• Energy Research

Livestock manure is a major source of the greenhouse gases (GHGs) methane (CH4) and nitrous oxide (N2O). The emissions can be mitigated by production of biogas through anaerobic digestion (AD) of manure, mostly together with other biowastes, which can substitute fossil energy and thereby reduce CO2 emissions and postdigestion GHG emissions. This paper presents GHG balances for manure and biowaste management as affected by AD for five Danish biogas scenarios in which pig and cattle slurry were codigested with one or more of the following biomasses: deep litter, straw, energy crops, slaughterhouse waste, grass–clover green manure, and household waste. The calculated effects of AD on the GHG balance of each scenario included fossil fuel substitution, energy use for transport, leakage of CH4 from biogas production plants, CH4 emissions during storage of animal manure and biowaste, N2O emissions from stored and field applied biomass, N2O emissions related to nitrate (NO3−) leaching and ammonia (NH3) losses, N2O emissions from cultivation of energy crops, and soil C sequestration. All scenarios caused significant reductions in GHG emissions. Most of the reductions resulted from fossil fuel substitution and reduced emissions of CH4 during storage of codigestates. The total reductions in GHG emissions ranged from 65 to 105 kg CO2-eq ton−1 biomass. This wide range showed the importance of biomass composition. Reductions were highest when straw and grass–clover were used as codigestates, whereas reductions per unit energy produced were highest when deep litter or deep litter plus energy crops were used. Potential effects of iLUC were ignored but may have a negative impact on the GHG balance when using energy crops, and this may potentially exceed the calculated positive climate impacts of biogas production. The ammonia emission potential of digestate applied in the field is higher than that from cattle slurry and pig slurry because of the higher pH of the digestate. This effect, and the higher content of TAN in digestate, resulted in increasing ammonia emissions at 0.14 to 0.3 kg NH3-N ton−1 biomass. Nitrate leaching was reduced in all scenarios and ranged from 0.04 to 0.45 kg NO3-N ton−1 biomass. In the scenario in which maize silage was introduced, the maize production increased leaching and almost negated the effect of AD. Methane leakage caused a 7% reduction in the positive climate impact for each percentage point of leakage in a manure-based biogas scenario.

Reutilizing biomass ashes in agriculture can substitute inputs of P from finite primary sources. However, recycling of ashes is disputed due to their content of toxic substances such as heavy metals. This study evaluates the potential risk of replacing easily soluble inorganic P fertilizer with P in biomass ashes in a barley crop grown on soil with adequate P status. Two contrasting doses of three different types of ashes were applied to an agricultural field with spring barley and compared to similar doses of triple-superphosphate fertilizer. In the second growing season after biomass ash application, grain, straw and root dry matter yield, and P and Cd uptake were determined. Resin-extractable P was measured in soil and the symbiotic arbuscular mycorrhizal fungal activity, colonization, and community composition were assessed. Crop yield was not affected by ash application, while P-uptake and mycorrhizal status were slightly enhanced with high ash applications. Changes to the mycorrhizal community composition were evident with high ash doses. Cadmium uptake in aboveground plant tissue was unaffected by ash treatments, but increased in roots with increasing doses. Consequently, we conclude that fertilization with biomass ashes can replace conventional fertilizers without risk to barley crops in the short term.

Ammonia (NH3) emissions resulting from the field application of livestock slurry has both negative human health and environmental impacts. However, decreasing the exposed surface area (ESA) of slurry upon application can reduce NH3 volatilization by limiting its atmospheric exposure. In the present study, three strategies for depositing slurry within a growing crop were studied, including: 1. standard trailing hoses (SAhose), 2. trailing shoes (SAshoes), and 3. the combination of rigid tines and trailing shoes (SAtines+shoes). Application methods interact with the soil to varying degrees and were evaluated within the context of contemporary weed management practices, namely in cereals receiving inter-row hoeing. SAhose, SAshoes, and SAtines+shoes were compared in three coinciding experiments that assessed slurry ESA, NH3 emissions, and crop and weed effects. SAtines+shoes resulted in smallest ESA, 70–72% and 61–66% less than SAhose and SAshoes, respectively. However, in only one of three site–years did SAshoes and SAtines+shoes reduce NH3 emissions compared to SAhose, by 46% and 29%, respectively. Crop yields, nitrogen (N) accumulation in crop biomass, and intra-row weed biomass were unaffected by the placement method. In heavily crusted soils, the SAtines+shoes prototype worked well; however, the functional differences among placement strategies were not great enough to detect crop and NH3 effects.

Cattle slurry is an important nitrogen source for maize on dairy farms. Slurry injection is an effective measure to reduce ammonia emissions after field application, but with higher risk of nitrous oxide emission than surface application. This study compared soil mineral nitrogen dynamics and nitrous oxide emissions with two ways of application. First, traditional injection at 25 cm spacing between rows followed by ploughing (called “non-placed slurry”), and second, injection using a new so-called goosefoot slurry injector that placed the slurry in ploughed soil as a c. 30 cm broad band at 10 cm depth below maize crop rows with 75 cm spacing (named “placed slurry”). Furthermore, the effect of treating slurry with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) in Vizura® was tested with both application methods. The field experiment was conducted on a sandy loam soil in a temperate climate. Both nitrous oxide emissions, and the dynamics of soil mineral nitrogen, were monitored for eight weeks after slurry application and seeding of maize using static chambers. The level of nitrous oxide emissions was higher with non-placed compared to placed slurry, mainly due to higher emissions during the first four weeks. That might be due to higher rates of nitrification rate and in turn stimulation of denitrification process. In both placed and non-placed slurry treatments, Vizura® caused higher soil ammonium concentrations and lower nitrate concentrations, particularly from 3 to 8 weeks after slurry application. The final level of soil nitrate was similar with and without the nitrification inhibitor, but higher with placed compared to non-placed slurry. Adding Vizura® to non-placed slurry reduced nitrous oxide emissions by 70 % when compared to placed slurry. Surprisingly, there was a non-significant trend towards higher cumulative emissions from placed slurry with the nitrification inhibitor compared to untreated slurry, which were due to higher emissions in the last part of the monitoring period (5-7 weeks after slurry application). Possibly degradation of the nitrification inhibitor, and nitrification activity inside the slurry band as the soil dried, promoted nitrous oxide emissions by this time. In summary, placement of untreated slurry in a broad band under maize seeds reduced nitrous oxide emissions compared to non-placed slurry with more soil contact. A comparable reduction was achieved by adding a nitrification inhibitor to non-placed slurry. The pattern of nitrous oxide emissions from placed slurry treated with the inhibitor was complex and requires more investigation.

For biomass combustion to become a sustainable energy production system, it is crucial to minimise landfill of biomass ashes, to recycle the nutrients and to minimise the undesirable impact of hazardous substances in the ash. In order to test the plant availability of phosphorus (P) and cadmium (Cd) in four biomass ashes, we conducted two pot experiments on a P-depleted soil and one mini-plot field experiment on a soil with adequate P status. Test plants were spring barley and Italian ryegrass. Ash applications were compared to triple superphosphate (TSP) and a control without P application. Both TSP and ash significantly increased crop yields and P uptake on the P-depleted soil. In contrast, on the adequate-P soil, the barley yield showed little response to soil amendment, even at 300-500kgPha(-1) application, although the barley took up more P at higher applications. The apparent P use efficiency of the additive was 20% in ryegrass - much higher than that of barley for which P use efficiencies varied on the two soils. Generally, crop Cd concentrations were little affected by the increasing and high applications of ash, except for relatively high Cd concentrations in barley after applying 25Mgha(-1) straw ash. Contrarily, even modest increases in the TSP application markedly increased Cd uptake in plants. This might be explained by the low Cd solubility in the ash or by the reduced Cd availability due to the liming effect of ash. High concentrations of resin-extractable P (available P) in the ash-amended soil after harvest indicate that the ash may also contribute to P availability for the following crops. In conclusion, the biomass ashes in this study had P availability similar to the TSP fertiliser and did not contaminate the crop with Cd during the first year.

Digested, undigested and green organic amendments were used to improve soil fertility and minimize any harmful effects. To understand the events in soil, we evaluated the nitrogen (N) and sulfur (S...

Biogas operation parameters are usually optimised to enhance biogas yields while ignoring digestate quality. This study evaluated the effects of varying operation parameters and input feedstocks used in co-digestion with manure on digestate properties and N turnover in soil for 80 days. Net inorganic N release (% of N input) from digestates varied significantly (p < 0.001) due to their contrasting properties. Temperature, hydraulic retention time (HRT), manure type and input feedstocks significantly influenced soil inorganic N release. Animal slurry co-digested with energy crops had the highest average net soil inorganic N release at 67 %, while co-digestion with straw-based feedstock had the lowest at 56 %. Feedstock type greatly influenced the choice of digesting temperature and HRT, with most energy crop-based feedstocks digested at <50 days HRT. Digestate properties NH4+ -N/total N, C/N and total carbon concentration significantly influenced net inorganic N release in soil. A better understanding of the effects of biogas operation variables on nutrient availability and digestate fertilising properties could help design and optimise the anaerobic digestion process to increase biogas yields and N fertiliser value of the digestates.