Is short rotation forestry biomass sustainable?

Despite the negative effects of fossil fuels on the environment, these remain as the primary contributors to the energy sector. In order to mitigate global warming risks, many countries aim at reducing greenhouse gas emissions. Bioenergy crops are being used as a substitute for fossil fuels and short rotation forestry is a prime example. In order to examine the sustainability of energy crops for fuel, typical European short rotation forestry (SRF) biomass, willow (Salix spp.) and poplar (Populus spp.) are examined and compared to rapeseed (Brassica napus L.) in respect to various aspects of soil respiration and combustion heat obtained from the extracted products per hectare. Various approaches are used to look at an As-contaminated site not only in the field but also in a soil-column experiment that examines the fate of trace elements in SRF soils, and in an analysis using MICMAC to describe the driving factors for SRF crop production. Based on the cause-effect chain, the impacts of land-use change and occupation on ecosystem quality are assessed when land-use is changed from degraded land (grassland) to willow and poplar SRF. A manual opaque dynamic closed chamber system (SEMACH-FG) was utilized to measure CO2 emissions at a willow/poplar short rotation forest in Krummenhennersdorf, Germany during the years 2013 and 2014, and at a rapeseed site in 2014. Short rotation forest soils showed higher CO2 emission rates during the growing season than the dormant season – with a CO2 release of 5.62±1.81 m-2 s-1 for willows and 5.08±1.37 µmol CO2 m-2 s-1 for poplars in the growing season. However, during the dormant season the soil sites with willow emitted 2.54±0.81 µmol CO2 m-2 s-1 and with poplar 2.07±0.56 µmol CO2 m-2 s-1. The highest emission rates for the studied plantations were observed in July for both years 2013 and 2014, during which the highest air and soil temperatures were recorded. Correlations between soil emission of CO2 and some meteorological parameters and leaf characteristics were investigated for the years 2013 and 2014. For example, for the willow clone (Jorr) and poplar clone (Max 3), high correlations were found for each between their soil emission of CO2 and both soil temperature and moisture content. Fitted models can explain about 77 and 75% of the results for Jorr and Max 3 clones, respectively. Moreover, a model of leaf area (LA) can explain about 68.6% of soil CO2 emission for H275. Estimated models can be used as a gap-filling method, when field data is not available. The ratio between soil respiration and the combustion heat calculated from the extracted products per hectare was evaluated and compared for the study’s willow, poplar and rapeseed crops. The results show that poplar and willow SRF has a very low ratio of 183 kg CO2 GJ 1 compared to rapeseed, 738 kg CO2 GJ 1. The soil-column experiment showed that by continuing the SRF plantation at the As-contaminated site, remediation would need only about 3% of the time needed if the site was left as a fallow field. In order to understand the complex willow and poplar short rotation forestry production system, 50 key variables were identified and prioritized to describe the system as a step to enhance the success of such potentially sustainable projects. The MICMAC approach was used in order to find the direct and the indirect relationships between those parameters and to classify them into different clusters depending on their driving force and interdependency. From this, it can be summarized that in order to enhance the success of a SRF system, decision makers should be focussing on: ensuring a developed wood-fuel market, increasing farmers’ experience/training, improving subsidy regulations and recommending a proper harvesting year cycle. Finally, the impacts of land-use change and occupation on the ecosystem quality were assessed. Results show that establishing SRF plantations on degraded lands improved the ecosystem structural quality (ESQ) by about 43% and ecosystem functional quality (EFQ) by about 12%. Based on overall results, poplar and willow SRF biomass can be recommended as renewable and sustainable sources for bioenergy.:Table of Contents Acknowledgements VI Abstract VII List of Figures IX List of Tables XI List of Appendix Tables XII List of Abbreviations XIII List of Abbreviations ...continued XIV 1. Background 1 1.1. General introduction 1 1.2. Soil organic carbon (SOC) 2 1.3. Soil respiration 4 1.4. Energy and bioenergy crops 5 1.5. Willow and poplar short rotation forestry 8 1.6. Degraded lands 10 1.8. Challenges 17 1.9. Objectives of this study 18 2. Methodology 19 2.1. Site Description 19 2.2. Environmental variables 22 2.3. Measuring CO2 emissions 23 2.3.1. Soil emission of CO2 23 2.3.2. Sensitivity of soil respiration to temperature (Q10) 25 2.4. Willow and poplar leaf traits 26 2.4.1. Measuring leaf area 26 2.4.2. Leaf Area Index (LAI) 27 2.4.3. Leaf sensitivity to high and low temperatures 28 2.5. Soil characteristics 30 2.5.1. Soil sampling 30 2.5.2. Soil Moisture Content % (SMC) by gravimetric method 31 2.5.3. Soil pH 31 2.5.4. Soil Cation Exchange Capacity (CEC) 31 2.5.5. Soil content of C, N, S, heavy metals and trace elements 31 2.5.6. Soil porosity 31 2.5.7. Soil pore water 32 2.5.8. Soil hydraulic conductivity (Kf) 32 2.6. Soil-column experiment 34 2.6.1. Experiment set-up 35 2.6.2. Distribution coefficients (Kd) 35 2.7. MICMAC approach 36 2.7.1. Selection of variables 36 2.7.2. Description of direct relationships 36 2.7.3. Classification of variables 37 2.8. Impacts of land-use change on the ecosystem quality 38 2.9. Computer software 40 3. Results and Discussion 41 3.1. Environmental conditions 41 3.1.1. Photosynthetically active radiation (PAR) 41 3.1.2. Soil temperature 42 3.1.3. Soil moisture content 43 3.2. Soil emission of CO2 46 3.2.1. CO2 emission from soil at the short rotation forestry site 46 3.2.2. Soil emission of CO2 during the day and the night 48 3.2.3. Cumulative emission of CO2 49 3.2.4. Comparison with other bioenergy crops 50 3.3. Q10 52 3.4. Willow and poplar Leaf Characteristics 54 3.4.1. Leaf Area Index (LAI) 54 3.4.2. Specific leaf area (SLA) 56 3.4.3. Leaf sensitivity to temperature 57 3.5. Correlations of soil CO2 emission with soil temperature and moisture content 59 3.6. Correlations of soil CO2 emission with plant parameters 65 3.7. Insights into soil respiration and combustion heat per area 67 3.7.1. Cumulative seasonal CO2 emission (CE) 68 3.7.2. Output energy 69 3.7.3. CO2(soil respiration) / Energy ratio 70 3.7.4. Global-warming potential (GWP) 72 3.8. Trace elements in soil 73 3.8.1. Solid-liquid partition coefficients (Kd) 74 3.8.2. Estimating time of remediation 78 3.9. Identification and Prioritization of Key Parameters for Willow and Poplar Short Rotation Forestry (SRF) Production System 82 3.9.1. Based on direct influence/dependence map: 85 3.9.2. Based on indirect influence/dependence map: 87 3.10. Impacts of Land-use Change on the Ecosystem Quality 93 4. Conclusions and Recommendations 101 5. References 102 Appendix 11