Are magnesium stable isotopes a valuable geochemical tool in agronomy?

A sustainable use of soil resources is urgently required to cope with the increasing demand for agricultural products. Advanced soil cultivation methods like subsoiling were suggested but analytical tools to measure changes in the nutrient use efficiency of crops are still missing. Here we tested the applicability of Mg stable isotopes as novel evaluation tool in agronomy and validated our results with the nutrient uptake depth proxy 87Sr/86Sr.

First, we conceptually demonstrated under which conditions changes in the Mg isotope composition (δ26Mg) of crops and the bioavailable fraction of Mg could be resolved from analytical uncertainty, when simulating subsoiling on soils with distinct Mg supply. Shifts in δ26Mg values are only detectable if i) the crop uptake-related Mg isotope fractionation factor is high, ii) a high Mg uptake flux of crops faces a low Mg supply in soil, and iii) subsoiling causes a considerable deepening of the nutrient uptake depth.

Second, we tested our concept on field trials, where deep loosening with and without compost incorporation was conducted. Although 87Sr/86Sr indicates a deepening of the nutrient uptake depth after subsoiling with compost, shifts in d26Mg values of crops and the exchangeable fraction were mostly unresolvable from the analytical uncertainty. Yet, systematic shifts in δ26Mg values among crops cultivated on and beside a melioration strip were found and attributed to the uplift of isotopically distinct compost-derived Mg. Still, Mg stable isotopes represent a valuable tool towards nutrient use efficiency measures on soils with low Mg supply, unless agricultural lime is applied.