Drying the surface soil reduces the nitrogen content of faba bean (Vicia faba L.) through a reduction in nitrogen fixation

Plant water extraction in the field often dries the soil surface while sufficient water for root functioning is available at depth. The shoot and root behaviour of nodulated Vicia faba (L) to drying the upper half of the soil profile was investigated in a glasshouse experiment. A segmented soil column system was used that hydraulically separated the root system into upper (0-0.35 m) and lower (0.35-0.70 m) soil layers without hindering root growth through both layers. The treatments imposed were (1) adequately watered in the upper and lower soil layers of the columns (well-watered) and (2) the upper soil layer unwatered with the lower layer kept adequately watered (water limited). The treatments were applied when the first flowers were visible at 61 days after sowing and continued for the following 20 days. Drying the upper 0.35 m soil layer slowly developed a plant water deficit despite well established roots and a well-watered lower soil layer. Fourteen days after withholding water to the upper soil layer, leaf conductance (gleaf) was reduced by 20% and net photosynthetic rate decreased by 25%. The expansion of leaflets formed during the period of drying was slightly reduced, but since leaf number was not affected, the total green area was only marginally reduced. Root and root nodule biomass were significantly reduced as a result of reductions in root length and root nodule number, respectively. The proportion of nitrogen derived from the atmosphere (Ndfa) was reduced by 37% and this contributed to a 21% reduction in total plant nitrogen (N). With some allowance for the effect of the reduced photosynthetic rate on Ndfa, the observation that drying the upper soil layer affected significantly root nodule number and biomass indicates that the reduction in Ndfa and total plant N was likely to be a consequence of the decay of root nodules.