Depth-dependent responses of soil organic carbon under nitrogen deposition.

Emerging evidence points out that the responses of soil organic carbon (SOC) to nitrogen (N) addition differ along the soil profile, highlighting the importance of synthesizing results from different soil layers. Here, using a global meta-analysis, we found that N addition significantly enhanced topsoil (0-30 cm) SOC by 3.7% (±1.4%) in forests and grasslands. In contrast, SOC in the subsoil (30-100 cm) initially increased with N addition but decreased over time. The model selection analysis revealed that experimental duration and vegetation type are among the most important predictors across a wide range of climatic, environmental, and edaphic variables. The contrasting responses of SOC to N addition indicate the importance of considering deep soil layers, particularly for long-term continuous N deposition. Finally, the lack of depth-dependent SOC responses to N addition in experimental and modeling frameworks has likely resulted in the overestimation of changes in SOC storage under enhanced N deposition.

Characterizing Soil Profile Salinization in Cotton Fields Using Landsat 8 Time-Series Data in Southern Xinjiang, China.

Soil salinization is a major obstacle to land productivity, crop yield and crop quality in arid areas and directly affects food security. Soil profile salt data are key for accurately determining irrigation volumes. To explore the potential for using Landsat 8 time-series data to monitor soil salinization, 172 Landsat 8 images from 2013 to 2019 were obtained from the Alar Reclamation Area of Xinjiang, northwest China. The multiyear extreme dataset was synthesized from the annual maximum or minimum values of 16 vegetation indices, which were combined with the soil conductivity of 540 samples from soil profiles at 0~0.375 m, 0~0.75 m and 0~1.00 m depths in 30 cotton fields with varying degrees of salinization as investigated by EM38-MK2. Three remote sensing monitoring models for soil conductivity at different depths were constructed using the Cubist method, and digital mapping was carried out. The results showed that the Cubist model of soil profile electrical conductivity from 0 to 0.375 m, 0 to 0.75 m and 0 to 1.00 m showed high prediction accuracy, and the determination coefficients of the prediction set were 0.80, 0.74 and 0.72, respectively. Therefore, it is feasible to use a multiyear extreme value for the vegetation index combined with a Cubist modeling method to monitor soil profile salinization at a regional scale.

Estimation of mercury emissions from the forest floor of a pine plantation during a wildfire in central Portugal.

Mercury (Hg) concentrations in soils and Hg releases from soils during wildfires are not well characterised in Portugal, even though wildfire activity continues to increase around the Mediterranean. This study focused on the low to moderate severity wildfire in Pombal (Portugal) in 2019, which consumed 12.5 ha of maritime pine (Pinus pinaster Ait.). We evaluated Hg concentrations in soil profiles and Hg pools in organic horizons to assess the fire-induced Hg emissions. Moreover, impacts of the fire on forest floor properties were estimated. Four soil profiles were sampled, two at the burned area and two at a nearby unburned area. The soil profiles displayed a typical Hg distribution, with higher Hg concentrations (156 µg kg-1) in the organic horizons with a sharp decrease in the mineral layers. The bond between organic matter and Hg was evident along the profiles, with a strong correlation between TOC and Hg. Ratios of Hg/TOC in the surface layers of the soil were similar in all profiles. The mean organic Hg pool at the studied site was calculated at 10.6 g ha-1. The fire did not seem to affect the topsoil properties based on visual indicators and the lack of statistical differences (p > 0.05) among measured fire-sensitive chemical soil properties (pH, CEC, TOC, TS) between the topsoils of the burned and unburned areas. If we consider a hypothetical complete combustion of the organic layer (743 Mg) and unaffected topsoil, we estimated a release of 133 g of Hg from the burned area. The study emphasised the importance of the forest floor for Hg retention and its crucial role in Hg emissions during wildfires in a country increasingly affected by climate change.

Depth-dependent soil organic carbon dynamics of croplands across the Chengdu Plain of China from the 1980s to the 2010s.

Agricultural soils have tremendous potential to sequester soil organic carbon (SOC) and mitigate global climate change. However, agricultural land use has a profound impact on SOC dynamics, and few studies have explored how agricultural land use combined with soil conditions affect SOC changes throughout the soil profile. Based on a paired soil resampling campaign in the 1980s and 2010s, this study investigated the SOC changes of the soil profile caused by agricultural land use and the correlations with parent material and topography across the Chengdu Plain of China. The results showed that the SOC content increased by 3.78 g C/kg in the topsoil (0-20 cm), but decreased in the 20-40 cm and 40-60 cm soil layers by 0.90 and 1.26 g C/kg respectively. SOC increases in topsoil were observed for all types of agricultural land. Afforestation on former agricultural land also caused SOC decreases in the 20-60 cm soil layers, while SOC decreases only occurred in the 40-60 cm soil layer for agricultural land using a traditional crop rotation (i.e. traditional rice-wheat/rapeseed rotation) and with rice-vegetable rotations converted from the traditional rotations. For each agricultural land use, SOC decreases in deep soils only occurred in high relief areas and in soils formed from Q4 (Quaternary Holocene) grey-brown alluvium and Q4 grey alluvium that had a relatively low soil bulk density and clay content. The results indicated that SOC change caused by agricultural land use was depth dependent and that the effects of agricultural land use on soil profile SOC dynamics varied with soil characteristics and topography. Subsoil SOC decreases were more likely to occur in high relief areas and in soils with low soil bulk density and low clay content.

Mapping rootable depth and root zone plant-available water holding capacity of the soil of sub-Saharan Africa.

In rainfed crop production, root zone plant-available water holding capacity (RZ-PAWHC) of the soil has a large influence on crop growth and the yield response to management inputs such as improved seeds and fertilisers. However, data are lacking for this parameter in sub-Saharan Africa (SSA). This study produced the first spatially explicit, coherent and complete maps of the rootable depth and RZ-PAWHC of soil in SSA. We compiled geo-referenced data from 28,000 soil profiles from SSA, which were used as input for digital soil mapping (DSM) techniques to produce soil property maps of SSA. Based on these soil properties, we developed and parameterised (pedotransfer) functions, rules and criteria to evaluate soil water retention at field capacity and wilting point, the soil fine earth fraction from coarse fragments content and, for maize, the soil rootability (relative to threshold values) and rootable depth. Maps of these secondary soil properties were derived using the primary soil property maps as input for the evaluation rules and the results were aggregated over the rootable depth to obtain a map of RZ-PAWHC, with a spatial resolution of 1 km2. The mean RZ-PAWHC for SSA is 74 mm and the associated average root zone depth is 96 cm. Pearson correlation between the two is 0.95. RZ-PAWHC proves most limited by the rootable depth but is also highly sensitive to the definition of field capacity. The total soil volume of SSA potentially rootable by maize is reduced by one third (over 10,500 km3) due to soil conditions restricting root zone depth. Of these, 4800 km3 are due to limited depth of aeration, which is the factor most severely limiting in terms of extent (km2), and 2500 km3 due to sodicity which is most severely limiting in terms of degree (depth in cm). Depth of soil to bedrock reduces the rootable soil volume by 2500 km3, aluminium toxicity by 600 km3, porosity by 120 km3 and alkalinity by 20 km3. The accuracy of the map of rootable depth and thus of RZ-PAWHC could not be validated quantitatively due to absent data on rootability and rootable depth but is limited by the accuracy of the primary soil property maps. The methodological framework is robust and has been operationalised such that the maps can easily be updated as additional data become available.

Unraveling soil geochemical, geophysical, and microbial determinants of the vertical distribution of organic phosphorus pesticide pollutants.

Pesticide contamination has emerged as a global threat to humans. Here, we investigate the soil distribution pattern of organic phosphorus pesticide contamination at a pesticide manufacturing site in northern China, exploring their relationships with soil properties and microbial communities. The concentrations of four organic phosphorus pesticides (i.e., phorate, terbuthion, fenitrothion, and parathion) decreased substantially with soil depths from the surface down to 2 m. However, terbuthion, fenitrothion, and parathion had second-peak concentrations at a depth of 8 m. The concentrations of those organic phosphorus pesticides were negatively correlated with soil water content, but positively correlated with sulfide, pH, and total phosphorus. The distribution patterns of organic phosphorus pesticides closely aligned with that of soil organic matter and clay minerals, especially in the presence of montmorillonite, kaolinite, and chlorite. Various bacterial genera known to degrade organic phosphorus pesticides, such as Flavobacterium, Bacillus, Acinetobacter, Lactobacillus, Pseudomonas, Sphingomonas, and Thiobacillus, were correlated with these pesticides. Since these genera were among the top 50 abundant genera in our samples, they might play a significant role in the degradation of organic phosphorus pesticides. Together, this study unveils previously unrecognized pesticide-soil-microbe interactions, thus providing an important knowledge basis for environmental remediation strategies.

Vertical distributions and potential contamination assessment of seldom monitored trace elements in three different land use types of Yellow River Delta.

The Yellow River Delta (YRD) is the second largest petrochemical base in China and the impact of human activities has been continuously increasing in recent decades, however, the contamination status of seldom monitored trace elements (SMTEs) in YRD has rarely been reported. This study evaluated the levels, vertical distributions, contamination status and sources of SMTEs in soil samples of three different land use types in YRD. The results indicated that the vertical distributions of SMTEs contents showed a gradually upward increasing trend for the soil profiles of black locust forest, while the SMTEs contents displayed a gradually upward decreasing trend for the soil profiles of cotton field. However, the SMTEs contents in the oil field area showed no significant difference among different depths. The vertical distributions of SMTEs were very likely related to the anthropogenic disturbance in the later stage. The environmental pollution status assessment of SMTEs showed obvious enrichment of Cs, Sn, and U in the soils of YRD. Moreover, the potential source analysis based on multivariate statistical methods indicated that Ga, Rb, Cs, Sc, Sn, Tl, Be, Bi, Ca and Mo were clustered together and positively correlated with Al, Fe, Mg and K, and may be mainly associated with geochemical weathering process, while the Ce, La, Th, U, Nb, Ta, and Hf may be impacted by both natural process and human activities. Though the SMTEs pollution status was not very serious, our results highlighted the non-negligible influence of anthropogenic activities on vertical distributions of SMTEs in three different land use types from YRD. Our results provide valuable information for understanding the vertical distribution and pollution status of SMTEs in YRD.

Deep soil microbial carbon metabolic function is important but often neglected: a study on the Songnen Plain reed wetland, Northeast China.

Soil microbial carbon metabolism is critical in wetland soil carbon cycling, and is also a research hotspot at present. However, most studies focus on the surface soil layer in the wetlands and the microorganisms associated with this layer. In this study, 0-75 cm soil profiles were collected from five widely separated reed wetlands in the Songnen Plain, which has a large number of middle-high latitude inland saline-sodic wetlands. The Biolog-ECO method was used to determine the carbon metabolic activity and functional diversity of soil microorganisms. The results showed that soil carbon metabolic activity decreased with increasing soil depth. The carbon metabolic activity of soil microorganisms in the 60-75 cm layer was approximately 57.41%-74.60% of that in the 0-15 cm layer. The soil microbial Shannon index and utilization rate of amines decreased with an increase in soil depth, while the Evenness index and utilization rate of polymers tended to increase with soil depth. Dissolved organic carbon (DOC) is the most important factor affecting microbial carbon source utilization preference, because microorganisms mainly obtain the carbon source from DOC. The result of the correlation analysis showed that the soil microbial carbon metabolic activity, Shannon index, and Evenness index significantly correlated with soil total carbon (TC), microbial biomass carbon (MBC), DOC, total nitrogen (TN), ammonium nitrogen (NH4 +-N), nitrate nitrogen (NO3 --N) contents, and electrical conductivity (EC). This study emphasized the important role of microbial carbon metabolic function in deep soil.

Temporal and spatial changes of Pb in soils in Cuihu wetland, Beijing, China.

Protecting soil from Pb contamination has been a programme for a long time in China. However, research on lead pollution in wetlands remains rare. To understand the characteristics of Pb pollution in Beijing and the sources of contamination of different soil samples in wetlands, we collected samples during all four seasons from two soil horizons and analyzed their Pb concentrations and Pb isotope ratios. The average concentrations of Pb varied from 65.44 mg/kg in spring to 106.11 mg/kg in winter. Concentrations in autumn were significantly lower than those in spring and summer and were notably different between A and B Horizons (59.42 and 71.47 mg/kg, respectively). The Pb isotopic compositions show that Pb pollution results from a mixing of geogenic and anthropogenic materials. The ratios of 206Pb/207Pb and 206Pb/208Pb evidenced that coal combustion and vehicle exhaust are the main sources of lead contamination in the two horizons. These results will help in reducing lead contamination in soil by knowing the temporal and spatial variations and sources of lead in Beijing.

Hazardous toxic metal(loid)s in top- and deep-soils during the transformation of aquaculture ponds restored to farmland.

The pollution risks due to the soil migration of toxic metal(loid)s (TMs) are a greatly hazard to ecological environment as well as animal and human health. Previous studies have primarily focused on surface contamination while deep soil layers often contain dangerous levels of TMs. We used restored wheat and rice farmlands from aquaculture ponds as a case study to examine the ecological risk and distribution of TMs in soil profiles. The elements Cu, Zn, Cr, Cd, Hg and As were markedly enriched in the 60-180 cm soil layers of restored farmland, and their concentrations decreased in the several depths as follows: 120-180 cm > 60-120 cm > 0-60 cm. Concentrations of TMs were 9.5-128 % greater in the restored farmlands relative to farmlands not exposed to aquaculture practices. Cadmium and mercury were the most serious contaminants and increased the overall ecological risk. The subsoil of wheat farming system had the highest pollution risk versus the restored rice farmland at 60-120 cm due to elevated levels of Cu, Zn and Pb. Toxic metal(loid)s might be derived from natural sources in deep soil of conventional farmland whereas aquaculture practices were found to constitute the major contribution in the subsoil of restored farmland. Our results indicated that the TMs that were buried in deep soil layers migrated upward and were a significant pollution risk. Urgent actions should be taken to identify and alleviate the contamination sources of these deep soils in addition to the conventional leaching and migration processes of surface contaminants.

Sulfur dynamics in forest soil profiles developed on granite under contrasting climate conditions.

Sulfur (S) dynamics in soils formed from granite remain poorly understood despite its importance as an essential plant macronutrient and component of soil organic matter. We used stable S isotope ratios to trace the sources and biogeochemical processes of S in four forest soil profiles developed on granite under contrasting climate conditions. The soil S is derived mainly from decomposing litter; no significant geogenic contribution to its content is noted as a result of the low S concentration of the granite (~ 5 µg/g). Colder/drier climate results in high organic S retention at the surface due to weak mineralization of organic S. Although warmer/wetter climate increases the S mineralization and leaching loss, SO42- adsorption is an important S retention process in the subsurface. The vertical distribution of S isotope compositions in the soil profiles across the four sites indicates (i) a downward increase in δ34S values in the upper profiles due to continuous mineralization of organic S with an occasional decrease in δ34S values in the subsurface due to dissimilatory sulfate reduction (DSR), (ii) constantly high δ34S values in the middle profiles due to the low water permeability, and (iii) a downward decrease in δ34S values in the low profiles due to increased contribution of bedrock with depth. Regardless of the variation in soil depth and climate, the total S concentration is proportional to the pedogenic Fe/Al minerals, suggesting the important role of secondary Fe/Al minerals in retaining S in soils. This study provides an integration and synthesis of controls of climatic and edaphic variables on S dynamics in forest soil profiles developed on granite.

Dynamics of phosphorus fractions and potential bioavailability along soil profiles from seasonal-flooding wetlands in a Chinese estuary.

Soil phosphorus fractions in wetland ecosystems have received increasing attention due to its high eutrophication risks. Soil samples were collected to 40 cm depth in three sampling seasons to investigate the seasonal dynamics of organic and inorganic phosphorus fractions, bioavailability, and relationship between those and soil properties in a seasonal-flooding wetland in the Yellow River Estuary. The results showed that inorganic phosphorus (IP) and organic phosphorus (OP) contents exhibited much higher levels in the top 10 cm soils, and declined along soil profiles in spring. IP kept constant along soil profiles in fall, while OP decreased in summer and fall. They were greatly affected by water content (WC), pH, Cl-/SO42-, soil organic matter (SOM), and electrical conductivity (EC). Middle labile organic phosphorus (MLOP) and non-labile organic phosphorus (NLOP) accounted for higher percentages of total OP in summer and fall respectively than labile organic phosphorus (LOP) in spring. MLOP and NLOP levels showed a decrease along soil profiles in spring and in spring/fall, respectively, while NLOP significantly increased with depth in summer. Ca-P was the dominant IP fraction in all soils in three sampling seasons, declined with depth in spring/fall and increased in summer. Comparatively, soluble/loosely-P(S/L-P) generally decreased with depth along soil profiles in three sampling seasons. And residual P (Res-P) kept little change with depth in spring. Fe/Al-P levels decreased firstly and then increased with depth in spring and summer. Available phosphorus and potential bioavailable phosphorus contents decreased with depth in spring and summer not in fall, and had a strong significant positive correlation with WC and SOM. Alkaline phosphatase not acid phosphatase was the key factor influencing soil MLOP levels. Generally, the fractions and bioavailability of phosphorus as well as phosphatase in this region were affected by soil depth, sampling seasons, and soil properties (e.g., WC, pH, Cl-/SO42-, SOM, and EC).

[Migration, Transformation Characteristics, and Ecological Risk Evaluation of Heavy Metal Fractions in Cultivated Soil Profiles in a Typical Carbonate-Covered Area].

In China, high heavy metal concentrations in cultivated soil are mainly distributed in carbonate-covered areas. The migration and transformation of heavy metals in such soils are influenced by interactions between natural processes and human activities. This study examined the profiles of nine paddy soils, derived from carbonate rocks in Guangxi. The Cd, As, Zn, Cr, Cu, Hg, Ni, and Pb contents we determined, and soil properties such as pH, Corg content, and fractions of Cd, As, Zn, and Cr were tested. Based on the above data, we assessed the vertical distribution of heavy metal fractions, as well as the ecological risks and factors affecting the migration ability of heavy metals, under the influence of human activities and natural soil formation. The results show that compared with the carbonate rocks in Guangxi, the soil profile of the study area is significantly enriched with all eight heavy metals. Among them, Cd, As, Zn, and Cr exceeded China's agricultural land (paddy field) pollution risk screening values, and Cd and As partially exceed the risk intervention values. All fractions of Cd in the soil profiles are widely distributed. The proportion of water-soluble and ion-exchange Cd fractions with high ecological risk decreased significantly from the surface to greater depths in the soil profile. As, Zn, and Cr in the soil profile were mainly in residual states, and the proportion of water-soluble and ion-exchange fractions did not change considerably with increasing depth. The evaluation results of RAC and RSP show that Cd pollution risk in the study area is relatively high, whereas As, Zn, and Cr are generally pollution-free or risk-free. In naturally developed soil in the study area, the clay mineral content and degree of soil development have significant effects on Cd migration, whereas in the cultivated layer affected by human activities, the soil pH and organic matter content are the main controlling factors. The migration ability of soil As is mainly related to soil organic matter, Fe2O3 content, and soil development degree, but organic matter has an obviously enhanced effect in the tillage layer. The main controlling factor of Zn and Cr migration in soil is pH, and the effects are more intense under the disturbance of human activities.

Vegetation canopy effects on total and dissolved Cl, Br, F and I concentrations in soil and their fate along the hydrological flow path.

Although halogens are omnipresent in the environment, detailed understanding of processes involving chlorine (Cl), bromine (Br), fluorine (F) and iodine (I) in the terrestrial halogen cycle is still sparse. Our objectives were to (i) assess vertical depth profiles of total and water-extractable inorganic halogen concentrations (Cltot, Brtot, Ftot, Itot) in solid soil, (ii) test the effect of a tree canopy, and (iii) follow the fate of dissolved inorganic halogens along the hydrological flow path. More than 200 soil samples and ecosystem solutions (rainwater, soil solution, adit and creek water) collected in the Schwarzwald, SW Germany, were analyzed by combustion ion chromatography and ion chromatography for total and inorganic halogen concentrations. We found decreasing Cltot concentrations with increasing soil depth which were indicative of biological chlorination of organic matter and nutrient uplift, both associated with Cl accumulation in upper soil horizons. Vertical patterns of total Br, F and I were contrary to Cltot concentrations and were related significantly (positively) to pedogenic oxides, revealing their dependence on abiotic processes. The presence of a canopy at our study site resulted in significantly higher halogen concentrations in throughfall compared to rainfall and higher Brtot concentrations in the organic layer. We attribute this difference to leaching from leaves and needles and wash-off of dry deposition. There were hardly any differences in halogen concentrations along the hydrological flow path except for significantly higher inorganic I concentrations in soil solution compared to rainfall due to equilibrium reactions between the soil solution and the solid soil phase. Highest inorganic F concentrations of up to 0.2 mg L-1 were detected in creek water samples and may originate from the weathering of fluorite-bearing veins. Our study indicates halogen-specific processes underlying Cl, Br, I and F cycling in ecosystems.

Factors affecting coupled degradation and time-dependent sorption processes of tebuconazole in mineral soil profiles.

This laboratory degradation and adsorption study aimed to determine the tebuconazole degradation parameters for 6 profiles of Polish mineral soils and to find links between the tebuconazole degradation rate, its adsorption, soil microbial activity and other significant soil properties. The values of the adsorption distribution coefficient Kd, obtained in batch experiments after 96 h of shaking were in the range of 6.2-34.6 mL g-1. In both batch experiments and incubation experiments at 20 °C, the typical course of adsorption processes was observed, an initial rapid stage followed by a slow stage. In 3 of the 18 soils examined, adsorption was not reached within 51 days. The range of the half-life values was 201-433 days for the Ap horizon and up to 3904 days for subsoils, which were estimated using the two-site nonequilibrium adsorption model coupled with first-order degradation for dissolved and adsorbed pesticide. It was found that modeling the degradation of tebuconazole on the basis of the coefficients of microbial biomass activity for topsoil and two subsoils explained almost 96% of the variance of the estimated pore water degradation rate coefficients in examined soils. The degradation rate was also negatively correlated with the amount adsorbed in the time dependent adsorption sites. This fraction was the least available for soil microorganisms because it was strongly adsorbed in soil pores with a radius <2.5 nm, determined from the H2O desorption isotherm. The degradation rate was also affected by the ratio of the water content in soil during degradation experiments to the water content at field capacity. The results indicated that degradation occurred in the soil liquid phase only.

Distribution of nutrients and trace elements in forest soils of Singapore.

Information on chemical fertility status and on trace element concentrations for Singapore soils is sparse. In this study, concentration and distribution of nutrients and trace elements in forest soils of Singapore, and the effect of geology on the current conditions of soils, were evaluated. Litter and mineral soil samples (0-10 cm, 10-20 cm, 20-50 cm) were divided into three groups according to geology (sedimentary rocks, Neogene-Quaternary sediments and igneous rocks). Basic soil properties, exchangeable cations including cation exchange capacity (CEC), and pseudototal concentrations of 17 elements including nutrients and potentially toxic elements (PTE; As, Cd, Cu, Cr, Ni, Pb, Zn) were determined. Results showed that the soils of Singapore were acidic (pH 3.7-6.2) and characterised by a low CEC (maximum mean value: 100 mmolc kg-1) and low P concentrations (mean values of 0.28 g kg-1 or less) irrespective of geology. Pseudototal concentrations of PTE were also low, except for As. However, significantly (p < 0.05) higher concentrations of soil organic carbon, total P, base forming elements, exchangeable base cations and PTE (except As and Cd) were found in soils developed on sedimentary rocks and Neogene-Quaternary sediments compared to those developed on igneous rocks. Although total N and S concentrations did not differ significantly between the three geological groups, the differences in nutrient status were more pronounced than in trace element levels across geology. The properties of forest soils, which are mainly unaffected by anthropogenic activities, vary depending on the parent material, i.e. igneous or sedimentary rocks and Neogene-Quaternary sediments.

Interrelation of Proctodrilus species (Oligochaeta: Lumbricidae) with lessivage and layering in European soil profiles.

The objective of this paper is to relate species occurrences of the earthworm genus Proctodrilus to soil properties, in particular the horizons and layers in the soil profile. Data on the occurrence in the soil profile of the earthworm species Proctodrilus antipai (Michaelsen, 1891), P. opisthoductus Zicsi, 1985, P. thaleri Höser Zicsi 2009, and P. tuberculatus (Cernosvitov, 1935) were collected in central Germany, northern Bohemia, Hungary, and Romania. Species of Proctodrilus were collected from 138 sites in total, P. antipai from 48, P. opisthoductus from 24, P. thaleri from 4, and P. tuberculatus from 97 sites. The field data show that species inhabiting the same site do not form communities but rather inhabit different soil layers. Furthermore, species of Proctodrilus prefer to live close to the boundaries between layers, a feature that can be regarded as a taxonomic characteristic of the genus and that is probably connected with their enteronephric excretory system. The habitats of the different species differ in the degree of soil leaching. P. thaleri and P. opisthoductus are restricted to the region of recent lessivage in warm-humid parts of Europe. Occurrence of the four Proctodrilus species is therefore not related to soil type but rather to soil stratification and soil leaching. Differences in the ecological behaviour of the species can be understood as taxonomic characters.

Vertical distribution of denitrification end-products in paddy soils.

Knowledge of denitrification process and its end-product at various depths of paddy soil is very important for our understanding of its role in mitigating reactive N and indirect nitrous oxide (N2O) emission. In this study, the end-products of denitrification were determined at four depths in a long-term field lysimeter experiment in southeast China over a rice season. Three treatments were included: (1) chemical fertilizer (NPK); (2) NPK plus pig manure (NPKM); and (3) NPK plus straw (NPKS). The concentration of dissolved N2O increased with soil depth across all treatments and the highest concentration of excess dinitrogen (N2) was observed at 0.2m depth, as was the highest dissolved organic carbon (DOC) content. Denitrification reduced the amount of nitrate by 48-54% in the paddy soil profile, especially at 0.2m depth (68-88%), whereas the lower reduction of NO3- (17-44%) in the subsoil (at 0.6 and 0.8m depth) was accompanied by a higher concentration of NO3-. Our results demonstrated that DOC was the major limiting factor of denitrification in the subsoil. The application of pig manure markedly increased the amount of DOC in the surface soil, resulting in a high rate of denitrification, whereas the addition of straw had no effect on denitrification. The indirect emission factors for N2O (EF5-g, 0.001-0.006) were comparable with the default value (0.0025) reported by the Intergovernmental Panel on Climate Change. The low N2O production was probably caused by the complete reduction of N2O to N2, as reflected by the lower N2O/(N2O+N2) ratios in the paddy soil profile. Although the surface soil was identified as a hotspot for denitrification, a considerable amount of excess N2 was observed in the subsoil for all three treatments. We therefore conclude that the loss of N through denitrification may be significantly underestimated if only the surface soil is considered.

Weathering processes and dating of soil profiles from São Paulo State, Brazil, by U-isotopes disequilibria.

This study reports the use of the U-series radionuclides 238U and 234U for dating two soil profiles. The soil horizons developed over sandstones from Tatuí and Pirambóia formations at the Paraná sedimentary basin, São Paulo State, Brazil. Chemical data in conjunction with the 234U/238U activity ratios (AR's) of the soil horizons allowed investigating the U-isotopes mobility in the shallow oxidizing environment. Kaolinization and laterization processes are taking place in the profiles sampled, as they are especially common in regions characterized by a wet and dry tropical climate and a water table that is close to the surface. These processes are implied by inverse significant correlations between silica and iron in both soil profiles. Iron oxides were also very important to retain uranium in the two sites investigated, helping on the understanding of the weathering processes acting there. 238U and its progeny 234U permitted evaluating the processes of physical and chemical alteration, allowing the suggestion of a possible timescale corresponding to the Middle Pleistocene for the development of the more superficial soil horizons.

Vertical profiles of pentachlorophenol and the microbial community in a paddy soil: influence of electron donors and acceptors.

Vertical variations of pentachlorophenol (PCP) dissipation and microbial community were investigated in a paddy soil with the addition of electron acceptors (NO3(-), SO4(2-)) and donors (crop residues). Crop residues enhanced PCP dissipation by supplying dissolved organic carbon (DOC) as an electron donor, whereas NO3(-) and SO4(2-) inhibited it. The dissipation of PCP in electron donor treatments resulted in the accumulation of 3,4,5-trichlorophenol (3,4,5-TCP) except for wheat residues. The abundance and diversity of phospholipid fatty acids (PLFAs) decreased with increasing soil depth. The succession of predominant PLFAs shifted from aerobic bacteria to anaerobic bacteria when electron acceptors were changed to electron donors. The saturated/monounsaturated fatty acids (S/M) ratio increased with soil depth, which probably implied that nutrient turnover rate declined after the accumulation of 3,4,5-TCP. The results showed that the addition of electron donors and acceptors modified the microbial communities, which then further influenced the degradation pathway of PCP.