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Biogas in organic agriculture—effects on productivity, energy self-sufficiency and greenhouse gas emissions
- Siri Pugesgaard, Jørgen E. Olesen, Uffe Jørgensen, Tommy Dalgaard
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- Journal:
- Renewable Agriculture and Food Systems / Volume 29 / Issue 1 / March 2014
- Published online by Cambridge University Press:
- 24 January 2013, pp. 28-41
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Anaerobic digestion of manure and crops provides the possibility of a combined production of renewable energy and organic fertilizer on organic farms and has been suggested as an option to improve sustainability of organic agriculture. In the present study, the consequences of implementation of anaerobic digestion and biogas production were analyzed on a 1000 ha model farm with combined dairy and cash crop production, representing organic agriculture in Denmark. The effects on crop rotation, nitrogen flows and losses, yield, energy balance and greenhouse gas (GHG) emissions were evaluated for four scenarios of biogas production on the farm. Animal manure was digested for biogas production in all scenarios and was supplemented with: (1) 100 ha grass–clover for biogas, (2) 100 ha maize for biogas, (3) 200 ha grass–clover for biogas and reduced number of livestock, and (4) 200 ha grass–clover for biogas, reduced number of livestock and import of biomass from cuttings made in ungrazed meadows. These four scenarios were compared with the current situation in organic agriculture in Denmark and to a situation where slurry from conventional agriculture is no longer imported. Implementation of anaerobic digestion changed the nitrogen flows on the farm by increasing the slurry nitrogen plant availability and introducing new nitrogen sources from legume-based energy crops or meadows. The amount of nitrogen available for application as fertilizer on the farm increased when grass–clover was used for biogas production, but decreased when maize was used. Since part of the area was used for biogas production, the total output of foodstuffs from the farm was decreased. Effects on GHG emissions and net energy production were assessed by use of the whole-farm model FarmGHG. A positive farm energy balance was obtained for all biogas scenarios, showing that biomass production for biogas on 10% of the farm area results in an energy surplus, provided that the heat from the electricity production is utilized. The energy surplus implies a displacement of fossil fuels and thereby reduced CO2 emission from the farm. Emissions of N2O were not affected substantially by biogas production. Total emissions of methane (CH4) were slightly decreased due to a 17–48% decrease in emissions from the manure store. Net GHG emission was reduced by 35–85% compared with the current situation in organic agriculture. It was concluded that production of biogas on organic farms holds the possibility for the farms to achieve a positive energy balance, provide self-sufficiency with organic fertilizer nitrogen, and reduce GHG emissions.
C and N mineralization of composted and anaerobically stored ruminant manure in differently textured soils
- INGRID K. THOMSEN, JØRGEN E. OLESEN
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- Journal:
- The Journal of Agricultural Science / Volume 135 / Issue 2 / September 2000
- Published online by Cambridge University Press:
- 10 November 2000, pp. 151-159
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Three animal manures cross-labelled with 15N in either the urine, faeces or straw fractions were prepared. After a storage period of 86 days when the manures were exposed to either composting or to anaerobic storage, portions of the manures were incubated in six differently textured soils with clay contents ranging from 11 to 45%. Evolved CO2-C was determined during a 266 day incubation and inorganic N and 15N in soil were measured at the termination of the incubation. The mineralization of C was analysed using first-order kinetics, and two C pools with fast (P1) and slow (P2) turnover rates were estimated. The total conversion of added C (Ps) was estimated as Ps=P1+P2.
The cumulated CO2 production was considerably higher from soils incubated with anaerobically stored manure compared with soils amended with composted manure. CO2 production levelled off after c. 60 days in the three sandier soils whereas CO2 continued to be produced throughout the incubation from the three soils with the highest clay content. More C was assigned to the easily decomposable P1 pool in the sandiest soils whereas the more recalcitrant P2 pool was larger in the soils with higher clay content. Because of the different relationships between soil texture and C pools, Ps ended up being similar for five of the six soils. When taking C losses during the preceding storage into account, the accumulated C losses during storage and after incubation in soil accounted for 60 and 54% of C initially present in the composted and anaerobically stored manure, respectively.
Net N mineralization which averaged 16% of applied organic N took place in all soils amended with composted manure. Soils with anaerobically stored manure showed net immobilization after the 266 days of incubation. The amount of N immobilized accounted for up to 30% of the inorganic N applied with the manure. As anaerobically stored manure generally loses less inorganic N during storage, it may contain more inorganic N than composted manure at the time of field application. Because of the immobilization that takes place after application of anaerobically stored manure to soil, the immediate levels of plant available N in soil may not be as different from soil supplied with composted manure as could be expected from the inorganic N content in the two types of manure. However, when considering the manure as a N resource, anaerobic storage is superior to composting.