The impact of nitrogen source and crop rotation on nitrogen mass balances in the Mississippi River Basin

Abstract

Abstract. Nitrogen (N) leaching to surface waters from grain farms in the Mississippi River Basin (MRB), USA, is the primary cause of hypoxia in the Gulf of Mexico. Regionalscale N mass balances indicate that a small, intensively cropped area of the upper MRB contributes disproportionately to nitrate loading. These aggregate balances miss small-scale variability, especially that caused by differences in farm management. We constructed N mass balances for a gradient of farm types, from corn-soybean monocultures to diversified grain farms that rely on biological N fixation (BNF) as a primary N source, to compare the relative efficiency of diverse farming systems in the MRB. Five-year N balances were calculated for a most and least productive field on each farm using data collected from interviews with 95 grain farmers in Iowa, Ohio, Minnesota, and Wisconsin; from legume biomass and corn grain samples collected from a subset of farms; and published values from the literature. Nitrogen balances ranged from high average annual surpluses (149 kg NÁha ) N rates. Diversified farmers were more likely to adjust their management practices in response to environmental variability compared to fertilizer-based farmers. Taken together, results from this study suggest that significantly reducing surplus N in agroecosystems will require reducing N inputs and increasing C availability to support the internal biological mechanisms for storing N in farm fields.

Relationships for estimating N2 fixation in legumes: incidence for N balance of legume‐based cropping systems in Europe

“…Therefore, N surplus could constitute a robust ecological metric for agricultural policy makers (Blesh and Drinkwater 2013). Nonetheless, many questions remain to accurately estimate BNF mainly due to uncertainties in the belowground contribution.…”

“…The challenge is to better understand and manage the N turnover in the soil to maximize crop yields (N productivity) while minimizing losses of reactive N to the environment. The literature analyzing the N balance in organic farming, at farm or field scales, has often reported very low or even negative surplus (Kelm et al 2008, Blesh andDrinkwater 2013). The common sense considers these results as an indication of insufficient N supply in the cropping system (''N mining'').…”

Cowpea yield under the influence of cropping system, location and nitrogen fertilization

“…According to Martellotto (2010) increasing yield by practicing crop rotation has been known for many years. It has been assumed that the positive effects of rotations arise from the added N from legumes in the cropping system (Blesh and Drinkwater, 2013).…”

A reassessment of the contribution of soybean biological nitrogen fixation to reactive N in the environment

“…Estimates of N r production due to soybean cultivation are uncertain, although agricultural systems with BNF as a major source of new N tend to have lower N surplus and thus appear to lose less nitrate than systems with high N fertilizer inputs (Blesh and Drinkwater 2013;Syswerda et al 2012). Current global estimates of N r loss from soybeans are based on reported yields, the grain to biomass ratio, and the contribution of BNF to total soybean biomass N. Four major assumptions are inherent: 1) the N content of non-grain above-and belowground biomass is 3 %; 2) the harvest index of soybean plants (the grain: total aboveground biomass ratio) is 0.4; 3) the ratio of aboveground N to total crop N content is 1.5; and 4) 68 % of total plant N is provided from BNF (N delivered from the atmosphere, or Ndfa) (Peoples et al 1995a, b;Herridge et al 2008).…”