The importance of canopy-derived dissolved and particulate organic matter (DOM and POM) - comparing throughfall solution from broadleaved and coniferous forests

Dissolved organic matter (DOM) and its main constituents carbon (DOC) and nitrogen (DON) represent an important part of the C and N cycles in forest ecosystems. Although many investigations have been addressing this issue, the knowledge on particulate organic matter (0.45 mu m < POM < 500 mu m) dynamics, its origin and involvement in organic matter cycling in forest ecosystems is still imperfect. In this paper, we report on dissolved and particulate organic carbon and nitrogen fractions in throughfall solutions collected from a broadleaved and coniferous forest stand in Central Germany. Over a period of 2.5 y (2005-2007) we followed the concentrations and fluxes of DOM and POM at a mature beech (Fagus sylvatica L.) and a Norway spruce (Picea abies L.) forest site. Bulk and throughfall precipitation were sampled in weekly (2005) and fortnightly (2006-2007) intervals and analyzed for dissolved (< 0.45 mu m, filtered) and total (< 500 mu m, unfiltered) amounts of organic carbon (DOC, TOC, POC) and nitrogen (TN, DN, PON, NO3-N) species. Proportions of particulate organic C and N were determined by difference between total and dissolved fractions. Under spruce, throughfall concentrations of most C and N fractions were twice as high as under beech. At both sites, concentrations and fluxes were significantly higher during the growing than the dormant season. At the broadleaved site, 80% of the annual fluxes of the DOC and TOC and 70% of the DN and TN were released during the growing season, compared to 60% for C and N at the coniferous site. POC under beech contributes with up to 30% to TOC compared to less than 20% at the spruce site. We suggest that pollen deposition, insect excretions and accumulated organic matter mobilised by dry/wet precipitation patterns play a supreme role for the formation of DOM and POM in forest canopies. The study demonstrates that the canopy is an important source for POM. Dynamics of DOM and POM are mainly driven by tree species effects and seasonality as well as by biotic agents.