Fluxes of dissolved organic carbon and nitrogen throughout Andisol, Spodosol and Inceptisol profiles under forest in Japan

Leaching of dissolved organic matter (DOM) is an important process in the translocation and stabilization of organic carbon (C) and in influencing nitrogen (N) availability in forest soils. The roles of DOM in soil carbon and nitrogen cycles were evaluated by quantifying the fluxes of dissolved organic carbon (DOC) and nitrogen (DON) entering and leaving the organic (O), A and B horizons. In Spodosol and Inceptisol soils, DOC fluxes were highest in the O horizon (149 to 344 kg C ha(-1) yr(-1)), decreasing in the A and B horizons. In Andisol soils, DOC fluxes were low throughout the profile because of low DOC production in the O horizon (53 kg C ha(-1) yr(-1)) and the high adsorption capacity of amorphous aluminum (Al) and iron (Fe) (hydr)oxides in the mineral horizons. In Spodosol soil, DOC from the O horizon represented a large proportion of C input into the mineral soil, whereas this contribution appeared to be small in Andisol soil. The DOM was enriched in nitrogen during decomposition and humification of the foliar litter, but DON was a small proportion (5-31%) of total dissolved nitrogen (TDN) in surface soil solutions. The narrow DOC/DON and DON/TDN rarios were attributable to the low C/N ratios of the foliar litter (33-40). It was quantitatively shown that the importance of DOM in C and N cycles in forest soils varied depending on soil types and litter C/N ratio.