Altitudinal gradient of dissolved organic carbon in headwater

Altitudinal gradient of dissolved organic carbon in headwater streams of the Wuyi Mountains in China Wei 1 1 Huang , Honghua 1＊ Ruan , Xiaoming 1, 2 Zou , William H. 3 Mc. Dowell , Jiashe 4 Wang , Liguang 1 Li Faculty of Forest Resources and Environmental Science, Nanjing Forestry University, China; 2 Institute for Tropical Ecosystem Studies, University of Puerto Rico, USA; 3 Department of Natural Resources and the Environment, University of New Hampshire, USA; 4 Administrative Bureau of Wuyishan National Nature Reserve, China. Question Result Dissolved organic carbon (DOC) in stream water is positively correlated with soil organic carbon (SOC) at regional scales in which climate and vegetation are relatively invariant. • Does this correlation hold when variations of temperature, precipitation and vegetation are large across altitudinal gradient? • Annual mean concentration of stream DOC exhibited a decreasing pattern with increasing altitude. • In contrast, annual mean contents of SOC showed an increasing trend with altitude. Contents of SOC correlated negatively with stream DOC along the altitudinal gradient. Objective • To examine the altitudinal patterns of headwater stream DOC concentrations along an altitudinal gradient in the Wuyi Mountains of China. • To investigate the relationship between headwater stream DOC concentrations and SOC contents along an altitudinal gradient. • To find the environmental factors that can be used to predicted headwater stream DOC concentrations. Fig. 2 Annual mean values in headwater stream (a) DOC concentrations, (b) SOC contents, and (C) temperature (S 1: 0 -10 cm soil layer; S 2: 10 -25 cm soil layer; S 3: 25 -40 cm soil layer). Significant differences between the means are marked with different letters. Site description and experimental design Wuyi Mountain National Nature Reserve, with distinct vertical zones of vegetation along the altitudinal gradient, is located in the subtropical monsoon region of China. Fig. 3 Correlations between annual mean headwater stream DOC concentrations and (a) altitude, (b) SOC contents, and (C) temperature (n=12; * indicated significant level at 0. 05). • Positive correlations were found between stream DOC concentrations and temperature. Stream DOC concentrations peaked in February at the end of the dry season and were lowest in August at the end of the rainy season. Seasonal variation of stream DOC concentrations correlated positively with stream water nitrate concentrations in the antecedent two months. Fig. 1 Map of headwater streams in the Wuyi Mountain National Nature Reserve of southeastern China (WNR, Wuyi Mountain National Nature Reserve; FJ, Fujian Province; JX, Jiangxi Province. ). Table 1 Mean annual precipitation (mm). Subtropical evergreen broadleaf forest (EBF) Coniferous forest (CF) Subalpine dwarf forest (SDF) Alpine meadow (AM) 1700 2000 2200 3100 Nanjing Forestry University *Corresponding author; e-mail: honghuaruan@gmail. com Fig. 4 Seasonal variations of (a) headwater stream DOC, (b) nitrate, and (c) correlation between headwater stream DOC concentrations and stream nitrate concentrations of the antecedent two months (n=72; ** indicated significant level at 0. 01). ☼ Conclusion ☼ • Concentrations of DOC in headwater streams are not solely controlled by SOC contents. Factors influencing the decomposition of SOC, such as temperature, as well as the dilution of stream DOC source by precipitation, can also play an important role in determining DOC concentrations in headwater streams along an altitudinal gradient that across vegetation types. • Annual mean values in headwater stream DOC can be largely predicted by temperature alone across the altitudinal gradient, and seasonal variation in stream DOC levels can be predicted by stream nitrate concentrations two months in advance.