Two pathways drive enhanced nitrogen acquisition via a complementarity effect in long-term intercropping

Context or problem: The complementarity effect (CE) that leads to more efficient utilization of resources has been invoked to explain the biodiversity-ecosystem functioning (BEF) relationships. Dinitrogen (N-2) fixation by legumes is a widely-accepted pathway to enhance N acquisition in diverse species communities. However, the relationships among positive CE, N-2 fixation, and soil N cycling are still poorly understood, especially in agroecosystems. Objective or research question: This study tested the hypothesis that positive CE enhances nitrogen (N) acquisition, driven by increased N-2 fixation of legumes and enhanced soil N cycling in intercropping systems. Methods: We used a long-term (12 years) field experiment of maize intercropped with both legumes (faba bean, chickpea, and soybean) and a non-legume (oilseed rape) under three phosphorus application rates and a meta-analysis, to explore the main processes for a positive CE of N acquisition in intercropping. Results: Intercropping increases grain yield and N acquisition compared with monocultures, with an increase of 35.1% and 28.0% of the average annual yield and N acquisition, respectively. This N acquisition gain (72 kg N ha(-1) in field studies and 14 kg N ha(-1) in a meta-analysis) was largely due to the complementarity effect (65 kg N ha(-1) in field studies and 25 kg N ha(-1) in the meta-analysis), which was significantly correlated with enhanced N-2 fixation in legume-based intercropping systems, in both field studies and the meta-analysis. After 12 years, the soil N concentration of intercropping systems was 4.0-6.3% higher than that of monocultures as a result of enhanced N-2 fixation and N-acquiring enzyme activities, which accounted for positive complementarity in both legume-based and non-legume-based systems. However, those benefits were only observed in later years. We provide a novel mechanism that a positive feedback effect of crop diversity on soil fertility enhances CE over time. Conclusions: Our study demonstrates that enhanced CE is related to N-2 fixation by legumes and a positive feedback effect of crop diversity on soil fertility which highlights that increasing soil N cycling aided by crop diversity may strengthen BEF relationships and enhance grain yield on a decadal scale.