Nitrogen rhizodeposition by legumes and its fate in agroecosystems: A field study and literature review

Quantification of legume nitrogen (N) rhizodeposition (N derived from roots) and its fate in agroecosystems is crucial for managing soil fertility, land productivity, and agriculture sustainability. In contrast to forage legumes, the N rhizodeposition by grain legumes is nearly unknown. Therefore, N rhizodeposition of four grain legumes and its transfer to subsequent wheat crops was quantified using the 15N stem labeling method under field conditions. The N rhizodeposition of the grain legumes: peanut, soybean, mungbean, and adzuki bean amounted to 25, 51, 20, and 63 kg N ha−1, respectively. N rhizodeposition was not affected by fertilization, and it was 53–257% more accumulated in topsoil (0–20 cm) than that in subsoil (20–40 cm). However, N rhizodeposition per unit of root biomass in subsoil was 3.5-times as much as that in topsoil (p < 0.05), indicating the importance of legumes for soil fertility and exploration in subsoil. Remarkably, subsequent wheat utilized 13–85% of legume N rhizodeposition, which contributed to 4–20% of total wheat N uptake. Combining the present data with the literature review, the average N rhizodeposition of legumes (both grain and forage legumes) is 83 kg N ha−1 (n = 75), and one-fourth of which was utilized by subsequent cereals. Increasing root biomass by 1 g increases rhizodeposition by 53 mg N. In conclusion, legume N rhizodeposition is crucial for the sustainability of legume-based crop rotations resulting in soil N build-up and is an important N source for subsequent crops. © 2020 John Wiley & Sons, Ltd.