Modeling carbon cycles and estimation of greenhouse gas emissions from organic and conventional farming systems

The paper describes the model software REPRO (REPROduction of soil fertility) designed for analyzing interlinked carbon (C) and nitrogen (N) fluxes in the system soil–plant–animal–environment. The model couples the balancing of C, N and energy fluxes with the target to estimate the climate-relevant CO2, CH4 and N2O sources and sinks of farming systems. For the determination of the net greenhouse effect, calculations of C sequestration in the soil, CO2 emissions from the use of fossil energy, CH4 emissions from livestock keeping and N2O emissions from the soil have been made. The results were converted into CO2 equivalents using its specific global warming potential (GWP). The model has been applied in the experimental farm Scheyern in southern Germany, which had been divided into an organic (org) and a conventional (con) farming system in 1992. Rather detailed series of long-term measuring data are available for the farm in Scheyern, which have been used for validating the software for its efficiency and applicability under very different management yet nearly equal site conditions. The organic farm is multi-structured with a legume-based crop rotation (N2 fixation: 83 kg ha−1 yr−1). The livestock density (LSU=Livestock Unit according to FAO) is 1.4 LSU ha−1. The farm is oriented on closed mass cycles; from the energetic point of view it represents a low-input system (energy input 4.5 GJ ha−1 yr−1). The conventional farm is a simple-structured cash crop system, based on mineral N (N input 145 kg ha−1 yr−1). Regarding the energy consumption, the system is run on high inputs (energy input 14.0 GJ ha−1 yr−1). The organic crop rotation reaches about 57% (8.3 Mg ha−1 yr−1) of the DM yield, about 66% (163 kg ha−1 yr−1) of the N removal and roughly 56% (3741 kg ha−1 yr−1) of the C fixation of the conventional crop rotation. In the organic rotation, 18 GJ per GJ of fossil energy input are bound in the harvested biomass vis-à-vis 11.1 GJ in the conventional rotation. The strongest influence on the greenhouse effect is exerted by C sequestration and N2O emissions. In Scheyern, C sequestration has set in under organic management (+0.37 Mg ha−1 yr−1), while humus depletion has been recorded in the conventional system (−0.25 Mg ha−1 yr−1). Greenhouse gas emissions (GGEs) due to fuel consumption and the use of machines are nearly on the same level in both crop rotations. However, the conventional system emits an additional 637 kg CO2 eq ha−1 yr−1, which had been consume