Underestimated role of nodulated roots for soil fertility and quality

Nitrogen (N) storage in legumes is usually estimated by N₂ fixation in shoots, whereas there is little knowledge on the contribution of roots and nodules to legume N and soil N. Here, we studied the contribution of recovered roots and nodules of grain and pasture legumes to plant N and soil N in Mediterranean fields. Experiments were run under rainfed conditions for a 2-yr period in three regions of Portugal (central and south west, and south inland). Complete plants including top plant and visible roots and nodules were sampled at the end of the growing seasons for grain legumes, sweet and yellow lupines, and over two harvests in case of pastures. N₂ fixation was measured by the ¹⁵N techniques. Results showed that aboveground N concentration did not vary significantly among species, but differed in the belowground tissues (Carranca et al., 2015). Roots and nodules contributed to 7 – 11% of total legume N, with an allocation of 11 – 14 kg N fixed t⁻¹ belowground dry weight (DW) in indeterminate legumes, representing half amount of aboveground plant. This finding demonstrated that investigation relying only on shoot N underestimates the role of legumes for soil N fertility. Further long - term studies on the contribution of belowground tissues to the soil N fertility are fully encouraged. Rhizodeposits also contribute strongly for stable organic matter in soils improving the soil structure (Pinto et al., 2021). Caddish et al. (2002) reinforced this statement and referred that nodulated roots and rhizodeposits may be protected from mineralization then playing a more important role in building of soil structure rather than in soil N supply by its high C concentration.