Abstract
Sustainable farming is one proposed method to reduce nitrogen pollution incurred by farming, but reactive nitrogen loss rates from sustainable farming vary and require further study. The overarching objective of this study was to determine how the nitrogen use efficiency of a permaculture livestock farm compares to that of conventional farms. Two comparison metrics were used: farm nitrogen budgets and virtual nitrogen factors.
For the first comparison method, a farm nitrogen budget was constructed for Timbercreek Organics Farm (TCO), a permaculture livestock farm in Albemarle Country, Virginia. The budget found that the total intended farm N inputs (e.g., feed, legume biological nitrogen fixation, purchased livestock) increased from 32 kg N ha-1 yr-1 in 2012 to 49 kg N ha-1 yr-1 in 2013. The intended farm N outputs (e.g., meat products, slaughter by-products) tripled from 5 to 14 kg N ha-1 yr-1. The overall TCO farm nitrogen use efficiency (NUE) doubled from 14% in 2012 to 28% in 2013. When compared to conventional farms from the literature, the 2013 TCO NUE (28%) was comparable or exceeded that of beef farms, but the TCO NUE was less than that of conventional pork, poultry, and layer farms. The TCO N surplus (difference between nitrogen inputs and outputs) at TCO was 2-10 times lower than conventional farms, suggesting that TCO has a lower local environmental impact than conventional. TCO required more 15-60% more time and 37-720% more land area to produce meat and animal products than conventional farms, although beef production at TCO utilized 95% less land than conventional.
For the second comparison method, virtual nitrogen factors (VNF) were used to compare the nitrogen efficiency of a permaculture livestock farm to conventional farms. VNF, which describe the reactive nitrogen lost to the environment per unit of nitrogen contained in a food product, were calculated for beef (0.1), pork (3.6 in 2012 and 2.5 in 2013), poultry (5.7 in 2012 and 4.4 in 2013), and egg (4.0 in 2012 and 5.5 in 2013) production at TCO. The TCO beef VNF was substantially lower than the beef VNF for conventional farms because TCO beef is grass-fed and natural forage inputs were not considered. When considering calculated uncertainty, the TCO pork, poultry, and egg VNF were as efficient as conventional production.
An exploratory field monitoring exercise was conducted between June 2013 and April 2014 to begin studying whether TCO has an influence on the inorganic nitrogen flux in the streams that flow through the property. The highest measured stream water nitrate and nitrite concentration (2.5 mg N L-1) was well below the US EPA drinking water standard for nitrate (10 mg N L-1). On more than half of the 13 sampling events, the flux of inorganic N entering the property exceeded the flux of inorganic N exiting the property. Although more research is needed, a likely explanation for the nitrogen stream inputs exceeding the nitrogen stream outputs is the low TCO N losses per unit area coupled with an on-farm holding pond that is effective for stormwater management.
