Natural organic matter contribution to throughfall acidity in French forests

This study examined the natural organic acidity of throughfall in French forests in order to determine the natural and anthropogenic contributions to the total acidity. The first results presented here were achieved from 64 open field precipitation and throughfall samples, collected from June to December 1996 at 7 RENECOFOR stations (French permanent plot network for the monitoring of forest ecosystems). A titration method was developed to quantify the organic contribution to the acid neutralizing capacity (ANC), to the charge balance and to the weak acidity. The natural organic acids occurring in throughfall samples were characterized by a mean site density of either 7.3 or 10.2 mu eq/mg of C, considering only titratable organic acidity or total organic acidity, respectively. The mean charge density was 5.2 mu eq/mg of C and, as a linear relationship exists between the charge density and pH, an estimate of the organic anion concentration can be obtained directly from dissolved organic carbon (DOC) and pH measurements. The organic contribution to the ANC, and thus to the buffering capacity, is of prime importance for those waters which have pH values between 3.8 and 6.6, and characterized by a low bicarbonate content and a high DOC concentration. The mean organic contribution to the titrated ANC was 2.2 mu eq/mg of C, which is less than half the charge density. These first results show that the acid-base properties of DOC are similar for all studied samples, despite different geographical situations, climates, and vegetation types. Furthermore, when comparing these results to literature data (obtained for stream, lake, and soil solution samples), it appears clearly that, unless a methodological artifact exists, the organic acids dissolved in throughfall are weaker. (C) 1998 Elsevier Science Ltd.