The effects of nitrogen deposition on carbon sequestration in temperate forest soils

Transcription

1 The effects of nitrogen deposition on carbon sequestration in temperate forest soils Preliminary results from studies of C in short N gradients (edges) Shimon Ginzburg 1

2 Content Uncertainties in N deposition effects on the C cycle SOC accumulation in N deposition gradients in forest edges Objectives of study Methods: Sites, experimental set-up etc. Results Discussion? Conclusions? Temperature effects on SOM decomposition in C/N gradients Objectives Methods Results???? 2

3 N deposition effects on the C cycle Estimations of above-ground C accumulation: kg C per kg N added (de Vries et al., 2009) Uncertainty increase when we include soils: kg C per kg N added (de Vries et al., 2008;Magnani et al., 2007 ;Magnani et al., 2008) But most estimates for soils: 5-35 kg C per kg N added (de Vries et al., 2009) 70 kg C per kg N added (Gundersen et al., 2006a) 100 kg C ha -1 year -1 (Gundersen et al., 2006a). Source: Gundersen et al

4 Objectives of study Overall objective: Quantify the effect of N deposition on C sequestration in temperate forest soils More specifically: Identify N deposition gradients in edge-interior transects in temperate conifer forests Quantify the effect the excess N deposition has on the C stock in the top soil. Hypothesis 1: Intermediate, long-term N deposition retards the decomposition of SOM and leads to C accumulation in the forest soils. Hypothesis 2: Under N saturated conditions the soil loses its capacity to sequester C. Focus is on the litter-fermentation-humus layer (LFH) and 0-5 cm soil layer We define intermediate deposition as kg N ha -1 year -1 compared with high deposition > 40 kg N ha -1 year -1 4

5 Study site: Conifer forests in Belgium and Denmark Tree species Belgium: Austrian pine (P. nigra) Sønder Omme: Sitka spruce (Picea sitchensis) Klosterhede: Norway spruce (Picea abies) Thyregod: Norway spruce (Picea abies) 5

6 Experimental set-up and analysis 6

7 Forest edge in Sønder Omme 7

8 Transect in Sønder Omme 8

9 Sampling of throughfall in Klosterhede 9

10 Sampling of organic and mineral soil 10

11 Results (1): N deposition gradient Vloethemveld N in throughfall (kg N/ha/year) r = p< Distance from edge (m) Pinus nigra P l (Pi i ) Vloethemveld: Data from Wuyts et al Klosterhede Thyregod N in throughfall (kg N/ha/year) Norway spruce and Silver fir r = p< Distance from edge (m) N in throughfall (kg N/ha/year) r = p< Distance from edge (m) Norway spruce P l (N ) Klosterhede: data based on measurements during 3 summer months 11

12 Results (2): Soil C/N ratio 12

13 Results (3): C content So far little evidence to support hypothesis 1: intermediate, long-term N deposition retards the decomposition of SOM and leads to C accumulation in the forest soils. 13

14 Results (4): N content 0.8 Vloethemveld 2.1 Sønder Omme N content (t N/ha) N content (t N/ha) r= p < Distance from edge (m) Distance from edge (m) N content LFH fine 1.6 Klosterhede 1.4 Thyregod N content mineral soil fine N content (t N/ha) N content (t N/ha) Distance from edge (m) Distance from edge (m) 14

15 Results (5): C accumulation and C/N ratio Stronger evidence to support hypothesis 2: Under N saturated conditions the soil loses its capacity to sequester C. 15

16 Discussion: C/N ratio and C content better correlation was found between the C/N ratio of the soil and its C content Supports the second hypothesis: under N saturated conditions the soil loses its capacity to sequester C, perhaps due to: reduced input of C originating from fine root growth (Persson et al. 1998) reduced input of C originating from soil bacterial biomass (Liu and Greaver, 2010) Sønder Omme? Vloethemveld? Thyregod? 16

17 What do we do now? Isolate the effect of N on soil processes from other factors: Identify trends in root growth Estimation of litterfall input along the transects Measurements of light and temperature along the transects Work on more edge sites (Ulfborg, Denmark) N deposition and microbial populations (MSc. Thesis Ludovica Dímperio) 17

18 Fine root growth as a function of distance from the forest edge in Sønder Omme Roots (g DW/ soil g FW) r = 0.80 p < roots MS roots LFH Linear (roots LFH) Distance from edge 60 Sønder Omme 50 Fine roots: < 2 mm in diameter High C close to edge in LFH layer not because of roots! Organic C content (t C/ha) 40 r= r= Distance from edge (m) 18

19 Litterfall measurements 19

20 Annual litterfall input as a function of distance from the forest edge In Klosterhede Litterfall (g dry weight/ha/year) TR A TR B Distance from edge (m) Data based on 5 months measurements in 2 transects No trend so far No results yet for Sønder Omme 20

21 Mean annual temperature and light intensity as a function of distance from forest edge in Klosterhede Mean annual temperature (C) r = p < 0.05 Temperature: 2 replicates per distance measured in one transect Small but significant difference Distance from edge (m) 1200 Light intensity (LUX) Light: No gradient Distance from edge (m) 21

22 Soil respiration in LF layer (mg CO2/ g C DW/hour) along transects A and B in Klosterhede Source: MSc. thesis Ludovica Dimperio (2011) Water content (%) r = p < LFH Fine MS Fine Poly. (MS Fine) Distance from edge (m) 22

23 Conclusions?????????? Little evidence to support the assumption that high N deposition leads to increased soil C content (Sønder Omme forest edge) Stronger evidence to support the assumption that under N saturated conditions the soil loses its capacity to sequester C (Vloethemveld, Thyregod) C/N ratio can be related to C content in sites suffering from high long-term N pollution 23

24 Temperature effects on SOM decomposition in C/N gradients Hypothesis: Effects of enhanced N deposition on decomposition of SOM can limit temperature effects on SOM in forest soils 40 Klosterhede Samples taken from edge and N-addition experiment 35 r=0.762 C/N ratio r=0.889 p< Distance from edge (m) We will use the Q10 expression to show that decomposition rate of SOM is expected to increase with temperature. However, we also expect lower Q10 values for soils characterised by low C/N ratio compared with those of high C/N ratio (control) 24

25 Incubation in a respirometer: 10 days under each of the following temperatures regimes: 5,10,15,20,25ºC In practice: 8,24,15ºC Basal respiration measurements 25

26 Temperature effects on respiration of humus Results respiration (mg CO2/g dry soil/hour) H 8 degrees high N H 15 degrees high N 0.02 H 24 degrees high N H 8 degrees low N H 15 degrees low N 0.01 H 24 degrees low N Help! Temperature Temperature effects on respiration of the LF layer 0.06 respiration (mg CO2/g dry soil/hour) LF 8 degrees high N LF 15 degrees high N LF 24 degrees high N LF 8 degrees low N LF 24 degrees low N Temperature 26

27 One important thing to take home! My Or call me: Thank you Questions? 27