Abstract
Planetary boundaries for terrestrial inputs of reactive nitrogen (Nr) are transgressed and reducing the input of new Nr and its environmental impacts are major global challenges. Grain legumes fix dinitrogen (N2) in symbiosis with soil bacteria and use soil N sources, but often less efficient than cereals. Intercropping grain legumes with cereals may be a means of increasing use efficiency of soil N. Here, we estimate the global sole cropped grain legume acquisition of N from soil to approximately 14.2 Tg N year−1, which corresponds to one-third of the global synthetic fertilizer N use (109 Tg N year−1) for all crops, assuming that grain legumes recover on average 40% of the fertilizer N. Published data from grain legume-cereal intercrop experiments, employing stable 15N isotope methods, have shown that due to competitive interactions and complementary N acquisition in intercrops, the cereals recover a more than proportional share of the soil N sources. As a consequence, the intercropped legume derives more of its N from the atmosphere, compared with when it is grown as legume sole crop. We estimated that the increased N use efficiency in intercropping can reduce the requirements for fossil-based fertilizer N by about 26% on a global scale. In addition, our estimates indicate that if all current grain legume sole crops would instead be intercropped with cereals, a potential net land saving would be achieved, when also replacing part of the current cereal sole crop area with intercropping. Intercropping has additional potential advantages such as increased yield stability and yield per unit area, reduced pest problems and reduced requirements for agrochemicals, while stimulating biodiversity. It is concluded that crop diversification by intercropping has the potential to reduce global requirements for synthetic fertilizer N and consequently support the development of more sustainable cropping systems.
Highlights
Assuming an average N recovery rate of plant-available soil N of 40% in grain legumes, i.e., similar to cereals, we find that global soil N accumulation in grain legumes corresponds to (14.2*100%/40%=) 35.5 Tg fertilizer N applied to crops
Reducing the global input of fertilizer N by 26% would significantly reduce the requirement for fossil energy to produce and use the fertilizer N, and less N would be lost as greenhouse gas N2O or as nitrate leaching to aquatic environments and improve sustainability of agricultural systems
We estimated that global sole crop grain legume soil-derived N accumulation equals approximately one-third of global fertilizer N use, with soybean as the dominant species
Summary
Anthropogenic inputs of new reactive nitrogen (Nr) are damaging terrestrial and aquatic ecosystems, causing climate change and imposing risks to human health (MEA 2005; Rockström et al 2009; Steffen et al 2015; Sutton et al 2011). From 1860 to 2005 the annual flux of Nr to the land surface has doubled, with an anthropogenic supply of roughly 187 Tg of Nr per year on top of the natural flux of N from the atmosphere to land (Galloway et al 2008). The anthropogenic inputs of Nr are mainly industrial fossil-based fertilizer N and legume crops fixing atmospheric dinitrogen (N2) in symbiosis with soil bacteria, collectively referred to as rhizobia. Much of the growth in the emissions of the important greenhouse gas (GHG) nitrous oxide (N2O), since the pre-industrial era, is attributed to the expansion in agricultural land area and increase in fertilizer N use (Reay et al 2012)
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