Large scale spatial modelling of soil organic carbon dynamics

Under the Kyoto Protocol, participating nations are required to reduce National CO₂emissions according to their 'reduction commitment' or 'quantified emissions limitation', over the first commitment period, 2008-2012. One way in which nations could achieve this would be by increasing soil carbon storage through different management practices. Most former estimates of regional scale C sequestration potential have made use of either linear regressions based on long-term experimental data, whilst some have used dynamic soil organic matter (SOM) models linked to spatial databases. Few studies have compared these two methods, and none have compared regressions with two different SOM models. This thesis presents a case study investigation of the potential of different land management practices to sequester carbon in soil in arable land, and preliminary estimates of other potential C savings. Two dynamic SOM models were chosen for this study, RothC (a soil process model) and CENTURY (a general ecosystem model). RothC and CENTURY are the two most widely used and validated SOM models world-wide. Methods were developed to enhance use and comparability of the models in a predictive mode. These methods included a) estimation of the IOM pool for RothC, b) estimation of C inputs to soil, c) investigation of pool size distributions in CENTURY, and d) creation of a program to allow use of C inputs derived from CENTURY with the RothC model. This thesis has also investigated the importance of errors in C inputs to soil for predictive SOM modelling, and performed sensitivity analyses to investigate how errors in setting the size refractory SOM pools might affect predictions of SOC. RothC and CENTURY were compared at the site scale using datasets from seven European long-term experiments, in order to a) verify their ability to predict SOC changes under changes in land use and management relevant to studies of C sequestration potential, b) evaluate model performance under European climatic conditions, and c) compare the performance of the two models. Finally, a Geographic Information System (GIS) containing soil, land use and climate layers, was assembled for a case study region in Central Hungary. GIS interfaces were developed for the RothC and CENTURY models, thus linking them to spatial datasets at the regional level. This allowed a comparison of estimates of the C sequestration potential of different land management practices obtained using the two models and using regression-based estimates. Although estimates obtained by the different approaches were of the same order of magnitude, differences were observed. Encouragingly, some of the land management scenarios studied here showed sufficient C mitigation potential to meet Hungarian CO₂reduction commitments

Simulating changes in the terrestrial biosphere during the last glacial/interglacial transition

The state of the terrestrial biosphere during the Holocene and the Last Glacial Maximum (LGM) was estimated from pollen data bases and steady state simulations in former studies. However, the amount of carbon bound in the terrestrial stocks varied considerably.Here, we narrow down this range of terrestrial carbon at the LGM by a transient simulation study over the last glacial cycle (125 kyr) and try to determine the amplitudes of the possible different driving forces (temperature, atmospheric carbon dioxide partial pressure and sea level).We developed a simple model of the terrestrial biosphere consisting of seven well-mixed boxes.By applying well defined boundary conditions of the total terrestrial carbon stock, average isotopic signature, and net primary production, the range of the terrestrial carbon at LGM can be focused to 1500--1700 PgC, equivalent to a reduction from interglacial times to the LGM of 500--700 PgC. This falls well within the range of former studies (LGM: 1100--1900 PgC) but reduces the range of uncertainty significantly. Simulation results were biased towards higher carbon stocks (+120--150 PgC) if we abstained from our transient modeling approach and analyzed steady states. This disequilibrium effect give us reasons to argue for considering the time-dependent nature of any driving forces, since fast temperature changes in the northern hemisphere, where 2/3 of all land area is situated, did prevent the system from reaching equilibrium