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Long-term elevated atmogenic deposition (~5 g m −2 year −1) of reactive nitrogen (N) causes N saturation in forests of subtropical China which may lead to high nitrous oxide (N 2O) emissions. Recently, we found high N 2O emission rates (up to 1,730 μg N 2O–N m −2 h −1) during summer on well-drained acidic acrisols (pH = 4.0) along a hill slope in the forested Tieshanping catchment, Chongqing, southwest China. Here, we present results from an in situ 15N–NO 3− labeling experiment to assess the contribution of nitrification and denitrification to N 2O emissions in these soils. Two loads of 99 at.% K 15NO 3 (equivalent to 0.2 and 1.0 g N m −2) were applied as a single dose to replicated plots at two positions along the hill slope (at top and bottom, respectively) during monsoonal summer. During a 6-day period after label application, we found that 71–100 % of the emitted N 2O was derived from the labeled NO 3− pool irrespective of slope position. Based on this, we assume that denitrification is the dominant process of N 2O formation in these forest soils. Within 6 days after label addition, the fraction of the added 15N–NO 3− emitted as 15N–N 2O was highest at the low-N addition plots (0.2 g N m −2), amounting to 1.3 % at the top position of the hill slope and to 3.2 % at the bottom position, respectively. Our data illustrate the large potential of acid forest soils in subtropical China to form N 2O from excess NO 3− most likely through denitrification. [ABSTRACT FROM AUTHOR]

Leaching of dissolved organic carbon (DOC) from the O layer is important for the carbon cycling of forest soils. Here we study the role of the Oi, Oe and Oa horizons in DOC leaching from the forest floor in field manipulations carried out in a Norway spruce forest stand in southern Sweden. The manipulations involved the addition and removal of litter and the removal of Oi, Oe and Oa horizons. Our data suggest that both recent litter and humified organic matter contribute significantly to the leaching of dissolved organic matter from the O layer. An addition of litter corresponding to four times annual litterfall resulted in a 35% increase in DOC concentrations and fluxes although the specific UV absorbance remained unchanged. The removal of litterfall and the Oi horizon resulted in a decreased DOC concentration and in a significant increase in the molar UV absorptivity. The DOC concentration under the Oa horizon was not significantly different from that under the Oe horizon and there were no increase in DOC flux, but rather a decrease, from the bottom of the Oe horizon to the bottom of the Oa horizon, suggesting that there is no net release of DOC in the Oa horizon. However, significant leaching of DOC occurred from the Oa horizon when litterfall and the Oi and Oe horizons were removed. This indicates that there is both a removal of DOC from the Oi and Oe horizons and a substantial production of DOC in the Oa horizon. Quantitatively, we suggest that the Oi, Oe and Oa horizons contributed approximately 20, 30 and 50%, respectively, to the overall leaching of DOC from the O layer. [ABSTRACT FROM AUTHOR]