B. Stenberg

Microbial biomass and activities in soil as affected by frozen and cold storage

Abstract When monitoring microbial biomass and activities in soil, the storage conditions of the soil samples prior to analysis may be decisive for the results. Studies made on freshly collected soils are generally preferred but these are not always possible for practical reasons, since sampling is often restricted to short periods of the year, e.g. due to climatic conditions. The most commonly used methods to store soils for microbiological analyses are refrigeration or freezing of field moist soil. There are, however, studies that warn against any kind of storage, although other studies do not indicate any drawbacks to it. We have compared the microbial biomass and activities in 12 different, annually frozen, agricultural mineral soils when fresh and when stored at +2±2°C and at −20±2°C for 1xa0d and for 1, 3, 6 and 13 months. The results showed that the effects of freezing generally were smaller than those of refrigeration. The biomass estimated by chloroform fumigation–extraction and biomass index estimated by substrate induced respiration differed in that chloroform fumigation extracted carbon had decreased with 27% after 3 months at +2°C, while substrate induced respiration showed only small deviations from the results from fresh soils. Basal respiration rate and potential denitrification activity showed a similar pattern, with a pronounced decrease in values for refrigerated soils. The nitrogen mineralisation capacity was the only measure that was greatly influenced by freezing. After 6 months N mineralisation in the frozen soils was 25% higher than that of the fresh soils. Potential ammonium oxidation and the degradation rate of the herbicide linuron were affected only a little or not at all by storage for 13 months. We concluded that storage at −20°C for 13 months does not affect the microflora in annually frozen soils in any decisive way. We have also discussed the possible reasons for the contradictory results between different studies made on storage effects.

Monitoring Soil Quality of Arable Land: Microbiological Indicators

The aim of this review is to present a foundation of present knowledge and to describe existing strategies for the development and implementation of monitoring systems for soil quality of arable land, based on soil-quality indicators. In the review viewpoints are discussed on the soil-quality concept, suggested frameworks for its assessment, and possible methods for the selection and evaluation of quality indicators. These sections are in general terms, without emphasis on chemical, physical or biological soil properties. The potential value of including microbiological indicators in a monitoring system is discussed thoroughly. Several possible microbiological indicators are suggested, and their advantages and disadvantages are discussed. Strategies for the selection and sampling of monitoring sites are reviewed, and the treatment of samples before analyses is also discussed. In conclusion, a framework for using microbiological indicators in a Swedish monitoring system is suggested.

Potential denitrification activity assay in soil—With or without chloramphenicol?

Abstract A common way to characterize denitrification in soil is to determine the potential denitrifying activity (PDA). Our objectives were to compare different techniques of mathematically describing experimental data obtained in the PDA assay, both with and without use of chloramphenicol (CAP), and to quantify the effect of CAP on the process. The PDA assay was carried out in the presence of acetylene in slurries of three agricultural soils containing 1 m m glucose and 1 m m KNO 3 . When CAP was not used in the assay, growth related curves of N 2 O-formation were obtained for all three soils. These data were used to calculate the initial rate by: (1) assuming the initial phase to be linear and using the four first data points for linear regression; and by (2) using a growth-assciated product formation equation. The good fit to the data that was obtained with the latter method suggests that PDA is a continuous process without distinct phases. Moreover, our results clearly show that denitrifying activity is inhibited by CAP even at the lowest concentration tested, 20 mg 1 −1 . The inhibiting effect increased with increasing concentrations of CAP. The PDA was 17–42% lower at 1 g CAP 1 −1 compared with assays without CAP. This shows that not only synthesis of new enzymes is affected but also that the activity of already existing enzymes is decreased. Results from our study strongly suggest that single concentrations of CAP must not be used in PDA assays. An alternative strategy could be to use multiple CAP concentrations and then extrapolate to the rate at 0 g CAP 1 −1 . However, we recommend assays without CAP and that data should be fitted to the growth-associated product formation equation. By using this method, values of the PDA and the growth rate of the denitrifying bacterial population are objectively obtained.

Use of near infrared reflectance spectra of soils for objective selection of samples

A low cost strategy for objective and rapid selection of soil samples from a large population was evaluated. The purpose of the strategy was to retain a maximum of the original variation in important soil properties with only a small selection of samples. The evaluation was made with emphasis on clay content, soil organic matter, cation exchange capacity, and base saturation, all of which are important factors for biochemical activities in the soil and, therefore, for soil fertility. The strategy involved use of near infrared (NIR) spectroscopy combined with principal component analysis (PCA). A 2-nm interval spectrum between 1300 and 2398 nm was recorded on 146 air-dried soil samples from the most important cultivated areas in Sweden. The samples were considered mainly Cambisols and Regosols. The first derivative of each NIR spectrum was used for PCA. Twenty soils were selected by visual examination of two-dimensional score-plots from PCA. Score-plots were made from NIR data alone, from NIR data combined with pH, and from the eight significant score vectors from PCA on NIR data, combined with pH. Two criteria for selection from these plots were applied: (i) one sample from each apparent group was selected and (ii) samples evenly distributed at the periphery of the total sample population, and one in the center, were selected. In all, six selections were made. The distributions in soil properties in the selections were compared with random selection and with the original population. It was clear that NIR could help to improve the diversity in sample selections compared with random selection. In general, peripheral selections generated a higher recovery of range and a more even distribution in soil parameters than cluster selections. For clay content and cation exchange capacity, PCA on NIR data alone gave the best results, but to improve the distribution in pH and the pH-dependent base saturation, pH had to be included in PCA. To select soil samples that are distributed in all five soil parameters to the best extent possible, we propose peripheral selection from a two-dimensional PCA plot calculated from score vectors and pH data. In the present study, this method would have reduced costs about 70% compared with wet chemistry analyzes.

Microbiological and chemical changes in two arable soils after long-term sludge amendments

Abstractu2002Sludge amendments increase the input of carbon and nutrients to the soil. However, the soil concentrations of heavy metals and xenobiotica can also increase due to sludge amendments, with possible effects on soil microorganisms and soil fertility. Therefore, we studied the effects on soil microorganisms and soil chemistry in two arable soils after 12 and 16 years of sewage sludge amendment (0, 1 and 3 dry matter ha–1 year–1). The sludge amendments were combined with nitrogen addition at three rates according to crop requirements, and all combinations were replicated 4 times, giving a total number of 36 parcels at each experimental site in a non-randomised block design. Univariate data evaluation as well as principal component analysis and discriminant function analysis (DFA) were used to identify differences between treatments in microbial and chemical parameters. The DFA showed that acid and alkaline phosphatase, potential ammonium oxidation and total nitrogen were the most important parameters to discriminate between a priori defined groups of sludge treatments. Among the heavy metals, copper showed the highest increase in soil concentration with sludge amendments, but this increase was still not high enough to have a significant influence on the measured parameters. None of the xenobiotica investigated was found in high soil concentrations. In conclusion, the present study showed that the sewage sludge affected several of the biological and chemical parameters investigated. However, no severe negative effects on soil microorganisms were detected at these moderate levels of sludge amendment.

Effects of reduced tillage and liming on microbial activity and soil properties in a weakly-structured soil

Abstract The effects of reduced tillage and lime on crop yield and soil physical and microbial properties were studied in a weakly-structured silty clay loam soil. Two autumn primary tillage practices were compared, mouldboard ploughing to 20–25xa0cm and cultivation to 12xa0cm. Seedbed preparation was carried out by several harrowing operations in the mouldboard ploughed treatment, and with a PTO-driven harrow in the same operation as sowing in the shallow cultivation treatment. The tillage treatments were applied alone or were combined with liming aimed at soil structural improvement. Lime was added as 6.5xa0Mgxa0CaOxa0ha−1 before the start of the experiment and mixed into the top 12xa0cm of soil with a disc cultivator. A 4-year crop rotation was used: spring barley, spring oilseed rape, spring/winter wheat and oats, and all crops were compared each year. Crop residues were retained in the experiment and incorporated at cultivation. Aggregate stability was improved by the shallower tillage depth, probably as an effect of an increase in soil organic matter and a more active microbial biomass. Liming had little effect on soil structure variables but increased microbial activity to some extent. This was reflected in higher crop yields, especially when the shallow tillage depth was combined with liming. Penetration resistance in the seedbed subsoil was highest when mouldboard ploughing was carried out in plots without liming. Data were examined with principal component analyses, and the structures in the data were presented as scores and loading plots, which revealed groupings between samples and relationships between variables, respectively.

NIR spectroscopy, mineral nitrogen analysis and soil incubations for the prediction of crop uptake of nitrogen during the growing season

To predict the amount of N taken up in above-ground plant parts during the growing season, initial mineral soil N, a soil incubation method, soil organic matter and NIR data were compared as predictors. Soil samples were taken from 15 plots cropped with winter wheat on a farm in south-western Sweden. The plots were not fertilized with N during the 1997 growing season. N contents in above-ground plant parts were measured in mid-June and in mid-August. All methods were capable of predicting the crop uptake of N reasonably well. NIR data gave at least as good predictions as the best traditional method, initial soil NO3-N. The most important wavelengths, around 1400 and 1700 nm, and above 2000 nm, coincide with the wavelengths reported earlier to be important for the prediction of soil organic matter. However, the data suggest that other soil components influencing mineralization are also spectrally active. Since very few samples were taken, the studies need to be extended in order to be able to use the method in practice. It is recommended that further studies be instigated for the possibility of using the same NIR calibration over several years and to clarify the spatial regions that the calibrations can cover.

Soil attributes as predictors of crop production under standardized conditions

Abstractu2002To assess soil fertility or quality three controlling components – its physical, chemical and biological nature – have to be considered. In this study a broad spectrum of agricultural soils from Sweden were cropped with ryegrass in pots under standardized conditions in climate chambers. Measurements of physical, chemical and biological attributes of soil were used to predict C and N yields by simple correlation and the multivariate calibration techniques, principal component analysis combined with multiple linear regression, and partial least squares (PLS) regression. The N yields were typically more accurately predicted than the corresponding C yields. The best single predictor of yields was always total soil N, but estimates produced by multivariate models including organic C, total N, C/N ratio, coarse silt, potential denitrification activity, N mineralization, substrate-induced respiration and sample site humidity were, in all cases, substantially more accurate. Coefficients of correlation between predicted and measured C or N yields ranged between 0.61 and 0.80 with total N as predictor, and between 0.69 and 0.97 with the multivariate models. Both quantitative and qualitative aspects of the organic matter were considered to be important with respect to the predictive ability. Both these aspects were accounted for by the multivariate models. The multivariate technique, PLS regression, facilitated the classification of soils into categories of good, normal or poor fertility in relation to their organic matter content.