Limiting Nitrogen Immobilization in Cover Crop Systems
Cover crops have significant potential to improve water quality by limiting erosion when row crops are not growing, and by limiting the loss of nitrogen by leaching. Yet widespread adoption has been limited, as the yield of subsequent-year corn crops may be negatively affected by cover crops. One reason for this is the potential for immobilization of nitrogen from soil organic matter as cover crop residues decompose.
This project will look at ways to fine-tune cover crop management so immobilization is unlikely and nitrogen can be mineralized for crop use. Two hypotheses will be tested. One is that limited nitrogen amendments to the soil at the time crop residues begin to decompose will promote rapid residue decomposition, yet limit immobilization of soil nitrogen. Two is the amount and composition of mineralized nitrogen will depend on the concentration of residues in the soil, the carbon-to-nitrogen ratios of the residues, and water content of the soil during decomposition.
This study will be based on soil and cropping systems in the current Comparison of Biofuel Cropping Systems (COBS) experiment at Iowa State University. The COBS project was established in 2007 to provide a large-scale, side-by-side comparison of several annual and perennial cropping systems, including continuous corn grown with and without a rye cover crop. This laboratory-based, soil incubation experiment involves the collection of soil samples and root residues from eight plots in the COBS experiment. The incubation experiment will employ specially designed glass columns. Three levels of residue concentrations and three levels of nitrogen amendment will be mixed with collected soil materials and root residues. Over an eight-week period, the samples will receive a range of treatments, and various measurements will be taken.
The laboratory experiments were completed within the first year of the project. But some of the analyses could not be completed until the summer of 2019, due to closure of the Soil and Plant Analysis Laboratory at ISU. The visiting scientist who conducted the study has returned to her home institution. At present, the researchers are in the process of completing the necessary calculations and developing interpretations. These will be incorporated into an upcoming journal article submission.
Progress this quarter has been slow. Personnel changes as well as problems associated with the closure of the Soil and Plant Analysis Laboratory set back the original schedule. Analytical work was completed and statistical analysis is being conducted to explore the results of the study.
Progress on this project slowed during the last quarter. The post-doctoral research associate who was overseeing the lab activities accepted a permanent position in California and left ISU in early July and the undergraduate assistant was not available during the summer. In addition, samples that we had submitted to the Soil and Plant Analysis Laboratory in the spring languished there while the lab caught up on a large backlog of other samples. Unfortunately, the lab was closed before our soil and plant samples could be run. We have now identified another lab where the samples can be submitted, but new samples will need to be prepared for analysis by a different instrument than the one used for our previous samples. The change in analytical method is worrisome, but at this point, we have no choice. By the end of the next quarter, we should have completed all of the analytical work, and we will be conducting statistical analyses of the results.
The laboratory incubation experiment for this project got underway this quarter. The design of the project was changed slightly from the original plan when researchers found the C:N ratios of a variety of root residues collected from the field research site were not much different from one another. So the incubation experiment was modified to allow researchers to look at the impact of different kinds of N fertilizers (ammonium chloride, urea and potassium nitrate) on N mineralization and immobilization. Now incubated soil samples are analyzed periodically to determine inorganic and organic extractable N, total N, organic C, particulate organic matter and other parameters.