Dissolved organic matter (DOM) is important for the cycling and transport of carbon (C) and nitrogen (N) in soil. In temperate forest soils, dissolved organic N (DON) partly escapes mineralization and is mobile, promoting loss of N via leaching. Little information is available comparing DOC and DON dynamics under tropical conditions. Here, mineralization is more rapid, and the demand of the vegetation for nutrients is larger, thus, leaching of DON could be small. We studied concentrations of DOC and DON during the rainy seasons 1998–2001 in precipitation, canopy throughfall, pore water in the mineral soil at 5, 15, 30, and 80 cm depth, and stream water under different land-use systems representative of the highlands of northern Thailand. In addition, we determined the distribution of organic C (OC) and N (ON) between two operationally defined fractions of DOM. Samples were collected in small water catchments including a cultivated cabbage field, a pine plantation, a secondary forest, and a primary forest. The mean concentrations of DOC and DON in bulk precipitation were 1.7 ± 0.2 and 0.2 ± 0.1 mg L–1, respectively, dominated by the hydrophilic fraction. The throughfall of the three forest sites became enriched up to three times in DOC in the hydrophobic fraction, but not in DON. Maximum concentrations of DOC and DON (7.9–13.9 mg C L–1 and 0.9–1.2 mg N L–1, respectively) were found in samples from lysimeters at 5 cm soil depth. Hydrophobic OC and hydrophilic ON compounds were released from the O layer and the upper mineral soil. Concentrations of OC and ON in mineral-soil solutions under the cabbage cultivation were elevated when compared with those under the forests. Similar to most temperate soils, the concentrations in the soil solution decreased with soil depth. The reduction of OC with depth was mainly due to the decrease of hydrophobic compounds. The changes in OC indicated the release of hydrophobic compounds poor in N in the forest canopy and the organic layers. These substances were removed from solution during passage through the mineral soil. In contrast, organic N related more to labile microbial-derived hydrophilic compounds. At least at the cabbage-cultivation site, mineralization seemed to contribute largely to the decrease of DOC and DON with depth, possibly because of increased microbial activity stimulated by the inorganic-N fertilization. Similar concentrations and compositions of OC and ON in subsoils and streams draining the forested catchments suggest soil control on stream DOM. The contribution of DON to total dissolved N in those streams ranged between 50% and 73%, underscoring the importance of DOM for the leaching of nutrients from forested areas. In summary, OC and ON showed differences in their dynamics in forest as well as in agricultural ecosystems. This was mainly due to the differing distribution of OC and ON between the more immobile hydrophobic and the more easily degradable hydrophilic fraction.
Geloster organischer Kohlenstoff und Stickstoff in Freiland- und Bestandesniederschlag, der Bodenlosung sowie in primaren Vorflutern in der tropischen Bergregion Nord-Thailands
Geloste organische Substanz (DOM) spielt eine entscheidende Rolle beim C- und N-Kreislauf und -transport im Boden. In Waldboden der gemasigten Zonen scheint geloster organischer N (DON) die mobilste N-Bindungsform darzustellen. Auch im Vergleich zum DOC ist DON leichter verlagerbar. Nur wenig ist jedoch uber die DOC- und DON-Dynamik unter tropischen Bedingungen bekannt. In dieser Studie wurden wahrend der Regenzeiten 1998–2001 die DOC- bzw. DON-Konzentrationen in Niederschlag, Bestandesniederschlag, Bodenlosung in 5, 15, 30 und 80 cm Tiefe und Abfluss kleiner Wassereinzugsgebiete in reprasentativen Landnutzungsformen des tropischen Berglands Nordthailands untersucht. Zusatzlich wurde die Verteilung von DOC und DON in zwei operational definierten DOM-Fraktionen bestimmt. Proben wurden auf einer Ackerflache (Kohlanbau), in einer Kiefernaufforstung, einem Sekundarwald und einem Primarwald genommen. Die mittleren DOC- und DON-Konzentrationen betrugen im Niederschlag 1.7 ± 0.2 bzw. 0.2 ± 0.1 mg L–1, wobei die hydrophile Fraktion dominierte. Der Bestandesniederschlag wies bis zu dreifach hohere DOC-Konzentrationen auf, vornehmlich in der hydrophoben Fraktion. Stickstoff lies dagegen keine Anreicherung erkennen. Die maximalen Konzentrationen (7.5–13.9 mg C L–1 und 0.9–1.2 mg N L–1) wurden in den Lysimeterproben aus 5 cm Bodentiefe gemessen. Die Freisetzung in den Auflagen und im obersten Mineralboden erfolgte bei C grostenteils in Form hydrophober Substanzen, wahrend bei Stickstoff die Zunahme der hydrophilen Fraktion entscheidend war. Wie in den Boden der gemasigten Breiten sanken die Konzentrationen mit zunehmender Bodentiefe. Hierbei wurde v. a. der hydrophobe DOM-Anteil in der Bodenlosung stark reduziert. Die Veranderung der DOC-Zusammensetzung im Kronenraum der Baume und in der Auflage zeigt, dass hier primar Substanzen der hydrophoben Fraktion, die N-arm sind, freigesetzt werden. Diese Substanzen werden im Mineralboden sorbiert und so der Bodenlosung entzogen. Im Gegensatz dazu ist DON mehr der labilen hydrophilen Fraktion zuzuordnen. Zumindest auf der Ackerflache mit Kohlanbau scheint auch Mineralisation einen Beitrag zur Abnahme der DOC- und DON-Konzentrationen zu leisten. Dies ist vermutlich Ergebnis einer Stimulation der mikrobiellen Aktivitat durch Mineral-N-Dungung. Die ahnlichen Konzentrationen an DOC und DON sowie deren ahnliche Verteilung auf DOM-Fraktionen in Unterboden und im Abfluss der Waldstandorte zeigen, dass hier Prozesse im Boden den DOM-Austrag bestimmen. Der DON-Anteil am Gesamt-N in den Bachlaufen der bewaldeten Einzugsgebiete liegt zwischen 50 und 73 %, was die Bedeutung von DOM fur den Nahrstoffaustrag aus Waldsystemen unterstreicht. Zusammenfassend zeigen DOC und DON Unterschiede in ihrer Dynamik sowohl unter Wald als auch unter Acker. Dies geht uberwiegend auf die unterschiedliche Verteilung auf die stark sorbierende hydrophobe bzw. die leicht zersetzbare hydrophile Fraktion zuruck.
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