Low-intensity surface fire effects on carbon and nitrogen cycling in soil and soil solution of a Scots pine forest in central Germany

Dissolved and particulate organic carbon (DOC, POC) and nitrogen (DN, PN) are important constituents and indicators of the C and N dynamics in forested ecosystems, but little is known about fire effects on the fluxes of these elements. Biweekly fluxes at three different soil depths (organic layer O, mineral soil A, mineral soil B) were measured with zero-tension lysimeters before and after (prescribed) low-intensity surface fires in a Scots pine forest in central Germany. Measurements of soil organic C and total N concentrations, cold (soluble) and hot (labile) water-extractable DOC and DN, and soil respiration also were conducted for both pre- and post-burn bulk soils. Linear mixed-effect modelling (LMM) revealed that repeated low-intensity fire reduced DOC (− 64%) and DN (− 11%) fluxes in the organic layer, but increased soil CO2 fluxes (+ 7%). A nutrient flush from the charred material into the A horizon, as indicated by an enhanced solution pH (+ 11%) and electrical conductivity (+ 68%), may have stimulated microbial activity, leading to enhanced DOC (+ 47%) and DN (+ 202%) production and fluxes, respectively. The B horizon was unaffected by the fire treatment and retained DOC and DN. In contrast to DOC and DN fluxes, POC and PN fluxes were less affected by the fire treatment and decoupled from those of dissolved organic matter (DOM). Our findings indicate that low-intensity surface fires can significantly affect generally nutrient-poor soil systems by causing a short-term flush (“hot moment”) of DOM in the mineral A horizon (vertical “hot spot”) and by sorption in the mineral B horizon.