2013 Annual Report
1a.Objectives (from AD-416):
To determine the optimum N application rate that will minimize nitrate leaching and N2O emissions when these quantities are expressed on a yield-scaled basis, to determine how the timing of N fertilizer application (i.e., single pre-plant versus split application) affects yield-scaled rates of nitrate leaching and N2O emissions, and to compare how the above effects differ in long-term corn-corn versus corn-soybean.
1b.Approach (from AD-416):
A two-year field experiment will be conducted at the University of Minnesota Research Station in Rosemount in plots that have been maintained under both continuous corn and corn-soybean rotation with conventional tillage and natural drainage for the past 20 years. Each year, we will utilize long-term plots from both the continuous corn treatment and the corn-phase of the corn-soybean rotation. Each main plot will be divided into two fertilizer-timing treatments consisting of either (i) a single pre-plant application of granular urea occurring 1 to 5 days prior to planting, or (ii) two applications of urea, with half occurring at pre-plant (per above) and half side-dressed at the V6-V8 leaf stage. Immediately following each application, the urea will be incorporated into the soil using a cultivator. Each of the two fertilizer timing treatments will be divided into 5 different rate treatments. The N rates will be 40, 80, 120, 160, and 200 lb N per acre. All treatments (including the control) will be replicated three times each year. Immediately following planting the corn, water sampling devices will be installed 60 cm below the soil surface in each treatment, which generally corresponds with the bottom of the silt-loam soil layer. Water collecting in the tubes will be collected at 1- to 2-week intervals and analyzed in the lab for nitrate concentration. A water balance approach will be used to calculate the rate of water and nitrate leaching. Soil samples will also be taken at regular intervals during the growing season, once after harvest, and once in the spring, to determine nitrate concentration in the soil to a depth of 2 feet. Soil nitrate concentration will be used as a measure of the nitrate leaching potential. Emissions of N2O gas will be measured using chamber methods. The total amount of N2O emitted during the growing season will be estimated by integrating the results from each sampling event. Once the corn has reached physiological maturity, samples of grain and stover will be collected from each treatment to determine yield of grain, stover, and above-ground N yield in both grain and stover. The nitrogen fertilizer recovery efficiency (NFRE) will be determined by subtracting the total grain plus stover N yield in the zero N control from the N yield in each treatment and then dividing by the N application rate. The grain yield versus N rate data will be used to conduct a maximum return to N (MRTN) analysis which will determine the profitable N rate range. We will also construct graphical plots showing the yield-scaled nitrate leaching (or N2O emissions) on the vertical axis (y-axis) versus the N application rate on the horizontal axis (x-axis). We will use standard analysis of variance (ANOVA) statistical method to evaluate the effects of N rate and timing on nitrate leaching, soil nitrate concentrations, N2O emissions, grain yield, N yield, and NFRE.
A field experiment at the Rosemount, MN experimental station was successfully completed during the 2012 growing season and is being repeated during the current (2013) growing season. The experiment consists of 66 individual sub-plots where we are quantifying nitrous oxide emissions, nitrate leaching potential, grain yield, and nitrogen use efficiency under 6 rates of N fertilizer application, each applied both in a single pre-plant application and in three split applications distributed over the growing season, and with each set of treatments replicated in a monoculture corn and corn-soybean rotations which have been in place since 1991. Results from the first growing season showed that split N application resulted in greater N2O emissions than single N application, which is the opposite of what was expected. Similarly, split N application did not reduce soil nitrate leaching potential. These results may have been due to the prolonged dry conditions in late June and early July followed by rainfall events later in July. This appeared to cause a burst of mineralization, which combined with the 3rd split application to increase N losses. These results point out the potential difficulty of optimizing N fertilizer timing especially during prolonged dry periods. Waiting to apply N under wetter soil conditions in order to minimize volatilization losses may also stimulate nitrate and N2O losses. This progress contributes to meeting Objective 3 of our NP212 Project Plan which is to “Enable reduced nitrous oxide emissions from fertilized cropping systems through improved understanding of controlling mechanisms, as a contributor to the ARS Greenhouse Gas Reduction through Agricultural Carbon Enhancement network (GRACEnet).”