Lowering river nitrogen load requires major action
Assessing trends in nitrogen (N) loss is much trickier than exploring how erosion and sediment transport in Iowa have been affected by changing precipitation regimes and extreme events. This article looks at studies of the Raccoon River Watershed in Iowa.
Although a lot of N data is available for the Raccoon River, the data is spotty until 1974, quite a while after large changes in river N loads began (load is the total amount of a substance transported by the river over a defined time period). And because nitrogen cycles into and out of the environment and is consumed by plants and microorganisms, linking river N levels to specific actions on the landscape is very difficult.
We know that production in the corn-soybean system is maximized when the landscape is essentially saturated with nitrogen. Because the most common form of N in the environment, nitrate (NO3), is very soluble in water, the system is vulnerable to loss. If we liken the Iowa landscape to a glass filled to the brim with nitrogen, it’s easy to imagine disruptions causing the N to slosh out of the glass, escaping the farm into the stream network. Anyone involved with farming knows that N loss is closely linked to precipitation. And we all know that predicting weather is uncertain business. Even the pros are wrong much of the time. We try to manage inputs based on “average” weather. But average is not what we should expect in any given year; rather average is merely a mathematical construct that illustrates what the Iowa climate is like over time.
What we get from one year to the next may be far from average. About 2/3 of the N loss in Iowa occurs during only 1/3 of the year: April through July. We get about half of our annual precipitation during this period. If this four month period gets wetter relative to the rest of the year, all things being equal, N loss will increase. And as we have seen in previous segments of this series, the frequency of extreme precipitation events is increasing in Iowa.
To learn about N loss, we need to “follow the water.” Can we learn anything from evaluating how N loss has changed with extreme precipitation over time? It’s much harder to make conclusions about N than it is sediment and erosion. If a large rainfall follows a period of dry weather, there may be adequate water storage capacity in the soil and N may not move to the stream. On the other hand, even a moderate rain falling on wet soils can initiate or increase water flow through the soil profile and into tiles, causing large N delivery to streams.
When we look at the precipitation and nitrate-N data for the Raccoon Watershed dating to 1974, we can make a few cautious statements about river N loading as it relates to extreme precipitation. April-July N loss does not appear to be increasing, and may be declining some, in the wettest years (wettest 1/3 of 40 years, 1974-2013). It does seem that N loss may have increased in the other years (i.e. below-and near-average precipitation years).
There are many ways to interpret this and what exactly it means is not clear. These possible trends could be linked to the improvement and expansion of tile drainage in the watershed.
During dry and average years, one can imagine that tile delivers water (and nitrate) to the river that otherwise would have remained on the landscape. On the other hand, in very wet years, when overland runoff becomes a more important water delivery mechanism relative to tile drainage, expansion of the tile system may have had relatively smaller impact on N loss. This may explain differences in N loss trends for wet years versus the other years. It also appears that N loading is being increasingly concentrated into two months: May and June. This is important because it means that we may continue to see very high river N concentrations during these months (important from the perspective of downstream water utilities) while overall, annual N river loads are declining.
So what does this mean for N management? We know that what is best for one farm may not be best for others. That said, the 4R fundamentals of right rate, right timing, right place, and right form will always be important. Monitoring on the farm—soil N tests, cornstalk N testing, water testing, manure nutrient content assessment, and other types of analysis can help the producer efficiently manage inputs and reduce loss. Beyond input management, edge of field treatments such as bioreactors and wetlands
work effectively to reduce N loading to streams. Although published research shows that we can still lose as much as 70 pounds of N per acre at the drainage district scale during wet years, we will not achieve our water quality goals just by improving input management. This is because our soils contain so much nitrogen in the organic matter. Thus, we have to more effectively manage water. Cover crops clearly have potential to help us do this, buffering the effects of extreme events and trapping some nutrients. Drainage water management also holds potential.
Some are now imagining a future where climate change makes irrigation cost effective, and springtime drainage water is impounded and then used to water crops later in the summer. Most farmers instinctively know that solving the problem of nutrient loss is much more difficult than preventing erosion. To succeed, researchers and farmers must work together to collect solid scientific evidence about what works, and what doesn’t work. This is crucially important as we try to cope with the uncertainties and extremes of climate that are characteristic of our mid‐continent location.
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