Spatial and vertical distribution of <Superscript>137</Superscript>Cs in soils in the erosive area of southeastern Serbia (Pčinja and South Morava River Basins)

Purpose

The area of southeastern Serbia, the P?inja and South Morava River Basins, is under the influence of very strong erosion, and the aim of this study was to investigate the vertical and spatial distribution of the ¹³⁷Cs in the eroded soils of this area.

Materials and methods

Vertical soil profiles were collected with 5-cm increments from the uppermost layer down to 20, 25, 30, 40, and 50 cm of depth, depending on the thickness of the soil layers, i.e., down to the underlying parent rocks. Measurements of ¹³⁷Cs activity concentration were performed by using the HPGe gamma-ray spectrometer ORTEC-AMETEK (34 % relative efficiency and high resolution 1.65 keV at 1.33 MeV for ⁶⁰Co), from its gamma-ray line at 661.2 keV.

Results and discussion

The mean ¹³⁷Cs activity concentration across all 18 soil profiles (for all soil layers) was found to be 20 Bq kg^?1. In the greatest number of soil profiles, the ¹³⁷Cs activity concentration was generally highest in the first soil layer (0–5 cm) and decreased with soil depth, while in a few soil profiles, the peak of either the ¹³⁷Cs activity concentration occurred in the second soil layer (5–10 cm) or the ¹³⁷Cs activity concentration was almost equal throughout the entire soil profile. The mean ¹³⁷Cs activity concentration in the first soil layer (0–5 cm) was found to be 61 Bq kg^?1, and the high coefficient of variation of 92 % pointed out high spatial variability and large range of the ¹³⁷Cs activity concentrations in the study area.

Conclusions

The obtained results indicate that in the greatest number of soil profiles, ¹³⁷Cs is present in the upper layers, with concentration decreasing with depth, as is typical in uncultivated soil. Its spatial distribution was very uneven among the surface soil layers of the investigated sites. One of the main reasons for such pattern of ¹³⁷Cs in the study area may be soil erosion. Additional investigations which would support this hypothesis are required.

     

Assessment of soil erosion rate trends in two agricultural regions of European Russia for the last 60 years

Purpose

Forest–steppe and the southern forest ecotones of European Russia (ER) are the most productive agricultural areas in Russia. Both climate and land use changes have occurred within the ER during last 30 years. These changes can lead to changes in the timing, magnitude, and spatial distribution of soil erosion rates on cultivated lands. The objective of this research was to assess the trends in soil erosion rates since the 1960s for two agricultural regions of ER.

Materials and methods

Rates of soil erosion were estimated for two time windows (1963–1986 and 1986–2015) within the two agricultural regions. Both regions are characterized by a high proportion of cropland (>?60%), and within each region, one river basin and one 1st–3rd-order agricultural catchment were selected for a detailed assessment of soil erosion rates. Erosion models and visual interpretation of satellite images were used for the evaluation of the erosion rates for the river basins. Sediment budget assessments, ¹³⁷Cs dating, geomorphologic mapping, and erosion models were used for the evaluation of the sediment redistribution for the two time windows in agricultural catchments.

Results and discussion

At the river basin scale, the mean annual erosion rate did not change in the western part of forest–steppe ecotone; however, there was a weak negative trend in the mean annual erosion rate for the eastern part of the southern forest ecotone. A large negative trend in the erosion rate was found for both small agricultural catchments. In all cases, the reduction in the erosion rates was mainly associated with a decrease of surface runoff during snowmelt, as a result of an increase in both the air and soil temperatures during winter season. The soil loss reduction during snowmelt was counteracted by an equal increase in rainfall erosion due to increase of rainfall intensity in western part of forest–steppe ecotone.

Conclusions

Reduction of surface runoff during spring snowmelt was the main reason the erosion rates declined on cultivated lands within the forest–steppe and southern forest ecotones of ER. Evaluation of ephemeral gully erosion rate was not incorporated into State Hydrological Institute erosion model used for the evaluation of the soil losses during snowmelt. This has led to an underestimation of the total soil losses for the 1963–1986 time window for all study sites.

     

Inherent interreplicate variability during small-scale rainfall simulations

Purpose

The validity of soil erosion data is often questioned because of the variation between replicates. This paper aims to evaluate the relevance of interreplicate variability to soil and soil organic carbon (SOC) erosion over prolonged rainfall.

Materials and methods

Two silty loams were subjected to simulated rainfall of 30 mm h^?1 for 360 min. The entire rainfall event was repeated ten times to enable statistical analysis of the variability of the runoff and soil erosion rates.

Results and discussion

The results show that, as selective removal of depositional particles and crust formation progressively stabilized the soil surface, the interreplicate variability of runoff and soil erosion rates declined considerably over rainfall time. Yet, even after the maximum runoff and erosion rates were reached, the interreplicate variability still remained between 15 and 39 %, indicating the existence of significant inherent variability in soil erosion experiments.

Conclusions

Great caution must be paid when applying soil and SOC erosion data after averaging from a small number of replicates. While not readily applicable to other soil types or rainfall conditions, the great interreplicate variability observed in this study suggests that a large number of replicates is highly recommended to ensure the validity of average values, especially when extrapolating them to assess soil and SOC erosion risk in the field.

     

Soil carbon dynamics in successional and plantation forests in subtropical China

Purpose

The purpose of this study was to better understand how both the content and flux of soil carbon respond to forest succession and anthropogenic management practices in forests in subtropical China.

Materials and methods

We assembled from the literature information on soil organic carbon (SOC) and soil respiration (Rs) covering the forest successional chronosequence from pioneer masson pine (Pinus massoniana) forest (MPF) to medium broadleaf and needleleaf mixed forest (BNMF) and the climax evergreen broadleaf forest (EBF), along with the two major forest plantation types found in subtropical China, Chinese fir (Cunninghamia lanceolata) forest (CFF) and Moso bamboo (Phyllostachys pubescens) forest (MBF).

Results and discussion

Both SOC and Rs increased along the forest successional gradient with the climax EBF having both the highest SOC content of 33.1?±?4.9 g C kg^?1(mean?±?standard error) and the highest Rs rate of 46.8?±?3.0 t CO₂?ha^?1 year^?1. It can be inferred that when EBF is converted to any of the other forest types, especially to MPF or CFF, both SOC content and Rs are likely to decline. Stand age did not significantly impact the SOC content or Rs rate in either types of plantation.

Conclusions

Forest succession generally increases SOC content and Rs, and the conversion of natural forests to plantations decreases SOC content and Rs in subtropical China.

     

The impact of different forest types on phytolith-occluded carbon accumulation in subtropical forest soils

Purpose

Occlusion of carbon in phytoliths is an important biogeochemical carbon sequestration mechanism and plays a significant role in the global biogeochemical carbon cycle and atmospheric carbon dioxide (CO₂) concentration regulation at a millennial scale. However, few studies have focused on the storage of phytolith and phytolith-occluded carbon (PhytOC) in subtropical forest soils.

Materials and methods

Soil profiles with 100-cm depth were sampled from subtropical bamboo forest, fir forest, and chestnut forest in China to investigate the variation of phytoliths and PhytOC storage in the soil profiles based on amass-balance assessment.

Results and discussion

The storage of phytoliths in the top 100 cm of the bamboo forest soil (198.13?±?25.08 t ha^?1) was much higher than that in the fir forest (146.76?±?4.53 t ha^?1) and chestnut forest (170.87?±?9.59 t ha^?1). Similarly, the storage of PhytOC in the bamboo forest soil (3.91?±?0.64 t ha^?1) was much higher than that in the fir forest soil (1.18?±?0.22 t ha^?1) and chestnut forest soil (2.67?±?0.23 t ha^?1). The PhytOC percentage in the soil organic carbon pool increased with soil depth and was the highest (4.29 %) in the bamboo forest soil. Our study demonstrated that PhytOC in soil was significantly influenced by forest type and the bamboo forest ecosystem contributed more significantly to phytolith carbon sequestration than other forest ecosystems.

Conclusions

Different forest types have a significant influence on the soil PhytOC storage. Optimization of bamboo afforestation/reforestation in future forest management plans may significantly enhance the biogeochemical carbon sink in the following centuries.

     

Quantification of vertical solid matter transfers in soils during pedogenesis by a multi-tracer approach

Purpose

Vertical transfer of solid matter in soils (bioturbation and translocation) is responsible for changes in soil properties over time through the redistribution of most of the soil constituents with depth. Such transfers are, however, still poorly quantified.

Materials and methods

In this study, we examine matter transfer in four eutric Luvisols through an isotopic approach based on ¹³⁷Cs, ²¹⁰Pb_(xs), and meteoric ¹⁰Be. These isotopes differ with respect to chemical behavior, input histories, and half-lives, which allows us to explore a large time range. Their vertical distributions were modeled by a diffusion-advection equation with depth-dependent parameters. We estimated a set of advection and diffusion coefficients able to simulate all isotope depth distributions and validated the resulting model by comparing the depth distribution of organic carbon (including ^12/13C and ¹⁴C isotopes) and of the 0–2-μm particles with the data.

Results and discussion

We showed that (i) the model satisfactorily reproduces the organic carbon, ¹³C, and ¹⁴C depth distributions, indicating that organic carbon content and age can be explained by transport without invoking depth-dependent decay rates; (ii) translocation partly explains the 0–2-μm particle accumulation in the Bt horizon; and (iii) estimates of diffusion coefficients that quantify the soil mixing rate by bioturbation are significantly higher for the studied plots than those obtained by ecological studies.

Conclusions

This study presents a model capable of satisfactorily reproducing the isotopic profiles of several tracers and simulating the distribution of organic carbon and the translocation of 0–2-μm particles.

     

Rainfall/runoff/erosion relationships and soil properties survey in abandoned shallow soils of NE Spain

Purpose

Shallow soils previously cultivated under terraced systems may change their properties after agricultural release and spontaneous plant colonization. Investigations were conducted in terraced fields (NE Spain) to prove that vegetation installed after the abandonment may generally improve soil properties by the formation of stable organic horizons. However, restriction in plant species along the natural vegetation succession and intensification of erosion processes may occur after abandonment depending on fire frequency and soil use history.

Materials and methods

Ten environments with different plant covers under a Lithic Xerorthent were selected and erosion plots (Gerlach type) installed providing their best adaptability at the terrace scale. Selected soil environments represented the sequence of abandonment: from current poorly cultivated soils, soils under pasture, soils under shrubs, and soils under stands of pine and cork trees. Relevant rainfall events producing runoff and erosion were recorded from November 2011 to May 2012. Erosion rates and erosion components were analysed in sediments and water in order to monitor carbon, nitrogen and other nutrient removal by overland flow. Similarly, the physical and chemical properties of the soil environments under study were determined at the same time interval of runoff erosion.

Results and discussion

Soils under pasture, vines and recently burnt pine forest produced the highest runoff followed by soils under shrubs and forest. However, eroded soil yields and nutrient removal were much higher in cultivated soils and soils in recently burnt sites, which had shown poorer soil properties with respect to soils abandoned for longer and preserved by fire. Fire-affected soil environments also showed a thinner organic horizon and reduced water retention. Although erosion rates and nutrient depletion were low in all environments with respect to other areas of Spain, higher splash than water erosion was an early warning indicator of the high susceptibility to degradation of these shallow soils.

Conclusions

Results outlined that the renaturalization dynamics after agricultural abandonment are complex biophysical processes involving the parent material, depth to bedrock and other soil properties as well as the succession of vegetative cover and plant associations responsible for building a new soil mantle contrasting with erosion processes. Planning for management of land abandonment is strongly recommended.

     

Distribution of artificial radioisotopes in granulometric and organic fractions of alluvial soils downstream from the Krasnoyarsk Mining and Chemical Combine (KMCC), Russia

Purpose

The purpose of this study was to compare the distribution of the most significant medium and long-lived radioisotopes, i.e., ⁶⁰Co, ¹³⁷Cs, and ¹⁵²Eu, in granulometric and organic fractions of alluvial soils downstream from the Krasnoyarsk Mining and Chemical Combine (KMCC), Russia, to reveal natural patterns of their behavior and accumulation.

Materials and methods

Soil samples collected at different elevations and depths in a floodplain of the Yenisey River downstream from the KMCC (20–250 km) were subjected to granulometric analysis by dry screening and a modified Petelin method. Fractions <0.05 mm were collected by a pipette method. Radionuclide activity in the different soil layers and in their fractions was determined using a spectrometer equipped with an HPGe detector. Concentration of total C (Ctot) and C of carbonates (Ccarb) was determined using a CHN analyzer before and after elimination of carbonates, organic C (Corg) being calculated as the difference between the obtained values. Organic fractions were separated by saturation of the air-dry sample with 0.1 M NaOH and further precipitation of humic acid from filtrate by 1 M HCl at pH 1. The separation resulted in three fractions of the fulvic acids, humic acids, and the residue containing the denuded mineral phase and the refractory organic residue. The selected bulk samples and fractions were analyzed for radionuclide activity.

Results and discussion

Based on earlier results, the distribution of the pelite (<0.01 mm) and aleurite (0.01–0.1 mm) fractions in alluvium and soil samples have been analyzed to evaluate the grain-size contribution to radionuclide fixation. A positive correlation between radionuclide activity and the portion of pelite fraction was established for ⁶⁰Co and ¹⁵²Eu, while ¹³⁷Cs accumulation was not related with this fraction. In organic matter (OM) extracts, more than 90 % of ¹³⁷Cs, at proportions similar to those attributable to ²³⁸Th and ⁴⁰K, were associated with the residue fraction, while 72 % of ¹⁵²Eu and 46 % of ⁶⁰Co were found in the mobile fraction of the low molecular fulvic acids. In successive layers of the soil vertical profile, approximately 94 % of the ¹⁵²Eu variation may be explained by a linear model with Corg and Ccarb values as independent variables.

Conclusions

Different associations of ¹³⁷Cs, ⁶⁰Co, and ¹⁵² Eu with particulate and organic fractions in river sediments and floodplain soils could be explained by the dominating discharge form (water soluble or particulate), affinity to organic substances of different mobility, sorption by minerals and their aggregates, and chemisorption.

     

Nitrogen addition impacts on the emissions of greenhouse gases depending on the forest type: a case study in Changbai Mountain,Northeast China

Purpose

Anthropogenic-induced greenhouse gas (GHG) emission rates derived from the soil are influenced by long-term nitrogen (N) deposition and N fertilization. However, our understanding of the interplay between increased N load and GHG emissions among soil aggregates is incomplete.

Materials and methods

Here, we conducted an incubation experiment to explore the effects of soil aggregate size and N addition on GHG emissions. The soil aggregate samples (0–10 cm) were collected from two 6-year N addition experiment sites with different vegetation types (mixed Korean pine forest vs. broad-leaved forest) in Northeast China. Carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) production were quantified from the soil samples in the laboratory using gas chromatography with 24-h intervals during the incubation (at 20 °C for 168 h with 80 % field water capacity).

Results and discussion

The results showed that the GHG emission/uptake rates were significantly higher in the micro-aggregates than in the macro-aggregates due to the higher concentration of soil bio-chemical properties (DOC, MBC, NO₃ ^?, NH₄ ⁺, SOC and TN) in smaller aggregates. For the N addition treatments, the emission/uptake rates of GHG decreased after N addition across aggregate sizes especially in mixed Korean pine forest where CO₂ emission was decreased about 30 %. Similar patterns in GHG emission/uptake rates expressed by per soil organic matter basis were observed in response to N addition treatments, indicating that N addition might decrease the decomposability of SOM in mixed Korean pine forest. The global warming potential (GWP) which was mainly contributed by CO₂ emission (>98 %) decreased in mixed Korean pine forest after N addition but no changes in broad-leaved forest.

Conclusions

These findings suggest that soil aggregate size is an important factor controlling GHG emissions through mediating the content of substrate resources in temperate forest ecosystems. The inhibitory effect of N addition on the GHG emission/uptake rates depends on the forest type.

     

Impact of climate and lithology on soil phytolith-occluded carbon accumulation in eastern China

Purpose

Occlusion of carbon (C) within phytoliths, biogenic silica deposited in plant tissues and returned to the soil, is an important mechanism for long-term terrestrial biogeochemical C sequestration and might play a significant role in mitigating climate change.

Materials and methods

Subtropical and tropical soil profiles (to 100 cm depth) developed on granite and basalt were sampled using a mass-balance approach to explore the influence of climate and lithology on soil phytolith-occluded carbon (PhytOC) accumulation.

Results and discussion

Soil PhytOC storage in the subtropics was significantly greater than in the tropics, with the soil profiles developed on granite storing greater PhytOC than soils derived on basalt. Phytolith and PhytOC content decreased with depth in all soil profiles. Phytolith content showed a positive correlation with the soil bio-available silicon in the soil profiles developed on basalt, while a negative correlation was observed in soil profiles developed on granite.

Conclusions

Climate and lithology have a significant impact on soil PhytOC sequestration. The management of forests (e.g., afforestation and reforestation) and external silicon amendments (e.g., basalt powder amendment) in soils, especially those developed on granite, have the potential to enhance PhytOC accumulation in forest ecosystems.

     

Soil organic carbon as a key predictor of N in forest soils of Brazil

Purpose

Despite the ancillary knowledge that soil N is chiefly retained as soil organic matter, little is known about how it is affected by other soil and environmental factors, especially in the tropics. In this study, we performed a comprehensive survey of soils under native vegetation in Minas Gerais, Brazil, aiming to (a) measure soil Kjeldahl-N concentrations to a 1-m depth, (b) identify the main affecting factors of soil N retention, and (c) predict N through soil profile based on organic C (SOC) and its main conditioning factors.

Materials and methods

Soils under 36 fragments of native forest and savanna were sampled at five depths (0–10, 10–20, 20–40, 40–60, and 60–100 cm) and characterized by physical and chemical analyses, including total N determined by the micro-Kjeldahl method. Single and multivariate regressions were used to predict N concentrations based on soil properties and climatic factors.

Results and discussion

The average N concentrations ranged between 0.12 and 7.54 g kg^?1, decreasing with depth, and can be predicted using SOC concentrations (R ² = 0.86). Multivariate regressions using more input data, namely texture, cation exchange capacity (CEC), and altitude increased slightly R ² values (0.68–0.90) for separate soil depths, but not for the whole dataset (R ² = 0.85).

Conclusions

We demonstrated that N can be adequately predicted based on SOC concentrations, for any depth and forest type. The implications of the stable SOC/N relation and their coupled cycles and the environmental factors affecting N retention in Brazilian weathered soils are further discussed.

     

Stability of soil organic carbon and potential carbon sequestration at eroding and deposition sites

Purpose

This study aims to explore the dynamics of the factors influencing soil organic carbon (SOC) sequestration and stability at erosion and deposition sites.

Materials and methods

Thermal properties and dissolved aromatic carbon concentration along with Al, Fe concentration and soil specific surface area (SSA) were studied to 1 meter depth at two contrasting sites.

Results and discussion

Fe, Al concentrations and SSA size increased with depth and were negatively correlated with SOC concentration at the erosion site (P?<?0.05), while at the deposition site, these values decreased with increasing depth and were positively correlated with SOC concentration (P?<?0.05). TG mass loss showed that SOC components in the two contrasting sites were similar, but the soils in deposition site contained a larger proportion of labile organic carbon and smaller quantities of stable organic carbon compared to the erosion site. SOC stability increased with soil depth at the erosion site. However, it was slightly variable in the depositional zone. Changes in SUVA₂₅₄ spectroscopy values indicated that aromatic moieties of DOC at the erosion site were more concentrated in the superficial soil layer (0–20 cm), but at the deposition site they changed little with depth and the SUVA₂₅₄ values less than those at the erosion site.

Conclusions

Though large amounts of SOC accumulated in the deposition site, SOC may be vulnerable to severe losses if environmental conditions become more favorable for mineralization in the future due to accretion of more labile carbon. Deep soil layers at the erosion site (>30 cm deep) had a large carbon sink potential.

     

Quantifying soil macropore networks in different forest communities using industrial computed tomography in a mountainous area of North China

Purpose

Macropores have important effects on the movement of soil water, air, and chemical substances. However, the quantitative relationship between complex 3D soil macropore networks and forest communities remains unclear in the northern mountainous area in China. The objectives of this study were to (1) use industrial computed tomography (CT) scanning and image analysis to quantitatively analyze macropore networks in intact soil columns and (2) identify characteristics of soil macropore networks in different forest communities.

Materials and methods

Intact soil columns (100-mm diameter, 300 mm long) were taken from six local forest communities with three replicates for a total of 18 samples. Industrial X-ray CT was used to scan soil samples; then, the scanned images were used to obtain the 3D images of rock fragments and macropore structures. Next, the macropore structure was quantified, including volume, diameter, surface area, length, angle, tortuosity, and number of macropores. This technique provided an accurate method to quantify the structure of macropores.

Results and discussion

The analysis and results revealed that different forest communities influence soil macropore 3D structure significantly and in different ways. Macropores in mixed Pinus tabulaeformis, Castanea mollissima, and Ulmus pumila forest had the largest diameter, surface area, network density, and length density of macropores as well as the smallest mean tortuosity of soil macropores. This is caused by the fact that mixed forest soils had more complex root systems, better soil structure, and more biotic activity. Within the soils of a single forest community, macropore porosity, network density, surface area density, and length of macropores decreased with increased soil depth, because more roots and more biological activity were present in the surface soil.

Conclusions

Advanced industrial CT technology can allow an accurate quantification of soil macropore structure. This is important because this type of structure has significant effects on soil water, air, and chemical transport. The results suggest that mixed forest is the best afforestation model in the northern mountainous area in China because of its ability to improve soil structure.

     

Effects of simulated wind followed by rain on runoff and sediment yield from a sandy loessial soil with rills

Purpose

Severe soil erosion is caused by wind and water acting separately or in combination or sequentially and is an important factor affecting dryland ecosystems, especially in the severely eroded “water–wind erosion crisscross region” on the Loess Plateau. Thus, the aim of the study was to determine the magnitudes of wind and water erosion under simulative conditions and explore the mechanisms of their interactions.

Materials and methods

We analyzed the interaction between these two types of erosion by exposing a sandy loessial soil with an artificial rill to simulated wind at four speeds (0, 1, 8, and 15 m s^?1) and then to simulated rainfall, measuring runoff, sediment yield, and characterizing changes in rill morphology. This simulated the transition period between the dry (windy) and wet seasons.

Results and discussion

The time to runoff initiation depended on both wind speed and rainfall intensity, but rainfall had a larger impact on runoff. At the 15 m s^?1 wind speed, the total runoff significantly (P?<?0.05) increased by 33.3 kg when the rainfall intensity was increased to 120 from 60 mm h^?1. Under the 120 mm h^?1 rainfall intensity, the total sediment yields increased significantly (P?<?0.05) with increasing wind speed. Erosion sediment yields increased by 9.7, 16.3, and 70.4 % with increasing wind speed under all three rainfall intensities when compared with a no wind case. Changes in rill morphology caused by wind erosion were a factor that affected the erosion processes of subsequent rainstorms.

Conclusions

Our results provide a basis for hypothesizing trends of wind and water erosion, highlight the importance of wind and water erosion acting in conjunction in semi-arid ecosystems, and are conducive for developing a more integrated perspective of wind–water dynamics on the Loess Plateau.

     

Early detection of the effects of compaction in forested soils: evidence from selective extraction techniques

Purpose

Soil compaction resulting from mechanisation of forest operations reduces air permeability and hydraulic conductivity of soil and can result in the development of hydromorphic and/or anoxic conditions. These hydromorphic conditions can affect physico-chemical properties of the soils. However, early detection of these effects on mineralogical portion of soils is methodologically difficult.

Materials and methods

To analyse the effects of soil compaction on iron minerals in loamy Luvisol, three compacted and three non-compacted soil profiles up to the depth of 50 cm were collected from an artificially deforested and compacted soils after 2 years of treatment. Soil was compacted with the help of 25 Mg wheeler’s load to increase the dry bulk density of soil from 1.21?±?0.05 to 1.45?±?0.1 g cm^?3. Soil samples were analysed by X-ray diffraction (XRD) and were treated by citrate bicarbonate (CB) and dithionite citrate bicarbonate (DCB) under controlled conditions. Major and minor elements (Fe, Al, Mg, Si and Mn) were analysed by ICP-AES in the CB and DCB extracts.

Results and discussion

It was found that X-ray diffraction is not an enough sensitive method to detect the quick mineralogical changes due to soil compaction. Results obtained from CB-DCB extractions showed that soil compaction resulted in larger CB and smaller DCB extractable elements as compared to non-compacted soil. Labile Fe was found 30 % of total Fe oxides in compacted soil against 10–14 % in non-compacted soils. Compaction thus resulted in Fe transfer from non-labile to labile oxides (s.l.). Results showed that soil compaction leads to the reduction of Fe³⁺ to Fe²⁺. The effects of hydromorphic conditions due to soil compaction were observed up to the depth of 35 cm in forest soil profile. Furthermore, a close association of Al with Fe oxides was observed in the soil samples, while Mn and Si were mainly released from other sources, Mg showing an intermediate behaviour.

Conclusions

Hydromorphic conditions owing to soil compaction affect the mobility and crystallisation process of iron mineral. CB-DCB selective extraction technique, in contrast to XRD technique, can be effectively used to examine the possible effects of soil compaction on iron minerals.

     

Temperature sensitivity of gross N transformation rates in an alpine meadow on the Qinghai-Tibetan Plateau

Purpose

Alpine ecosystems on the Qinghai-Tibetan Plateau are sensitive to global climatic changes. However, the effects of temperature change resulting from global warming or seasonal variation on soil N availability in those ecosystems are largely unknown.

Materials and methods

We therefore conducted a ¹⁵N tracing study to investigate the effects of various temperatures (5–35 °C) on soil gross N transformation rates in an alpine meadow (AM) soil on the Qinghai-Tibetan Plateau. A natural secondary coniferous forest (CF) soil from the subtropical region was chosen as a reference to compare the temperature sensitivity of soil gross N transformation rates between alpine meadow and coniferous forest.

Results and discussion

Our results showed that increasing temperature increased gross N mineralization and NH₄ ⁺ immobilization rates and overall enhanced N availability for plants in both soils. However, both rates in the CF soil were less sensitive to a temperature change from 5 to 15 °C compared to the AM soil. In both soils, different N retention mechanisms could have been operating with respect to changing temperatures in the different climatic regions. In the CF soil, the absence of NO₃ ^? production at all incubation temperatures suggests that in the subtropical soil which is characterized by high rainfall, an increase in N availability due to increasing temperature could be completely retained in soils. In contrast, the AM soil may be vulnerable to N losses with respect to temperature changes, in particular at 35 °C, in which higher nitrification rates were coupled with lower NH₄ ⁺ and NO₃ ^? immobilization rates.

Conclusions

Our results suggest that increased soil temperature arising from global warming and seasonal variations will most likely enhance soil N availability for plants and probably increase the risk of N losses in the alpine meadow on the Qinghai-Tibetan Plateau.

     

Topography-soil relationships in a hilly evergreen broadleaf forest in subtropical China

Purpose

Topography-soil relationships usually vary with climate, vegetation type, degree of human disturbance, type of parent material, and the scale being studied. In this paper, we studied the topography-soil relationship in a hilly forest in subtropical China.

Materials and methods

The influence of topography on soil properties (soil moisture, organic carbon (C), total nitrogen (N) and total phosphorus contents, C:N ratio, and pH) was evaluated using a recursive partitioning conditional inference tree (CIT) as well as a multiple linear regression (MLR) method.

Results and discussion

The CIT models generally performed better than MLR in describing the topography-soil relationships. Topographic parameters chosen by the CIT models, which indicate the mechanisms at play for the spatial variation of the soil properties, varied with the soil property of concern. The soil moisture, organic C, and total N models contained only primary terrain attributes, the soil C:N ratio and pH models contained both primary and secondary terrain attributes, while the total phosphorus model contained mostly secondary terrain attributes.

Conclusions

The CIT method worked well for exploring the topography-soil relationships in the studied undisturbed hilly forest. We conclude that (1) soil moisture, organic C, and total N were strongly affected by location-specific topographic features such as gravitational potential, the amount of precipitation, temperature, and vegetation type; (2) total phosphorus was affected by catchment-related hydrological activities and soil C:N ratio; and (3) pH was affected by location-specific topographic features and catchment-related hydrological activities.

     

Binding,fractionation, and distribution of Cs,Co, and Sr in a US coastal soil under saturated and field capacity moisture regimes

Purpose

The concerns of the public on safe handling of nuclear energy power facilities have increased due to the recent nuclear plant accidents in Japan and others. Cesium, cobalt, and strontium are a few of the major radionuclides released from nuclear power plant accidents. The objectives of this study are to investigate binding, distribution, fractionation, and transformation of cesium (Cs), cobalt (Co), and strontium (Sr) in a US coastal soil under saturated paste (SP) and field capacity (FC) moisture regimes.

Materials and methods

There are four major nuclear power plants in the coast region around the northern Gulf of Mexico where coastal soil often undergoes soil moisture change. A coastal soil was taken from the middle region of these major nuclear power plants and spiked with different concentrations of cesium, cobalt, and strontium salts. The sequential selective dissolution technique was used to investigate the transformation and fractionations of these metals in the coastal soils affected by moisture regime, a key factor in the coastal environment.

Results and discussion

The adsorption kinetics showed that both Co and Sr reached the adsorption plateau even after 5 h of adsorption, indicating a fast initial adsorption process in the coastal soil. Cesium, cobalt, and strontium were dominantly presented in the soluble and exchangeable form (EXC) (Cs?>?Co and Sr), which linearly increased with the addition levels, possessing the high bioavailability, mobility, and ecotoxicity. Saturated regime significantly reduced the soluble and exchangeable form compared to field capacity moisture regime.

Conclusions

The current study provides the fundamental understanding for designing the cost-effective remediation technology to remediate these metals in coastal soil by targeting on the soluble and exchangeable forms and better prepare the USA for future potentially nuclear power plant accidents.

     

Impact of fire on soil gross nitrogen transformations in forest ecosystems

Purpose

Forests play a key role in the global carbon (C) and nitrogen (N) cycling. Fire is a global phenomenon occurring in many forest ecosystems, which has several environmental and ecological effects. The objective of this review was to improve our understanding of the effect of fire on soil gross N transformations in forest ecosystems.

Methods and results

We have reviewed the published studies using ¹⁵N pool dilution technique with analytical data analysis method to study the effect of fires on gross N transformations in forest ecosystems. Wildfires increased gross N mineralization rates in the short term and the effect disappeared from 3 years after the fire, while the effect of prescribed fires disappeared from 2 years after the burning. Both wildfires and prescribed fires reduced gross nitrification in the short term, while their effects varied from 6 months following the burning.

Conclusions

The different responses of gross N transformations to the fires in forest ecosystems depended on many factors including forest types, the intensity and frequency of fires, the time elapsed between the fires and sampling events, incubation conditions (field or laboratory incubation), climatic conditions and so on. In view of many factors influencing the effect of fires on gross N transformations, more comprehensive studies with physical, chemical, microbial and ecological characterization are needed to improve our knowledge about the effect of fires on soil gross N transformations and then N cycling in forest ecosystems.