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

Abstract

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 consumed in the manufacture of mineral N and pesticides in the upstream industry.Besides the analyses in the experimental farm Scheyern, the model has been applied in 28 commercial farms (18 org and 10 con) with comparable soil and climate conditions in the surroundings of Scheyern (mean distance 60 km). The program calculations are aimed at benchmarking the results obtained in the farming systems Scheyern; they are expected to disclose management-specific variations in the emission of climate-relevant gases and to rate the suitability of the model for describing such management-specific effects. In order to make the situation in the farms comparable, only the emissions from cropping systems were analyzed. Livestock keeping remained unconsidered. Due to lower N and energy inputs, clearly lower N₂O and CO₂ emissions were obtained for the organic farms than for the conventional systems.The analyses have shown possibilities for the optimization of management and the mitigation of GGE. Our findings underline that organic farming includes a high potential for C sequestration and the reduction of GGEs. Currently, the model REPRO is tested by 90 farms in the Federal Republic of Germany with the aim to apply it in the future not only in the field of research but also in the management of commercial farms.