Location: Soil, Water & Air Resources Research2019 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.
Manipulation of the canopy architecture will affect the distribution of light into the canopy. Analysis of the distribution of carbon dioxide within the canopy revealed the effect of canopy architecture on the movement of wind into the canopy. Considering the combined distribution of sunlight and wind into the canopy explains the dynamics of carbon dioxide exchange in lower leaves of the canopy. These findings show that we can change the overall canopy architecture and have a positive impact on plant growth. Surface soil temperatures in the early spring often exceed 40C. This occurs in fields where there is no residue and large amounts of soil water evaporation occur from the surface. The dry, hot soil surface conditions limit the biological activity at the surface and lead to unstable soil aggregates causing crusting. Observations in the 2019 spring with wet soil conditions followed by warm temperatures produced crusting in tilled fields but no crusting in fields with cover crops or crop residue greater than 50% cover of the soil surface. There was no significant soil erosion on these fields. Modifying the soil surface to protect against temperature and soil moisture extremes benefits the soil biology and the developing plants in terms of increased vigor. Observations from previous years across the Midwest have shown the importance of the soil water depth in the soil profile. Growing seasons with high water tables that extend into the late stages of vegetative growth have reduced yields when combined with less than normal rainfall in the remainder of the growing season. A study is underway to manipulate the water table depth in rhizotrons and soil columns to evaluate the molecular genetic response and overall growth response in terms of phenology, leaf area, and biomass. These studies are instrumental to quantify the response over a range of genetic material. Soybean yield in 2018 as for corn in 2017, was not significantly different between the open and sheltered sites in Mead, Nebraska. Additional analyses of yield maps are underway to investigate the spatial patterns of yield and relate those patterns to observed microclimate parameters. Persistent heavy rains and flooding have delayed installation of biophysical sensors at both windbreak and silvopasture research sites in 2019. The full complement of sensors and supporting equipment have been acquired and will be installed as soon as field conditions allow. Additional groundwater depth sensors were installed over the winter at the silvopasture site in Fayetteville, Arkansas, to better document groundwater depth fluctuation and water availability to trees and forage. Eddy covariance data collection and analysis at both sites is undergoing an upgrade to commercial software to enhance the efficiency of data processing and quality control.
1. Adoption of no-till and cover crops shifts carbon balance. Conversion of a conventionally tilled field to a cover crop no-till management system revealed that this change improves the carbon balance with minimal impact on the water balance. Previous research has shown that conventionally tilled corn-soybean fields had a negative carbon balance and reducing tillage and adding a cover crop shifted the carbon balance to a positive net ecosystem productivity. The primary factor responsible for this change was the reduction in soil disturbance. Intensive grid soil sampling in this field showed that after two years of cover crops and no-till, there was a doubling of the microbial biomass in the upper 15 cm of the soil profile. Understanding the coupling of carbon and water in agricultural systems provides a framework for quantifying how changes in agricultural management will have a positive or negative effect. This is providing producers with information on soil management practices that will enhance their soil to increase productivity across fields.
2. Assessing crop productivity with remote sensing and field validation. The Midwest region of the United States is dominated by corn and soybean production systems and the carbon dynamics of the region is dominated by the dynamics of these two crops. Observations of the carbon and water balance over corn and soybean are often made at local scales using different measurement techniques; however, the impact of these systems need to be evaluated at the regional scale. A study was conducted to determine whether measurements from field-scale instruments could be upscaled using remote sensing data obtained from satellite platforms. Products from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite were used to estimate the interception of the light by the corn and soybean canopies and subsequently used to determine the gross primary productivity of the crops. These results were compared with the direct observations from the measurements of the carbon balance in the field. The agreement was good for both the overall biomass and the grain yield of both crops. This methodology can be used with confidence to determine the regional scale gross primary productivity and grain yield. This method will allow for use of ground-based field observations in a regional context.
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