[Performance Assessment of Field-scale Dry Grass Swale with Shallow Substrate Layer].

A field-scale dry grass swale with impermeable bottom and shallow substrate layer was built in Shanghai, where the groundwater table is very high, to avoid groundwater pollution caused by runoff infiltration. The underdrain pipe of the facility was up bended to avoid outside water flowing backward because local ground elevation is very low. Performance of the facility under the actual precipitation conditions was evaluated in the rainy season of 2019. The average runoff volume reduction is 39.4%, and the peak flow is reduced effectively when rain intensity is lower than 8.0 mm ·h-1. Influent mass load reduction of TSS, COD, TP, and TN are 95.4%, 83.1%, 90.0%, and 57.7%, respectively. Wood chips in the substrate layer and the saturated zone are effective for denitrification during the wet and dry periods, respectively. Hydraulic loading rate and antecedent drying period are the main factors affecting denitrification. Improved influent quality combined with the storage volume supplied by the local urban river network could meet the goal of annual runoff volume reduction and annual pollution load removal in districts with high groundwater levels.

[Nitrogen Removal Efficiencies from Road Runoff by Dry Grass Swales with a Shallow Substrate Layer].

To investigate nitrogen removal efficiencies and mechanisms from road runoff by dry grass swales with a shallow substrate layer, we constructed six dry grass swale columns with different structures and media composition. In order to enhance the nitrogen removal efficiencies during the whole process, fermented woodchips were added into the substrate layer, and saturated zones were established. Semi-synthetic road runoff was used as the influent water. The influent and effluent quality were analyzed, and the change in volumetric water content and ORP of the media were monitored. The results showed significant nitrogen removal by these columns under unfavorable conditions. The range of the average removal rate of TN by the dry grass swales with saturated zones was 67%-78%. The nitrogen removal process mainly occurred during the wet period of the substrate layer. The saturated zones enhanced nitrogen removal efficiencies during the dry period, and also promoted the quick establishment of anoxic conditions in the substrate layer during the wet period. The water-holding transition layer with organic matter was effective at providing a carbon source for denitrification in the saturated zone, and for avoiding the leaching of pollutants caused by organic decomposition.

Short-term low-severity spring grassland fire impacts on soil extractable elements and soil ratios in Lithuania.

Spring grassland fires are common in boreal areas as a consequence of slash and burn agriculture used to remove dry grass to increase soil nutrient properties and crop production. However, few works have investigated fire impacts on these grassland ecosystems, especially in the immediate period after the fire. The objective of this work was to study the short-term impacts of a spring grassland fire in Lithuania. Four days after the fire we established a 400m2 sampling grid within the burned area and in an adjacent unburned area with the same topographical, hydrological and pedological characteristics. We collected topsoil samples immediately after the fire (0months), 2, 5, 7 and 9months after the fire. We analysed soil pH, electrical conductivity (EC), major nutrients including calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K), and the minor elements aluminium (Al), manganese (Mn), iron (Fe) and zinc (Zn). We also calculated the soil Na and K adsorption ratio (SPAR), Ca:Mg and Ca:Al. The results showed that this low-severity grassland fire significantly decreased soil pH, Al, and Mn but increased EC, Ca, Mg, and K,. There was no effect on Na, Fe, and Zn. There was a decrease of EC, Ca, Mg, and Na from 0months after the fire until 7months after the fire, with an increase during the last sampling period. Fire did not significantly affect SPAR. Ca:Mg decreased significantly immediately after the fire, but not to critical levels. Ca:Al increased after the fire, reducing the potential effects of Al on plants. Overall, fire impacts were mainly limited to the immediate period after the fire.