Soil organic C and nutrient contents under trees with different functional characteristics in seasonally dry tropical silvopastures

Aims

The selection of tree characteristics is critical for the outcome of the tree effects on soil fertility in silvopastoral pastures. This study aims to quantify the effects of trees on soil nutrient and C stocks, as well as assessing differences on the effects between legume (Albizia saman; Enterolobium cyclocarpum) and non-legume tree species (Tabebuia rosea; Guazuma ulmifolia).

Methods

In Central Nicaragua, soil was sampled (0–10 cm deep) in paired plots, under both a canopy and in open grassland, in 12 sites per tree species and analysed for organic C, total N stocks, available P and extractable K⁺, Ca²⁺ and Mg²⁺. To assess the effects of herbaceous composition and cattle to soil proprieties, we recorded the cover of plant groups and assessed the mass of dung in each plot.

Results

Soil organic C and N, available P and extractable K⁺ and Ca²⁺ were higher under the tree canopy than under paired open grassland. The basal area of trees was positively related with the canopy effect on soil variables, thus suggesting that the age or sizes of the trees are relevant factors associated with the content of soil C and nutrients. No specific effects related to the legume species group were detected.

Conclusions

Our results indicate that in fertile seasonally dry subtropical pastures, scattered trees have an overall effect on soil fertility, and that the magnitude of the effect depends more on the tree characteristics (i.e. basal area, crown area) than on whether the species is a legume or not.     

Soil organic carbon and root distribution in a temperate arable agroforestry system

Aim

To determine, for arable land in a temperate area, the effect of tree establishment and intercropping treatments, on the distribution of roots and soil organic carbon to a depth of 1.5 m.

Methods

A poplar (Populus sp.) silvoarable agroforestry experiment including arable controls was established on arable land in lowland England in 1992. The trees were intercropped with an arable rotation or bare fallow for the first 11 years, thereafter grass was allowed to establish. Coarse and fine root distributions (to depths of up to 1.5 m and up to 5 m from the trees) were measured in 1996, 2003, and 2011. The amount and type of soil carbon to 1.5 m depth was also measured in 2011.

Results

The trees, initially surrounded by arable crops rather than fallow, had a deeper coarse root distribution with less lateral expansion. In 2011, the combined length of tree and understorey vegetation roots was greater in the agroforestry treatments than the control, at depths below 0.9 m. Between 0 and 1.5 m depth, the fine root carbon in the agroforestry treatment (2.56 t ha^-1) was 79% greater than that in the control (1.43 t ha^?1). Although the soil organic carbon in the top 0.6 m under the trees (161 t C ha^?1) was greater than in the control (142 t C ha^?1), a tendency for smaller soil carbon levels beneath the trees at lower depths, meant that there was no overall tree effect when a 1.5 m soil depth was considered. From a limited sample, there was no tree effect on the proportion of recalcitrant soil organic carbon.

Conclusions

The observed decline in soil carbon beneath the trees at soil depths greater than 60 cm, if observed elsewhere, has important implication for assessments of the role of afforestation and agroforestry in sequestering carbon.     

Introducing Acacia mangium trees in Eucalyptus grandis plantations: consequences for soil organic matter stocks and nitrogen mineralization

Background and aims

Eucalyptus plantations cover 20 million hectares on highly weathered soils. Large amounts of nitrogen (N) exported during harvesting lead to concerns about their sustainability. Our goal was to assess the potential of introducing A. mangium trees in highly productive Eucalyptus plantations to enhance soil organic matter stocks and N availability.

Methods

A randomized block design was set up in a Brazilian Ferralsol soil to assess the effects of mono-specific Eucalyptus grandis (100E) and Acacia mangium (100A) stands and mixed plantations (50A:50E) on soil organic matter stocks and net N mineralization.

Results

A 6-year rotation of mono-specific A. mangium plantations led to carbon (C) and N stocks in the forest floor that were 44% lower and 86% higher than in pure E. grandis stands, respectively. Carbon and N stocks were not significantly different between the three treatments in the 0–15?cm soil layer. Field incubations conducted every 4?weeks for the two last years of the rotation estimated net soil N mineralization in 100A and 100E at 124 and 64?kg?ha^?1?yr^?1, respectively. Nitrogen inputs to soil with litterfall were of the same order as net N mineralization.

Conclusions

Acacia mangium trees largely increased the turnover rate of N in the topsoil. Introducing A. mangium trees might improve mineral N availability in soils where commercial Eucalyptus plantations have been managed for a long time.     

The role of harvest residues to sustain tree growth and soil nitrogen stocks in a tropical Eucalyptus plantation

Aims

Tropical plantations are likely to supply a growing share of the increasing world demand for forest products. We aimed to gain insight into the role of the nitrogen (N) contained in harvest residues (HR) for tree growth and soil N stocks.

Methods

We used 15N-labeled harvest residues to (1) study the dynamic of N release throughout decomposition, (2) determine the vertical transport pathways of N from the forest floor to the upper soil layers, and (3) quantifying the contributions of HR to soil N stocks and the supply of N to young Eucalyptus trees.

Results

Almost all of the 15N initially contained in the HR was recovered 27 months after deposition, with 21 % remaining in HR, 38 % being transferred to the underlying O layer, 21 % being transferred to the 0–15 cm soil layer, and approximately 15 % accumulating in the tree biomass. Our results supported the presence of two pathways of N transfers from the O layer to the mineral soil: (1) the leaching of dissolved 15N from fresh litter during the first year after planting which actively contributed to Eucalyptus N nutrition and (2) the transport of particulate organic matter in percolating water which contributed to maintain N stocks in the first 15 cm of the soil. Approximately 40 % of the N content in 2-year-old Eucalyptus trees was derived from the labeled HR.

Conclusions

The sustainability of fast-growing Eucalyptus trees established on N-poor sandy tropical soils largely relies on organic residues, as an early source of mineral N for tree and as a source of organic N in the top soil.     

Impact of historic grazing on steppe soils on the northern Tibetan Plateau

Aims

Few studies have focused on changes in the physical and chemical properties of soils that are induced by grazing at high altitudes. Our aim was to identify potential responses of soil to grazing pressure on the semiarid steppe of the northern Tibetan Plateau and their probable causes.

Methods

Fractal geometry to describe soil structure, soil dynamics, and physical processes within soil is becoming an increasingly useful tool that allows a better understanding of the performance of soil systems. In this study, we sampled four experimental areas in the northern part of the Tibetan Plateau under different grazing intensities: ungrazed, lightly grazed, moderately grazed and heavily grazed plots. Fractal methods were applied to characterise particle-size distributions and pore patterns of soils under different grazing intensities.

Results

Our results reveal a highly significant decrease in the fractal dimensions of particle size distributions (D ₁ ) and the fractal dimensions of all pores (D ₂ ) with increasing grazing intensity. Soil organic carbon (SOC), total N and total P concentrations increased significantly with decreasing grazing intensity. We did not find differences in soil pH in response to grazing.

Conclusions

Grazing induced a significant deterioration of the physical and chemical topsoil properties in the semiarid steppe of the northern Tibetan Plateau. Fractal dimensions can be a useful parameter for quantifying soil degradation due to human activities.     

The age of managed heathland communities: implications for carbon storage?

Background and aims

Shrublands are ecosystems vulnerable to climate changes, with key functions such as carbon storage likely to be affected. In dwarf shrublands dominated by Calluna vulgaris, the aboveground carbon allocation is associated with community age and phase of development. As the Calluna community grows older, a shift to net biomass loss occurs and it was hypothesized this would result in carbon stock increases within the soil.

Methods

The interaction of community age with ecosystem carbon stocks was investigated through a chronosequence study on three Calluna communities, aged 11, 18 and 27 years.

Results

Aboveground Calluna carbon stock increased significantly from the 11 year community (0.73 kg C m^?2) to the 18 year community (1.11 kg C m^?2) but did not significantly change from 18 to 27 years (1.0 kg C m^?2), indicating a net carbon gain that corresponded with the growth phase of the Calluna plants. Moss was also found to be a relatively large contributor to aboveground carbon stock (e.g. 30 % in the Young community). Moss has often been excluded in aboveground assessments on Calluna heathlands which may have led to previous stock underestimation. Belowground carbon stocks to 25 cm were six to nine times greater than in the aboveground pools. For example in the Young community, 8 % of the carbon stock was located aboveground, 35 % in the organic layer and 55 % in the mineral soil.

Conclusions

Increased heathland age resulted in increased aboveground carbon stock until peak production was reached at approximately 18 years of age. However, the proportionally large belowground carbon stock eclipsed any aboveground effect when total carbon stocks were considered. The investigation emphasized both the importance of including the mineral soil in sampling programs and of consider all major species, such as bryophytes, and vegetation age in carbon stock assessments.     

Fine root and aboveground carbon stocks in riparian forests: the roles of diking and environmental gradients

Aims

We analysed current carbon (C) stocks in fine root and aboveground biomass of riparian forests and influential environmental parameters on either side of a dike in the Donau-Auen National Park, Austria.

Methods

On both sides of the dike, carbon (C) stock of fine roots (CFR) under four dominant tree species and of aboveground biomass (CAB) were assessed by topsoil cores (0–30 cm) and angle count sampling method respectively (n?=?48). C stocks were modeled, performing boosted regression trees (BRT).

Results

Overall CFR was 2.8 t ha^?1, with significantly higher C stocks in diked (DRF) compared to flooded riparian forests (FRF). In contrast to CFR, mean CAB was 123 t ha^?1 and lower in DRF compared to FRF. However, dike construction was consistently ruled out as a predictor variable in BRT. CFR was influenced by the distance to the Danube River and the dominant tree species. CAB was mainly influenced by the magnitude of fluctuations in the groundwater table and the distances to the river and the low groundwater table.

Conclusions

Despite pronounced differences in FRF and DRF, we conclude that there is only weak support that dikes directly influence C allocation in floodplain forests within the time scale considered (110 years).     

Harvest residue management effects on tree growth and ecosystem carbon in a Chinese fir plantation in subtropical China

Aims

This study aimed to determine the influence of different harvest residue management strategies on tree growth, soil carbon (C) concentrations, soil nitrogen (N) availability and ecosystem C stocks 15 years after replanting second rotation Chinese fir (Cunninghamia lanceolata), an important plantation species in subtropical China. Such information is needed for designing improved management strategies for reforestation programmes in subtropical environments aimed at mitigating CO₂ emissions.

Methods

Four harvest residue management treatments including slash burning, whole tree, stem-only and double residue retention were applied to sixteen 20 m?×?30 m plots in a randomized complete block design with four replicates. Tree growth was measured annually and soil properties were measured at 3 year intervals over a 15 year period after re-planting.

Results

Cumulative diameter growth at age 15 years was significantly smaller in the slash burning than the whole tree and double residue harvest treatments. Hot water extractable N concentrations increased with the increased organic residue retention levels and significant differences were observed between double residue and slash burning treatments. Harvest residue management had no significant effect on the soil C concentrations to 40 cm depth. ANOVA showed that harvest residue management had no significant effect on total biomass carbon at age 15, but the plantation ecosystem (soil C at 0–40 cm depth plus forest biomass C) had significantly lower C mass in the slash burning treatment compared with whole tree, stem only harvest and double residue harvest treatments.

Conclusions

These observations suggest that organic residue retention during the harvesting could improve the growth and ecosystem C stocks of Chinese fir in second rotation forest plantations in subtropical China and highlight the importance of viewing the ecosystem as a whole when evaluating the impact of harvest residue management on C stocks.     

Impact of the invasive tree black locust on soil properties of Mediterranean stone pine-holm oak forests

Background and aims

Invasion by N₂-fixing species may alter biogeochemical processes. We hypothesized that the grade of invasion by the N₂-fixer black locust (Robinia pseudoacacia L.) could be related to the distribution and pools of carbon (C) and nitrogen (N) along the profile of two Mediterranean mixed forests of stone pine (Pinus pinea L.) and holm oak (Quercus ilex L.).

Methods

A low-invaded (LIN) and a high-invaded (HIN) mixed forest were studied. We assessed: N concentration in green and in senescent leaves; C and N pools along the soil profile; seasonal changes of soluble C and N fractions, and microbial activity.

Results

Compared to coexisting holm oak and stone pine, black locust had higher N content in green and in senescent leaves. In the mineral soil: N stocks were similar in LIN and HIN; water soluble C and microbial activity, were lower in HIN compared to LIN; water soluble N showed seasonal changes consistent with tree growth activity in both HIN and LIN. In the organic layer of HIN, C and N stocks were about twofold larger than expected on the basis of stand density.

Conclusion

Black locust increased C and N stocks in the upper organic layers that are more vulnerable to disturbance. However, it did not increase N stocks in the mineral soil.     

Do soil enzymes respond to long-term grazing in an arid ecosystem?

Background and aims

Our objective was to assess the effects of long-term continuous grazing on soil enzyme activities in relation to shifts in plant litter attributes and soil resources in an arid ecosystem, considering both spatial and temporal variations.

Methods

We randomly extracted soil samples with the respective litter cover at 5 modal size plant-covered patches (PCP) and the nearest inter-canopy areas (IC) at Patagonian Monte sites with low, medium and high grazing intensity in winter and summer from 2007 to 2009. We analyzed enzyme activities (dehydrogenase, ß-glucosidase, protease, alkaline and acid phosphatase), microbial biomass-C, organic-C, total soil-N, and moisture in soil and mass and quality in plant litter. We assessed faeces density and plant cover in the field.

Results and conclusions

Grazing led to reduced grass cover, decreasing plant litter mass with increasing soluble phenolics, and reduced phosphatases, ß-glucosidase and microbial biomass-C at PCP. A localized nutrient input from animal excreta seems to promote microbial biomass-C, alkaline phosphatase and dehydrogenase activities but only at IC from the site with high grazing intensity. Plant heterogeneous distribution, plant litter quantity and quality, nutrient inputs from grazers and seasonal variation in soil moisture, also affecting soil resources and microbial biomass, modulate soil enzyme responses to long-term grazing in the arid Patagonian Monte.     

Spatial patterns of total and available N and P at alpine treeline

Background and aims

Vegetation can have direct and indirect effects on soil nutrients. To test the effects of trees on soils, we examined the patterns of soil nutrients and nutrient ratios at two spatial scales: at sites spanning the alpine tundra/subalpine forest ecotone (ecotone scale), and beneath and beyond individual tree canopies within the transitional krummholz zone (tree scale).

Methods

Soils were collected and analyzed for total carbon (C), nitrogen (N), and phosphorus (P) as well as available N and P on Niwot Ridge in the Colorado Rocky Mountains.

Results

Total C, N, and P were higher in the krummholz zone than the forest or tundra. Available P was also greatest in the krummholz zone while available N increased from the forest to the tundra. Throughout the krummholz zone, total soil nutrients and available P were higher downwind compared to upwind of trees.

Conclusions

The krummholz zone in general, and downwind of krummholz trees in particular, are zones of nutrient accumulation. This pattern indicates that the indirect effects of trees on soils are more important than the direct effects. The higher N:P ratios in the tundra suggest nutrient dynamics differ from the lower elevation sites. We propose that evaluating soil N and P simultaneously in soils may provide a robust assay of ecosystem nutrient limitation.     

Dwarf pine invasion in an alpine tundra of discontinuous permafrost area: effects on fine root and soil carbon dynamics

Key message

Recent invasion of Pinus pumila , a highly productive shrub-like tree species, into alpine tundra does not significantly modify the dynamics of fine root and soil carbon in the tundra.

Abstract

Climate warming may directly and indirectly affect the large carbon stock in discontinuous permafrost soil at high latitudes. In recent decades, Siberian dwarf pine [Pinus pumila (Pall.) Regel] has been invading dry heath alpine tundra in the northern Amur region of Far East Russia. Siberian dwarf pine is known to have high aboveground productivity, comparable to that of tall coniferous trees. We hypothesised that the invasion of Siberian dwarf pine into alpine tundra could increase soil carbon stocks via an increase in fine roots. Contrary to our expectations, the invasion of dwarf pine did not significantly increase the fine root biomass and productivity of the tundra, probably due to the belowground competitive exclusion between the dwarf pine and alpine tundra plants. Furthermore, the invasion of the dwarf pine did not affect soil carbon in the alpine tundra ecosystem. These results show that the recent invasion of Siberian dwarf pine into tundra did not influence the fine root dynamics or the soil carbon stock in the study site. Together, these results implied that (1) it takes a long time for pine invasion to change the belowground ecosystem properties of tundra vegetation to that of pine thickets and therefore (2) the lack of an increase in soil carbon from recent tree invasion should be taken into account when modelling future carbon dynamics in alpine tundra.

     

Response of soil microbial activity to grazing, nitrogen deposition, and exotic cover in a serpentine grassland

Background and aims

Exotic species, nitrogen (N) deposition, and grazing are major drivers of change in grasslands. However little is known about the interactive effects of these factors on below-ground microbial communities.

Methods

We simulated realistic N deposition increases with low-level fertilization and manipulated grazing with fencing in a split-plot experiment in California’s largest serpentine grassland. We also monitored grazing intensity using camera traps and measured total available N to assess grazing and nutrient enrichment effects on microbial extracellular enzyme activity (EEA), microbial N mineralization, and respiration rates in soil.

Results

Continuous measures of grazing intensity and N availability showed that increased grazing and N were correlated with increased microbial activity and were stronger predictors than the categorical grazing and fertilization measures. Exotic cover was also generally correlated with increased microbial activity resulting from exotic-driven nutrient cycling alterations. Seasonal effects, on abiotic factors and plant phenology, were also an important factor in EEA with lower activity occurring at peak plant biomass.

Conclusions

In combination with previous studies from this serpentine grassland, our results suggest that grazing intensity and soil N availability may affect the soil microbial community indirectly via effects on exotic cover and associated changes in nutrient cycling while grazing directly impacts soil community function.     

Soil nutrients and microbial biomass in three contrasting Mediterranean forests

Aims

The extent to which the spatial and temporal patterns of soil microbial and available nutrient pools hold across different Mediterranean forest types is unclear impeding the generalization needed to consolidate our understanding on Mediterranean ecosystems functioning.

Methods

We explored the response of soil microbial, total, organic and inorganic extractable nutrient pools (C, N and P) to common sources of variability, namely habitat (tree cover), soil depth and season (summer drought), in three contrasting Mediterranean forest types: a Quercus ilex open woodland, a mixed Q. suber and Q. canariensis woodland and a Pinus sylvestris forest.

Results

Soil microbial and available nutrient pools were larger beneath tree cover than in open areas in both oak woodlands whereas the opposite trend was found in the pine forest. The greatest differences in soil properties between habitat types were found in the open woodland. Season (drought effect) was the main driver of variability in the pine forest and was related to a loss of microbial nutrients (up to 75 % loss of N_mic and P_mic) and an increase in microbial ratios (C_mic/N_mic, C_mic/P_mic) from Spring to Summer in all sites. Nutrient pools consistently decreased with soil depth, with microbial C, N and P in the top soil being up to 208 %, 215 % and 274 % larger than in the deeper soil respectively.

Conclusions

Similar patterns of variation emerged in relation to season and soil depth across the three forest types whereas the direction and magnitude of the habitat (tree cover) effect was site-dependent, possibly related to the differences in tree species composition and forest structure, and thus in the quality and distribution of the litter input.     

Pinyon pine (Pinus edulis) mortality and response to water addition across a three million year substrate age gradient in northern Arizona, USA

Background and aims

Pinyon pine (Pinus edulis Engelm.) is an important tree species in the western United States that has experienced large-scale mortality during recent severe drought. The influence of soil conditions on pinyon pine response to water availability is poorly understood. We investigated patterns of tree mortality and response of tree water relations and growth to experimental water addition at four sites across a three million year soil-substrate age gradient.

Methods

We measured recent pinyon mortality at four sites, and tree predawn water potential, leaf carbon isotope signature, and branch, leaf, and stem radial growth on 12 watered and unwatered trees at each site. Watered trees recieved fifty percent more than growing season precipitation for 6 years.

Results

Substrate age generally had a greater effect on tree water stress and growth than water additions. Pinyon mortality was higher on intermediate-aged substrates (50–55%) than on young (15%) and old (17%) substrates, and mortality was positively correlated with pinyon abundance prior to drought.

Conclusions

These results suggest high soil resource availability and consequent high stand densities at intermediate-age substrates predisposes trees to drought-induced mortality in semi-arid regions. The response of tree water relations to water addition was consistent with the inverse texture hypothesis; watering reduced tree water stress most in young, coarsely textured soil, likely because water rapidly penetrated deep in the soil profile where it was protected from evapotranspiration.     

Conventional tillage versus cover crops in relation to carbon fixation in Mediterranean olive cultivation

Background and Aims

For croplands, controversy persists concerning the adequacy of the soil use and the management of environmental problems such as soil erosion and fertility in a context of climate change. In this study, we used the RothC model to evaluate the capacity of carbon fixation by the soil in a Mediterranean olive grove for two different scenarios: the land-use change from native vegetation (NV) to conventional tillage (T) in the olive grove, and for the change in soil management from conventional tillage to cover crop (CC).

Methods

In three experimental olive groves in Andalusia (S Spain) two soil-management systems were sampled: T and CC. Areas of NV adjacent to the grove were also sampled as indicative of the initial state of the soil without olive trees. We measured the aboveground biomass production of the cover and the clay content, bulk density, and soil organic carbon (SOC) for 0–5, 5–15 and 15–30 cm depth.

Results

The removal of NV to implement T resulted in a significantly loss of SOC that depended mainly on the relief of the terrain. However, the use of CC increased the SOC because of greater inputs (above-and belowground plant inputs) to the soil. The final concentration at each location was related to the carbon inputs and the clay content. The CC resulted in carbon storage during the first year of 4.02?±?1.65 Mg C ha^?1, and a total carbon fixation by the soil of 5.91?±?2.06 Mg C ha^?1.

Conclusion

The use of cover crops in Mediterranean olive groves proved to be a suitable strategy to increase the carbon storage into the soil and then to decrease the CO₂ concentration in the atmosphere.     

Is plant genetic control of ectomycorrhizal fungal communities an untapped source of stable soil carbon in managed forests?

Background

Ectomycorrhizal (ECM) fungi provide one of the main pathways for carbon (C) to move from trees into soils, where these fungi make significant contributions to microbial biomass and soil respiration.

Scope

ECM fungal species vary significantly in traits that likely influence C sequestration, such that forest C sequestration potential may be driven in part by the existing community composition of ECM fungi. Moreover, accumulating experimental data show that tree genotypes differ in their compatibility with particular ECM fungal species, i.e. mycorrhizal traits of forest trees are heritable. Those traits are genetically correlated with other traits for which tree breeders commonly select, suggesting that selection for traits of interest, such as disease resistance or growth rate, could lead to indirect selection for or against particular mycorrhizal traits of trees in forest plantations.

Conclusions

Altogether, these observations suggest that selection of particular tree genotypes could alter the community composition of symbiotic ECM fungi in managed forests, with cascading effects on soil functioning and soil C sequestration.     

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 125th anniversary of the first postulation of the soil origin of endophytic bacteria – a tribute to M.L.V. Galippe

Aims

In spite of the important role played by herbivory in plant community structure and the fact that it constitutes one of the most important ecological disturbance processes in the dynamics of both natural and anthropic systems, few studies have concentrated on the consequences of browsing on vegetation spatial patterns. The main objective of this study was to examine the role of domestic livestock pressure in the spatial distribution pattern and interspecific competition of two sand dune shrubs: Retama monosperma and the endangered species Thymus carnosus.

Methods

We compared three areas featuring different intensities of livestock pressure. Two 25 × 25 m plots were established in each study area, and the crown heights, diameters and x, y coordinates of every individual in each plot were recorded.

Results

Livestock activity produced a considerable effect, not only on plant cover and size (larger and higher crowns of both species in non herbivory plots) but also on spatial pattern. Whereas light herbivory lead to a random spatial pattern for both species, high herbivory resulted in a repulsion one.

Conclusions

Under high and no livestock pressure, T. carnosus is displaced by R. monosperma, creating a repulsion point pattern distribution between the two species. However, when livestock disturbance was low, our data reveal relatively higher T. carnosus cover values than in the other livestock pressure scenarios, along with a random distribution pattern.     

The relationship between the diversity of arbuscular mycorrhizal fungi and grazing in a meadow steppe

Aims

To study the relationship between changes in soil properties and plant community characters produced by grazing in a meadow steppe grassland and the composition and diversity of spore-producing arbuscular mycorrhizal fungi (AMF).

Methods

A field survey was carried out in a meadow steppe area with a gradient of grazing pressures (a site with four grazing intensities and a reserve closed to grazing). The AMF community composition (characterized by spore abundance) and diversity, the vegetation characters and soil properties were measured, and root colonization by AMF was assessed.

Results

AMF diversity (richness and evenness) was higher under light to moderate grazing pressure and declined under intense grazing pressures. Results of multiple regressions indicated that soil electrical conductivity was highly associated with AMF diversity. The variation in AMF diversity was partially associated to the density of tillers of the dominant grass (Leymus chinensis), the above and below-ground biomass and the richness of the plant community.

Conclusions

We propose that the relationship between plants and AMF is altered by environmental stress (salinity) which is in turn influenced by animal grazing. Direct and indirect interactions between vegetation, soil properties, and AMF community need to be elucidated to improve our ability to manage these communities.