Stability and composition of soil organic matter control respiration and soil enzyme activities

Relationships between soil organic matter (SOM) molecular composition, thermal stability and decomposability by soil enzymes and microbes are largely unknown. We incubated soils from unfertilized and NPK-fertilized neighboring field plots of a long-term rye (Secale cereale) monoculture experiment and investigated relationships between changes in the molecular-chemical composition of SOM, the CO₂ flux and the activities of enzymes. Pyrolysis-field ionization mass spectrometry (Py-FIMS) showed larger ion intensities in the NPK-fertilized than in the unfertilized soil at start of the incubation, only small changes in composition and thermal stability in the unfertilized soil, and a preferential reduction in thermally stable components as well as general shifts towards lower pyrolysis temperature after three weeks of incubation in the NPK-treatment. We found evidence that thermally labile and stable proportions of various compound classes were differently susceptible to decomposition, depending on the fertilization history of the soil. Irrespective of fertilization treatment, peaks in xylanase activity after 7 days of incubation followed by decreasing values were reflected by the ratio of xylan (m/z 114) to xylose (m/z 132) marker signals in the Py-FI mass spectra. Thus, the study proved that (1) SOM composition was changed due to long-term rye cropping without and with NPK-fertilization, (2) the modified SOM composition affected the decomposability and microbial parameters under optimized conditions and (3) the thermal properties of individual compound classes derived from Py-FI mass spectra can be sensitive predictors of microbial decomposition.     

Synergistic antifungal interactions of zinc or copper with anilazine

Two isolates of Botrytis cinerea and one of Colletotrichum coccodes were used to test for synergistic or antagonistic interactions (a) between pairs of 11 common fungicides and (b) between mixtures of individual fungicides with components of foliar fertilisers. Two screening assays monitored the effects with time of inhibitor combinations on spore germination and subsequent mycelial growth. The combinations that gave positive results for interaction in the screening assays were further investigated by a third quantitative assay, which compared the observed inhibition of mycelial growth with the net inhibition predicted by a non-interaction model. This model assumed that the components of a combination of substances each inhibited the test fungus independently of one another. Only mixtures of anilazine with zinc or copper resulted in appreciable deviation from the predicted behaviour based on a non-interaction model. The inhibition of mycelial growth by combinations of anilazine with Zn²⁺ (inorganic salt or formulated with urea) was consistently greater than predicted for each of three fungi on either of the two test media. Similar effects were observed with Cu²⁺, although the synergism was less marked.     

Modification of porespace by tillage in two stagnogley soils with contrasting management histories

The soil porespace was studied in two long-term tillage experiments on two clayey stagnogleys in Southern England. The soils differed in respect of mineral and organic composition and previous management history. In both soils the total volume of pores and the volume fraction of macropores in the topsoil horizon declined with direct drilling compared with annual ploughing. This difference between tillage treatments appeared to develop more slowly in the soil that was formerly under continuous arable cultivation than in the soil that was previously in long-term grassland. Fluid transport coefficients were greater in ploughed topsoil in both soils; however, at the boundaries between topsoil and subsoil, and in the upper subsoil, permeability and gaseous diffusivity were greater after direct drilling. At a long-term arable site, soil was more consolidated below the depth of ploughing or shallow tillage, whereas in a former grassland soil ploughing disrupted the continuity of channel-type macropores.     

Movement of added clay through calcareous materials

A study using Cs-134 tagged sediment suspended in infiltrating water has indicated that clay can move in calcareous materials. The study indicated that the time and conditions under which clay movement is initiated in calcareous materials are functions of the availability of clay for movement, channels or pores for the clay to move through, and the quantity of precipitation available to move the clay.     

Prediction of the water sorption isotherm in air dry soils

Within this work we present a revised pedotransfer function (PTF) that predicts water sorption isotherms for dry soils based on the clay content of the soils. When the water sorption isotherm is plotted as a water retention curve (log water potential plotted against the water content) it typically results in a log linear function as described by Campbell and Shiozawa (1992). The linear function is defined by its slope and a fixed endpoint at zero water content. The reciprocal of the slope shows a strong correlation with the clay fraction. For the calibration of a PTF we measured water sorption data for 18 soils with clay contents from 2% to 61%. The final predictions of the water sorption isotherms from the clay mass fraction were very good if the clay content was higher than 7%. The use of a revised theoretical endpoint at the dry end of the WRC did improve the prediction as compared to the endpoint that has been used in the literature before. In addition Literature data for 22 soils and 3 pure clay minerals were used for validation. The good performance of the PTF only occurred if the clay fraction was dominated by 2:1 clay minerals. The water retention isotherm of soils rich in the 1:1 clay mineral kaolinite could not be predicted by this approach; the actual water content was strongly over-predicted and the water retention curve did not follow a log linear relationship.     

Assessing the progress of colonization by arbuscular mycorrhiza of four plant species under different temperature regimes

To capitalize on the benefits offered by arbuscular mycorrhizal fungi (AMF) in agricultural systems, the effect of low soil temperature has to be taken into consideration over large areas of the planet. However, the effect of suboptimal root‐zone temperatures on AMF colonization is poorly understood. It has been suggested that it depends on the host plant species. We hypothesized that this interdependence is a function of the parameter used to assess the presence of AMF in the roots. In a pot experiment with non‐sterilized soil, we investigated the influence of three soil temperatures (10, 15, and 20°C) on the progress of root colonization of four host plant species (Ornithopus compressus, Lolium rigidum, Triticum aestivum, and Zea mays) by indigenous AMF. Plant root density, arbuscular colonization rate (AC) and colonized root density (CRD) were assessed 14, 28, and 42 d after sowing. Based on CRD, the effect of temperature on the progress of root colonization by AMF was independent of the host plant species. The apparent influence of the host plant species was only due to the species‐specific effect of soil temperature on root growth and therefore on AC. The host plant species only determined the minimum temperature for the AM colonization initiation, possibly due to species‐specific response of root growth and exudation to cool temperatures.