Soil moisture effects on gross nitrification differ between adjacent grassland and forested soils in central Alberta, Canada

Background and aims

Changes in soil moisture availability seasonally and as a result of climatic variability would influence soil nitrogen (N) cycling in different land use systems. This study aimed to understand mechanisms of soil moisture availability on gross N transformation rates.

Methods

A laboratory incubation experiment was conducted to evaluate the effects of soil moisture content (65 vs. 100% water holding capacity, WHC) on gross N transformation rates using the ¹⁵N tracing technique (calculated by the numerical model FLUAZ) in adjacent grassland and forest soils in central Alberta, Canada.

Results

Gross N mineralization and gross NH ₄ ⁺ immobilization rates were not influenced by soil moisture content for both soils. Gross nitrification rates were greater at 100 than at 65% WHC only in the forest soil. Denitrification rates during the 9 days of incubation were 2.47 and 4.91 mg N kg^-1 soil d^-1 in the grassland and forest soils, respectively, at 100% WHC, but were not different from zero at 65% WHC. In the forest soil, both the ratio of gross nitrification to gross NH ₄ ⁺ immobilization rates (N/IA) and cumulative N₂O emission were lower in the 65 than in the 100% WHC treatment, while in the grassland soil, the N/IA ratio was similar between the two soil moisture content treatments but cumulative N₂O emission was lower at 65% WHC.

Conclusions

The effect of soil moisture content on gross nitrification rates differ between forest and grassland soils and decreasing soil moisture content from 100 to 65% WHC reduced N₂O emissions in both soils.     

Rhizoreduction of arsenate and chromate in Australian native grass, shrub and tree vegetation

Background and aims

Rhizosphere influences the dynamics of nutrients and contaminants through increased microbial activity, release of root exudates and alteration of pH. The objective of this study was to evaluate the rhizosphere-induced reduction (i.e. rhizoreduction) and redistribution of arsenate [As(V)] and chromate [Cr(VI)] in Australian native vegetation in relation to their bioavailability.

Methods

The reduction of As(V) and Cr(VI) was examined using rhizosphere soils from a number of Australian native vegetation (Acacia pubescens, Eucalyptus camaldulensis, Enchylaena tomentosa, Templetonia retusa, Dichantheum sericeum and Austrodanthonia richardsonii). Naturally contaminated As and Cr soils were used to examine the effect of Dichantheum sericeum on the redistribution and bioavailability of these metal(loid)s.

Results

The rhizosphere soil contained higher levels of microbial activity, dissolved organic carbon and organic acid content than the non-rhizosphere soil. The rhizosphere soil caused up to 2.4 and 5.1 fold increases in the rate of reduction of As(V) and Cr(VI), respectively. There was a significant relationship between rhizosphere-induced increases in microbial activity (Δ basal respiration) and As(V) and Cr(VI) reduction (Δ rate of reduction), indicating the role of increased microbial activity in rhizosphere soil on metal(loid) reduction. In the plant growth experiment, Dichantheum sericeum enhanced the reduction of metal(loid)s in the naturally contaminated soils, thereby increasing the bioavailability of As but decreasing that of Cr.

Conclusions

Depending on the nature of metal(loid)s present in soil, the rhizosphere-induced reduction by plant species such as Dichantheum sericeum and Templetonia retusa has implications to both their bioavailability to higher plants and microorganisms, and remediation of contaminated soils. While rhizoreduction decreases Cr bioavailability it increases that of As.     

Linkages between grazing history and herbivore exclusion on decomposition rates in mineral soils of subalpine grasslands

Background & aims

Herbivore-driven changes to soil properties can influence the decomposition rate of organic material and therefore soil carbon cycling within grassland ecosystems. We investigated how aboveground foraging mammalian and invertebrate herbivores affect mineral soil decomposition rates and associated soil properties in two subalpine vegetation types (short-grass and tall-grass) with different grazing histories.

Methods

Using exclosures with differing mesh sizes, we progressively excluded large, medium and small mammals and invertebrates from the two vegetation types in the Swiss National Park (SNP). Mineral soil decomposition rates were assessed using the cotton cloth (standard substrate) method between May and September 2010.

Results

Decomposition displayed strong spatio-temporal variability, best explained by soil temperature. Exclusion of large mammals increased decomposition rates, but further exclusion reduced decomposition rates again in the lightly grazed (tall-grass) vegetation. No difference among treatments was found in the heavily grazed (short-grass) vegetation. Heavily grazed areas had higher decomposition rates than the lightly grazed areas because of higher soil temperatures. Microbial biomass carbon and soil C:N ratio were also linked to spatio-temporal decomposition patterns, but not to grazing history.

Conclusions

Despite altering some of the environmental controls of decomposition, cellulose decomposition rates in the SNP’s subalpine grasslands appear to be mostly resistant to short-term herbivore exclusion.     

The contribution of crop residues to changes in soil pH under field conditions

Background and Aims

Crop residues are important for the redistribution of alkalinity within soils. A net increase in pH following residue addition to soil is typically reported. However, effects are inconsistent in the field due to confounding soil processes and agronomic practises.

Methods

A column experiment investigated the effects of canola, chickpea and wheat residues, differing in alkalinity content and C:N ratio, on soil pH changes in a Podosol (Podzol; initial pH 4.5) and Tenosol (Cambisol; initial pH 6.2) under field conditions.

Results

Residues (10 g dry matter kg^-1 soil; 0–10 cm) increased soil pH, and temporal changes in alkalinity depended on the residue and soil type. Alkalinity was generated via abiotic association reactions between H⁺ and added organic matter and via ammonification and decarboxylation processes during decomposition. Alkalinity from canola and chickpea residues moved down the soil profile (10–30 cm) and was attributed to nitrate immobilisation and organic anion decomposition by soil microbes.

Conclusions

The application of residues to acid and moderately acid soils increased the pH of both topsoil and subsoils, which persisted over 26 months. Maximal increase of pH observed at 3 months was correlated with the concentration of excess cations in the residues.     

Runoff and erosion from volcanic soils affected by fire: the case of Austrocedrus chilensis forests in Patagonia, Argentina

Aims

We characterized the runoff and erosion from a volcanic soil in an Austrocedrus chilensis forest affected by a wildfire, and we evaluated the effects of a mitigation treatment.

Methods

Rainfall simulations were performed in the unburned and burned forest, with and without vegetation cover, and under a mitigation treatment.

Results

After the wildfire, the mean infiltration rate decreased from 100 mm?h^?1 in unburned soils to 51 and 64 mm?h^?1 in the burned with and without litter and vegetation cover, respectively. The fast establishment of bryophytes accelerated the recovery of soil stability. Sediment production was negligible in the control plots (4.4 g?m^?2); meanwhile in the burned plots, it was 118.7 g?m^?2 and increased to 1026.1 g?m^?2 in the burned and bare plots. Total C and N losses in the control plots were negligible, while in the burned and bare plots the organic C and total N removed were 98.25 and 1.64 g?m², respectively. The effect of mitigation treatment was efficient in reducing the runoff, but it did not affect the sediment production.

Conclusions

These fertile volcanic soils promoted the recovery of vegetation in a short time after the wildfire, diminishing the risk of erosion.     

Reductive soil disinfestation (RSD) alters gross N transformation rates and reduces NO and N2O emissions in degraded vegetable soils

Background and aims

Continuous vegetable cultivation in greenhouses can easily induce soil degradation, which considerably affects the development of sustainable vegetable production. Recently, the reductive soil disinfestation (RSD) is widely used as an alternative to chemical soil disinfestations to improve degraded greenhouse vegetable soils. Considering the importance of nitrogen (N) for plant growth and environment effect, the internal N transformation processes and rates should be well investigated in degraded vegetable soils treated by RSD, but few works have been undertaken.

Methods

Three RSD-treated and three untreated degraded vegetable soils were chosen and a ¹⁵?N tracing incubation experiment differentially labeled with ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ was conducted at 25 °C under 50 % water holding capacity (WHC) for 96 h. Soil gross N transformation rates were calculated using a ¹⁵?N tracing model combined with Markov Chain Monte Carlo Metropolis algorithm (Müller et al. 2007), while the emissions of N₂O and NO were also measured.

Results

RSD could significantly enhance the soil microbial NH₄ ⁺ immobilization rate, the heterotrophic and autotrophic nitrification rates, and the NO₃ ^? turnover time. The ratio of heterotrophic nitrification to total inorganic N supply rate (mineralization + heterotrophic nitrification) increased greatly from 5.4 % in untreated vegetable soil to 56.1 % in treated vegetable soil. In addition, low release potential of NO and N₂O was observed in RSD-treated vegetable soil, due to the decrease in the NO and N₂O product ratios from heterotrophic and autotrophic nitrifications. These significant differences in gross N transformation rates, the supply processes and capacity of inorganic N, and the NO and N₂O emissions between untreated and treated vegetable soils could be explained by the elimination of accumulated NO₃ ^?, increased pH, and decreased electrical conductivity (EC) caused by RSD. Noticeably, the NO₃ ^? consumption rates were still significantly lower than the NO₃ ^? production rates in RSD-treated vegetable soil.

Conclusions

Except for improving soil chemical properties, RSD could significantly alter the supply processes of inorganic N and reduce the release potential of N₂O and NO in RSD-treated degraded vegetable soil. In order to retard the re-occurrence of NO₃ ^? accumulation, acidification and salinization and to promote the long-term productivity of greenhouse vegetable fields, the rational use of N fertilizer should be paid great attention to farmers in vegetable cultivation.     

Modifying the LEACHM model for process-based prediction of nitrate leaching from cropped Andosols

Aims

Nitrogen (N) management strategies for reducing the risk of groundwater contamination around agricultural fields require precise prediction of N leaching using a process-based model. We modified LEACHM model for use in Andosols, which are characterized by slow soil organic carbon (SOC) mineralization and nitrate adsorption.

Methods

The modification was made with regard to the SOC mineralization of incoming plant-residue/manure and humus following the RothC model, as well as for nitrate adsorption. Empirical equations were employed to determine the parameters of the modified model. The ability of the modified LEACHM to predict N leaching was tested against existing data from a 4-year lysimeter study for cropped Andosol and sandy soils and compared with that of the original model.

Results

The modified model improved the prediction of leached N concentrations and the loss of N from Andosol with relative improvements of 63.5 and 76.5 %, respectively, over the original model, while retaining model applicability in sandy soil. This effective modeling was achieved by using precise predictions of N mineralization in the humus pool along with SOC mineralization processes that were based on the RothC model.

Conclusions

The modification extended the applicability of LEACHM and may provide better N management strategies for reducing leaching from cultivated Andosols.     

Coupled soil-availability and tree-limitation nutritional shifts induced by N deposition: insights from N to P relationships in Abies pinsapo forests

Background and aims

Interacting effects of atmospheric N deposition on the degree to which tree demand for other nutrients is met by soil supply has seldom been explored in Mediterranean-type ecosystems. We hypothesized that patterns for the relative availability of N and P in soils will be matched by variations in process rates related to soil organic P cycling and by shifts from N to P limitation of tree growth.

Methods

We examined N/P relationships in Mediterranean-fir (Abies pinsapo) forests from two nearby regions differing in N deposition levels.

Results

N pools and transformation rates and the contribution of organic fractions to the labile P pool in soils showed increasing trends toward the pollution source. Phosphomonoesterase activity (PME) in bulk soils, root PME per unit biomass (but not per unit soil volume) and biomass accumulation in P-fertilized root-in-growth cores incubated in situ were also the highest at the sites receiving elevated N deposition, indicating P limitation. In contrast, forest stands in the region farther from the pollutant source were N-limited (preferential root growth in N-rich soil microsites) and showed lower PME activities and higher total fine root biomass.

Conclusions

In the forests under elevated N deposition, higher values for an overall indicator of soil N status matched with indications of an accelerated soil organic P subcycle and P-limitation of tree growth.     

Soil N forms and gross transformation rates in Chinese subtropical forests dominated by different tree species

Background and aims

Knowledge related to extent of differing soil N forms and N transformation rates in subtropical southern China is severely limited. Accordingly, the purpose of this study was to investigate if and how tree species of different foliage types (coniferous, deciduous, and evergreen broadleaved) influence N forms and microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) content as well as gross N transformation rates in the organic and mineral soils of three distinct subtropical forests in China.

Methods

Chloroform fumigation extraction was used to determine MBC and MBN content while ¹⁵N-isotope dilution techniques were used to measure gross N transformation rates. Canonical correspondence analysis (CCA) was used to quantify relationships between soil chemical characteristics and changes in soil N transformation rates.

Results

Soil N forms, MBC and MBN content, and N transformation rates were found to be significantly different between tree species. Deciduous forest soil exhibited the highest N transformation rates. Soil N transformation rates were closely associated with total soil C and N and MBC and MBN content.

Conclusions

Soil substrate quantity and soil microbial activity play a more important role in soil N transformation processes than does soil quality in China’s subtropical forests. Tree species type should therefore be taken into account when trying to determine ecosystem N cycling.     

Positive feedbacks between decomposition and soil nitrogen availability along fertility gradients

Background and aims

We determined the relationship between site N supply and decomposition rates with respect to controls exerted by environment, litter chemistry, and fungal colonization.

Methods

Two reciprocal transplant decomposition experiments were established, one in each of two long-term experiments in oak woodlands in Minnesota, USA: a fire frequency/vegetation gradient, along which soil N availability varies markedly, and a long-term N fertilization experiment. Both experiments used native Quercus ellipsoidalis E.J. Hill and Andropogon gerardii Vitman leaf litter and either root litter or wooden dowels.

Results

Leaf litter decay rates generally increased with soil N availability in both experiments while belowground litter decayed more slowly with increasing soil N. Litter chemistry differed among litter types, and these differences had significant effects on belowground (but not aboveground) decay rates and on aboveground litter N dynamics during decomposition. Fungal colonization of detritus was positively correlated with soil fertility and decay rates.

Conclusions

Higher soil fertility associated with low fire frequency was associated with greater leaf litter production, higher rates of fungal colonization of detritus, more rapid leaf litter decomposition rates, and greater N release in the root litter, all of which likely enhance soil fertility. During decomposition, both greater mass loss and litter N release provide mechanisms through which the plant and decomposer communities provide positive feedbacks to soil fertility as ultimately driven by decreasing fire frequency in N-limited soils and vice versa.     

Accumulation rates of soil organic matter in wet dune slacks on the Dutch Wadden Sea islands

Background and aims

A long-term monitoring program (ranging from 16 to 77 years) on the Dutch Wadden Sea Islands provided well-documented examples of vegetation succession in wet dune slacks. We used this opportunity to study soil organic matter (SOM) accumulation in relation to vegetation succession. The aim of this paper is to identify the factors which regulate accumulation rates of SOM in wet dune slacks.

Methods

We used several soil chronosequences using data from the monitoring program together with data from a long-term research activity and more recent measurements. We used several soil chronosequences using data from the monitoring program together with data from a long-term research (up to 150 years) and more recent measurements. Field measurements included pH, soil organic matter, above ground standing crop and water levels. Water level regimes (inundation duration and mean minimum water level), were simulated using a hydrological model. Capable of simulating inundation duration and water-level fluctuations, this model used field measurements collected over more than 5 years, as well as precipitation and evapotranspiration data collected over a period of 25 years.

Results

Sampling two synchronic chronosequences showed that SOM accumulations increased linearly during the first 50–60 years and then levelled off. Sampling various diachronic chronosequences over time showed a wide variation in accumulation rates. Slacks with low productive species, such as Littorella uniflora, showed low accumulation rates (0.02–0.08 kg/m²/year), and persisted even over a period of more than 90 years. In contrast, slacks dominated by high productive species, such as Phragmites australis, showed ten times higher accumulation rates (0.17–0.26 kg/m²/year) over a similar time period and comparable annual inundation periods (176–240 days). A multiple linear regression showed that variation in SOM accumulation rates was best explained by above-ground biomass of the vegetation.

Conclusions

We conclude that the rate of SOM accumulation in wet dune slacks is primarily controlled by plant above-ground biomass. Both above-ground biomass and SOM accumulation can remain very low over a long period of time when dune slacks are flooded during most of the year and plants with adaptive traits are able to maintain vegetation succession at a pioneer stage.     

Savanna soil fertility limits growth but not survival of tropical forest tree seedlings

Background and Aims

Cerradão (Brazilian woodland savannas) and seasonally dry forests (SDF) from southeastern Brazil occur under the same climate but are remarkably distinct in species composition. The objective of this study was to evaluate the role of soil origin in the initial growth and distribution of SDF and Cerradão species.

Methods

We conducted a greenhouse experiment growing Cerradão and SDF tree seedlings over their soil and the soil of the contrasting vegetation type. We evaluated soil nutrient availability and seedling survivorship, growth and leaf functional traits.

Results

Despite the higher nutrient availability in SDF soils, soil origin did not affect seedling survivorship. The three SDF species demonstrated home-soil advantage, enhanced growth with increasing soil nutrient availability and had higher growth rates than Cerradão species, even on Cerradão soils. Growth of Cerradão seedlings was not higher on Cerradão soil and, overall, was not positively correlated with soil nutrient availability.

Conclusions

SDF species are fast-growing species while Cerradão trees tend to be slow-growing species. Although savanna soil reduces growth of forest species, our findings suggest that soil chemical attributes, alone, does not exclude the occurrence of SDF seedlings in Cerradão and vice-versa.     

Photoassimilated carbon allocation in a wheat plant-soil system as affected by soil fertility and land-use history

Background and aims

Carbon (C) cycling in terrestrial ecosystems is influenced by the distribution of photo-assimilated C in the plant-soil system. Photo-assimilated C allocation in a wheat cropping system was examined to identify the links between soil fertility, C partitioning and soil C sequestration.

Methods

A pulse labelling experiment was conducted where ¹⁴CO₂ was introduced to wheat plants grown in two groups of soils of varying fertility: arable soils spiked with nutrients, and soils with differing land-use histories. Wheat shoot, root and soil samples were taken 1, 14 and 28 days after pulse labelling to examine the fluxes of ¹⁴C in the plant-root-soil system.

Results

The partitioning of ¹⁴C in wheat plant-root-soil system was found to vary with time, nutrient spiked soil fertility and land-use history. At the end of the experiment using spiked soils, a positive correlation was observed between the allocation of ¹⁴C in the shoots and soil fertility, whereas in the roots, this relationship was negative. The overall allocation of ¹⁴C in the plant-root system differed significantly between the land-use histories; while in the spiked arable soils ¹⁴C allocation in the shoots and roots systematically followed their fertility status.

Conclusions

There was a weak relationship between C allocation and soil fertility in the soils of different land-use history compared to the strong relationship in the spiked arable soils. This suggests that other factors in the soils under different land uses were more important than nutrient status alone in driving photo-assimilated C allocation. This study demonstrated that soil fertility and land-use history have a crucial role in the allocation of photo-assimilated C in the plant-soil system and are important factors by which C sequestration in soil may be impacted.     

Effects of nano-sized zero-valent iron on DDT degradation and residual toxicity in soil: a column experiment

Background and aims

Nanoscale zero-valent iron (nZVI) application is a promising technology for degradation of chlorinated contaminants in soil. Plants also play an important role in soil remediation and nZVI should not adversely affect plants growing on treated soils. Large amounts of DDT are still found in certain soils and means to remediate these soils are limited. Our aims were to investigate the effect of nZVI on DDT degradation and evaluate possible negative effects of nZVI on plants.

Methods

Columns with spiked (20 mg DDT kg^?1) soil were percolated with nZVI (1 g nZVI L^?1) and leached with five pore volumes of water to assess leaching of nZVI and residual toxicity of leachates and soil to plants using seed germination and plant growth tests (barley, flax).

Results

Addition of nZVI led to degradation of 45 % of the added DDT. Percolation with water significantly oxidized and transported iron through the columns. The first leachates had negative effects on plant development, but after leaching with 4 pore volumes, neither soil nor leachates affected plant negatively.

Conclusions

nZVI is efficient for degradation of DDT and adverse effects of nZVI on plants seem ephemeral and are alleviated after oxidation mediated by percolating water.     

High surface area biochar negatively impacts herbicide efficacy

Background and Aims

Amendment of soil by biochar may reduce efficacy of soil-applied herbicides due to sorption.

Methods

Bioassays with Green Foxtail (Setaria viridis) tested the influence of two biochars on phytoavailability of S-metolachlor and sulfentrazone under biochar amendment of 0, 13, 26 and 52?Mg?ha^-1.

Results

Adsorption of both herbicides was an order of magnitude greater on a high specific surface area (SSA) biochar (EUC-800; SSA 242?m²?g^-1) than on a low SSA biochar (BC-1; SSA 3.6?m²?g^-1). Herbicide doses near the lowest recommended label rates controlled the weed at 13 and 26?Mg?ha^-1 of BC-1; sulfentrazone was also effective at 52?Mg BC-1?ha^-1. These same herbicide doses controlled weed germination and development only at 13?Mg?ha^-1 of EUC-800; at herbicide doses near the highest label rates, weed control was also achieved at 26?Mg EUC-800?ha^-1, but not at 52?Mg EUC-800?ha^-1.

Conclusions

Increased doses of soil-applied herbicides cannot necessarily offset decreases in herbicide phytoavailability in biochar-amended soils, particularly if the biochar has a high SSA. Considering the long half-life of biochar in soil, pest control needs will be best served by low SSA biochars.     

Preferential use of root litter compared to leaf litter by beech seedlings and soil microorganisms

Background and aims

Litter decomposition is regulated by e.g. substrate quality and environmental factors, particularly water availability. The partitioning of nutrients released from litter between vegetation and soil microorganisms may, therefore, be affected by changing climate. This study aimed to elucidate the impact of litter type and drought on the fate of litter-derived N in beech seedlings and soil microbes.

Methods

We quantified ¹⁵N recovery rates in plant and soil N pools by adding ¹⁵N-labelled leaf and/or root litter under controlled conditions.

Results

Root litter was favoured over leaf litter for N acquisition by beech seedlings and soil microorganisms. Drought reduced ¹⁵N recovery from litter in seedlings thereby affecting root N nutrition. ¹⁵N accumulated in seedlings in different sinks depending on litter type.

Conclusions

Root turnover appears to influence (a) N availability in the soil for plants and soil microbes and (b) N acquisition and retention despite a presumably extremely dynamic turnover of microbial biomass. Compared to soil microorganisms, beech seedlings represent a very minor short-term N sink, despite a potentially high N residence time. Furthermore, soil microbes constitute a significant N pool that can be released in the long term and, thus, may become available for N nutrition of plants.     

Contrasting nitrogen and phosphorus dynamics across an elevational gradient for subarctic tundra heath and meadow vegetation

Aims

This study explores soil nutrient cycling processes and microbial properties for two contrasting vegetation types along an elevational gradient in subarctic tundra to improve our understanding of how temperature influences nutrient availability in an ecosystem predicted to be sensitive to global warming.

Methods

We measured total amino acid (Amino-N), mineral nitrogen (N) and phosphorus (P) concentrations, in situ net N and P mineralization, net Amino-N consumption, and microbial biomass C, N and P in both heath and meadow soils across an elevational gradient near Abisko, Sweden.

Results

For the meadow, NH₄ ⁺ concentrations and net N mineralization were highest at high elevations and microbial properties showed variable responses; these variables were largely unresponsive to elevation for the heath. Amino-N concentrations sometimes showed a tendency to increase with elevation and net Amino-N consumption was often unresponsive to elevation. Overall, PO₄-P concentrations decreased with elevation and net P immobilization mostly occurred at lower elevations; these effects were strongest for the heath.

Conclusions

Our results reveal that elevation-associated changes in temperature can have contrasting effects on the cycling of N and P in subarctic soils, and that the strength and direction of these effects depend strongly on dominant vegetation type.     

Nitrate-use traits of understory plants as potential regulators of vegetation distribution on a slope in a Japanese cedar plantation

Background and aims

Plant physiological traits and their relation to soil N availability was investigated as regulators of the distribution of understory shrub species along a slope in a Japanese cedar (Cryptomeria japonica) plantation in central Japan.

Methods

At the study site, previous studies demonstrated that both net and gross soil nitrification rates are high on the lower slope and there are dramatic declines in different sections of the slope gradient. We examined the distributions of understory plant species and their nitrate (NO ₃ ^? -N) use traits, and compared the results with the soil traits.

Results

Our results show that boundaries between different dominant understory species correspond to boundaries between different soil types. Leucosceptrum stellipilum occurs on soil with high net and gross nitrification rates. Hydrangea hirta is dominant on soil with high net and low gross nitrification rates. Pieris japonica occurs on soil with very low net and gross nitrification rates. Dominant understory species have species-specific physiological traits in their use of NO ₃ ^? -N. Pieris japonica lacks the capacity to use NO ₃ ^? -N as a N source, but other species do use NO ₃ ^? -N. Lindera triloba, whose distribution is unrelated to soil NO ₃ ^? -N availability, changes the extent to which it uses NO ₃ ^? -N in response to soil NO ₃ ^? -N availability.

Conclusions

Our results indicate that differences in the physiological capabilities and adaptabilities of plant species in using NO ₃ ^? -N as a N source regulate their distribution ranges. The identity of the major form of available soil N is therefore an environmental factor that influences plant distributions.     

Soil organic matter dynamics in density and particle-size fractions following destruction of tropical rainforest and the subsequent establishment of Imperata grassland in Indonesian Borneo using stable carbon isotopes

Background and aims

Large portions of the deforested areas in Southeast Asia have been ultimately replaced by the invasive grass Imperata cylindrica, but the dynamics of soil organic matter (SOM) during such land transitions are poorly understood. This study presents SOM dynamics in density and particle-size fractions following rainforest destruction and the subsequent establishment and persistence of Imperata grassland.

Methods

We examined soil C stock and natural ¹³C abundance in these fractions to depths of 100 cm. We predicted future soil C storage and evaluated C turnover rates in these fractions using a simple exponential model. Because soil texture strongly affects soil C storage, two chronosequences of soils differing in soil texture were compared (n?=?1 in each chronosequence).

Results

The clay-associated SOM increased in all soil layers (0–100 cm) along the forest-to-grassland chronosequence, whereas light-fraction SOM in the surface soil layer (0–5 cm) decreased.

Conclusions

In the surface layer, all SOM fractions exhibited rapid replacement of forest-derived C to grassland-derived C, indicating fast turnover. Meanwhile, δ¹³C values of the light fraction in the surface layer indicated that forest-derived charcoal and/or occluded low-density organic matter constituted unexpectedly large proportions of the light fraction. Mathematical modelling (0–50 cm) showed that grassland-derived C in the clay and silt fractions in all soil layers increased almost linearly for at least 50 years after grassland establishment. In the meantime, the forest-derived C stock in the clay fraction constituted 82 % of the total stable C pool at 0–50-cm depths even under steady-state conditions (t = ∞), indicating that residue of forest-derived SOM associated with clay largely contributed to preserving the soil C pool. Comparing soils with different soil textures, clay and silt particles in coarse-textured soil exhibited a substantially higher degree of organo-mineral interactions per unit volume of clay or silt compared to fine-textured soils.     

Topsoil organic matter properties in contrasted hedgerow vegetation types

Background and aims

Hedges, semi-natural landscape components, have the ability to integrate both agronomic and environmental functions and to provide several ecosystem services. The aim of this study was to test whether hedgerow vegetation is a determinant of soil organic matter properties in ancient agricultural lands.

Methods

We complemented cluster analysis and ordination to determine the extent to which two types of hedges that were distinct in character-plant species also differed between each other in concentration and composition of two major constituents of soil organic matter, namely humic substances and dissolved organic matter.

Results

The two types of hedges were associated with significant differences in humic carbon content, hormone-like activity and molecular size of humic substances, which, in general, were more similar to those typical of forest than of agricultural soils. Moreover, we detected between-group differences in several phenolic acids.

Conclusions

Variation of the topsoil biochemical properties of hedges may be explained by variation in their vegetation characteristics, similar to other ecosystems. Spontaneous vegetation in hedges perform an important role in controlling the variability of surface soil properties that influence the evolution of soil organic matter and nutrient availability in agricultural lands.