What is the Potential for Nitrogen Losses from Extreme Summer Rainfall?

Generally speaking, excess moisture in the soil profile is one of the main causes of nitrogen (N) losses in crop production. These types of losses are due to either leaching or denitrification of the nitrate form of N. Leaching losses occur when water percolating through the soil profile carries nitrate below the rooting zone. Denitrification losses occur when anaerobic soil conditions cause microbes to convert nitrate to nitrous oxide or di-nitrogen gases which escape from the soil into the atmosphere.  The extent to which these types of N losses occur after extreme precipitation events in mid-summer depends on many factors. These factors include the timing of precipitation relative to crop N uptake patterns, the quantity of N in the soil profile in the nitrate form during the precipitation events, and soil drainage patterns.

If a majority of the N fertility program for a corn crop is coming from inorganic fertilizer applications, it is likely that most of the fertilizer N applied has been converted to the nitrate form by mid-July. The exception to this is if a nitrification inhibitor was used with the fertilizer, which will provide about 6 weeks of protection against the conversion of N from ammonium to nitrate forms. The nitrate remaining in the soil profile is what will be susceptible to leaching and denitrification losses. If a significant portion of the N fertility program for a corn crop is coming from the residual manure history, a previous legume crop, and current year manure applications, these organic sources of N will continue to mineralize through the summer. Mineralization of these organic N sources creates the ammonium form of N, which later in the season may be taken up directly by the crop before it is nitrified and susceptible to loss. Moist soil conditions in late summer may create more favorable conditions for mineralization, possibly increasing the amount of N available to the crop. Therefore, the severity of N losses due to mid-summer extreme rainfall will be greater in systems where inorganic fertilizer is the primary N source for the crop as compared to systems where N mineralization from organic materials is a major source.

Extreme rain events that occur before corn has reached reproductive stages will have a greater impact on N losses because there is potentially a larger pool of soil inorganic N in the nitrate form that will be vulnerable to losses. When extreme precipitation events occur later in the growing season, much of the N needed by a crop such as corn has already been taken up by the plant. For instance, corn in the silking stage (R1) has already taken up about 60% of the nitrogen it needs and corn in the blister stage (R2) has taken up 75% of the nitrogen that it needs. The remaining N requirement of the crop may come from fertilizer that has been applied earlier in the season, or from N mineralized from organic sources, such as soil organic matter, cover crop residues, or manure applications. 

Another factor affecting losses is soil drainage patterns. For leaching to occur, nitrate in the soil profile must be flushed below the rooting zone. This requires enough precipitation to recharge the soil profile with water to exceed field capacity, as well as enough precipitation to exceed the evapotranspiration removed by the corn crop. For a silt loam soil with a 24-inch rooting depth that has been previously dried out by a growing crop, it will take about 4 inches of rainfall for the soil to reach field capacity. Any additional rainfall above that amount would then theoretically trigger an equal amount of leaching of water and nitrate from the bottom of the root zone. For nitrate that accumulates in the topsoil during the growing season, it could take several drainage events of this magnitude to push the nitrate below the root zone. Counteracting these drainage events is evapotranspiration, which is removal of water from the soil profile by evaporation and crop usage. A corn crop at full canopy on a sunny day will evapostranspire about one third inch of water each day.

Another aspect of soil drainage that will affect nitrogen losses is whether the soil remains saturated long enough for anaerobic conditions to occur and trigger denitrification. Denitrification will occur when microbes use up oxygen in the soil. When a dry soil is wetted to saturation, it may initially take a couple of days for oxygen levels to be depleted to the point of denitrification occurring. If the soil was already wet to begin with, anaerobic conditions could occur even faster. These anaerobic soil conditions need to be paired with nitrate existing in the top soil where the microbes are most active for denitrification to occur. So, the severity of denitrification losses is to some extent tied back to how much nitrate is actually in the topsoil when the extreme summer precipitation occurs, which depends on crop uptake patterns and the predominant sources of N fertility in the system. Denitrification losses are also likely to be patchy within a field, corresponding to low lying spots with poor drainage where rainfall may accumulate and pond.

For those producers trying to decide about whether to add more nitrogen to their corn crop, the answer will certainly be site and scenario specific.  Excessive rain in June has the highest potential for losses, in early- to mid-July the risk for N losses become moderate, and by late-July the risk for N losses is lowest. Areas within a field that will suffer the worst will probably be low lying areas where water ponds, causing denitrification. Accessing fields with equipment that can apply additional N fertilizer into a tall corn crop also creates its own limitations. Since the areas within a field that need to be treated with additional N may be patchy, the economics of treating small areas with large equipment, or the economics of treating whole fields where much of the crop may not be responsive to additional N, may not be favorable. As always, understanding the processes that lead to N losses and integrating this understanding into adaptive management tactics will help you make a decision that best fits your individual scenario.