1. Soil CO 2 production and transport in the drought experiment in Caxiuana National Forest, Para, Brazil. Eleneide Doff Sotta Antonio Carlos Lola Rosiene Keila da Paixao Edzo Veldkamp Brenda Rocha Guimaraes Patrick Meir Alessandro Rosario Maria de Lourdes Ruivo Luitgard Schwendenmann

2. Purpose of the study Estimate soil CO 2 production in deep soil Test which model of soil gas diffusivity better work for Caxiuana soils Show how CO 2 production rate varies with seasons and under drought conditions Background: CO 2 Transport CO 2 Emission Microbial respiration CO 2 Diffusion CO 2 concentration in soil air Root respiration Soil water content Soil total porosity Soil air pore space Field capacity Soil water content Soil temperature Litterfall Radiation Environmental factors

3. Drought experiment: site description Period of the measurements: from Jan to Dec 2002 Bi-weekly and from Jan to Nov 2003 monthly Control Treatment ~ 90% water exclusion Experimental design: 2 plots of 1 ha each 16 efflux chambers per plot 4 soil shafts per plot Localization: Caxiuana National Forest, Para, Brazil Forest type: Terra firme Soil type: oxisol and ultisol

4. Sampling and sample processing Soil CO 2 efflux PVC rings covered with a lid for 5 min. Closed dynamic system (LiCor 6262) Soil air CO 2 concentration 30 mL plastic syringes 0.5 mL samples injected in a gas chromatograph Rn measurements Rn concentration: soil gas samples same depth (Lucas cells) Impulses counted with a Radon monitor Rn activity: incubation of soil samples for 4 weeks (wet and dry conditions) Pits instrumentation Stainless steel tubing Thermocouple T-probes Depths (cm) : 5 10 25 50 100 200 300 Soil moisture sensors (TDR) Depths (cm): 0-30 (vertically) 50 100 200 300

5. 1) Calculation of diffusion coefficient through empirical formulas: Non-aggregated porous media Millington & Quirk (1961) Aggregated porous media Millington & Shaerer (1971) Parameters given: Total pore space, soil water content, Air pore space, field capacity Method: CO2 production rates P CO 2 production (mg C m -2 h -1 ) 2) Validation of diffusion coefficient with the help of Radon concentration profile Numerical solution of transport for Radon in the soil air Parameters given: Radon production, diffusion coefficient P CO2 per layer is calculated from Fick’s first law Topsoil P CO2 (including litter layer) P CO2 topsoil = soil CO 2 efflux – P CO2 modelled. 0 – 50 cm 50 - 300 cm P CO2 topsoil P CO2 modelled Soil CO 2 efflux d Diffusion coefficient (m 2 h -1 ) PiPi F i+1 [C i+1 ] [Ci][Ci] [C i-1 ] FiFi F i-1 d i-1 didi d i+1 [C] CO 2 concentration in soil air (mg C m -3 ) F CO 2 flux between layers (mg C m -2 h -1 ) i Layers Davidson & Trumbore (1995)

6. Simulated vs. measured Radon concentration measured Rn concentration Simulated Rn concentration non-aggregated model aggregated model

7. Results: Soil CO 2 concentration Seasonality Both plots had higher concentration during wet season Wet = up to 2.0 % upper layers Drought effect Treatment plot had lower CO 2 concentration Control = 3.2 % Treatment = 1.0 %

8. Results: CO 2 concentration profile Wet season: Big difference on the first 5 cm depth Almost no change in [CO 2 ] in the profile Drought effect: ~50 % lower [CO 2 ] in the profile

9. Results: estimates of soil CO 2 production Average CO 2 production rate: Control = 170.1 ± 4.7 mg C m -2 h -1 Treatment = 137.3 ± 5.8 mg C m -2 h -1 No difference during wet season

10. Results: Profile CO 2 production rate Seasonality Control - no difference between wet and dry season Control Wet and Dry season 0 – 50 cm = 76 % 60 – 300 cm = 24 % Treatment Wet season Dry season 0 – 50 cm = 84 %71 % 60 – 300 cm = 16 %29 % 168.7 159.9 171.5 113.4 Drought effect Treatment – almost 30% less during dry season

11. Correlations: P CO2 and environmental factors P CO2 Soil Moisture Soil Temperature Air TemperatureRadiationRainfall Litter (1 month lag) Soil depthTotalLeavesFlowersTwigs 0 - 0.5 m -0.23n.s.0.33n.s.-0.16n.s.-0.08n.s.-0.21n.s.0.23n.s.-0.04n.s.0.59*0.47n.s. 0.6 - 1 m 0.82**-0.60**-0.22n.s.0.63**-0.14n.s.-0.61**-0.55*-0.28n.s.-0.24n.s. 1.1 – 2 m -0.12n.s.0.24n.s.0.72**-0.51*0.72**-0.37n.s.-0.16n.s.-0.68**-0.06n.s. 2.1 – 3 m -0.55**0.47*0.81**-0.81**0.66**0.11n.s.0.34n.s.-0.58*0.03n.s. P CO2 Soil Moisture Soil Temperature Air TemperatureRadiationRainfall Litter (without lag) Soil depthTotalLeavesFlowersTwigs 0 - 0.5 m0.76**-0.41n.s.-0.70**0.84**-0.73**-0.37n.s.-0.62*0.52*0.67** 0.6 - 1 m0.57**-0.47*-0.26n.s.0.59**-0.35n.s.0.14n.s.-0.05n.s.0.65**0.00n.s. 1.1 – 2 m-0.43*0.47*0.81**-0.67**0.71**0.63*0.68**-0.10n.s.-0.35n.s. 2.1 – 3 m-0.53*0.45n.s.0.91**-0.76**0.78**0.45n.s.0.60*-0.33n.s.-0.45n.s. (a) Control plot (b) Treatment plot ** P < 0.01 * P < 0.05 n.s. no significance

12. Conclusions 1.There was seasonality in [CO 2 ] in soil profile 2.[CO 2 ] was lower in drought conditions 3.The cumulative P CO2 in topsoil was affected by drought but not by season 4.~76 % of the P CO2 happened in the topsoil 5.During the dry season in the drought plot the deep soil compensated for the CO 2 production. 6.Apparently the capacity of water storage is limited, which makes the forest more susceptible to drought. The P CO2 in deep soil may not recover during the wet season. 7.A lower soil respiration can be expected during El Nino due to the limited capacity of compensation of the deep soil.