Investigations on the binding mechanism of the herbicide simazine to dissolved organic matter in leachates of compost

14C-labelled simazine was composted together with biowaste on a pilot (m3) scale. The herbicide was quickly bound to the compost matrix. By aqueous extraction of 29 and 200 days old compost (equivalent to thermophilic and mesophilic phase of composting) only 4.2% and 3.1% respectively of the radioactivity in the compost samples could be extracted with water. Analysis of the extracts using high-performance size exclusion chromatography (HPSEC) revealed that the dissolved organic matter (DOM) had molecular weights ranging between 2 and 28 kDa. The amount of DOM-associated radioactivity increased from 53% (day 29) to 65% (day 200) of total extractable radioactivity. The type of binding of the 14C-labelled residues and the DOM was elucidated by silylation of humic matter and subsequent HPSEC. The data demonstrated that besides polar metabolites also intact simazine was bound to the DOM. A distinct shift from rather weak interactions to strong covalent linkages of simazine and its metabolites with increasing age of the compost was observed. The results showed that only low amounts of free simazine and its degradates can be extracted with water. We concluded that the shift towards stable covalent linkages is equivalent to a detoxification of the contaminant in aged compost. Consequently, the use of the analysed compost in its mature stage should not pose an environmental risk to the groundwater or the subsoil.     

Leaching of dissolved organic carbon and carbon dioxide emission after compost application to six nutrient-depleted forest soils

The objective of this study was to assess the effect of compost application on soil respiration and dissolved organic carbon (DOC) output of nutrient-depleted forest soils. An amount of 6.3 kg m(-2) mature compost was applied to the forest floor of European beech (Fagus sylvatica L.), Norway spruce (Picea abies Karst.), and Scots pine (Pinus sylvestris L.) stands at Soiling and Unterlüss, Germany. Cumulative soil respiration significantly increased by 499 g C m(-2) in the spruce stand at Unterlüss and by 274 g C m(-2) in the beech stand at Soiling following compost application whereas soil respiration of the other four stands was not affected. The increases in soil respiration of the two stands were explained by improved microbial decomposition of soil organic matter. The DOC concentrations and fluxes in throughfall and seepage water at 10- and 100-cm depths were determined from August 1997 to March 2000. In the control plots, cumulative DOC outputs at 10 cm ranged between 57 and 95 g C m(-2), with the highest rates in the pine stands. Compost treatment significantly increased cumulative DOC outputs by 31 to 69 g C m(-2) at 10 cm and by 0.3 to 6.6 g C m(-2) at 100 cm. The mineral soils between the 10- and 100-cm depths acted as significant sinks for DOC, as shown by much lower cumulative outputs at 100 cm of 3 to 11 g C m(-2) in the control and 6 to 16 g C m(-2) in the compost plots. Our results suggest that a single, moderate application of mature compost to nutrient-depleted forest soils has little effect on C losses to the atmosphere and ground water.