Phil J. Hobbs

Optimisation of the anaerobic digestion of agricultural resources

It is in the interest of operators of anaerobic digestion plants to maximise methane production whilst concomitantly reducing the chemical oxygen demand of the digested material. Although the production of biogas through anaerobic digestion is not a new idea, commercial anaerobic digestion processes are often operated at well below their optimal performance due to a variety of factors. This paper reviews current optimisation techniques associated with anaerobic digestion and suggests possible areas where improvements could be made, including the basic design considerations of a single or multi-stage reactor configuration, the type, power and duration of the mixing regime and the retention of active microbial biomass within the reactor. Optimisation of environmental conditions within the digester such as temperature, pH, buffering capacity and fatty acid concentrations is also discussed. The methane-producing potential of various agriculturally sourced feedstocks has been examined, as has the advantages of co-digestion to improve carbon-to-nitrogen ratios and the use of pre-treatments and additives to improve hydrolysis rates or supplement essential nutrients which may be limiting. However, perhaps the greatest shortfall in biogas production is the lack of reliable sensory equipment to monitor key parameters and suitable, parallelised control systems to ensure that the process continually operates at optimal performance. Modern techniques such as software sensors and powerful, flexible controllers are capable of solving these problems. A direct comparison can be made here with, for instance, oil refineries where a more mature technology uses continuous in situ monitoring and associated feedback procedures to routinely deliver continuous, optimal performance.

Seasonal changes in soil microbial communities along a fertility gradient of temperate grasslands

This study aimed to: (1) determine whether soil microbial communities along a gradient from intensive (fertilized) to low-input (unfertilized) grassland management, shift in their composition as shown by an increase in the abundance of fungi relative to bacteria and (2) whether these shifts in soil microbial communities vary depending on season. At all sample dates soil microbial biomass-C and -N, and the total abundance of phospholipid fatty acids (PLFA) were highest in unfertilized, undrained treatments and lowest in fertilized and drained grassland. Similarly, microbial activity, measured as CO2-C respiration, was found to be at its lowest in the most intensively managed grassland. Measures of microbial biomass showed a high degree of seasonality, having summer maxima and winter minima. In contrast, PLFA measures had spring maxima and autumn minima. Seasonal and management differences were also observed within the microbial community. PLFA profiles revealed that most individual fatty acids were highest in the unfertilized treatments, and lowest in fertilized grassland. The fungal-to-bacterial biomass ratio was also highest in the unfertilized and lowest in the fertilized soils, suggesting that higher microbial biomass in former were more due to the growth of fungi than bacteria. As with total PLFA, the abundance of individual fatty acids showed a spring maximum and an autumn minimum. Seasonal differences in PLFA patterns were shown to be related to soil mineral-N and soil moisture contents. Factors controlling shifts in microbial community structure between sample dates and sites are discussed in relation to other studies. A critical assessment of the different measures of microbial biomass is also given. Overall, the findings of this study support the thesis that fungi play a more significant role in soil biological processes of low-input, unfertilized grasslands, than in intensively managed systems.

Bioenergy from permanent grassland--a review: 2. Combustion.

The aim of this review is to summarize current knowledge on suitability and sustainability of grassland biomass for combustion. In the first section grassland management for solid biofuel as well as information on harvest, postharvest and firing technology are described. An extensive grassland management system with one late cut and low level of fertilization is favored for grass as a solid biofuel. The grass harvest usually involves drying in the field and clearing with conventional farm machinery. Pelleting or briquetting improves the biofuel quality. Grass combustion is possible as stand-alone biomass-firing or co-firing with other fuels. Firing herbaceous biomass requires various specific adaptations of the different combustion technologies. In the second section economic and environmental aspects are discussed. Costs for biomass supply mainly depend on yields and harvesting technologies, while combustion costs are influenced by the size and technical design of the plant. Market prices for grass and possible subsidies for land use are crucial for profitability. Regarding biogeochemical cycles a specific feature of combustion is the fact that none of the biomass carbon and nitrogen removed at harvest is available for return to the grassland. These exports can be compensated for by fixation from the air given legumes in the vegetation and sufficient biomass production. Greenhouse gas emissions can be considerably reduced by grass combustion. Solid biofuel production has a potential for predominantly positive impacts on biodiversity due to the extensive grassland management.

Abiotic drivers and plant traits explain landscape-scale patterns in soil microbial communities

The controls on aboveground community composition and diversity have been extensively studied, but our understanding of the drivers of belowground microbial communities is relatively lacking, despite their importance for ecosystem functioning. In this study, we fitted statistical models to explain landscape-scale variation in soil microbial community composition using data from 180 sites covering a broad range of grassland types, soil and climatic conditions in England. We found that variation in soil microbial communities was explained by abiotic factors like climate, pH and soil properties. Biotic factors, namely community-weighted means (CWM) of plant functional traits, also explained variation in soil microbial communities. In particular, more bacterial-dominated microbial communities were associated with exploitative plant traits versus fungal-dominated communities with resource-conservative traits, showing that plant functional traits and soil microbial communities are closely related at the landscape scale.

Bioenergy from permanent grassland--a review: 1. Biogas.

Grassland biomass is suitable in numerous ways for producing energy. It is well established as feedstock for biogas production. The aim of this review is to summarize current knowledge on suitability and sustainability of grassland biomass for anaerobic digestion. In the first section grassland management for biogas feedstock as well as specifics of harvest, postharvest and digestion technology are described. Methane yields from grass are influenced by many factors. While the effects of some parameters such as grass species, cutting period and management intensity can be regarded as well known, other parameters such as preservation and processing still need investigation. In the second section economic aspects and environmental impacts are discussed. Profitability can be achieved depending on grass silage supply costs and the concept of anaerobic digestion and energy use. Grassland biomass for biogas production competes with other feedstock and other forms of grassland use, in particular animal husbandry. In developed countries a growing production of milk and meat is achieved with decreasing ruminant numbers, resulting in an increasing amount of surplus grassland with a remarkable bioenergy potential. In emerging and developing countries a rapidly rising demand for and production of milk and meat induce growing pressure on grasslands, so that their use for animal feed presumably will take priority over use for bioenergy. Grasslands provide a variety of essential environmental benefits such as carbon storage, habitat function, preservation of ground and surface water quality. When producing biogas from grassland these benefits will remain or even grow, providing appropriate grassland management is implemented. In particular, greenhouse gas emissions can be considerably reduced.

Soil microbial community patterns related to the history and intensity of grazing in sub-montane ecosystems

Long-term variations in the frequency and intensity of sheep (Ovis aries) grazing have led to the development of ubiquitous plant successional transitions in sub-montane regions of the UK. In this study, we measured a range of soil microbial properties across these successional transitions in three biogeographic regions of the UK, to establish how gradients of grazing-influence (in terms of the history and intensity of sheep grazing) alter the biomass, activity, and structure of soil microbial communities. We also measured soil physicochemical variables to relate changes in soil microbial community arrangement along these grazing-related successional transitions to key soil properties. Our results from three locations show that microbial communities of soils display some consistent and ‘broad-scale’ trends along successional transitions that are related to the history and intensity of grazing. We show that microbial biomass of soil is maximal at low-to-intermediate levels of grazing influence and that the phenotypic evenness (a component of diversity) of the microbial community declines as the intensity of grazing increases. We also provide evidence that soil microbial communities of heavily grazed sites are dominated by bacterial-based energy channels of decomposition, whereas in systems that are less intensively grazed, or completely unmanaged, fungi have a proportionally greater role. Further studies are needed to establish the significance of these changes in relation to soil-level ecosystem processes of decomposition and nutrient cycling. The data show that human disturbances can have profound effects on the biomass and structure of the soil communities that regulate soil processes in these ecosystems and that these effects are consistent across sites.

Heterotrophic microbial communities use ancient carbon following glacial retreat.

When glaciers retreat they expose barren substrates that become colonized by organisms, beginning the process of primary succession. Recent studies reveal that heterotrophic microbial communities occur in newly exposed glacial substrates before autotrophic succession begins. This raises questions about how heterotrophic microbial communities function in the absence of carbon inputs from autotrophs. We measured patterns of soil organic matter development and changes in microbial community composition and carbon use along a 150-year chronosequence of a retreating glacier in the Austrian Alps. We found that soil microbial communities of recently deglaciated terrain differed markedly from those of later successional stages, being of lower biomass and higher abundance of bacteria relative to fungi. Moreover, we found that these initial microbial communities used ancient and recalcitrant carbon as an energy source, along with modern carbon. Only after more than 50 years of organic matter accumulation did the soil microbial community change to one supported primarily by modern carbon, most likely from recent plant production. Our findings suggest the existence of an initial stage of heterotrophic microbial community development that precedes autotrophic community assembly and is sustained, in part, by ancient carbon.

Parasitic plants indirectly regulate below-ground properties in grassland ecosystems.

Parasitic plants are one of the most ubiquitous groups of generalist parasites in both natural and managed ecosystems, with over 3,000 known species worldwide. Although much is known about how parasitic plants influence host peformance, their role as drivers of community- and ecosystem-level properties remains largely unexplored. Parasitic plants have the potential to influence directly the productivity and structure of plant communities because they cause harm to particular host plants, indirectly increasing the competitive status of non-host species. Such parasite-driven above-ground effects might also have important indirect consequences through altering the quantity and quality of resources that enter soil, thereby affecting the activity of decomposer organisms. Here we show in model grassland communities that the parasitic plant Rhinanthus minor, which occurs widely throughout Europe and North America, has strong direct effects on above-ground community properties, increasing plant diversity and reducing productivity. We also show that these direct effects of R. minor on the plant community have marked indirect effects on below-ground properties, ultimately increasing rates of nitrogen cycling. Our study provides evidence that parasitic plants act as a major driver of both above-ground and below-ground properties of grassland ecosystems.

Management influences on soil microbial communities and their function in botanically diverse haymeadows of northern England and Wales.

The effects of management intensification on the size, activity and structure of soil microbial communities in botanically diverse haymeadows were examined. Paired traditionally managed and intensively managed haymeadows, at three submontane regions in northern England and north Wales, were sampled over four seasons. Management intensification had no significant effect on soil nutrient status, soil microbial biomass and soil microbial activity. Management intensification did influence soil microbial community structure, resulting in a significant reduction in soil fungal biomass, measured as soil ergosterol content, and a decline in the proportion of fungi relative to bacteria in the soil microbial community. Fungi of the genera Fusarium, Mucor, Absidia, Cladosporium, Trichoderma, Acremonium, Zygorhynchus, Phoma and Paecilomyces were commonly isolated from litter and soil of both the traditionally and intensively managed haymeadows of the site tested. Management had a significant effect on the relative isolation frequency of these fungi at this site. All commonly isolated species had proteolytic and urease activity and approximately half had cellulolytic and lignolytic activities. These findings were taken to suggest that although management improvements to submontane haymeadows will induce changes in the size and composition of the fungal community, they do not necessarily influence the functioning of the soil microbial community with respect to soil ecosystem-level processes of organic matter decomposition and nutrient cycling. We suggest that changes in soil microbial communities are related primarily to changes in plant productivity and composition or the form and quantity of fertiliser applied to the site.

Below-ground microbial community development in a high temperature world

The response of above-ground plant and ecosystem processes to climate change are likely to be influenced by both direct and indirect effects of elevated temperature on soil biota and their activities. This study examined the effects of elevated atmospheric temperature on the development of the soil microbial community in a model terrestrial ecosystem facility. The model system was characterized by a soil of low nutrient availability, a condition that simulates most native terrestrial plant communities. The experiment was run over three plant generations, broadly mimicking the early stages of a plant succession, and showed that microbial biomass, measured using phospholipid fatty acid (PLFA) analysis, increased significantly in response to elevated temperature during the first generation only. This increase was unrelated to changes in plant productivity or soil C-availability, and was largely due to a direct effect of elevated temperature on fast-growing Gram-positive bacteria. Slow growing soil micoorganisms such as fungi and actinomycetes were unaffected by elevated temperature throughout the experimental period. Measures of microbial biomass, microbial respiration and N-mineralization were also unaffected by elevated atmospheric temperature over the three generations. The lack of effects on the soil microbial community is thought to be due to the fact that elevated temperature did not influence root biomass or soil C-availability. We suggest that the observed reductions in above-ground plant productivity, in response to elevated temperature, will become apparent in the longer term when litter decomposition pathways are more established. The temporal measures of PLFA and microbial biomass indicated that over the experimental period rapid initial changes occurred in most soil biological characteristics, followed by periods of stabilization during later plant succession. These changes were associated with increases in above-ground plant productivity and amounts of available C in the soil. In contrast, total microbial biomass declined during the last plant generation. Reductions in the diversity of PLFAs in later plant generations appeared to be associated with an increase in the proportion of fatty acids associated with fungi, relative to those from bacteria. These changes are likely to be related to increased competition for resources within the soil, and an associated reduction in N- and C-availability. These changes appear to be broadly consistent with those reported for other studies on the successional development of soil microbial and plant communities. Overall, our data suggest that elevated atmospheric temperature has little effect on the development of below-ground microbial communities and their activities in soils of low nutrient status.