Anaerobic digestion (AD) for biogas production leads to several changes in the composition of the resulting digestates compared to the original feedstock (ammonia content, pH, carbon to nitrogen ratio, etc.), which are relevant for the plant availability of macro- and micronutrients after field application. Increased NH4+-N content in digested slurries compared to undigested slurries does not guarantee improved uptake efficiency of slurry nitrogen and increased savings in fertilizer nitrogen. AD of crop residues and cover crops leads to an increase in the total amounts of mobile organic manures within the farming system, resulting in a higher nitrogen use efficiency and an increased scope for target-oriented nitrogen application in time and space, when needed by the crop, as an alternative to the site-bound soil incorporation as green manures. AD of dairy manure appears to reduce the fraction of immediate plant available phosphorus and micronutrients. This does, however, not affect short-term crop availability under field conditions. More studies are needed to improve current knowledge on sulfur losses during AD and fertilizer value of digestates.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/elsc.201100085

Effects of anaerobic digestion on digestate nutrient availability and crop growth: A review

Soil organic matter (SOM) is a complex mixture, which influences a number of soil properties and nutrient cycling, and is itself influenced in kind and amount by land-use, soil type, climate and vegetation. There is considerable concern that, if SOM concentrations in soils are allowed to decrease too much, then the productive capacity of agriculture will be compromised by deterioration in soil physical properties and by impairment of soil nutrient cycling mechanisms. This has clear implications for the sustainable use of soil. We have focussed our discussion from the standpoint of the sustainability of UK agriculture, because we know that best, but similar concerns are equally valid elsewhere in the world. Although soil scientists would expect to find different behaviour in different soils at different ‘critical’ concentrations of SOM, it seems widely believed that a major threshold is 2% soil organic carbon (SOC) (ca. 3.4% SOM), below which potentially serious decline in soil quality will occur. This review summarises what is known about critical thresholds of SOC or SOM, mainly in soils of temperate regions. It examines critically the quantitative, rather than anecdotal or descriptive, evidence for such thresholds and their potential effects on soil quality, soil physical properties and crop nutrition, and the links between these. We conclude that the quantitative evidence for such thresholds is slight, although there is some evidence that there might be an desirable range of SOC covering a wide spectrum of soils, but again the quantitative evidence for this needs considerable development. There is also little quantitative evidence that reduction in SOC concentrations in the soils of England and Wales will have marked effects on other soil properties or crop yields. The data do suggest, however, that more research is required on the nature of SOC, particularly of the so-called ‘active’ or ‘fresh’ fraction and its influence on the properties of a range of soil types under different land uses. This is particularly relevant to the ongoing debate about soil quality, its definition, and the identification of appropriate indicators that relate soil quality to soil functions.

Is there a critical level of organic matter in the agricultural soils of temperate regions: a review

The microbial biomass and community structure of eight Chinese red soils with different fertility and land use history was investigated. Two community based microbiological measurements, namely, community level physiological profiling (CLPP) using Biolog sole C source utilization tests and phospholipid fatty acid (PLFA) profiles, were used to investigate the microbial ecology of these soils and to determine how land use alters microbial community structure. Microbial biomass-C and total PLFAs were closely correlated to organic carbon and total nitrogen, indicating that these soil microbial measures are potentially good indices of soil fertility in these highly weathered soils. Metabolic quotients and C source utilization were not correlated with organic carbon or microbial biomass. Multivariate analysis of sole carbon source utilization patterns and PLFAs demonstrated that land use history and plant cover type had a significant impact on microbial community structure. PLFAs showed these differences more than CLPP methods. Consequently, PLFA analysis was a better method for assessing broad-spectrum community differences and at the same time attempting to correlate changes with soil fertility. Soils from tea orchards were particularly distinctive in their CLPP. A modified CLPP method, using absorbance readings at 405 nm and different culture media at pH values of 4.7 and 7.0, showed that the discrimination obtained can be influenced by the culture conditions. This method was used to show that the distinctive microbial community structure in tea orchard soils was not, however, due to differences in pH alone.

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Microbial biomass and community structure in a sequence of soils with increasing fertility and changing land use

We investigated the influence of tillage (conventional, minimum and reduced) on selected soil microbial properties of a fine-sandy loamy Haplic Chernozem over a period of 8 years. The microbial biomass and soil microbial processes were affected mostly by type of tillage and to a lesser extent by the date of soil sampling. Whereas xylanase activity was significantly higher in the 0 to 10-cm soil layer of the reduced and minimum tillage systems within the first year of the experiment (protease and phosphatase activities were significantly higher in the second year), significant treatment effects on microbial biomass, N mineralisation and potential nitrification were observed after a 4-year period. The slow response of substrate-induced respiration to the change in type of tillage may have been due to the differences in the biomass C turnover rates. After a 4-year period, the stratification of the soil microbial biomass within the profile of reduced and minimum tillage systems was probably responsible for the more intensive soil microbial processes near the soil surface compared with conventional tillage. In the 20 to 30-cm layer, N mineralisation, potential nitrification and xylanase activity in the conventional treatment were significantly higher than in the minimum and reduced tillage plots due to buried organic materials. Discriminant analysis underlined the similarity of the enzyme activity patterns in the top layer of the reduced and minimum tillage treatments, and in both layers of the conventional tillage system. The trend towards a significant increase in functional diversity caused by reduced tillage became obvious within the first year of the experiment, and this effect was still manifest after 8 years. All relationships suggested that there were differences in available resources (e.g. organic matter) along the sequence of different tillage systems; this was reflected in part by enhanced enzymatic and microbial activities in the soil layers. In conclusion, this study showed that soils affected by tillage may be classified on the basis of their functional diversity. Therefore, the soil microbial properties chosen for microbiological soil monitoring (microbial biomass, N mineralisation and enzyme activities involved in C, N and P cycling) provide a reliable tool with which to estimate early changes in the dynamics and distribution of soil microbial processes within soil profiles.

Long-term monitoring of microbial biomass, N mineralisation and enzyme activities of a Chernozem under different tillage management

The impacts of crop rotations and N fertilization on microbial biomass C (Cmic) and N (Nmic) were studied in soils of two long-term field experiments initiated in 1978 at the Northeast Research Center (NERC) and in 1954 at the Clarion-Webster Research Center (CWRC), both in Iowa. Surface soil samples were taken in 1996 and 1997 from plots of corn (Zea mays L.), soybeans (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha–1 before corn and an annual application of 20 kg P and 56 kg K ha–1. The Cmic and Nmic values were determined by the chloroform-fumigation-extraction method and the chloroform-fumigation-incubation method, respectively. The Cmic and Nmic values were significantly affected (P<0.05) by crop rotation and plant cover at time of sampling, but not by N fertilization. In general, the highest Cmic and Nmic contents were found in the multicropping systems (4-year rotations) taken in oats or meadow plots, and the lowest values were found in continuous corn and soybean systems. On average, Cmic made up about 1.0% of the organic C (Corg), and Nmic contributed about 2.4% of the total N (Ntot) in soils at both sites and years of sampling. The Cmic values were significantly correlated with Corg contents (r≥0.41**), whereas the relationship between Cmic and Ntot was significant (r≤0.53***) only for the samples taken in 1996 at the NERC site. The Cmic : Nmic ratios were, on average, 4.3 and 6.4 in 1996, and 7.6 and 11.4 in 1997 at the NERC and CWRC sites, respectively. Crop rotation significantly (P<0.05) affected this ratio only at the NERC site, and N fertilization showed no effect at either site. In general, multicropping systems resulted in greater Cmic : Corg (1.1%) and Nmic : Ntot (2.6%) ratios than monocropping systems (0.8% and 2.1%, respectively).

Soil microbial biomass carbon and nitrogen as affected by cropping systems

Adenosine triphosphate (ATP) contents, enzymatic activities, cellulose decomposition in situ, aggregate stability, total organic C contents, C- and N-mineralization potentials and amounts of water-soluble C compounds were measured in the plough layer of field plots from a haplic Luvisol at the ‘Ihinger Hof’ near Stuttgart, SW Germany Plots had been treated differently for 14 years with regard to tillage (ploughing, rotary cultivation) and crop rotations (legume-cereals, rape-cereals) Soils were sampled from the 0–10 cm and 10–25 cm layer of the Ap horizon 4 times in 1990 during and after the vegetative growth of oats On the basis of the input and distribution of crop and root residues during the previous 8 years a model for the decomposition of organic substances was used to estimate the amounts of the microbially-decomposable organic substances added to the plots (decomposable young soil organic matter, DYSOM) Our aim was to study the effects of management practices on soil microbial properties and to relate those effects to changes in the microbially-available part of the soil organic matter The contents of total soil organic C, total N, ATP, and soil microbial activities in the 0–10 cm layer were mostly higher with rotary cultivation than with ploughing In contrast, either no differences between the tillage systems or higher values with ploughing occurred in the second soil layer (10–25 cm) In the upper soil layer of the rape-cereals crop rotation soil microbial biomass was higher and more active than in the legume-cereals crop rotation In the lower soil layer no uniform effects of the crop rotations were found For the 0–10 cm layer there was an overlapping effect of cultivation and crop rotation on soil organic matter fractions and microbial biomass Differences in soil microbial measurements were partly related to differences in humus contents However, variations in microbial properties with different treatments were mostly larger than variations in total organic C content, sometimes occurring without any difference in total organic C contents In these cases the contents of DYSOM and soil pH were found to have a dominant influence Soil physical properties either showed only little differences (texture, bulk density), or were highly correlated to total organic C contents (water contents) There seemed to be a contradiction between a tendency for higher microbial activities and at the same time larger humus contents on plots with rotary cultivation compared to ploughing This may be explained by a higher C efficiency in these plots, ie the soil microbial biomass-to-organic C ratio was relatively high and accompanied by small changes in biomass contents during the year

Soil microbial properties and the assessment of available soil organic matter in a haplic Luvisol after several years of different cultivation and crop rotation

The triphenyltetrazolium chloride (TTC) method described by Thalmann (1968) and the iodonitrotetrazolium chloride (INT) method described by Spothelfer-Magana and Thalmann (1992), used for measuring soil dehydrogenase activity, have been modified to overcome some methodical short-comings. Absorption maxima of 485 nm for triphenylformazan dissolved in acetone, 491 nm for iodonitrotetrazolium formazan (INTF) dissolved in tetrahydrofuran and 455 nm for INTF dissolved in N,N-dimethylformamide are recommended for measuring wavelengths. Extracting triphenylformazan twice with acetone is less toxic and proved to be at least as efficient as extraction with a mixture of 90% acetone and 10% carbon tetrachloride (Thalmann 1968 method). Tetrahydrofuran and dimethylformamide were equally good in extracting INTF from soils, but the former was less toxic. Anaerobic incubation resulted in the formation of higher amounts of triphenylformazan and INTF as well as reduced standard error. Both TTC and INT reduction showed high reproducibility and good differentiation of the microbial activity of six soils. For several reasons (more easily determined substrate dose depending on different soil types, better reduction, shorter incubation time), INT reduction seems to be a more suitable method of measuring soil microbial activity than TTC reduction.

Comparison and improvement of methods for determining soil dehydrogenase activity by using triphenyltetrazolium chloride and iodonitrotetrazolium chloride

W. R. Fischer

Papers

Soil microbial properties and the assessment of available soil organic matter in a haplic Luvisol after several years of different cultivation and crop rotation

Comparison and improvement of methods for determining soil dehydrogenase activity by using triphenyltetrazolium chloride and iodonitrotetrazolium chloride

Nitrogen mineralization of liquified, anaerobically fermented cattle manure in soil