Availability and Uptake of Potentially Toxic Elements by Rice as Influenced by Green Mulching

Italian rice farming, accounting for over 50% of European production, is struggling to balance environmental sustainability with food safety, driven by increasing demand for high-value organic products. Rice organic farming includes the use of cover crops for green manuring and mulching to enhance soil fertility and weed control, while minimizing chemical inputs. However, meeting stringent food safety standards is imperative, due to the greater concentration of potentially toxic elements (PTEs) such as arsenic (As), cadmium (Cd), and nickel (Ni) in rice compared with other cereals. Hence, understanding the biogeochemical mechanisms that influence the bioavailability, plant absorption, and grain accumulation of PTEs is essential. This study was aimed to assess the impact of green mulching on nutrient and PTE dynamics in the soil-water-plant system of Italian rice cultivation through a mesocosm experiment. The effect of using cover crops as green mulch was evaluated in comparison with green manuring and bare soil, focusing on two main variables: cover species (hairy vetch and ryegrass) and termination techniques (rolling and chopping). The findings revealed that cover crop fresh biomass significantly boosted microbial degradation, enhancing nutrient cycling, notably for nitrogen and phosphorus. Cover crops reduced Cd bioavailability compared to bare soil, which had higher concentrations both in plant tissues and grain. Green mulching released substantial amounts of N-NH4+, P-PO43-, and dissolved organic carbon (DOC) into standwater. Chopped treatments accelerated nutrient release more than rolled ones, with vetch increasing ammonium levels in solution. Conversely, biomass incorporation through green manuring rapidly established a highly reductive environment, enhancing the mobility of elements associated with iron oxides, resulting in significantly higher concentrations of dissolved As and Ni compared to green mulching, coupled with the behavior of divalent iron in solution. Interestingly, Ni levels initially peaked at decreasing redox potential (Eh), but then progressively diminished while reductive conditions further intensified, differently from iron. Unlike As and Cd, predominantly absorbed by rice during flowering and tillering stages, Ni seemed to accumulate during the early tillering phase. Differences in PTE levels in solution among the various mulching tests were reflected in the plant tissue concentrations, although no significant variations were observed in the grain contents. The adoption of green mulching did not affect As and Ni concentration in rice grain, while decreased Cd. However, standwater quality suggests to devote further attention to the environmental impacts of these techniques.