Understanding and predicting nitrogen dynamics in the Midwest United States Corn Belt

Mahal, Navreet Kaur
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Understanding nitrogen (N) dynamics in agricultural systems is very critical for profitable corn production while minimizing the N losses from agroecosystems which cause environmental degradation and increase cost of production. The nitrogen cycle in agricultural systems is complex; it includes interactions among many pools and fluxes. These pools and fluxes are controlled by climate, soil properties and crop management. The overall objective of this research is to better understand the impacts of crop management practices on cropping systems N dynamics.

Potentially mineralizable N (PMN), measures the release of plant-available N from soil organic matter, is controlled by the soil properties, climate and past crop management practices. However, the effect size and relationship with the crop yield across different conservation and conventional crop management practices remain uncertain. Using a meta-analysis approach, chapter two examined the effects of various conservation and conventional crop management practices on PMN in soil. This quantitative review suggests that, as compared with no fertilizer, cropping systems with inorganic N fertilizer had 22%, and systems with manure had 34% higher PMN. Three or more different crops in rotation had 44% higher PMN than continuous cropping systems. Cropping systems with leguminous cover crops had 211% higher PMN than systems without cover crops. Compared with till systems, no-till systems had 13% higher PMN. Although few studies reported PMN and crop yield, in those that did report both variables, conservation crop management practices consistently increased both PMN and yield. Consistent with the use of PMN as a soil health indicator, this study suggests that practices benefiting PMN also benefit yield.

Uncertainties about the effects of inorganic N fertilizer on soil organic matter has led to a great debate about the long-term sustainability of fertilized continuous corn production systems in the Corn Belt. Fertilizer application enhance the primary productivity and thereby increase the soil organic matter by adding higher residue inputs. Whereas, other studies have suggested N fertilizer application enhances the microbial activity and thereby decreasing the soil organic matter. The objective of chapter three was to quantify the effects of inorganic N fertilizer on soil organic matter mineralization using a combination of field and laboratory experiments. In the field, at the onset of rapid corn N uptake, N fertilizer reduced gross ammonification rate by 12-15%. A companion laboratory experiment suggests that the negative effect of N fertilizer was due to direct effect of ammonium fertilizer addition rather than indirect effects of N on crop growth that affect gross ammonification such as soil water content and temperature (i.e., well-fertilized crops use more soil water and shade the soil). Ammonium pool was negatively associated with the gross ammonification rate. This work demonstrates that optimum rates of inorganic N fertilizer does not enhance SOM mineralization and increases crop yield and residue input to the soil and therefore, does not contribute to reducing SOM content of conventionally managed continuous corn production systems of Midwestern US Corn Belt.

Careful management of N is agroecosystems require to match the N supply with crop N requirement. Late spring soil nitrate test and end of season corn nitrate test are commonly used methods to determine the N fertilizer recommendations in corn cropping systems of Midwestern US Corn belt. However, late spring soil nitrate test is an indicator of N supplying capacity of soil and the end of season test is a post hoc test which indicates the whether the N in corn was low, optimum or excessive and it cannot be used for in-season N fertilizer recommendation. Chapter four evaluated the potential for corn stalk sap nitrate concentration to make a useful tool to monitor crop N status and determine the need for supplemental N fertilizer input. Multiple N rate trials were conducted across the state of Iowa to determine the response of corn stalk sap nitrate concentration at the V7-V8 development stage to soil N availability and crop N demand. There was a positive linear or quadratic response of sap nitrate concentration to N fertilizer rate at each site. Grain yield had positive association with the sap nitrate concentration at V7-V8 developmental stage. This one-year study conducted at multiple locations suggested that 570-820 ppm N was the sap nitrate concentration sufficiency range at V7-V8 corn development stage to maximize net return from per unit of N applied per unit of land. Sap nitrate test can be used to make in-season fertilizer N recommendations based on the plant N status.