Exploring Non-traditional Metal(loid) Stable Isotope Tools for Agricultural Systems

Ratios of non-traditional metal(loid) stable isotopes are a well-established tool in geosciences, used to semi-quantitatively trace geological transformation processes and biological cycling of mineral nutrients in the soil-plant system. Even though these processes also occur in agricultural systems, non-traditional metal(loid) isotope ratios are rarely used in agronomy. Their potential lies in revealing variations in isotope composition of metal elements like Fe and Mg between soil compartments and crops due to isotope fractionation occurring along the solubilization-uptake-translocation pathway [e.g., 1-5]. Agricultural management practices may influence isotope ratios in plant-available soil pools and, consequently, in plants.

The BonaRes-project Soil³ aims to enhance crop yield by optimizing nutrient and water use efficiency for field crops through subsoil management. We hypothesized that creating favorable conditions for crops in subsoil, like reducing physical resistance for roots or creating nutrient-rich hotspots, will stimulate crops to develop deeper root systems than without subsoil management. To examine our hypothesis, we altered subsoil conditions in field trials by cultivating deep-rooting pre-crops and employing technical subsoil improvement techniques through strip-wise deep loosening and organic matter injection. To assess the influence of standard management practices, such as liming, and possible nutrient deficiencies on isotope ratios in soil compartments and plants, we also investigated the isotope composition of nutrient pools in the deep subsoil of long-term field experiments and set up controlled pot experiments with defined nutrient conditions.

In the context of subsoil management experiments, we first conceptually explored the extent to which the Mg isotope composition of soil compartments and crops would be influenced by subsoil management. The novel outcome of this concept is that the Mg use efficiency of crops can be solely quantified from Mg stable isotope ratios, provided that agricultural lime is not applied to the fields [2]. Secondly, we used ⁸⁷Sr/⁸⁶Sr ratios to assess alterations in nutrient uptake depth in the subsoil managed plots. Our findings indicate that deep loosening with compost incorporation indeed deepened the nutrient uptake depth, with crops reaching previously unused nutrient reservoirs [6].

Regarding the influence of liming on Fe and Mg isotope compositions in a 100-year field experiment, we found a shift towards heavier Fe isotopes in rye, indicating an upregulation of the phytosiderophore complexation mechanism to counteract reduced Fe solubility at higher pH [5], and a pronounced shift towards lighter Mg isotopes in the exchangeable Mg pool, mainly attributed to an increased removal of heavy Mg isotopes by plant uptake [3]. A controlled pot experiment revealed that Mg deficiency altered the Mg isotope composition in wheat organs, indicating stress-induced shifts in Mg translocation within the plant [4].

Non-traditional metal(loid) stable isotopes hence provide powerful insights into biogeochemical cycling of nutrients that conventional analyses cannot detect.

[1] Wu et al., Earth-Science Reviews 2019, 190:323-352.

[2] Uhlig et al., Chem. Geol. 2022; 611:121114.

[3] Wang et al., Eur. J. Soil Sci. 2021; 72:300–312.

[4] Wang et al., Plant Soil 2020; 455:93–105.p

[5] Wu et al., Eur. J. Soil Sci. 2021; 72:289-299.

[6] Uhlig et al., Plant Soil 2023; 489: 613–628.