m-2), respectively. This finding demonstrates the importance of quantifying C stored at deeper depths under tree-based systems when tree SOC sequestration is being assessed. Overall, our results indicate the potential of trees to store C in soils and at deeper depths in semi-arid climates, a finding that provides landowners and agency personnel with information needed to guide land use practices for soil health and climate change mitigation.
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