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The fate of 15N-labelled nitrogen deposition in N-saturated pinus massoniana forest, Southwest of China

Wenjing Liu1, Ting Zhang1, Jing Zhu2, Ronghua Kang2, Jan Mulder2, Lei Duan1
1 State Key Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, P. R. China
2 Department of Environmental Sciences, Norwegian University of Life Sciences, Box 5003, NO-1432 Ås, Norway

O 3.8 in Fluxes between the atmosphere and ecosystems

17.07.2014, 16:10-16:30, H17

To explore the response of N-saturated forest on high nitrogen deposition in subtropical China, a field-scale 15N-labelled tracer experiment was carried out in Tieshanping, Chongqing in Southwestern China. An on-going long-term N addition experiment (with non-labelled N) in eight (10m×10m) experiment plots began in January 2005, including three experiment plots with ammonium nitrate addition (AN), another three plots with sodium nitrate addition (SN), and the other two plots as blank control without any addition (R1 and R2). These treatments were made at the beginning of every month by back atomizer spraying evenly into the forest, with the total amount of 40 kg N ha-1 a-1, which was similar as the N deposition in the site. Based on these treatments, 15N-labelled nitrogen were added on June 2012, with 0.95g m-2 15NH4NO3 into AN plots , 1.90g m-2 Na15NO3 into SN plots , 0.15g m-2 15NH4NO3 into R1 plot, and 0.30g m-2 Na15NO3 into R2. The 15N enrichment of all the N fertilizers were 60 atom%. The fate of this applied 15N in the ecosystem, i.e., retaining in some important pools such as soil and vegetation, as well as releasing into soil water or atmosphere, was estimated, with N2O gas sampling quarterly, plant and soil sampling once a year in autumn, and soil water sampling once a week but combining every four week samples as a month. The 15N enrichment of gas, soils and plant were measured directly by isotope-ratio mass spectrometer (IRMS), and the 15N enrichment of soil water was measured by bacteria method 1.

Results showed that in AN plots (with 15NH4+ and long-term non-labelled high N addition), about 15% of the added 15N had still been retained in the soil until 2013, with 2% emitted as N2O, and less than 2% uptaken by vegetation. In contrast, the retention rates were only about half in SN plots (with 15NO3- addition and long-term non-labelled high N addition). 15N loss by leaching in both AN and SN plots were huge, indicating that N saturation should happen as N deposition was doubled. In the two control plots with no long-term N addition, the leaching of 15N were both much lower than the treated plots, and the recovery of 15N was close to 76% in soils for plot R1 (with 15NH4+ addition), and about 37% for plot R2(with 15NO3- addition), indicating soil under natural N deposition could still retain considerable amount of N, especially in the form of NH4+.

Reference

[1] Zhu Jing, Mulder Jan, Bakken Lars, Dörsch Peter. The importance of denitrification for N2O emissions from an N-saturated forest in SW China: results from in situ 15N labeling experiments. Biogeochemistry, 2013, 116: 103–117.



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last modified 2014-06-19