Local weather conditions determine DOC production and losses from agricultural fen soils affected by open-pit lignite mining

Abstract

Disturbance to peatlands via climate change and human activity affects their carbon storage potential. This leads to the loss of a significant part of their carbon stock as dissolved organic carbon (DOC) via fluvial pathways. The grassland fens in the Grójec Valley, Poland are dominated by organic soils, which degraded as a consequence of long-term agricultural use. Here, we assessed the seasonality of DOC production in these soils and its release to groundwater. To the best of our knowledge, this the first example of a seasonal DOC production assessment in agricultural fen soils that have also been affected by open-pit mining. All designated sites in the study area were under agricultural use (grasslands) and two of the sites were also located within predicted depression cones, caused by lignite mining operations. Samples were taken during the growing season (March–October) over three consecutive years 2017–2019. The observed DOC concentrations in the soils (1.00–4.99 g kg−1) and water (18.8–73.4 mg l−1) were similar to other studies conducted on agro-managed fens in central Europe. We found that the highest labile carbon concentrations in the soils and water occurred after dry/warm periods in the study area. However, the observed seasonal variability was especially evident in DOC concentrations in the groundwater, which indicates its dependence on weather conditions. Our preliminary assumption that DOC production and release would be greatest in sites located within predicted depression cones was not confirmed in this study. Overall, our results indicate that temperate grassland fens are endangered if future climate warming results in soil carbon losses via soil water deficiency and further acceleration of organic matter decomposition.