A note on sewage sludge - risk assessments and fertilization value

A number of recent studies of risk related to agricultural use of sewage sludge are reviewed, as a contribution to the discussion of potential for use in organic agriculture. Furthermore a very tentative assessment of the fertilization value of sewage sludge and its derived products is developed

Hard and soft science issues to be negotiated to improve urban metabolism

In the industrialised world, waste management systems have developed to maturity without primary concern for recycling. These systems have originally been designed to ensure human health and a high local hygienic standard. More recently environmental concerns have been the driving force behind a technological development of sewage treatment with biological removal of N, P and organic matter. This technology addresses some immediate problems in the aquatic environment, but the sewage sludge from the treatment plants contains considerable quantities of xenobiotic compounds and heavy metals, and only a fraction of the nutrients that entered the urban areas, thus making the sludge a non-attractive fertiliser source. In recent years there has been concern about the sustainability of this state of affairs as regards wastewater handling, as well as concern about the fate of the final waste deposits in the environment. In the mid 1990s Danish organic farmers made a point of refusing to accept sewage sludge as a source of nutrients. This sparked a heated debate, and for a time all farmer organisations refused to accept sewage sludge on their fields, leading to severe problems in urban areas. One of the consequences of this conflict is that municipalities are increasingly seeking alternatives to returning sewage sludge to the land (e.g. burning or dumping), in order to rid their dependence of farmers acceptance. Another consequence has been that the issue of ‘closing the urban-rural nutrient circle’ as part of a sustainable development has received increasing attention among Danish organic farmers. This issue had been identified already in the early days of the organic movement in Denmark, but has never been a top priority. It was accentuated by a strong Swedish emphasis of agricultural use of human urine from source separating toilets that provided inspiration to look at implementing such techniques in Danish urban areas. One additional factor that has increased the priorities of the issue was the growing realisation that current day organic farmers have a strong bias towards milk production, due to the natural integration of the clover-grass in the production system, that is essential for ensuring an ample supply of fixed atmospheric nitrogen. If more stockless organic farms (e.g. vegetable and grain production for human consumption) are to become economically sustainable, it is important to find ways of using the land with less emphasis on clover grass. One of the ways of doing this is to increase the amounts of nutrients that can be re-cycled from urban areas in a form that is acceptable to organic farms

Green manure crops for low fertility soils

Organic crop production is growing, but crop yields are still below potential. The purpose of our project “Nutrients for higher organic yields (NutHY)” is to increase yields and resource efficiency in organic crop production by optimizing nutrient supply. Growing green manure is an important tool to improve fertilization by biological nitrogen (N) fixation but also by mobilization and release of other nutrients such as phosphorus (P). However, development and performance of green manure are affected by low soil nutrient availability that is often reported as a problem in organic arable farms, especially with regard to P. Poster at DOK-Monte Veritá Conference, 6-11 October, 2019, Congressi Stefano Fanscini, Monte Veritá, Switzerlan

Measuring degradation of transgenic DNA and screening for horizontal gene transfer from GMO-plant material during composting

The experiments show that composting of GM plant residues greatly increases the rate of degradation of transgenic DNA compared to the rate for plant residues left in the soil. If the persistence of transgenic DNA in the environments is considered as the only risk factor, composting is a 'DNA-safe' method to treat GM plant residues. However, even though transgenic plant DNA was not detected in bacterial isolates in our experiments, we cannot conclude that horizontal gene transfer can not take place. The 300 isolates tested proved to be too low a number to be conclusive. The numbers of isolates tested were based on the screenings indicating high transfer, but the screenings were biased apparently because some Bacillus species gave PCR products matching the transgenic DNA. Thus, it is still an open question if composting constitutes a safe way of disposing of GM plant residues. Furthermore, these experiments give rise to other interesting questions, e.g., the behavior of GM plant materials decomposing in waste piles or manure yards under composting-like conditions and the possibility of horizontal gene transfer to indigenous bacteria at the comparably lower temperatures presentat these environments.These questions need to be assessed if the risk associated with the use of GM plants is to be thoroughly investigated

Urban nutrient balance for Bankok

To enhance agricultural sustainability, former linkages between agriculture and urban waste production should be reintroduced. Therefore, to explore the options for recycling of nutrients from mega-cities, a nutrient balance model was developed. The parameterization were established for the Bangkok Province and considers nitrogen (N) and phosphorous (P). To model the food supply, an online database (FAOSTAT) estimating supply at country levels, was employed. It is argued that desaggregation to urban level is reasonable after adjustments for different economy in Bangkok than the average in Thailand. The balance shows that only a small fraction of nutrients are recovered, currently about 7 and 12% respectively, of the amount of N and P in the total food supply. On the other hand most (about 95%), of the total loss of N can be accounted for by elevated N levels in the Chao Phraya River from where also much (about 38%) of the loss of P can be explained. That is, in- and out-flows of N is almost found in balance but a huge amount of P must be accumulated somewhere. However the balance also shows that the Bangkok Province throws out into the river (and the sea) very huge quantities of plant nutrients that could be recovered and reused. For future research is it of particular interest to explore the maximum nutrient recovery fraction in different waste management systems

Byernes affaldshåndtering og næringsstofkredsløb

I Danmark og den vestlige verden er systemer til håndtering af byernes affald i dag indrettet ud fra sundhedsbetragtninger, mens hensyn til recirkulering har været helt underordnet. Kapitlet diskuterer mulighederne for at ændre denne indretning, den betydning det kan have for jordbruget, og for dialogen mellem by og land

Dual-labelled (13C/15N) green manure to differentiate between plant uptake of organic and inorganic N

Experimental data is still lacking for determining whether plant uptake of organic nitrogen in agricultural soils contributes substantially to the total N uptake. Pulseinjection studies with dual-labelled amino acids have confirmed that non-mycorrhizal crops possess the capacity to take up organic N but failed to quantify the uptake relative to total N uptake

Catch crops have little effect on P and K availability of depleted soils

It is a well-known fact that catch crops have a significant effect on availability and loss of soil inorganic nitrogen (Thorup-Kristensen et al., 2003) and recently marked effects on soil inorganic sulphur dynamics have also been shown (Eriksen and Thorup-Kristensen 2002; Eriksen et al., 2004). However, we know much less about the effect of catch crops on phosphorous (P) and potassium (K) mobilisation and availability for the next crop. After several years of organic cash crop production, e.g. vegetables and cereals, yield levels may gradually be limited by soil P and K availability, depending on the initial status at conversion to organic production principles. This is particularly the case during the establishment phase of certain vegetable cultures with a limited rooting system (e.g. lettuce, leeks, onions). Therefore, it has often been hypothesized that certain catch crops are capable of increasing the availability of P and K when the soil status becomes low. In the VegCatch subproject 'Catch crops as a tool for increasing P bioavailability in soils' we have therefore studied the ability of different catch crop species to mobilise and take up P and K from soils of low availability, as well as the ability of the catch crops deliver P and K to the subsequent main crop

Collembola’s role in regulating mass fluxes in soil and the effects of contrasting life histories

The study determined metabolic rates and elemental pools for two Collembola species with contrasting life histories. The fittest of the two species, P. minuta, excreted the equivalent of 10–12% of the elemental body content per day, and P. armata 7–10%. Most elements are lost to excretion (CO2 and N-waste). These figures in combination with stoichiometry and life histories indicate that the cost of P. minuta’s better fitness is a requirement for a higher quality diet than P. armata. The data produced in this study can be used to estimate the collembolan contribution to C and N fluxes in the soil

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