Bachand & Associates, Davis, CA 95618, United States.
U.S. Geological Survey, California Water Science Center, Sacramento, CA 95819, United States.
Sci Total Environ. 2014 Feb 15;472:957-70. doi: 10.1016/j.scitotenv.2013.11.086. Epub 2013 Dec 15.
Concentration and mass balance analyses were used to quantify methylmercury (MeHg) loads from conventional (white) rice, wild rice, and fallowed fields in northern California's Yolo Bypass. These analyses were standardized against chloride to distinguish transport pathways and net ecosystem production (NEP). During summer, chloride loads were both exported with surface water and moved into the root zone at a 2:1 ratio. MeHg and dissolved organic carbon (DOC) behaved similarly with surface water and root zone exports at ~3:1 ratio. These trends reversed in winter with DOC, MeHg, and chloride moving from the root zone to surface waters at rates opposite and exceeding summertime root zone fluxes. These trends suggest that summer transpiration advectively moves constituents from surface water into the root zone, and winter diffusion, driven by concentration gradients, subsequently releases those constituents into surface waters. The results challenge a number of paradigms regarding MeHg. Specifically, biogeochemical conditions favoring microbial MeHg production do not necessarily translate to synchronous surface water exports; MeHg may be preserved in the soils allowing for release at a later time; and plants play a role in both biogeochemistry and transport. Our calculations show that NEP of MeHg occurred during both summer irrigation and winter flooding. Wild rice wet harvesting and winter flooding of white rice fields were specific practices that increased MeHg export, both presumably related to increased labile organic carbon and disturbance. Outflow management during these times could reduce MeHg exports. Standardizing MeHg outflow:inflow concentration ratios against natural tracers (e.g. chloride, EC) provides a simple tool to identify NEP periods. Summer MeHg exports averaged 0.2 to 1 μg m(-2) for the different agricultural wetland fields, depending upon flood duration. Average winter MeHg exports were estimated at 0.3 μg m(-2). These exports are within the range reported for other shallow aquatic systems.
采用浓度和质量平衡分析方法，量化了来自加利福尼亚州约洛旁路北部常规（白）水稻、野生水稻和休耕农田的甲基汞（MeHg）负荷。这些分析以氯化物为标准，以区分传输途径和净生态系统生产（NEP）。在夏季，氯化物负荷与地表水一起被输出，并以 2:1 的比例进入根区。MeHg 和溶解有机碳（DOC）与地表水和根区出口的行为相似，比例约为 3:1。这些趋势在冬季发生逆转，DOC、MeHg 和氯化物以与夏季根区通量相反且超过的速度从根区移动到地表水。这些趋势表明，夏季蒸腾将地表水中的物质经平流输送到根区，而冬季由浓度梯度驱动的扩散随后将这些物质释放到地表水中。这些结果对许多关于 MeHg 的范式提出了挑战。具体来说，有利于微生物 MeHg 产生的生物地球化学条件不一定转化为同步的地表水输出；MeHg 可能在土壤中得到保存，随后在以后的时间释放；植物在生物地球化学和运输中都发挥作用。我们的计算表明，夏季灌溉和冬季洪水期间均发生了 MeHg 的 NEP。野生水稻湿收割和冬季白稻田洪水是增加 MeHg 输出的具体做法，这两种做法都可能与增加的可利用有机碳和干扰有关。在这些时期进行流出物管理可以减少 MeHg 的输出。将 MeHg 流出物：流入物浓度比标准化为天然示踪剂（例如氯化物、EC）提供了一种简单的工具，可以识别 NEP 期。不同农业湿地的夏季 MeHg 平均出口量为 0.2 至 1 μg m(-2)，具体取决于洪水持续时间。估计冬季 MeHg 的平均出口量为 0.3 μg m(-2)。这些出口量在其他浅水生态系统报告的范围内。
