Significance and driving forces of dark CO 2 fixation for organic carbon inputs in temperate forest soils 1 Aquatic Geomicrobiology group, Institute of Biodiversity, Jena 2 Department of Biogeochemical Processes, Max Planck Institute for Biogeochemistry, Jena. Rachael Akinyede 1,2 , Martin Taubert 1 , Marion Schrumpf 2 , Susan Trumbore 2 , Kirsten Küsel 1 Soils of temperate forests are currently regarded as organic carbon sinks (1) . Although soils are also one of the largest terrestrial sources of atmospheric CO 2 (2) , this can by modulated by dark (non-phototrophic) CO 2 fixation by microbes (3) , contributing to microbial biomass (MB) and soil organic carbon (SOC) (4) . • Dark CO 2 fixation by autotrophs increases with soil depth. • Dark CO 2 fixation is enhanced by increase in soil CO 2 concentration. • Dark CO 2 fixation contributes to MB and SOC across the soil profile. Dark CO 2 fixation in soils 4. Dark CO 2 fixation rates Pine (SEW 17) Mixed spp. (Hainich) Beech (SEW 49) * (p<0.05), *** (p<0.001) cbbL IC (facultative) Vegetation type: (R 2 = 0.44, p < 0.01) Depth: (R 2 = 0.14, p < 0.05); Permanova. 0.0 0.2 0.4 0.6 0.8 1.0 0 5 10 15 20 25 30 13 C uptake rate / μg C g(dw) soil -1 d -1 Time / days 0 100 200 300 400 500 600 0 5 10 15 20 25 30 13 C uptake rate / μg C gMB -1 d -1 Time / days 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 5 10 15 20 25 13 C uptake rate / μg C g(dw) soil -1 d -1 A horizon B horizon C horizon +ve correlation -ve correlation 0.00 0.01 0.02 0.03 0.04 0.05 7 14 28 Time / days CO 2 derived carbon / % SOC 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 7 14 28 Time / days CO 2 derived carbon / % MB 0.000 0.005 0.010 0.015 0.020 0.025 0.030 2 20 CO 2 / % v:v CO 2 derived carbon / % SOC * *** CO 2 / % v:v A horizon C horizon B horizon Hypotheses: Microbial biomass level Soil level Microbial biomass level Soil level Soil level Microbial biomass level Soil level Water content explains 77.5% variability (R 2 = 0.79; p = 0.02); Multiple step-wise regression. cbbL IA (obligate) MB determines dark CO 2 fixation rates in soils from mixed spp. forests Dark CO 2 fixation contributes to SOC and MB Increase in CO 2 concentration enhance dark CO 2 fixation rates Dark CO 2 fixation rates differ with Vegetation type Autotrophs constitute less than 2% of the bacterial community δ- 13 C ny Summary • MB, CO 2 and water content are important factors driving dark CO 2 fixation in temperate forest soils. 1. Lorenz et al., 2011. AREPS. 29: 535-562. 2. Batjes, N.H. 2016. Geoderma. 269: 61-68. 3. Miltner et al., 2005. Plant and Soil. 269: 193-203. 4. Nowak et al., 2015. Biogeosciences. 12: 7169-7183. References • Autotrophs represents only a small fraction of the microbial community. A horizon B horizon C- horizon A horizon C- horizon B horizon μg C gMB -1 d -1 : expressed per g of microbial biomass * (p<0.05), *** (p<0.001) denotes significant differences across depth for the beech and pine plots, respectively. Pine (SEW 17) Mixed spp. (Hainich) Beech (SEW 49) Red arrows represent dark CO 2 fixation by microbes 0.0 1.0 2.0 3.0 Gene abundance / % Bacterial community Depth/ Horizon aclA (obligate) 13 C uptake rate / μg C g(dw) soil -1 d -1 13 C uptake rate / μg C gMB -1 d -1 5. Bacterial community structure and autotroph abundance 6. Effect of soil properties on dark CO 2 fixation rates Mixed spp. (Hainich) A horizon B horizon C horizon Mineral horizon (0 – 100 cm) Schorfheide-chorin exploratory (SEW 17 & SEW 49) (3 cores per plot) Hainich National Park Germany Berlin 1. Soil sampling 7, 14, 28 days incubation; 2, 5, 10, 20% (v:v) 13 CO 2 2. 13 C metabolic labelling Chloroform Fumigation Extraction (CFE) 3. 13 C quantification in MB Soil properties affect dark CO 2 fixation rates Bacterial community shift with vegetation type and depth μg C gdw -1 d -1 : expressed per g dry weight of soil *** (p<0.001) denotes significant difference across depth for all three soil plots. CRC 1076 AquaDiva www.geomicrobiology.de μg C g(dw) soil -1 d -1 : expressed per g dry weight of soil μg C gMB -1 d -1 : expressed per g of microbial biomass • Estimated fixation of ~0.4Gt C/yr (1m deep) reduces global temperate forest soil CO 2 emissions by 3%. Average SOC accumulation rate per unit area: 57.36 ± 16.75 g C m 2 yr -1 (1m deep) 0.0 0.2 0.4 0.6 0.8 1.0 C-h B-h A-h Depth / Horizon 0 150 300 450 600 C-h B-h A-h Depth / Horizon Temperate forests