Natural Reactive Iron Dynamics in the Agricultural Soil of Semiarid to Arid Systems

Quantifying redox-driven changes in iron chemistry in irrigated semiarid to arid soils and their relevance for the availability of nutrients and contaminants is critical for global food security. Data across three growing seasons and two different soil types in semiarid to arid climates indicate site-independent peaks of reactive iron in soil aligned with peaks in irrigation events. The reactive iron formed during irrigation was short-lived, and the concentration was back at baseline during harvest. The significant (p < 0.01) increase of reactive iron ranging from 1589.0 ± 172.3 to 1898.0 ± 201.1 μg g^–1 over the growing season triggered by reducing conditions due to transient water infiltration resulted in the mobilization of organic soil carbon and affected the mobility and plant availability of nitrogen, uranium, and arsenic. Porewater samples collected during irrigation events demonstrated increasing iron concentrations over time and positively correlated (p < 0.05) with arsenic and uranium levels. Geogenic arsenic mobilization into soil porewater during peak irrigation events contained significantly (p < 0.01) higher (∼90%) reduced inorganic arsenic species. Crop tissue analysis indicated that roots contained the highest concentrations of trace elements, followed by shoots and grains. Coupled (bio)geochemical redox cycles of iron, nutrients, and contaminants seem to play a critical but so far less recognized role for crop production in irrigated agroecosystems of semiarid to arid systems.