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There have been few long-term field studies on greenhouse gases measurement in organic crop rotations under temperate climatic conditions. Little is known about the extent to which the share of legumes in a crop rotation of organic farming affects the potentials for CO 2 emission and soil organic carbon sequestration. The current study was aimed to investigate soil physicochemical state and soil net CO 2 exchange rate in diverse organic crop rotations with different crop species and proportions of legumes. Four 5-year duration crop rotations were investigated. The best soil sustainability of the arable layer was found in a crop rotation enriched with red clover (Trifolium pratense L.). This rotation resulted in the highest soil mesoporosity and the lowest microporosity, ensured the best supply of plant-available water and revealed high soil resistance to dry conditions. Red clover secured the highest soil organic C sequestration, caused the increase in reserves of total N and available K, and slackened the decrease of soil-available P sources. Red clover-based cropping system exhibited the highest soil net CO 2 exchange rate during five experimental years. The effect of crop rotation, consisting of phacelia (Phacelia tanacetifolia Benth.), peas (Pisum sativum L.) and yellow lupin (Lupinus luteus L.), on soil sustainability was weaker than the effect of rotation with red clover. Non-legume rotations, i.e. binary (two-crop) rotation and the crop rotation involving four spring and one winter species, can be regarded as miners of soil nutrient resources rather than contributors. These rotations did not promote soil sustainability because the soil lost large amounts of macronutrients and caused 26-33% lower soil net CO 2 exchange rate, compared with leguminous rotations. For future, it could be recommended for ecological farming to rely more on crop rotations with red clover to improve ecosystems functioning.

Background: Climate mitigation is a major consideration when choosing bioenergy systems. Anaerobic digestion of biomass is a bioenergy system, which is normally used for the treatment of manure or other residues, but cultivated energy crops may also provide a potential feedstock. One of the main crops for biogas production is still maize, but it may be desirable to replace it by perennial grasses, which also have high yields, but do not require as much management. The aim of this study was to analyse greenhouse gas (GHG) emissions in a biogas production system based on a perennial grass, cocksfoot, grown under different fertiliser application systems. Methods: Field and laboratory experiments were carried out over a 7-year period to evaluate the influence of mineral nitrogen and digestate fertiliser application on cocksfoot biomass and biogas productivity. The obtained data were used to calculate GHG emissions and emission savings for the whole biogas production cycle, by use of the SimaPro v.8 software with the IMPACT 2002+ model, based on the climate change environmental impact category. Results: Productivity of cocksfoot (Dactylis glomerata L.) was influenced by the annual rate of nitrogen applied as fertiliser, the fertiliser type and year, as influenced by the weather conditions. The biomass yield of cocksfoot fertilised with 180 kg nitrogen ha −1 year −1 was similar regardless if the nitrogen was applied as digestate or mineral fertiliser. At higher rates of nitrogen application (360-450 kg ha −1 year −1), higher yields and lower net GHG emissions could be obtained from the biogas production cycle when the nitrogen was applied as digestate compared to mineral fertiliser, especially in dry years. Energy input over the whole cycle was mainly influenced by the type of fertiliser and the biomass yield. Mineral fertiliser generally caused higher levels of GHG emissions than organic digestate fertiliser, when the same amount of energy is produced. Conclusions: The cocksfoot grass-based biogas production system showed that a significant GHG emission saving potential exists, if nitrogen is added as digestate instead of mineral fertiliser, for similar yields of biomass and energy from biogas. Cocksfoot grass fertilised with digestate is thus a promising feedstock for biogas production.