Location: Soil, Water & Air Resources Research
2023 Annual Report
Objectives
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.
Approach
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.
Progress Report
Objective 1 Hypothesis 1.1.
Manipulation of canopy architecture will affect the distribution of light into the crop canopy. An initial 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, the analysis explains the dynamics of carbon dioxide exchange in lower leaves of the canopy. These initial findings show that the overall crop canopy architecture can be modified to benefit plant growth.
Surface soil temperatures in the early spring often exceed 40C in the Midwest. This occurs in fields where no crop residues lie on the soil and large amounts of soil water evaporate from the surface. The dry, hot soil surface conditions limit 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 both soil biology and the developing plants in terms of increased vigor.
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 was under conventional crop and soil management, while the second site was 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 and compared with conventional management. At this site the aspirational management had reduced tillage with a cover crop following the main crop harvest. All four fields were equipped with eddy-covariance stations and continuously recorded energy, water, and carbon flow between the atmosphere and the crop canopy. Water use efficiency in the aspirational crop management system was not improved, as evapotranspiration was similar in both cropping systems, while gross primary production (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 with the eddy covariance stations of conventional and aspirational managed fields were incorporated into the biogeochemical model, DayCent, to improve the mechanistic capabilities to simulate production and water and nutrient fluxes in response to different cropping systems and climatic scenarios.
Objective 1 Hypothesis 1b.
Observations from previous years across the Midwest showed the importance of soil water depth in the soil profile. Growing seasons with high water tables that extend into late stages of vegetative growth had reduced yields when combined with less than normal rainfall in the remainder of the growing season. A study was conducted to manipulate the water table depth in rhizotrons and soil columns in order to evaluate the molecular genetic response and overall growth response in terms of phenology, leaf area, and biomass. These studies were intended to quantify the response over a range of genetic material. Controlled environment chambers (rhizotrons) were used to compare the phenological development of corn, soil respiration, and soil water interactions under two temperature regimes: (a) 30-year average Iowa temperatures for each 24 hour cycle, and (b) 30-year average Iowa temperatures during daytime with nighttime temperatures increased 3 degrees C. Soil monoliths were maintained at a set moisture content to exert water stress on the maize by providing either too little or too much water. Corn development was monitored weekly until harvest, while soil respiration, soil water content, and temperature were measured continuously throughout the growing cycle.
Two growing cycles provided study replication. Upon completion of data collection after the second cycle, the data were 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 also compared phenological development of corn under imposed water stress conditions as in experiment 1 but with stress conditions applied at an early vegetative stage. Corn 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 corn plants were completed for this study. Data collected in these runs were used to identify water stress-related genes that could be used for selection of water-stress traits. The data from these experiments were incorporated into the biogeochemical model, DayCent, to improve performance in simulating water stress responses of plant production.
A meta data analysis of climate trends indicated that increasing average temperatures are driven by rising daily low temperatures rather than rising daily high temperatures. Precipitation trends suggest that rainfall events will be less consistent but greater in intensity.
Objective 2 Hypothesis 2.
As preparatory activities at the tree windbreak site near Mead, Nebraska, corn grain yield in 2017 and soybean grain yield in 2018 and were found not significantly different between the open and sheltered sites. Later additional analyses of yield maps investigated the spatial patterns of grain yield and related them to observed microclimate parameters. A full complement of micrometeorological sensors and supporting equipment was installed. As preparatory activities at the silvopasture site in Fayetteville, Arkansas, more than 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. Groundwater depth sensors were installed to better document groundwater depth fluctuations and water availability to trees and forage. A flux station and supporting microclimate sensors were installed in the open pasture to allow direct comparison with sensors deployed in the silvopasture. Light interception measurements were begun to enable measurements of light use efficiency of the forage under the trees and at the open control sight. The sensors deployed also allowed calculation of a livestock heat stress index.
Yield of pecan nuts was measured on one tree in each plot in the fall of 2019 to determine whether fertilizer treatments affected nut yield and to provide additional information on the economic viability of silvopasture systems in the Ozarks. Eddy covariance data collection and analysis at both sites were upgraded to commercial software to enhance the efficiency of data processing and quality control.
At the Mead, Nebraska site, eddy covariance flux data, weather data, and other environmental parameters from the tree windbreak site assisted in the comparison of water use efficiency and light use efficiency by the corn and soybean crops. Crop grain yield and its spatial variability, together with water use efficiency were compared between the sheltered and open sites, with extra care taken in coordinating data collection and analysis with University of Nebraska staff who managed the open site. The precautions included development of a standard data collection protocol. Wind direction and speed have a large impact on all parameters; thus, data were carefully screened to identify periods of similar wind conditions, enabling true assessment of the windbreak effect. As the trees at both the Nebraska and Arkansas sites represent air flow obstructions for the sonic anemometer of the eddy covariance system, a careful analysis of air flow was conducted to determine when conditions occur that result in flow distortion and errors with the eddy covariance technique. Eddy covariance data collection and analysis at both sites focused on footprint analysis, energy balance closure, and comparison of sensible heat flux measured by eddy covariance and estimated using the surface renewal technique.
At the Mead, Nebraska windbreak site, preliminary analyses in 2019 indicated relatively small differences in air temperature, relative humidity and other microclimate parameters with distance from the tree windbreak. Light use efficiency for corn ranged from 2.17 to 2.82 grams of grain per mega Joule of intercepted photosynthetically active light, with the highest value farthest from the windbreak. For soybean, the range was from 0.53 to 0.88, with the highest value nearer to the windbreak. At the Fayetteville, Arkansas silvopasture site, data from the 2021 growing season indicated that forage canopy development proceeded rapidly early in the spring before haying or grazing. The livestock heat stress index was found to be more sensitive to wind speed than to shade effects, indicating that the benefit of shade is reduced if the tree canopy significantly reduces the wind speed.
Accomplishments
