Recently, an article was published in The Gazette about a new publication on nutrient budgets in Iowa. The article discussed work published in the journal, Ambio, entitled, “Livestock manure driving stream nitrate.” The paper looked at watersheds in western Iowa and compared the flow weighted nitrate concentration against several watershed parameters including a “nitrogen surplus.” This was a rough nitrogen budget comparing nitrogen additions and removals, various sources of nitrogen application rate, and the percent of the crop land in corn and soybean acres.
While all showed some positive correlation with the flow weighted nitrogen application concentration in stream water, the highest correlations were found with the nitrogen surplus, manure nitrogen application, and then the area portion of the watershed in corn and soybean. Based on this, the authors proposed manure was a key contributor in driving the higher nitrogen losses. However, let’s delve deeper into some of the results and closely examine some of the results.
The first thing to note is a strong relationship exists between the fraction of a watershed alone that is row crop (corn and soybean) and the flow-weighted nitrate concentration. This factor alone explains 65% of the variation in nitrogen content. This is in line with field plot trials conducted at Iowa State University that have generally shown similar trends with nitrogen reductions coming from land use change towards perennial crops or the use of cover crops. A big part of why these practices have nothing to do with an annual nutrient balance, as suggested in this paper, but more temporal dynamics of when nutrients are released from soil organic matter. These cool weather plants uptake nitrogen at times when more traditional row crops (corn and soybean) aren’t actively growing and as a result, can reduce nitrogen losses to waters.
They also constructed a nitrogen surplus budget for the watersheds (comparison of nitrogen applications and removals) and found it did a slightly better job of describing the average flow-weighted nitrogen concentration in the watersheds, describing 80% of the variation, or 15% more than land use alone. One important thing to consider, the nitrogen surplus budget itself is strongly correlated with land use, so these two measures aren’t independent. This isn’t to say nitrogen application rate isn’t important, it is, but to say land use alone may be a bigger component than nitrogen application rate, i.e., the correlation coefficient alone isn’t able to suggest how much of that effect is from land use and how much is from the effect of nitrogen application rate. Despite this, the result is clear, the higher nitrogen application rates will tend to result in higher nitrogen leaching losses, especially when certain thresholds of exceedance are met.
One last thing I found very interesting about the manuscript was manure nitrogen in the watershed was a much stronger predictor than commercial fertilizer. This result needs to be used with some caution, as the correlation efficient can be impacted by the range in date and in the manure nitrogen application rates had a much higher range than the commercial fertilizer application rate. There are several interesting aspects that manure has a stronger relationship. The first of which, is this represents excreted manure and not necessarily available nitrogen applied to the land.
While in most cases we consider nitrogen application to be a single year fertilizer application, with some manure types offering several years of potential fertility benefits due to the organic fraction which can be slow release. Could the higher loss of nitrogen from the high manure watersheds be a result of improved soil health and mineralization of nitrogen from soil organic matter? Potentially, but without good research or understanding of how long-term manure applications (especially of high organic manures like solid cattle manure) impact soil health, it is hard to anticipate how nitrogen needs and losses from the soil would change with much certainty . Alternatively, the improved nitrogen concentration in streams with manures could be indicative of challenges of using manure as a fertilizer; things like timing of application, certainty of the fertility it provides, or even application uniformity can all be issues that make it harder to trust manure fertility and in the right weather conditions could increase losses.
The important point to consider is this doesn’t necessarily mean we have an application rate problem, it means we have to find better ways to focus on the challenges we have with manure and how to overcome them.