Boron isotope fractionation in soil-plant systems and its influence on biogeochemical cycling

Boron (B) is an essential mineral nutrient for higher plants. Although B plant nutrition is well studied, the B isotope fractionation at the soil-plant interface, within plant metabolism, and its influence on biogeochemical cycling is not fully understood. Boron concentrations and isotope variations (δ¹¹B) of the dicotyledonous plants of Chenopodium album and Brassica napus and their growing soils along a climatic gradient were analyzed to decipher these unresolved issues of the B behavior. The boron concentrations and δ¹¹B values show an increasing trend from roots to leaves for both plants, while a decreasing trend from flower to shell and to seed for Brassica napus. A large boron isotope fractionation occurs within the plants with median Δ¹¹B_leaf-root ≈ +20‰, which is related to different boron transporters and transportation ways. Formation of borate dimerized rhamnogalacturonan II in cell and B(OH)₃ transportation in xylem lead to heavier δ¹¹B values from root to stem and leaf while B(OH)₄⁻ transportation in phloem lead to lighter δ¹¹B values from flower to shell and seed. Although samples cover a distinct transect with systematically different climatic conditions, Δδ¹¹B within the individual plant compartments and between the bulk plants and the soil available B do not show any systematic variation. This suggests that B uptake from the soil into Chenopodium album and Brassica napus occurs without a distinct isotope fractionation at the soil-plant interface (median Δ¹¹B_{bulkplant-soil} = −0.2‰) and plants are able to regulate boron uptake. Both the observed large B fractionation within plant and low or absent B isotope fractionation at the soil-plant interface may have profound implications for the biological and geological B cycle. If this observed boron behavior also exists in other plants, their litters would be an important source for exporting ¹¹B-rich biological material from continental ecosystems via rivers to the global oceans. This may be helpful for the explanation of ocean B cycle and the increasing δ¹¹B values over the Cenozoic.