Legume versus fertilizer sources of nitrogen: ecological tradeoffs and human needs

Abstract During the 20th century, farmers around the world replaced legume rotations and other traditional sources of nitrogen (N) fertility with synthetic N fertilizers. A sizable percentage of the human population now depends on synthetic N fertilizers for survival. In recent decades, N fertilizers have been linked to numerous environmental hazards including marine eutrophication, global warming, groundwater contamination, and stratospheric ozone destruction. Some researchers suggest that legumes, which can support biological N 2 fixation, offer a more environmentally sound and sustainable source of N to cropping systems. This perspective is countered by researchers who argue that, (1) legume-derived N has equally negative environmental impacts as the N derived from synthetic fertilizers, and (2) the human population now exceeds the carrying capacity of agricultural systems that depend on legumes for N inputs. In this review, we compare the sustainability of obtaining N from legume versus industrial sources in terms of ecological integrity, energetics and food security. We conclude that obtaining N from legumes is potentially more sustainable than from industrial sources. We further suggest that while some countries are fundamentally dependent on synthetic N for food production, many countries have the capacity to greatly reduce or eliminate dependence on synthetic N through adoption of less meat-intensive diets, and reduction of food waste.