Mitigation of Nitrate, Nitrous Oxide, and Ammonia Loss with Time And Source of Nitrogen Application in Corn

Abstract

The large need of nitrogen (N) for crop production and the negative impacts of fertilizer on the environment result in a compelling need to identify new or advanced N management practices to reduce N losses while maintaining productivity and profitability. Nitrate (NO3) leaching, nitrous oxide (N2O) denitrification, and ammonia (NH3) volatilization are the most common pathways of N loss when synthetic N fertilizer is applied in agricultural lands. The objectives of this 7-year study (2014-2020) were to evaluate the use of traditional management (urea applied at pre-plant) and advanced [enhanced efficiency fertilizers and split applications] N management practices on i) N losses (NO3, N2O, and NH3); ii) corn yield and profitability; and iii) cropping system N balance including plant N removal and soil N status. The field experiment was conducted at the University of Minnesota Southwest Research and Outreach Center, near Lamberton MN. Four treatments were applied and replicated four times in a randomized complete block design: a pre-plant application of 202 kg N ha-1 of either urea (U) or polymer coated urea (PCU; ESN) (E) and 135 kg N ha-1 as urea with urease inhibitor (Agrotain) sidedressed at development stage V4-6 with 67 kg N ha-1 applied before planting as either urea (U/U+) or ESN (E/U+). Across the 7 years, the advanced management practice E/U+ increased corn grain yield (1 Mg ha-1 or 10%) and total N uptake (TNU, plant + grain N; 18 kg N ha-1) compared to the traditional management practice (U), while E and U/U+ had similar grain yields and TNU to the other treatments. Furthermore, while net economic returns (NER) were not statistically different between treatments, E/U+ generated numerically greater NER per hectare than U ($63), E ($52), and U/U+ ($46). Nitrate-N leaching (measured from 2015 to 2020) was highly influenced by weather. Excess precipitation, especially after fertilizer application, increased NO3-N loads, which was lower for E compared to the other treatments; however, flow-weighted NO3-N concentration was not different between treatments, with only a trend for lower NO3-N concentrations for E. Nitrous oxide emissions (measured from 2018 to 2020) also increased with excess precipitation, especially after fertilizer application. Split application treatments and U did not reduce N2O-N loss, but in wet years N2O-N loss was lower for E compared to the other treatments. This, however, was not observed in drier years or when precipitation was evenly distributed. Unlike NO3-N and N2O-N losses, ammonia volatilization (measured from 2019 to 2020), decreased as the result of excess precipitation that helped incorporate the fertilizer deeper into the soil. Overall, NH3-N loss was lower for E compared to other treatments. Integration of all the variables measured into a N balance calculation showed to be a poor approach to estimate N efficiency or impact on environmental quality because plant N uptake overshadow treatment influence on N loss measurements. Our findings indicate that pre-plant ESN can be considered a strategy to reduce N losses while maintaining crop yield. While split treatments increased corn yield, they did not reduce N losses, which contrasts the common assumption held by many that split applications are better for the environment. Nonetheless, the traditional management practice of pre-plant urea was the least efficient, producing lower crop yields and increasing N losses compared to pre-plant ESN in wet years. This demonstrates that there are N management practices that can improve production and environmental protection and their merit should be explored and refined further.