Recent rainfalls have brought about an abrupt change to soil conditions in some Ontario fields in just a few short weeks ago. Localized heavy rainfalls have left some fields with standing water in low-lying areas, while many fields are wet or saturated. This has raised questions about the potential losses of nitrogen (N) for the corn crop in these wet fields. Determining how much N has been lost is influenced by several factors including rainfall intensity and amount, soil texture, soil temperature, fertilizer source, and corn growth stage.
Nitrogen loss pathways in these significant rainfall events can happen via runoff, leaching, or denitrification.
Runoff
Intense, short-lived storms with significant rainfall can often cause heavy erosion and physical soil movement, especially in areas with complex topography. This eroded topsoil often contains high levels of soil organic matter, as well as fertilizer nutrients applied earlier in the spring. If recent N fertilizer applications have occurred on the soil surface without earlier rains to move N deeper into the soil, there can be substantial loss in eroded areas.
Nitrate-N
Leaching and denitrification are concerns when there is a significant concentration of the nitrate form of N in the soil. Nitrate is a portion of some nitrogen fertilizers when applied (for example, Urea Ammonium Nitrate or UAN), but all nitrogen fertilizers will naturally convert to the nitrate form relatively rapidly by soil microbial interactions. Very recent in-season applications of N may not be entirely in the nitrate form, but for most early season applications, it’s likely that most of the usable N in the field is nitrate-N.
Leaching
In well-drained, coarse-textured soils, the primary form of N loss may be leaching. Leaching occurs when excess water causes a downward movement of nitrate-nitrogen through the soil profile. The vast majority of leaching occurs out of the growing season.
Leaching is often overestimated in the field because of the soil’s rate of net infiltration from a single rainfall event. Net infiltration is the amount of precipitation minus the amount of runoff or evapotranspiration. In short, heavy rainfall events, a 50mm rain doesn’t mean that the soil takes in 50mm in every location. Prolonged rains, or several continuous rainfall events without drying time between may put the N at higher risk of leaching loss.
Figure 1 shows the variation in depth between soil textures that water can infiltrate into the soil, and the depth that it can wet the soil to field capacity and therefore move nitrate-N downward. Sandy soils are more prone to leaching loss than loamy or clay soils, as water moves more rapidly through the soil profile. Ten mm of water infiltration in a sandy soil can move the water and nitrogen down by almost 6x that distance through the soil. Once nitrate-N has leached below the corn’s rooting zone, it is unavailable to the plant, and can be an impact to groundwater or surface water through tile drainage. In general, substantial N losses out of the rooting zone through leaching are rare.
Denitrification
In fine-textured soils with saturated conditions, the primary form of N loss is often denitrification. Denitrification is a conversion of nitrate-N to gaseous forms like nitrogen gas (N2) or nitrous oxide (N2O). Nitrous oxide is a small proportion of the overall N loss but is a potent greenhouse gas and has a significant effect on the environment.
Denitrification occurs rapidly in saturated conditions, especially poorly drained clay soils, or areas where water ponds. Total N potentially lost is a function of length of soil saturation, soil temperature, and amount of nitrate-N in the soil. Table 1 gives an indication of potential denitrification losses as soils remain saturated. The longer the soils stay saturated, the larger the potential losses from denitrification.
Do I need to top up my crop with additional N?
To figure out how much crop-available N has potentially been lost from your field in extremely wet conditions, you must first know how much nitrate-N resides in your soil.
Figure 2 shows the nitrogen uptake pattern of corn, including the total percentage of nitrogen that the corn crop has accumulated. To be lost, the nitrate-N must be available in the soil, and not yet taken up by the plant. Prolonged saturated soil conditions in early growth stages can lose much more N than it can in late stages, as there has been more already taken up by the plant, and less sitting in the soil to be lost. By tassel timing, around 60% of the total nitrogen has already been utilized by the plant, meaning only 40% of the total nitrogen is available for loss.
Identifying your corn’s current growth stage is important to estimate how much nitrate-N could be in the soil in your management system. With poor drainage, compacted conditions, and prolonged wetness in the soil, denitrification losses can add up rapidly. If your crop is still at the V8 stage, you may want to assess how long the soil has been saturated and if an extra application will pay. If you are getting closer to the reproductive stages, the majority of N has already made it into the plant, and so it likely makes less sense to apply additional N.
Lastly, nitrate-N losses can be reduced through the use of nitrification inhibitors. Nitrification inhibitors decrease N losses through leaching and denitrification by slowing down the conversion of ammonium to nitrate. This means that less nitrate-N is available for loss in wet conditions. N stabilizers can often last 2-5 weeks, so an application with urea preplant might not be enough to prevent losses two months later.
Ultimately, leaching or denitrification losses of N will differ greatly between production systems. The decision to top up N application will differ between fields as well.
Sources:
Munroe, J., editor. (2018). Soil Fertility Handbook, Publication 611. Ontario Ministry of Agriculture, Food, and Rural Affairs. https://www.ontario.ca/page/soil-fertility-handbook