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

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 N2 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.