My last post focused on some comparisons of 2015 versus 2016 for Raccoon River nitrate. As part of updating that data, I decided to also update some of my nitrogen budget data for the watershed. Inputs of nitrogen into the watershed include fertilizer and manure N, deposition from rainfall, and fixation of N by legumes. Outputs of nitrogen are loss of nitrate to denitrification in the soil, export of nitrogen in the harvested grain, vaporization of nitrogen gases (N2, N2O and NO and ammonia gas-NH3) to the atmosphere, and finally, loss of N to the stream network. Some of these things are easy to quantify or estimate: rainfall deposition, grain N, and river N. Denitrification and vaporization are difficult because they vary with soil type and weather. Fertilizer and manure N can be estimated, but hard, credible data is scarce. Legume fixation can be credibly estimated, but is known to vary and there is uncertainty in the estimates.
I haven’t competed all of my budget, but I do have river N loads and grain N export and these numbers for 2015 are interesting. Firstly, more nitrogen left the watershed in the grain in 2015 than in any year dating back to 1995, indicating a robust harvest. The conventional wisdom usually is that good harvests are good for water quality (and bad harvests bad for water quality), because the crop was able to access the needed nutrients, leaving fewer nutrients on the landscape that are vulnerable to transport. However, 2015 was a bucket of cold water on that idea, as river N load was the biggest ever dating back to 1974. The second half of the year had an especially large river N load (see 8/18/16 blog post).
What this shows us is how exposed, vulnerable and unpredictable this production system is to N loss in Iowa’s climate. I think it is difficult for most people to grasp the enormity of solving this problem without negatively affecting crop yields. While degraded water quality due to nutrient pollution is probably not our most pressing societal problem, we should see it as a sign that the resilience and sustainability of our production system is an open question.
The table below shows nitrogen exports in the grain and river going back to 1995. All units are metric tons (or million grams, symbolized by Mg).
| All units are Mg | ||||||
| year | corn yield | soy yield | N removed in corn grain | N removed in Soy grain | total N removed in grain | River N Load |
| 1995 | 3,040,000 | 1,040,000 | 31,000 | 57,700 | 88,800 | 18,570 |
| 1996 | 3,370,000 | 1,070,000 | 34,400 | 59,500 | 93,900 | 19,715 |
| 1997 | 3,080,000 | 1,070,000 | 31,400 | 59,800 | 91,200 | 12,730 |
| 1998 | 3,210,000 | 1,070,000 | 32,700 | 59,300 | 92,000 | 27,110 |
| 1999 | 3,390,000 | 1,010,000 | 34,500 | 56,400 | 91,000 | 30,326 |
| 2000 | 3,270,000 | 922,000 | 33,400 | 51,300 | 84,700 | 1,099 |
| 2001 | 3,160,000 | 1,030,000 | 32,200 | 57,400 | 89,600 | 16,769 |
| 2002 | 3,650,000 | 1,110,000 | 37,200 | 62,000 | 99,200 | 11,839 |
| 2003 | 3,540,000 | 759,000 | 36,100 | 42,300 | 78,300 | 16,376 |
| 2004 | 4,360,000 | 1,070,000 | 44,500 | 59,700 | 104,000 | 18,968 |
| 2005 | 4,240,000 | 1,150,000 | 43,200 | 63,800 | 107,000 | 12,969 |
| 2006 | 3,740,000 | 1,040,000 | 38,200 | 58,000 | 96,200 | 9,309 |
| 2007 | 4,510,000 | 967,000 | 46,000 | 53,800 | 99,800 | 31,445 |
| 2008 | 4,330,000 | 951,000 | 44,200 | 52,900 | 97,100 | 25,688 |
| 2009 | 4,630,000 | 1,020,000 | 47,300 | 56,800 | 104,000 | 16,598 |
| 2010 | 4,210,000 | 991,000 | 43,000 | 55,200 | 98,200 | 31,753 |
| 2011 | 4,620,000 | 991,000 | 47,200 | 55,200 | 102,000 | 13,249 |
| 2012 | 3,400,000 | 736,000 | 34,700 | 41,000 | 75,700 | 2,333 |
| 2013 | 3,630,000 | 722,000 | 37,000 | 40,200 | 77,200 | 25,725 |
| 2014 | 4,620,000 | 1,020,000 | 47,100 | 56,600 | 104,000 | 21,919 |
| 2015 | 4,760,000 | 1,140,000 | 48,500 | 63,500 | 112,000 | 47,456 |