Sorption, persistence and leaching of clomazone in rice environments under varying severity of alternate wetting and drying irrigation management with and without biochar amendment

Clomazone (CLMZ) is one of the most effective and widely used herbicides in rice cultivation, but it has probable risks for environmental contamination. Alternate wetting and drying irrigation (AWDI), with or without holm oak biochar (B) amendment, is one of the sustainable alternatives to conventional flooding rice cropping. However, its implementation may induce changes in soil characteristics that can strongly affect the environmental behaviour of the CLMZ, although its extent will depend on the degree of severity of AWDI. This three-year study is the first to assess how two different severities of AWDI, without or with fresh or field-aged B, impact CLMZ's sorption, persistence and leaching. The treatments were: conventional continuous flooding (CCF), moderate AWDI (MAWDI) and intensive (IAWDI) in which fields were reflooded when the soil matric potentials at 0–15 cm depth were −20 kPa and −70 kPa, respectively, and the corresponding homologs with B addition (CCF-B, MAWDI-B, and IAWDI-B, respectively). The transition to AWDI increased CLMZ's sorption but only at the intensive severity level. Fresh and aged B-amendment increased CLMZ's sorption particularly in soils from MAWDI regime, with K_d values increasing by 1.4-fold (fresh case, measurements in the first year after B addition) and 1.3 and 1.2- fold (aged cases, measurements 2 and 3 years after B addition, respectively). The B addition leads to more CLMZ persistence, especially for the IAWDI-B case where the t_1/2 incremented by factors of 1.6 and 1.2 (fresh year) and 1.4 and 1.3 (second aged year) under flooding and AWDI conditions, respectively. The MAWDI and IAWDI regimes reduced CLMZ leaching by up to a factor on average of 2.3 and 2.8, respectively. The addition of B reduced CLMZ's leaching, particularly for the MAWDI regime where their losses were 2.3 and 4.3 times less in the fresh and second aged year, respectively. Therefore, the change in regime from flooding to AWDI alone or using B as amendment could be considered a useful strategy to greatly mitigate pollution of water by CLMZ in rice-growing environments, particularly in those under MAWDI management and after B aging.