An improved nitrogen difference method for estimating biological nitrogen fixation in legume-based intercropping systems

摘要

The nitrogen difference method (NDM) for quantifying N₂ fixation, based on the same amount of soil N exploited by N₂-fixing and non-N₂-fixing plant, may not be suitable to plants with different root traits. We tested the reliability of NDM in legume-based intercropping systems by two field experiments in Northwest China. In experiment 1, faba bean (Vicia faba), pea (Pisum sativum), and soybean (Glycine max) grew solely or intercropped with maize (Zea mays) with two N application rates (0, 225 kg ha−1). The biomass of faba bean, pea, and maize was significantly increased, whereas that of soybean was decreased when intercropped than solely grown. Aggressivity analyses demonstrate greater N competition ability of faba bean and pea, but not soybean, than maize. An improved NDM (INDM) could mitigate these effects: , where x and 1 − x are planting area of legume and non-legume in the intercropping system. Compared to traditional NDM (TNDM, ), %N _dfa (N derived from air) by INDM was decreased by 54.3% and 39.8% for faba bean, 44.7% and 5.0% for pea, but increased by 113.5% and 191.0% for soybean at the two N application rates, indicating different %N _dfa quantifications between the two methods. In experiment 2, %N _dfa of sole or intercropped faba bean was quantified by TNDM, INDM, and 15N natural abundance method (NA). The %N _dfa only by INDM correlated significantly with that from NA. Both interspecific root interactions and N loss affect %N _dfa estimation. Our results suggested that INDM could be more suitable than TNDM for quantifying %N _dfa of a N₂-fixing plant in intercropping systems.