Role of dissolved organic matter in translocation of nutrient cations from organic layer materials in coniferous and broad leaf forests

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Abstract
Dissolved organic matter (DOM) transports many kinds of substances in soil. In order to investigate the contribution of DOM to the transport of nutrient cations from forest organic layers to mineral soils, incubation experiments were conducted on organic layer materials of two Quercus species (broad leaves), Japanese cypress and Japanese cedar. Water leachates from the incubated organic layer materials contained far smaller amounts of mineral anions than cations. The deficit of negative charges was related to the organic anions of DOM. In the course of the decomposition, exchange sites formed on the surface of the plant residues retained some of the cations, which were prone to elution by organic anions of DOM. Potassium was least retained; 33 to 62% of the initial contents were leached with water during the 5-week incubation period. The divalent cations, Mg and Ca, were not substantially leached. Still at most 81 and 45%, for Mg and Ca, respectively, of the initial amounts were transformed into an ammonium acetate-extractable form" due to the elution by organic anions of DOM on a longer time scale. The amount of DOM controlled the total amount of eluted cations. The extent of cation mobilization differed among the four plant species, and decreased in the order of broad leaf species> cypress> cedar. A similar tendency was also observed in the distribution patterns of the cations between the organic layers and the mineral soils in fields. The production of DOM increased at higher (up to 30°C) temperature, while the increase in CO2 emissions was larger. The production of DOM and CO2 showed a linear relation for the Oe layer materials, while no correlation was detected for the Oi ones, presumably because water-soluble saccharides, the major substrates used in the Oi layer, did not produce efficiently DOM. The production of DOM and CO2 was larger in the broad leaf litter than in the coniferous one. The emission of CO2 was negatively correlated with the C/P and lignin/P ratio, while the production of DOM was positively correlated with the contents of hot water-soluble N and hemicellulose. The largest correlation coefficient was detected between CO2 and DOM production.