Intercropping of grain legumes and cereals improves the use of soil N resources and reduces the requirement for synthetic fertilizer N: A global-scale analysis

Agronomy for Sustainable Development (2020) 40: 5 https://doi.org/10.1007/s13593-020-0607-x REVIEW ARTICLE Intercropping of grain legumes and cereals improves the use of soil N resources and reduces the requirement for synthetic fertilizer N: A global-scale analysis Erik Steen Jensen 1 1 & Georg Carlsson & Henrik Hauggaard-Nielsen 2 Accepted: 14 January 2020 / Published online: 10 February 2020 # The Author(s) 2020 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. Keywords Biochemical flows of nitrogen . Crop diversification . Greenhouse gas emissions . Soil nitrogen use efficiency . Symbiotic N2 fixation Contents 1. Introduction 2. Global acquisition of N from the soil by grain legumes 3. Soil nitrogen dynamics and use in grain legume-cereal intercropping 4. Global soil N use by intercrops and potential fertilizer N and land sparing 5. Conclusions and perspectives * Erik Steen Jensen 1 Department of Biosystems and Technology, Swedish University of Agricultural Sciences, SE-23053 Alnarp, Sweden 2 Department of People and Technology, Roskilde University, DK-4000 Roskilde, Denmark 1 Introduction 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). 5 Page 2 of 9 Post-World War II the use of synthetic N fertilizer (hereafter referred to as N fertilizer) has led to significant increases in food production, due to the fossil-based Haber-Bosch process of N fertilizer production (Crews and Peoples 2004; Smil 2002). The current global use of N fertilizer was estimated to approx. 109 Tg of N per year in 2017 (FAOSTAT 2019) and the N2 fixed by grain legumes (including soybean and groundnut) to be approx. 21.5 Tg N and of pasture and forage legumes to between 12 and 25 Tg N per year (Herridge et al. 2008). Before the common introduction of N fertilizers, typically 25–50% of farmed land was cropped with legume-based pastures or cover crops in the global North to regenerate soil fertility or was fertilized with animal manures (Crews and Peoples 2004). The planetary boundary work highlights the zone of uncertainty or high risk of the biochemical flow of N (Rockström et al. 2009; Steffen et al. 2015). To be within the planetary boundary, new Nr inputs should be limited to less than half of present inputs at a global level, with even more drastic reductions in some regions (Steffen et al. 2015). The reduction of new Nr inputs will require new paradigms in terms of N use and cycling in global food production. More efficient N use and reduced N losses are key factors in reducing inputs of new Nr (MEA 2005). These factors have been and still are at the top of the global agricultural research agenda. The increased food production, availability of cheap fossil energy for N fertilizer production, increased meat consumption and lack of action on mitigating greenhouse gases (Foley et al. 2011) have counterbalanced knowledge gains, innovations and policies on the reduction of N losses and increased nitrogen use efficiency. Real transition towards closing the agricultural N cycles involves redesign of cropping systems for a more balanced use of new Nr, based primarily on enhanced recycling at several levels, e.g., from household organic waste to agriculture or between farms with or without animals, and on new crops and fertilization methods for more efficient use of different N sources, such as perennials grain crops (Crews et al. 2016) and differential fertilization based on field-scale variability. Currently, the N fertilizer production (incl. Transport and storage) requires between 65 and 100 MJ per kg N fertilizer (Kongshaug 1998; Wood and Cowie 2004) depending on factory and fossil fuel type. It results in an emission of 2.1–5.5 kg CO2 equivalents per kg−1 N fertilizer. The emission of greenhouse gasses from the production of fertilizer N can thus be estimated to between 229 and 545 Tg CO2 equivalents. Additional Nfertilizer driven emissions as N2O occur in agricultural fields, adding up to a total GHG emission of 703 Tg CO2 equivalents, or 13.4% of agriculture’s total GHG emissions caused by the production and use of N fertilizer (FAOSTAT 2018). Nitrogen fixation by legumes is not associated with fossil carbon costs and N2O emissions are seldom greater than emissions from bare soil or N fertiliz (...truncated)