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Temperature-dependent oxide removal from manganese- and iron oxide-coated soil redox bars

Purpose

Soil temperature is a fundamental parameter affecting not only microbial activity but also manganese (Mn^III,IV) and iron (Fe^III) oxide reduction rates. The relationship between Mn^III,IV oxide removal from oxide-coated redox bars is missing at present. This study investigated the effect of variable soil temperatures on oxide removal by Mn^III,IV and Fe^III oxide-coated redox bars in water-saturated soil columns in the laboratory.

Materials and methods

The Mn coatings contained the mineral birnessite, whereas the Fe coatings contained a mixture of ferrihydrite and goethite. Additionally, platinum (Pt) electrodes designed to measure the redox potential (E_H) were installed in the soil columns, which were filled with either a humic topsoil with an organic carbon (C_org) content of 85 g kg^?1 (pH 5.8) or a subsoil containing 2 g C_org kg^?1 (pH 7.5). Experiments were performed at 5, 15, and 25 °C.

Results and discussion

Although elevated soil temperatures accelerated the decrease in E_H after water saturation in the topsoil, no E_H decreases regardless of soil temperature occurred in the subsoil. Besides soil temperature, the importance of soil organic matter as an electron donor is highlighted in this case. Complete removal of the Mn^III,IV oxide coating was observed after 28, 14, and 7 days in the soil columns filled with topsoil at 5, 15, and 25 °C, respectively. Along the Fe redox bars, Fe^III reducing conditions first appeared at 15 °C and oxide removal was enhanced at 25 °C because of lower E_H, with the preferential dissolution of ferrihydrite over goethite as revealed by visual differences in the Fe^III oxide coating. Oxide removal along redox bars followed the thermodynamics of the applied minerals in the order birnessite > ferrihydrite > goethite.

Conclusions

In line with Van’t Hoff’s rule, turnover rates of Mn^III,IV and Fe^III oxide reduction increased as a result of increased soil temperatures. Taking into account the stability lines of the designated minerals, E_H-pH conditions were in accordance with oxide removal. Soil temperature must therefore be considered a master variable when evaluating the oxide removal of redox bars employed for the monitoring of soil redox status.

     

Repeated sublethal freshwater exposures reduce the amoebic gill disease parasite,Neoparamoeba perurans,on Atlantic salmon

Freshwater bathing is one of the main treatment options available against amoebic gill disease (AGD) affecting multiple fish hosts in mariculture systems. Prevailing freshwater treatments are designed to be long enough to kill Neoparamoeba perurans, the ectoparasite causing AGD, which may select for freshwater tolerance. Here, we tested whether using shorter, sublethal freshwater treatment durations are a viable alternative to lethal ones for N. perurans (2–4 hr). Under in vitro conditions, gill‐isolated N. perurans attached to plastic substrate in sea water lifted off after ≥2 min in freshwater, but survival was not impacted until 60 min. In an in vivo experiment, AGD‐affected Atlantic salmon Salmo salar subjected daily to 30 min (sublethal to N. perurans) and 120 min (lethal to N. perurans) freshwater treatments for 6 days consistently reduced N. perurans cell numbers on gills (based on qPCR analysis) compared to daily 3 min freshwater or seawater treatments for 6 days. Our results suggest that targeting cell detachment rather than cell death with repeated freshwater treatments of shorter duration than typical baths could be used in AGD management. However, the consequences of modifying the intensity of freshwater treatment regimes on freshwater tolerance evolution in N. perurans populations require careful consideration.     

Effect of the duration of inoculum exposure on development of citrus canker symptoms on seedlings of Swingle citrumelo

Citrus canker (Xanthomonas citri subsp. citri, Xcc) is one of the most serious diseases citrus in Florida, and elsewhere in the world. The disease causes yield loss and some fresh fruit trade restrictions may apply. Cultural management techniques such as windbreaks may work by not only reducing wind speed, but also reducing the period of exposure of susceptible foliage or fruit to those wind speeds that support infection from incoming inoculum. To investigate the effect of exposure period to inoculum of Xcc, seedlings of canker-susceptible Swingle citrumelo were exposed to sprayed inoculum for increasing periods at different wind speeds. The incidence and severity of citrus canker was assessed. In three experiments the incidence and severity of citrus canker most often increased with longer periods of exposure to inoculum, especially so at wind speeds of ≥16 m/s compared to wind speeds of ≤5 m/s (wind speed also increased disease incidence and severity). Regression analysis demonstrated relationships between period of exposure to inoculum and the percent infected leaves per plant, the number of lesions per plant, the number of lesions per infected leaf, and for the percent of infected leaves with lesions on the petioles at wind speeds of ≥16 m/s (R²?=?0.16–0.72). Due to the effect of inoculum exposure period and wind speed, attempts should be made to minimize exposure of canker-susceptible citrus when wind speed is highest and inoculum is available. Windbreaks should help minimize periods of exposure to splashed inoculum in high winds.