Kurt Möller

Influence of different manuring systems with and without biogas digestion on soil organic matter and nitrogen inputs, flows and budgets in organic cropping systems

Nitrogen (N) and carbon (C) cycles are closely linked in organic farming systems. Use of residues for biogas digestion may reduce N-losses and lead to higher farmland productivity. However, digestion is connected to large losses of organic C. It is the purpose of this paper (1) to compare farming systems based on liquid slurry and solid farmyard manure regarding the N, C and organic dry matter (ODM) inputs and flows, (2) to analyse the effect of digestion on soil N, C and ODM inputs and flows within the cropping system, (3) to assess the effects of organic manure management on biological N2 fixation (BNF), and (4) to assess the effect of biogas digestion on the sustainability of the cropping systems in terms of N and C budgets. The BNF by clover/grass-leys was the most important single N input, followed by the BNF supplied by legume cover cropping. Growth of crops in organic farming systems is very often N limited, and not limited by the soil C inputs. However, balances of N inputs showed that the implemented organic farming systems have the potential to supply high amounts of N to meet crop N demand. The level of plant available N to non-legume main crops was much lower, in comparison to the total N inputs. Reasons were the non-synchronized timing of N mineralization and crop N demand, the high unproductive gaseous N losses and an unfocussed allocation in space and time of the circulating N within the crop rotation (e.g. allocation of immobile manures to legumes or of mobile manures to cover crops). Simultaneously, organic cropping systems very often showed large C surpluses, which may be potentially increased the N shortage due to the immobilization of N. Soil organic matter supply and soil humus balance (a balance sheet calculated from factors describing the cultivation effects on humus increasing and humus depleting crops, and organic manure application) were higher in cropping systems based on liquid slurry than in those based on solid farmyard manure (+19%). Simultaneously, soil N surplus was higher due to lower gaseous N losses (+14%). Biogas digestion of slurry had only a very slight effect on both the soil N and the soil C budget. The effect on the N budget was also slight if the liquid slurry was stored in closed repositories. Digestion of residues like slurry, crop residues and cover crops reduced in a mixed farming system the soil C supply unilaterally (approximately −33%), and increased the amounts of readily available N (approximately +70–75%). The long-term challenge for organic farming systems is to find instruments that reduce N losses to a minimum, to keep the most limiting fraction of N (ammonia-N) within the system, and to enhance the direct manuring effect of the available manures to non-legume main crops.

Effects of anaerobic digestion on soil carbon and nitrogen turnover, N emissions, and soil biological activity. A review

Sustainability in agriculture means the inclusion of several aspects, as sustainable agriculture systems must not compromise not only their ability to satisfy future needs by undermining soil fertility and the natural resource base but also sustainable agriculture has had to address a range of other issues including energy use, efficient use, and recycling of nutrients, the effects on adjacent ecosystems including the effects on water bodies and climate change. Organic manures are an important factor to keep the soil fertility level of soils. However, their management is often related to large emissions. In this context, anaerobic digestion is—similarly to composting—a treatment option for stabilization of biogenic wastes leading to a residual product called digestates, enabling the sanitation and the recycling and use as fertilizer. It is also a means to obtain energy from wastes as well as from dedicated energy crops. Therefore, anaerobic digestion potentially addresses several aspects of agricultural sustainability. This review discusses the current state of knowledge on the effects of anaerobic digestion on organic compounds in digestates and the most important processes influencing N emissions in the field, as well as the possible long-term effects on soil microbial biomass and soil fertility. The main findings are that (1) the direct effects of anaerobic digestion on long-term sustainability in terms of soil fertility and environmental impact at the field level are of minor relevance. (2) The most relevant effects of anaerobic digestion on soil fertility as well as on N emissions will be expected from indirect effects related to cropping system changes such as changes in crop rotation, crop acreage, cover cropping, and total amounts of organic manures including digestates. Furthermore, (3) the remaining organic fraction after anaerobic digestion is much more recalcitrant than the input feedstocks leading to a stabilization of the organic matter and a lower organic matter degradation rate after field application, enabling a similar reproduction of the soil organic matter as obtained by direct application of the feedstock or by composting of the feedstock. (4) Regarding emissions, the main direct effect of anaerobic digestion on a farm level is the influence on gaseous emissions during manure or digestate treatment and handling, whereas the direct effects of anaerobic digestion on a field level on emissions (NH3− and N2O− emissions, NO3- leaching) are negligible or at least ambiguous. (5) The main direct effects of anaerobic digestion on the field level are short-term effects on soil microbial activity and changes in the soil microbial community. Therefore, in terms of the effects on agricultural sustainability, potential cropping system-based changes induced by introduction of biogas plants are probably much more relevant for the overall performance and sustainability of the cropping system than the direct effects triggered by application of digestates in comparison to the undigested feedstocks. Furthermore, to get the full potential advances from implementation of biogas plants in terms of improvement of the nutrient use efficiency and reduction of greenhouse gas emissions, there is the need to introduce more sophisticated techniques to avoid counteracting effects by pollution swapping, e.g., by gas-tight closure of the digestate stores and direct soil incorporation of the field-applied digestates.

Substrate inputs, nutrient flows and nitrogen loss of two centralized biogas plants in southern Germany.

In Germany, centralized biogas digestion plants (BGP) have been recently constructed. BGPs purchase the substrates from surrounding farmers and, in return, farmers receive the effluents. Substrate inputs, nutrient inputs and outputs were studied for two BGPs with effluent liquid–solid separation. Additionally, the path of the nitrogen (N) during manure handling was assessed. Silage maize (65–75% of the dry matter (DM) inputs) and grass (ca. 20% of the DM inputs) were the main inputs in both BGPs. During manure handling, it is estimated that 20–25% of the N in the effluents was lost via gaseous N emissions. From an environmental point of view the two main challenges are to reduce these gaseous N losses, and to provide N via the effluents mainly for spring manure application, and less so for autumn application. In solid effluents, gaseous N losses during storage are the main potential N loss pathway, whereas for liquid effluents gaseous N losses during and after field spreading are of great relevance. Current management indicated that approximately 50% of the N in the effluents was available for spring application and approximately 30% in autumn due to cleanout of stores before winter. Calculations show that the use of substrates with high DM content during autumn and winter would reduce the demand for storage capacity, thus reducing the demand for store’s cleanout in autumn. This leads to effluents with higher nutrient concentration that are very suitable for application to spring sown crops. Furthermore, some substrates like cereal grains and grass lead to effluents higher in N, whereas silage maize and other substrates lead to effluents low in N. An adapted substrate management would allow more N for spring application. The cycles of P and K are closed, enabling a complete replenishment of the P and K outputs.

Effects of organic wastes digestion for biogas production on mineral nutrient availability of biogas effluents

Organic farming systems are characterized by the strong regulation of the import of nutrients into the farming system to replace nutrient losses via sold products. In the present study mineral nutrient flows and balances of P, K and magnesium (Mg) were analysed for a mixed organic cropping system with dairy husbandry and for a stockless organic farming system. Also the influence of biogas digestion of farmyard residues (stable wastes, crop residues, etc.) as well as the effect of the import of substrates for biogas digestion on plant mineral nutrient uptake and farmgate nutrient balances was analysed. The objectives of the current study were; (1) to study the effects of anaerobic digestion of cattle manure and crop residues on plant mineral nutrient uptake; and (2) to model nutrient flows and balances related to the input of different kind of substrates for biogas digestion at the farmgate. Results indicated that slurry digestion did not influence plant P and K uptake. Import of single allowed substrates for digestion would lead to large imbalances in nutrient inputs compared to withdrawals. Most of the suited substrates for biogas digestion were associated with large K surpluses and insufficient P returns in comparison to mineral nutrient outputs via sold animal and plant products.

Effects of setup of centralized biogas plants on crop acreage and balances of nutrients and soil humus

An increasing number of biogas plants (BGPs) based on digestion of dedicated energy crops have been implemented in Germany. The objectives of this study were to assess the changes in (1) the acreage of different crops (silage maize, cereals, etc.) related to the setup of the BGP, (2) nutrient flows and budgets (N, P, K) due to the implementation of the BGPs, and (3) to assess the effluent N in the overall crop N supply. Data from 14 farmers before the setup of the BGPs were compared with data after implementation. Due to the setup of the BGPs, the acreage of silage maize greatly increased and there were significant negative effects on the weighted soil humus budgets, no effects on the weighted mean N and P budgets, and a negative trend regarding the K budgets. Results concerning the N release from organic manuring to maize crops showed that one third of the farmers considerably over-fertilize maize, indicating an underestimation of short- and long-term N supply of manure N. The implementation of centralized BGPs established very intensive nutrient cycles and, in the long-term higher risks of nutrient losses and environmental pollution are expected. One very effective measure to compensate for negative effects on the soil humus budgets and nitrate leaching is an enlargement of cover cropping, which will also offer economic revenue by providing aboveground biomass for digestion. If the amounts of effluents returned to a single farm or field are not adapted to the nutrient composition of the substrates delivered to the BGP, large nutrient imbalances can result. An effective measure to get a better allocation of the available nutrients is a solid-liquid separation of the effluents, enabling a more targeted allocation of the nutrients.

Chemical characterization of commercial organic fertilizers

A wide range of commercial organic fertilizers is currently available to organic farmers. Their use has numerous agronomic implications, including the supply of plant macro- and micronutrients, organic matter, but also pollutants such as toxic elements (metals (often referred to heavy metals), metalloids, and non-metals), persistent organic pollutants, pesticides, and antibiotic residues. For many organic fertilizers, the database concerning all these characteristics is weak. Therefore, in the frame of collections carried out throughout Germany, 77 samples of commercial organic fertilizers were analyzed for their nutrient and pollutant contents. Commercial organic fertilizers differ widely in nutrient concentration as well as in nutrient spectrum, meaning large differences among the fertilizers in their suitability to complement the nutrient supply by base organic fertilizers like composts, livestock manures, or digestates. The sampled commercial organic fertilizers show stable values for few characteristics (e.g. concentration of organic matter, carbon, and nitrogen), and high variation in the composition of the most other macro- and micronutrients as well as toxic metals. Consequently, the use of default values in budget calculation sheets can result in considerable errors of estimation. The concentrations of toxic elements, pesticides, and persistent organic pollutants are low, indicating the low risk of pollution related to their use.

Phosphorus bioavailability of sewage sludge-based recycled fertilizers

Six phosphorus (P) fertilizers recycled from sewage sludge [Struvite SSL, Struvite AirPrex,P-RoC, Mephrec, Pyrolysis coal and Ash (Mg-SSA)] were tested for their plant availability in potted soil of pH 7.2 under greenhouse conditions. The crop sequence simulated a rotation of red clover (Trifolium pratense L.), maize (Zea maize L.), and ryegrass (Lolium perenne L.). Other P fertilizer treatments included: Phosphate Rock (PR), Calcium dihydrogen phosphate [Ca(H2PO4)2], and an unfertilized control. Additionally, soil was regularly inoculated with two strains of plant growth-promoting rhizobacteria (PGPR; Pseudomonas sp. Proradix, and Bacillus amyloliquefaciens) to test their ability to increase P availability to plants. Sequential P fractionation was conducted to link the amount of readily available P in fertilizers to plant P acquisition. Shoot P content and dry matter of maize decreased in the following order: Struvite SSL >= Ca(H2PO4)2 > P-RoC >= Struvite AirPrex >= Mephrec > Pyrolysis coal >= Mg-SSA >= PR >= unfertilized. Rhizobacteria did not affect shoot biomass or P content. The results show that red clover might have mobilized substantial amounts of P. Sequential P fractionation was not suitable to predict the efficacy of the fertilizers. Generally, the sewage sludge-based fertilizers tested proved to be suitable alternative P sources relevant to organic farming systems. However, the efficacy of recycled fertilizers is strongly dependent on their specific production conditions.

Nutrient supply to organic agriculture as governed by EU regulations and standards in six European countries

Organic farming systems need to replace nutrients exported via farm products, especially phosphorus (P) which may otherwise become depleted in soil in the long term. In Europe, EU regulations for organic production are shaping the farming systems with respect to inputs of nutrients. Permitted off-farm P sources include conventional animal manure, composted or anaerobically digested organic residues, rock phosphate, and some animal residues such as meat and bone meal. The recent proposed revision of EU regulations for organic production (2014) puts less emphasis on closing nutrient cycles and instead aims at minimizing off-farm inputs, to reduce the risk of importing contaminants. This development, which has received little attention from the organic sector so far, is explained here. The paper further explores the regulatory conditions that govern the P supply to organic agriculture in six European countries: Austria, Denmark, Germany, Great Britain, Norway, and Switzerland. Organic farmers are subject to substantial variation in standards arising from the interpretation of EU regulations into national laws, restrictions imposed by private actors such as retailers, and private standards which may be stricter than EU regulations. In several countries, the majority of organic farmers are certified by private, stricter standards. We propose that EU regulations and private standards for organic production should not limit the use of recycled fertilizers in organic farming systems, as long as means are taken to ensure the quality and safety of these inputs. Awareness of the need to close nutrient cycles may contribute to adapting regulations and private standards to support recycling of nutrients from society to organic agriculture. A better definition of the term “natural substance” in organic regulations is required.

Sustainable Potato Production Worldwide: the Challenge to Assess Conventional and Organic Production Systems

Sustainable agriculture integrates environmental health, economic profitability as well as social and economic equity. Worldwide interest in potato as a valuable food security crop is increasing, because it is not globally traded, the prices are determined by local production costs and due to its beneficial impact on human nutrition. In the present review, organic and conventional potato productions were assessed by means of key indicators for sustainability. These indicators were fertility management and crop protection, yield level, tuber quality and environmental impact. The evaluation of several studies shows that each system has advantages and disadvantages. None of the production systems is per se more sustainable than the other. Each of them has potential for improvement of the system performance. In organic production, for example, by establishment of improved fertilization (e.g., application of more N-efficient base organic fertilizers) and crop management strategies (e.g., pre-sprouting of seed tubers, bio-based fungicides), in conventional farming by implementation of more target-oriented fertilization and pesticide spraying schedules. To meet the future challenges with increasing food demand while simultaneously decreasing its environmental impact, efforts on increasing the performance of both conventional and organic production systems, e.g., improving the nutrient use efficiency, are necessary.

Phosphorus availability on many organically managed farms in Europe

Maintaining sufficient soil phosphorus (P) levels for non-limiting crop growth is challenging in organic systems since off-farm inputs of P are restricted. This study assessed the status of P on organic farms in Europe using soil test results for extractable P. Data was obtained from published literature, unpublished theses, and various national and regional databases of soil test values. Most of the data (15,506 observations) came from field scale soil tests, but in some cases (1272 observations) values had been averaged across a farm. Farm scale and field scale data were analysed separately and the impact of farm type (arable, dairy, grassland, horticulture, mixed, poultry, unknown) was assessed. Soil test results were assigned to P classes from very low (P class 1) to very high (P class 5). The farm scale data came primarily from Norway, Sweden and Switzerland and did not indicate deficiencies in extractable P; 93% of farms fell into class 3 or above. The majority of the field scale data came from Germany and indicated sufficient or higher levels of P availability for arable and grassland systems on 60% of fields; the remaining fields had low or very low available P. Adaptations in organic systems may improve P uptake and utilization efficiency allowing yields to be maintained in the short-term, nevertheless there is cause for concern about the long-term P sustainability of some organic farming systems in Europe. This highlights the need to reassess allowable P inputs in organic farming systems to improve overall sustainability.