Transport of Nitrogen and Phosphorus through Grain,

For a problem to be global, it must either have local impacts that are suffered globally or globally distributed impacts.Air emissions, such as nitrous oxide that distribute themselves globally are clearly a global issue. But what about a local activity that primarily affects land and water? Even site-specific application of fertilizers to crops precisely measured to meet crop needs can still result in an environmental imbalance because some of these nutrients leave the site. But beyond this the production, harvest, and transport of grain provide a good Figure 5.3 How humans have affected the N cycle (Source: Nielsen, 2005).

example of how agricultural production in one location can affect global nutrient balances. This is because the export of grain from where it is grown to where it is consumed can result in the introduction of more nitrogen and phosphorus than can be accommodated by plants or crops where it has been imported.

Although humans benefited tremendously through our manipulation of the carbon, nitrogen, and phosphorus systems, we have also long paid a price in terms of degradation of our environment. Examples of these environmental problems are discussed in Chapter 6. Our responses to these challenges, the antidotes for the negative consequences, have not kept pace with the expanding scale of the generation of rN and bioavailable phosphorus. An apt analogy is Mickey Mouse as the Sorcerer’s Apprentice in Disney’s “Fantasia.” Mickey solves his work problem by creating a magical broom that does the work for him–that is carry water and fill the basin. Unfortunately, he fell asleep and therefore did not pay attention to when enough is enough. His attempts to quell the rising tide of water result in more brooms, only making the problem worse, with more flooding.

Human reactions to problems can be much like Mickey’s. We too often solve one problem by adopting technologies that, to our dismay, create other problems, although they may be different by nature. Artificial fertilizer illustrates this serial problem. Fertilizer use has improved the health and wellbeing of billions of people on this planet, yet the overflow, the increased availability of rN now poses a threat to many important ecosystems essential to human existence. The existential problem with nitrogen to be addressed is the imbalance between the global rate of production of reactive nitrogen and its rate of immobilization or removal and conversion back to N2.

To examine how local agriculture can lead to global imbalances, let us first ask a couple of very basic questions: Are there a lot of agricultural products imported or exported? If so, does what is transported represent much nitrogen and phosphorus?

We will focus on grain because grain export constitutes the most trade in raw agricultural products. More specifically, let us use corn as an example, because corn is widely traded worldwide and corn production is a major user of fertilizer.

Similar analysis could be conducted examining other grains and meats. According to U.S. Department of Agriculture’s Foreign Agricultural Service (2020), roughly 171 million metric tons (188 million U.S. tons) of corn were traded in 2019. The corn was harvested from a little more than 190 million hectares (409 million acres) of land. The United States accounted for 33 million hectares (81.5 million acres) of corn harvested and roughly 49 million metric tons (54 million U.S. tons) exported. Because only the kernels are traded, we examine only the nutrient content of the kernel, not the corn plant. The mean nitrogen content of the corn grain is roughly 1.54% (Booneet al., 1984). For phosphorus, the values can vary considerably, but for the sake of this analysis, let us use the average phosphate content of 0.195 kg of phosphate per bushel (or 0.43 lb.). There are 39.368 bushels per metric ton (MT) of corn (U.S. Grains Council, 2020). Doing the math Guide to Understanding the Principles of Environmental Management 86

(0.195×39.368) shows that there are 7.7 kg of phosphate per MT of grain (Nafziger, 2017). Multiplying the numbers gives us an estimate for global transport of 1.31 million metric tons of phosphate and 2.63 million metric tons of nitrogen.

(Global phosphate transported in corn=171 million MT×(7.7 kg/MT)/(1000 kg/MT)=1.31 million MT; Global nitrogen transported in corn=171 million MT×1.54%=2.63 million MT.)

This is a large amount of nitrogen and phosphorus, but do we know whether or not it is large enough to affect anything? Let us compare it to what occurs naturally.

Lightning fixes between 3 and 10 Tg per year. Since a teragram is equivalent to 1 million metric tons and using the midpoint, lightning fixes 6.5 million MT (7.1 million U.S. tons) of nitrogen. Therefore, we see that trade just in corn moves 2.63/6.5 or about 40% of what lightning provides in new reactive nitrogen globally. For phosphorus (i.e., phosphate), let us compare our estimate to the global consumption of 47 million MT (52 million U.S. tons) in 2018 (USGS, 2019). Thus, about 1.31 divided by 47 or roughly 2.8% of annual global production of phosphorus as phosphate is exported as grain (USGS, 2019). Are these numbers significant? In considering your answer, remember that the transport of these quantities of nitrogen and phosphorus is (1) going to concentrate the nitrogen and phosphorus in areas with either concentrated human or animal populations, (2) in locations that are likely to lack the capacity to process the additional nitrogen and phosphorus and (3) this transport occurs each year.

Most of the nitrogen and phosphorus will initially remain in the destination where it is consumed and transformed into animal or human waste. As explained earlier, this animal waste (along with human waste that has been processed through sewage treatment plants) will ultimately be disposed of, that is added as a fertilizer, on cropland U.S. EPA, 2020a). Because the ratio of nitrogen to phosphorus in human and animal waste is not consistent with their corresponding ratios in soil, their application without further processing to resolve the imbalance can result in too much of one or the other being applied. This can overload the capacity of the soil microbial systems to convert the nitrogen and phosphorus into new soil leading ultimately to the transport of nitrogen (and under certain circumstances, phosphorus) offsite into the air and/or water. The volume of waste makes the potential for overloading the soil capacity a concern in many areas. Unless, there is sufficient cropland or other vegetated land to accommodate the waste generated by large numbers of animals and humans, and unless the manure is applied at appropriate rates, there will be residual nutrients that can move offsite from land to water and/or air.

The amount of nutrients transported as animal products can be estimated by working backwards from the type and number of animals (generally, chickens, pigs, and beef and dairy cattle). The conversion rate, that is the ratio of grain to animal weight, for chickens is roughly 2 to 1. For pork, the number ranges from 3 to 1, to 4 to 1. And for beef the number is between 7 and 10 to one (Oros, 2020; Wikipedia, 2020). The grain that is not incorporated into the animals is

expelled as manure, and the manure contains a large amount of nitrogen and phosphorus.

Traditional farming practices that cycled nutrients locally from fertilizer, to crops, to grain, to animals that generate waste that is then returned to the land as fertilizer have been disrupted. Modern agricultural systems still cycle nutrients, from fertilizer to grain, meat, and manure, but the nutrients in grain and meat are transported, making the geographic area in which cycling occurs larger. Efficient use is correspondingly more difficult. Animal and human waste generally has a large water content which makes the waste heavy and expensive to transport any significant distance. The historical rule of thumb has been the following: for manure to be applied in an economically feasible manner (it needs to be applied within 40–50 miles (64 to 80 km) from where it is generated).

Even poultry waste which has a low water content and thus is relatively light given its mass of nitrogen and phosphorus is generally not transported more than 50 miles or 90 km from where it was produced. Poultry waste can also be burned and used as a fuel source, emitting its carbon into the atmosphere, but leaving the nitrogen and phosphorus as residue which can then be stored and transported at lower cost. However, care must be used when applying the waste as a fertilizer because of the relatively high phosphorus to nitrogen ratio and the variability in this ratio between different sources of chicken litter.

Regional and global transport of grain and the cycling of nitrogen (and phosphorus as well) are illustrated in Figure 5.4. The import of feed grains divorces livestock production from the land where livestock waste could be recycled, resulting in a concentrated mass of nitrogen and phosphorus. The residual nitrogen and phosphorus not taken up in crops, converted back to N2, in

Atmosphere Figure 5.4 Trade impact on the N cycle.

It Ain’t Magic: Everything goes Somewhere 88

the case of nitrogen, or incorporated into new soil is lost to the atmosphere and water.

5.6 LANDSCAPE MODIFICATIONS THAT AFFECT WATER,