Long-term litter input manipulation effects on production and properties of dissolved organic matter in the forest floor of a Norway spruce stand

Environmental factors such as climate and atmospheric CO2 control inputs of plant-derived matter into soils, which then determines properties and decomposition of soil organic matter. We studied how dissolved organic matter (DOM) in forest floors responded to six years of litter fall manipulation at a spruce site. Experimental treatments included (i) ambient litter fall, as well as (ii) reduction or (iii) increase of litter fall, each by 80%. Reduced litter input did not change fluxes of dissolved organic carbon (DOC), which suggests that much of the mobile DOM in forest floors is not from recent litter but older, partly degraded material. Litter addition increased DOC fluxes over 6 years by 68% (Oi), 23% (Oe) and 12% (Oa). This was mainly due to excessive DOM production in Oi horizons, while net DOM production in Oe and Oa horizons decreased. Upon litter addition, aromaticity and molecule complexity of DOM released from the Oi horizon increased, but decreased for DOM leaving Oa horizons. The results suggest that DOM production by lignin degradation was enhanced in Oi but reduced in Oe and Oa horizons upon litter addition. Recent litter is not an important source for DOM entering mineral soils, but increased litter input changes organic matter decomposition, thus affecting fluxes and properties of DOM within forest floors.