Field-aging biochar influence on bensulfuron-methyl behavior in rice under sprinkler irrigation in high water stress regions

The intensive use of bensulfuron-methyl (BM) in rice cultivation, together with its high toxicity to aquatic organisms and its frequent detection in water bodies, raises significant environmental concern. This study assesses the effect of fresh and aged biochar on the sorption–desorption, dissipation and leaching of BM under different irrigation (flooding and sprinkler) and tillage (conventional tillage and no-tillage) systems in rice cultivation, under semi-arid Mediterranean conditions. A two-year field experiment was carried out comparing the following treatments: permanent flooding irrigation with conventional tillage (PFT), sprinkler irrigation with conventional tillage (ST), sprinkler irrigation without tillage (S0T), and each of them amended with holm-oak pruning biochar (PFT-BHO, ST-BHO and S0T-BHO). The sorption-desorption, dissipation and leaching of BM were then assessed under laboratory conditions using soils collected from these field treatments. In all three amended treatments, biochar was applied only in the first year, allowing the assessment of its effects during the first year and during the second year, when the biochar had aged under field conditions. Compared with the traditional flooding system, sprinkler irrigation reduced the sorption capacity of BM and increased its persistence. Thus, the implementation of sprinkler managements increased BM persistence, extending its half-life to up to 68 days compared with approximately 30 days under flooded conditions, which might favour weed control under field conditions. Biochar consistently reduced BM sorption and enhanced its desorption through the rise in soil pH, although this effect was weaker in the second year. In addition, BHO reduced BM persistence under both the first and second years, which was associated with an increase in soil dehydrogenase activity. Furthermore, the addition of BHO reduced the losses of BM by leaching, which were higher under sprinkler management. Therefore, the use of BHO may represent an effective strategy to reduce BM contamination of water resources in rice crop, particularly under water-saving management practices such as sprinkler irrigation, however, this hypothesis needs to be confirmed under field conditions.