# The Carbon Sequestration Potential of Degraded Agricultural Land in the Amhara Region of Ethiopia
## Materials and Methods
### Model Description and Simulation Procedure  
We used the process-based biogeochemical mechanistic ecosystem model Biome-BGC [39; 40] version 4.1.2 [41] to assess the impact of reforestation/afforestation (hereafter called reforestation) and different forest management scenarios on carbon storage across the highly diverse landscape of Amhara. The model was chosen because it simulates forest growth and soil development in response to atmospheric (climate, CO\({}_{2}\) concentration, nitrogen deposition) and soil variables (i.e., total soil depth and texture) and different forest management measures, such as planting of specific forest types, clear cuts, and different types and intensities of thinning. The advantage of the model is that it simulates pools and, compared to other ecosystem models, it does not require tree populations to initialize a simulation run. Furthermore, in a previous study we evaluated Biome-BGC for different forest and agro-ecological zones (across elevation, temperature and precipitation gradients) in the Amhara region, using measured vegetation carbon, mineral soil carbon, and net primary production data [10]. Biome-BGC simulates the pools and fluxes of carbon, nitrogen, and water in a specific ecosystem. Carbon is stored in different plant and soil compartments, characterized by specific C/N-ratios, lifespan, and degradability. Carbon assimilation (gross primary production--GPP) is simulated with the Farquhar-photosynthesis routine. The supply with CO\({}_{2}\) for photosynthesis is regulated by stomatal conductance. Growth and maintenance respiration lower the amount of carbon that can be used for the build-up of the plant compartments (GPP-respiration = NPP). Dead organic matter is decomposed and builds up the soil carbon stocks. The velocity of decomposition depends on the content of labile organic carbon, cellulose, and lignin in the different plant compartments. Decomposition provides mineralized nitrogen for plant growth. The main water cycle processes are interception (leaf area index--LAI and leaf type dependent), storage in the soil (soil depth and texture dependent), and evapotranspiration from the soil or through the leaves (stomatal conductance dependent). The processes in Biome-BGC typically also are temperature and moisture dependent and are interconnected through various feedback-loops within and among the carbon, nitrogen, and water cycles. All details on the processes have been previously published [39; 40; 41; 42; 43]. Biome-BGC uses either biome or species specific eco-physiological parameters to simulate pools and fluxes of a given forest ecosystem [44; 45; 46]. Within Biome-BGC, a typical ecosystem or biome type is defined by 35 eco-physiological parameters, which can be obtained from the literature [43], or intensive monitoring sites [44]. For this study, we selected the eco-physiological parameters for the biome type  
Figure 1: (**a**) Land use, provided by the Bureau of Agriculture [37], and (**b**) elevation class (30 m by 30 m resolution) in the Amhara region in Ethiopia. 'tropical evergreen broadleaved forest', suggested by [PERSON] et al. [47] for several reasons: (1) We had already successfully used and evaluated this parameter set for its applicability on diverse Ethiopian dry Afromontane forest ecosystems [10]; (2) We assumed a reforestation with a natural species mix as opposed to frequent plantations with non-natives such as _Eucalyptus_ sp. ; (3) There is no other suitable parameter setting available for Ethiopian Afromontane forests. As a fully prognostic model, Biome-BGC has two main working steps: the'spin-up' and the current simulation run. The first step, the spin-up, initializes the model for a given ecosystem by mimicking the development of a theoretical virgin forest ecosystem [41; 42]. In the spin-up run, the model accumulates carbon and nitrogen until the carbon content of the mineral soil has reached a'steady state'.