Input and output of dissolved organic and inorganic nitrogen in subtropical forests of South China under high air pollution
Résumé
The nitrogen (N) emissions to the atmosphere and are thereby N deposition to forest ecosystems increasing rapidly in Southeast Asia, but little is known about the fate and effects elevated N deposition in warm and humid forests. Here we report the concentrations and fluxes of dissolved inorganic (DIN) and organic N (DON) in precipitation, throughfall, surface runoff and soil solution for three subtropical forests in a region of South China under high air pollution, to investigate how deposited N is processed and examined the importance of DON in N budget. The precipitation DIN input was 32?34 kg N ha?1 yr?1. An additional input of 18 kg N ha?1 yr?1 as DON was measured in 2005, which to our knowledge is the highest DON flux ever measured in precipitation. Dry deposition was of minor importance at the site. A canopy uptake of DIN was indicated in two young conifer dominated forests (72?85% of DIN input reached the floor in throughfall), whereas no uptake occurred in an old-growth broadleaf forest. The DON fluxes in throughfall of all forest were similar to that of precipitation. In the young forests DIN was further retained in the soil, but 41?63% of precipitation DIN was still leached. Additionally, about half of the DON input was retained in these forests. The N retention in the two young aggrading forests (21?28 kg N ha?1 yr?1) was in accordance with estimates of N accumulation in biomass and litter accretion. In the old-growth forest, no N retention occurred, but rather a net loss of 8?16 kg N ha?1 yr?1 from the soil was estimated. In total up to 60 kg N ha?1 yr?1 was leached, indicating that this forest was completely N saturated and could not retain additional anthropogenic N inputs. We found that the majority of DIN deposition and DIN leaching simultaneously occurred in the rainy season (March to August) and monthly DIN concentrations and fluxes in leaching were positively related to those in throughfall in all three forests, implying that part of the N leaching was hydrological driven by the abundant precipitation in the monsoon climate at the site. Our results suggest that long-term high N deposition has caused elevated N leaching in all studied forest types although most pronounced in the old-growth forest where wood increment was negligible or even negative. N availability even exceeded the biotic N demand in the young aggrading forests, albeit intensive rain in the growing season is likely to enhance N leaching in these forests.
Origine : Accord explicite pour ce dépôt
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