March 2014 Articles

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Making Sense of the Nitrogen Cycle

Making Sense of the Nitrogen Cycle
Source: University of Arkansas, Dirk Philipp, Assistant Professor Animal Science
Nitrogen (N) is the most limiting nutrient for plant growth and should be managed accordingly to make the most efficient use of synthetic fertilizer and manure. This is of importance to all dairy operations. In conventional dairies, manure and/or synthetic N fertilizer is applied to crops that may be used as feed such as silage, and in pasture-based dairies, cows may periodically graze pastures that are part of the overall forage management plan, under which most of the N is recycled. In either case, N from whatever source has to be converted to nitrate (NO3-) or ammonium (NH4 +) in the soil before it becomes plant available.
Making sure that much of the nitrogen is accessible to plants and can be used when needed depends on management decisions, for which basic knowledge of the N cycle is of benefit. In the case of applying mineral fertilizer such as urea, the enzyme urease is necessary to catalyze the reaction in which urea is converted to carbon dioxide and ammonia, which can be partly lost to volatilization. In the next reaction, ammonia is converted to ammonium, which then can be taken up by plant roots. This ammonium is even further converted to nitrate, which is also plant available and preferably taken up. This step is called nitrification and is one of the most sensitive soil processes. The process involves microbes (Nitrosomonas, Nitrobacter) and is heavily dependent on soil conditions, including optimum pH (around 7), temperature and oxygen levels.
Therefore, keeping up with soil testing for judging liming requirements and maintaining soil organic matter for sustained soil tilth is necessary. Unfortunately, nitrate can easily be lost to leaching, because the negatively charged soil particles cannot adsorb the equally charged nitrate. Long-term storage for N can be accomplished by increasing soil organic matter. The soil bacteria incorporate nitrate into humus.
Because dairy operations have to deal with large amounts of manure, nitrogen management strategies are different from a row crop situation, where synthetic fertilizer can be applied in a more targeted fashion. Recycling N in a grazing situation poses a different problem, because N in manure and urine is highly concentrated and thus is difficult to incorporate and mineralize by soil organisms. In a review paper by Rotz (J. Anim. Sci. 2004, 82:119- 137), nitrogen losses through volatilization from manure patches were estimated to be about 5 percent, a relatively minor amount. Nitrogen volatilization from urine can be much higher, as most of the excreted N (55 to 75 percent) is present in urine. Losses may be anywhere between 5 to 66 percent of urinary N, with greater losses during dry, hot weather and smaller losses under cool, moist conditions. Leaching losses of N can be higher under grazing than under manure spreading because of the high concentrations of N under a urine patch that can reach a concentration of 1,000 pounds N/acre. This amount obviously exceeds crop needs on that particular location, and much of it is lost through the soil profile. Leaching is less during spring compared with fall, as growing forage can take up N more readily. This paper also reported that runoff losses of N from pastures are small, which can be even further reduced by filter strips and rotational grazing, provided management is adjusted to this form of grazing method. Although management can only go so far in reducing N losses, moving feeding areas frequently can reduce manure and urine loads on particular areas in the pasture, ensuring a more even distribution of not just N but of other nutrients contained in animal excrement. Overstocking of animals should be avoided, and crop rotations, where possible, help make use of soil nitrogen in an efficient manner while reducing leaching losses and volatilization.