Evaluation of various tests for the diagnosis of soil contamination by 2,4,5 trichlorophenol (2,4,5-TCP).

The response of different types of soils to contamination with 2,4,5-triclorophenol was studied to test the validity of the concept of generic reference levels (GRL), the main criterion used to define soil contamination. Soil samples were artificially contaminated with doses of between 0 and 5000 mg kg-1of 2,4,5-triclorophenol, and analysed by various tests. Where possible, the response of soils to the contaminant was modelled by a sigmoidal dose-response curve in order to estimate the ED50 values. The tests provided different responses, but only microbial biomass-C and dehydrogenase and urease activities demonstrated soil deterioration in response to contamination. The results suggest that the diagnosis of soil contamination has been greatly simplified in the legislation by the provision of a single figure for each compound, and that the GRL concept could perhaps be substituted by measurement of ED50 values, which better reflect the alteration of a soil due to the presence of a xenobiotic substance.Peer reviewe

Translocation of soils to simulate climate change: CO2 emissions and modifications to soil organic matter.

The effect of climate change on CO2 emissions was studied on undisturbed soil monoliths (40-cm diameter, 25-cm high), which were translocated to warmer zones than their place of origin. Thirty-two months after the translocation, a climatic factor deduced from the moisture content of the soil and from the effective mean temperature (temperatures in excess of 5 ºC) revealed that translocation increased the potential of the climate to enhance the biological processes by between 73% and 26% compared with what the soil would support in its place of origin. At the end of the study, the transported soils had lost a large proportion of both total carbon and nitrogen (between 20 and 45%). During the experiment, the CO2 emissions from the soils, measured under field conditions, were quite variable, but were usually greater than from soils in situ. The variation in labile C in the soil throughout the experiment was calculated from a first-order kinetic equation for organic matter decay. The relative CO2 emissions, expressed in terms of the labile carbon fraction in the soils, were clearly greater in those translocated soils that underwent the most intensive climate change, which indicates that the variations in emissions over time are basically a function of the size of the labile organic matter pool.Peer reviewe

Modification of enzymatic activity in soils of contrasting pH contaminated with 2,4-dichlorophenol and 2,4,5-trichlorophenol.

According to previous studies, acidic soils may receive larger quantities of 2,4-dichlorophenol (2,4-DCP) and of 2,4,5-trichlorophenol (2,4,5-TCP) than the concentrations indicated in the prevailing legislation for defining a soil as contaminated, without any important changes in their biochemical properties. In this study, we investigated whether neutral or slightly alkaline soils behave in the same way as acidic soils in response to contamination by these compounds. For this purpose, a large number of acidic soils (pH between 4.2 and 5.9) and calcareous soils (pH between 6.5 and 8.0 ) were contaminated in the laboratory with different doses of 2,4-DCP (up to 10000 times the GRL) and of 2,4,5-TCP (up to 500 times the GRL). After an incubation period of three days, the activities of several enzymes (dehydrogenase, catalase, ß-glucosidase and phosphomonoesterase) were measured in the soils. The effects of 2,4,5-TCP were much greater than those of 2,4-DCP in both the acidic and calcareous soils, regardless of the dose applied. Phosphomonoesterase and ß-glucosidase activities were scarcely affected by either of the contaminants in any of the soils, whereas the catalase activity decreased slightly. The dehydrogenase and urease activities were strongly affected in all soils and in some cases even disappeared, particularly after the application of 2,4,5-TCP. Multiple regression analysis of the percentage reductions in dehydrogenase and urease activities in relation to contaminant dose and different soil properties indicated that the reduction in enzyme activity depended, in decreasing order, on the dose of contaminant applied, total carbon content and soil pH. We suggest that the processes that regulate the toxicity of these compounds in soils are their adsorption by soil organic matter and the dissociation of the non-adsorbed compound into phenolate ions (which are toxic to microorganisms). In fact, the chlorophenols scarcely affected the biochemical properties of the soils under study because of their high organic matter contents (A horizons with total carbon contents of up to 11%). Moreover, both chlorophenols had slightly stronger effects on the calcareous soils than on the acidic soils, probably because the dissociation process was favoured at higher pH. On the other hand, the 2,4,5-TCP had stronger effects on soil biochemical properties than 2,4-DCP, which may be explained by the lower pKa value of 2,4,5- 2 TCP (6.9) than that of 2,4-DCP (7.9). The results show that the GRL values established by the legislation are not appropriate for either of these chlorophenol compounds.Peer reviewe

Hydrolase enzyme activities in a successional gradient of biological soil crusts in arid and semi-arid zones.

In arid and semi-arid regions, pioneer organisms form complex communities that penetrate the upper millimetres of the bare substrate, creating biological soil crusts (BSC). These thin crusts play a vital role in whole ecosystem functioning because they enrich bare surfaces with organic matter, initiate biogeochemical cycling of elements, modify hydrological cycles, etc., thus enabling the ground to be colonized by vascular plants. Various hydrolase enzymes involved in the carbon (cellulase, ß-glucosidase and invertase activities), nitrogen (casein-protease and BAA- protease activities) and phosphorus (alkaline phosphomonoesterase activity) cycles were studied at three levels (crust, middle and deep layers) of three types of BSCs from the Tabernas Desert (SE Spain), representing an ecological gradient ranging from crusts predominated by cyanobacteria to crusts predominated by lichens (Diploschistes diacapsis, Lepraria crassissima). All enzyme activities were higher in all layers of all BSCs than in the bare substrate. The enzymes that hydrolyze low molecular weight substrates were more active than those that hydrolyze high molecular weight substrates (cellulase, casein-protease), highlighting the pioneering characteristics of the BSCs. The hydrolytic capacity developed in parallel to that of ecological succession, and the BSCs in which enzyme activity was highest were those under Lepraria crassissima. The enzyme activity per unit of total organic C was extremely high; the highest values occurred in the BSCs formed by cyanobacteria and the lowest in those formed by lichens, which 2 indicates the fundamental role that the primary colonizers (cyanobacteria) play in enriching the geological substrate with enzymes that enable degradation of organic remains and the establishment of more developed BSCs. The results of the study combine information on different enzyme activities and provide a clear vision of how biogeochemical cycles are established in BSCs, thus confirming the usefulness of enzyme assays as key tools for examining the relationship between biodiversity and ecosystem function in biological soil crusts.Peer reviewe

Hydrolytic enzyme activities in agricultural and forest soils. Some implications for their use as indicators of soil quality

Although a great deal of information exists about the effect of land use on soil enzyme activities, much of this is contradictory and brings into question the suitability of soil enzyme activities as indicators of how land use affects soil quality. The purpose of this study was to investigate the effect of land use on different soil biochemical properties, especially hydrolytic enzyme activities, with the aim of providing knowledge about the problems related to the use of enzymes as indicators of soil quality. The data presented derive from various studies in which a large number of soils under different types of forest or agricultural management were analysed by the same methods. All of the soil samples were characterized in terms of their main physical and chemical properties, the activity of several hydrolases, microbial biomass C and soil basal respiration. The results indicate that soil use causes a large reduction in organic matter content and that the effect on enzyme activity varies depending on the type of land use or management and the type of enzyme. Furthermore, the enzyme activities per carbon unit (specific activities) in soils affected by land use are almost always higher than in maximum quality soils (climax soils under oak vegetation or oak soils), and land use also generates greater increases in the specific activity as the C content decreases. The mechanism responsible for these increases probably involves loss of the most labile organic matter. Enzyme enrichment is not always produced to the same degree, as it varies as a function of the enzyme and the type of land use under consideration. It is concluded that the complexity of the behaviour of the soil enzymes raises doubts about the use of enzyme activities as indicators of soil degradation brought about by land use.This research was financed by the Xunta de Galicia (Project No. XUGA 40003B94) and by the Spanish Ministerio de Ciencia y Tecnología (Project No. BTE 2001-0987).Peer reviewe

Biochemical properties of soils under crop rotation

Although biochemical properties are commonly used to study soil degradation brought about by agricultural practices, the data obtained are often not comparable because only small numbers of soils and biochemical properties are analysed. The biochemical status of 45 cultivated soils (cropped soils) in Galicia (NW Spain) under rotation of maize, rape and potatoes was evaluated by characterising a large number of biochemical properties, and the results were compared with those from soils under oak forest (climax soils), i.e., natural soils of the highest possible quality. The organic matter content of the cropped soils and the values of the biochemical properties analysed were much lower than those corresponding to the climax soils. However, when the biochemical properties were expressed per unit of organic C, the values for both types of soils were very similar, and for some properties the values for the cultivated soils were even higher. The different biochemical properties in the cultivated soils were largely unrelated to each other and to the organic matter content. The results indicate that intensive crop rotation has led to the loss of a large portion of the labile organic matter and suggest that the residual organic matter acts to stabilize hydrolytic enzyme activities. The stabilizing action implies a saving in enzyme synthesis and to a certain extent should mitigate the biochemical degradation associated with the loss of soil organic matter.This study was financed by the Spanish Ministerio de Ciencia y Tecnología (Project No. BTE 2001-0987). The authors thank Ana I. Iglesias-Tojo and Carolina López-Cotón for their assistance in carrying out the analyses.Peer reviewe

Laboratory study of the availability of nutrients in physical fractions of cattle slurry

The characteristics of three fractions obtained by physical separation from each of 19 cattle slurries are reported, with the aim of investigating their behaviour in the soil. The fraction retained on a 1 mm sieve (F1) was the poorest in nutrient content. The fraction passing through a 1 mm sieve but retained in Richard's apparatus on a cellulose membrane of 2·4 nm pore radius (F2) contains most of the slurry's organic N and Pand most divalent cations (Ca2+ and Mg2+). F3, which passes through the cellulose membrane, contains most of the slurry's inorganic N and most monovalent cations. The individual fractions are both chemically and physically much more homogeneous than the slurry as a whole, and their relative proportions are well correlated with the slurry's density and dry-matter content (P < 0·001), so that this kind of fractionation may confidently be expected to provide an analytical scheme facilitating research on the behaviour of slurry after its application to soil.Peer reviewe