Nitrogen input strategies impact fertilizer nitrogen saving by intercropping: A global meta-analysis

Several meta-analyses have demonstrated that intercropping increases land use efficiency and fertilizer use efficiency of grain-producing crop species, but no overarching synthesis has been made on the effects of fertilizer management on fertilizer use efficiency. Here we assess the relative N use efficiency of species mixtures compared to sole crops under different N input strategies using global data. We built a global database of yield and fertilizer N input with 600 data records representing the results of 136 independent experiments from 80 publications with four main types of species functional combinations (SFCs), based on key traits that were found relevant in previous intercropping studies: C3-cereal/legume, maize/legume, C4-non-maize/legume and maize/C3-cereal. We found that the literature reports results for four main N input strategies in the intercrop and sole crops: (1) zero fertilizer N input (N ic =N 1 =N 2 =0), (2) equal fertilizer N input (N ic =N 1 =N 2 >0), (3) intermediate fertilizer N input (N 1 ≥N ic ≥N 2 and N 1 >N 2 ), and (4) transgressive fertilizer N input (N ic >N 1 ≥N 2 ), where N 1 , N 2 and N ic represent the N input in sole crop 1, sole crop 2 and the intercrop. With zero N input, high land equivalent ratio was found in cereal/legume intercrops but not in maize/C3-cereal intercrops. Intermediate N input (strategy 3) resulted in high LER (land equivalent ratio) and FNER (fertilizer N equivalent ratio) because of the high intercropping advantage of cereals in C3-cereal/legume, maize/legume, and C4-non-maize/legume intercrops and maize in maize/C3-cereal intercrops. Equal N input in the sole crops and the intercrops resulted in LER and FNER being equal and N saving was entirely due to land saving, regardless of SFCs. Transgressive N input resulted in high LER but low FNER. The study confirms that cereal/legume intercropping increases LER at zero N input. Strategies that tailor N input in intercropping to species demand (strategy 3) pair high productivity and LER to high fertilizer N use efficiency. The transgressive N input strategy maximizes LER at the expense of FNER, thus potentially generating high N losses.