Carbon and nutrients loos in aboveground biomass along a fire induced forest-savanna gradient in the Gran Sabana,southern Venezuela

Forest degradation and savannization are critical environmental issues associated with forest fires in the Gran Sabana, southern Venezuela. Yet little is known about the ecological consequences resulting from the conversion of forest to savanna in this region. In this study we quantified the change in C and nutrients in aboveground biomass along a fire induced gradient consisting of unburned tall primary forest (TF), slightly fire-affected medium forest (MF), strongly fire-affected low forest (LF) and savanna (S). Total aboveground biomass (TAGB) decreased from 411 Mg ha⁻¹ in TF to 313 Mg ha⁻¹ in MF, 13 Mg ha⁻¹ in LF and 5 Mg ha⁻¹ in S. The pools of C and nutrients in TAGB decreased 13–25% from TF to MF, 88–97% from TF to LF and 97–98% from TF to S. In TF and MF, about 40% of C and over 80% of base cations (Ca, K and Mg) was stored in TAGB, whereas the bulk of N and P were stored in the soil (90% of N and 72% of P). This distribution of elements was different in LF and S, where about 50% of base cations were stored in TAGB, and more than 94% of C, 98% of N and 87% of P were stored in the mineral soil. The large amount of elements stored in the biomass of the tall unburned forest demonstrates the high sensitivity of this ecosystem to fire. The change from tall forest to low forest and savanna implies large losses of C and nutrients stored in aboveground biomass and soils (namely 390–399 Mg C ha⁻¹, 11–13 Mg N ha⁻¹, 70–72 kg P ha⁻¹, 783–818 kg K ha⁻¹, 736–889 kg Ca ha⁻¹, and 200–225 kg Mg ha⁻¹). Such drain of C and nutrients in soils extremely low in silicates, which can replenish the lost nutrients by weathering reduces the recuperation chance of these ecosystems and therefore their future capacity to sequester C and accumulate nutrients.     

Tree encroachment into savannas alters soil microbiological and chemical properties facilitating forest expansion

Forests have been expanding over typical savanna sites for the past 3000 years in the Neotropics.Such invasion can produce a series of environmental modifications on typical savanna;however,it remains unclear how modifications in soil properties,caused by the encroachment of woody species,facilitate the expansion of forest ecosystems under dystrophic conditions.Here we examined chemical and microbiological changes associated with tree encroachment in oxisols of a Neotropical Savanna at Assis Ecological Station,Southeastern Brazil.We predicted that tree encroachment caused by typical forest species would cause significant changes in the chemical and microbiological properties of savanna soils.Soils were sampled at Assis Ecological Station,from savanna sites differing in tree encroachment(typical,dense and forested savanna) caused by decades of fire exclusion.We analysed vegetation leaf area index and leaf litter volume deposited in the studied plots and chemical(pH,organic matter,P,K,Ca,Mg,Al,NO_3~-,NH_4~+) and microbiological(microbial C biomass and dehydrogenase activity) properties of soils under distinct encroachment conditions.Most soil chemical properties did not change along the tree encroachment gradient;however,total P,soil organic matter,soil microbial C and dehydrogenase activity increased from typical savanna to forested savanna.The changes in soil organic matter and dehydrogenase activity were correlated with the values of leaf area index and litter volume along the encroachment gradient.Our results demonstrate that forest species can increase carbon and phosphorus supplies in tropical savanna soils.     

Ground floor vegetation assessment within the intensive (Level II) monitoring of forest ecosystems in Germany: chances and challenges

As a part of the ‘Intensive Forest Monitoring Programme’ of ICP Forests, ground floor vegetation has been surveyed along with parameters of other relevant components of the forest ecosystems and their environment at 80 permanent plots all over Germany. Its floristic composition and their changes can therefore be linked to a wide variety of potentially influencing factors, scrutinising recent hypotheses on floristic changes, mainly soil eutrophication and acidification due to air pollutants. Results of a broad-scaled feasibility study are presented and critically discussed with regard to future in-depth evaluations. After an overview on the most abundant species, the syntaxonomic allocation of the plots is given. An ordination reveals a gradient from nutrient and base rich soils to poor acidic soils. Floristic dynamics are mainly aligned with the main axis, but conclusions about the medium-term development cannot be stated yet. Ordination and subsequent statistics are recommended to open up a wide field for explorative investigations. Indicator values for soil acidity and nutrient supply corroborate the main floristic gradient. Based on an empirical species–area relationship (SAR), species numbers for a common plot size of 400 m² were calculated. Basic relationships among different diversity measures and between diversity measures and basic stand and site-related parameters were elaborated. Recommendations focus on enhancements of the assessment of ground floor vegetation within the context of the Level II monitoring like annual sampling or harmonisation of the plot sizes.     

Forest vegetation of Xishuangbanna, south China

1 Introduction The tropical area of southern China is climatically and biogeographically located at the northern edge of tropical Asia, including southeastern Xizang (Tibet, lower valleys of the southern Himalayas), southern Yunnan, southwestern Guangxi, southern Taiwan and Hainan Island. The largest tropical area still covered by forests is in southern Yunnan. Tropical forests of southern Yunnan were little known until the late 1950s because of poor access except for some brief descrip-…     

Edge effects on soil seed banks and understory vegetation in subtropical and tropical forests in Yunnan,SW China

Human-induced forest edges are common in many forest landscapes throughout the world. Forest management requires an understanding of their ecological consequences. This study addressed the responses of three ecological groups (non-forest species, secondary forest species and primary forest species) in edge soil seed banks and edge understory vegetation, and explored the relationship between the invasion of non-forest species in edge understory vegetation and the accumulation of their seeds in edge soil seed banks. The soil seed banks and understory vegetation were sampled along transects established at the edges of a continuous subtropical evergreen broad-leaved forest tract (Lithocarpus xylocarpus forest) bordering anthropogenic grasslands and three tropical seasonal rain forest fragments (Shorea wantianshuea forest) bordering fallows. Species composition in both soil seed banks and understory vegetation showed great difference among edge sites. In soil seed banks, the dominance (relative abundance and relative richness) of each ecological group did not change significantly along the edge to interior gradient. In understory vegetation, the invasion of non-forest species concentrated on the first several meters along the edge to interior gradient. The dominance of secondary forest species decreased with distance from the edge, while the dominance of primary forest species increased with distance from the edge. In forest edge zones, the invasion of a majority of non-forest species in understory vegetation lags behind the accumulation of their seeds in soil seed banks. Forest edges do not act as a good barrier for the penetration of non-forest species seeds. The lack of non-forest species in understory vegetation must then be due to conditions that are not appropriate for their establishment. Therefore, to prevent germination and survival of non-forest species further into the forest, management should focus on maintaining interior forest conditions.     

自然因子对中国森林土壤碳储量的影响分析

文中分析了森林植被、土壤属性、立地条件、气候条件以及凋落物和根系输入等自然因素对中国森林土壤碳储量的影响。森林植物种类组成决定了进入土壤的植物残体量和分解速率,导致土壤有机碳的含量及分布有很大差异。随着林龄的增加,土壤碳储量会呈现增加或产生波动2种情况。土壤理化性质影响土壤有机碳的含量,而土壤碳储量又影响着土壤结构、根系深度、土层特性、有效水分保持能力、土壤生物多样性等; 海拔、坡度、坡向、坡位等立地条件对森林土壤有机碳储量的影响各不相同; 温度、水分、CO₂浓度等气候因子在森林土壤有机碳的蓄积过程中起着至关重要的作用; 凋落物和根系对土壤的输入也可以改变土壤碳库。     

Microbial properties and litter and soil nutrients after two prescribed fires in developing savannas in an upland Missouri Ozark Forest

On some landscapes periodic fire may be necessary to develop and maintain oak-dominated savannas. We studied the effects of two annual prescribed burns to determine their effect on microbial activity and soil and litter nutrients 1 year after the last burn. Surface litter and soil from the upper 0–5 cm soil layer in three developing savannas (oak-hickory, Quercus-Carya), oak-hickory-pine (Quercus-Carya-Pinus), and pine (Pinus) were collected one year after the second of two annual prescribed burns. Surface litter was analyzed for nutrients and soil was analyzed for phospholipid fatty acids (PLFAs) and nutrients. Surface litter chemistry differed across the three savannas for potassium (K) and boron (B), being significantly (P < 0.05) higher for unburned forest than for burned forest. Among savannas, only sulfur (S) was higher for the pine savanna and B for the oak-hickory savanna, both were higher for unburned forest than for burned forest. For soil, calcium (Ca) and B differed across savannas, being higher for burned forest than for unburned forest. Among savannas, soil pH, Ca, and B concentrations were higher in soil from burned forest than from unburned forest. Total PLFA differed among savannas, but was not affected by burning treatments. However, the amounts of biomarkers for Gram-positive and Gram-negative bacteria were higher while the amount of biomarker for fungal PLFA was lower for burned forest than for unburned forest. Our results indicate that the two annual prescribed burns moderately affected PLFA microbial community structure and litter and soil nutrient concentrations. However, the long-term effects of fire on these study sites are not known and merit further study.     

Indices of soil nitrogen availability for an ecological site classification of British forests

An adequate supply of nitrogen (N) is essential for the successful establishment and sustainable productivity of forest stands. N deficits may necessitate the use of artificial fertilisers. Availability of N in the inorganic forms, and the relative abundance of the NO₃-N and NH₄-N components, influences the species composition of natural forest vegetation. Hence it is essential to use reliable measures of soil N supply that fully reflect its ecological significance. The new Ecological Site Classification (ESC) used in British forestry employs a multi-factorial definition of soil nutrient regime (SNR), including soil N. To develop this, a soil and vegetation study was made at 89 forest sites throughout Great Britain covering the major soil types used for forestry. “Total N” levels were compared with separate pre- and post-incubation measures of the two inorganic N components as potential indices of soil N supply. Multivariate statistical analysis showed that the major discriminant chemical variables for the sampled soils were pH, calcium and NO₃-N and that these were also the main variables influencing the species composition of the ground vegetation. Total N and NH₄-N were less effective discriminant variables for these sites. In some infertile soils the levels of NH₄-N or total N may be of greater importance, as NO₃-N is usually in very limited supply. A multivariate gradient of SNR, which incorporates the NO₃-N measures, has been adopted for use within the ESC system. The position of a site on this gradient can be estimated quantitatively from soil type, ground vegetation species composition and humus type. This enables soil N supply and overall SNR to be assessed in a simple but effective way that guides the operational management of British forest soils for sustainable productivity. It will also be possible to use these techniques to monitor the nutritional status of forest sites over time.     

Effect of recurrent fires on soil nutrient dynamics in a tropical dry deciduous forest of Western Ghats,India

The role of forest fires in the soil dynamics and global carbon cycle has not been comprehensively studied in tropical forests as the effects of fire on tropical forest soils can be extremely variable. This study was aimed to understand how repeated fires affect physical and chemical properties of soil in a tropical dry deciduous forest and alter soil fertility and health. The study was carried out in the dry deciduous forest of Mudumalai Tiger Reserve. Soil samples were collected from unburned (B0) to six-time burned (B6) plots. Samples were collected from each plot from three different depths viz. 0–10 (Top), 10–20 (Middle), and 20–30 cm (Bottom) and analyzed for soil physical and chemical properties. Soil pH, EC, WHC decreased with increasing fire frequencies while bulk density increased. Organic Carbon, Total N, and available P decreased with increasing fire frequencies whereas extractable K initially increased but decreased with the very high frequency of fires. NO₃^?N slightly decreased with high fire frequencies but NH₄^?N decreased significantly with increasing fire frequency. These results provide a new insight regarding the influence of repeated fires on soil that will be valuable to understand the effect of fire on the recovery of soils and nutrient dynamics.     

Change in soil macrofauna and vegetation when fast-growing trees are planted on savanna soils

Many forest species can be found in understory vegetation of old plantation plots, despite the fact that the native vegetation was a poor savanna growing on highly nonfertile sandy soils. The aim of the present paper is to describe the changes that occur in the environmental conditions when savanna is planted with fast-growing trees, and is particularly concerned with vegetation and soil macrofauna. The study was carried out in industrial eucalyptus plantations, and in experimental Acacia and pine plantations. Most plots were located on sandy soil, but some measurements were also carried out on clay soil planted with the same species in order to assess the influence of soil type.

A strong correlation was shown between the age of the eucalyptus trees and the percentage of forest species in undergrowth, emphasizing the progressive change from savanna vegetation towards forest vegetation.

Biomass and density of macrofauna were very low in both sandy and clayey savanna soils, total biomass being 3.3 and 5.8 g/m² respectively. Soil macrofauna became more important as the age of plantations increased, and biomass reached 29 g/m² in the 20-year-old eucalyptus plot on sandy soil, and 74 g/m² in 26-year-old eucalyptus plantation on clay soil, compared to 33 g/m² in the natural forest plot on sandy soil; however, frequency of occurrence and number of taxa were lower in old eucalyptus plot as compared to forest. Large differences in the abundance of macrofauna were observed in relation to planted species. Acacia was most favourable to soil macrofauna, with a total biomass of 60 g/m² on sandy soil and many taxa present. Pine plantations had a poor macrofauna and several taxa were lacking, particularly in the sandy soil.

Total macrofauna frequency was significantly correlated with the percentage of forest species in understory vegetation. Both were correlated with soil pH and soil organic-matter content. The results suggest that soil organic matter and litter quality are of main importance in changing the above- and below-ground habitat in plantations.     

Seasonal variations in phosphorus fractions in semiarid sandy soils under different vegetation types

We investigated the seasonal patterns of soil phosphorus (P) fractions under five vegetation types – Ulmus macrocarpa savanna, grassland, Pinus sylvestris var. mongolica plantation, Pinus tabulaeformis plantation, and Populus simonii plantation – in the southeastern Keerqin Sandy Lands of China. The measured P fractions (0–20 cm depth) included: soil total P (TP), total organic and inorganic P (TPo and TPi), bicarbonate extractable organic and inorganic P (BPo and BPi), microbial biomass P (MBP), and in situ resin-adsorbed P (resin-P). Soil TP and TPo concentrations in the savanna and grassland were significantly lower in summer than in spring and autumn. However, they were relatively stable in three forest plantations. Soil labile P fractions showed a significant seasonal pattern under all vegetation types with the peak in summer, except soil MBP that was constant in the savanna and grassland and BPo that decreased over time in the savanna. This pattern of labile P fractions was attributed to a combination of seasonal climatic changes, low P availability, as well as the biological controls of soil P transformation in the study area. Litter decomposition played a key role in soil P availability. The monthly resin-P released from litter decomposition in summer was 2.6–7.4 times greater than in other seasons, and was 1.7–3.4 times of that in the 10 cm depth soil. Concentrations of soil P fractions were obviously affected by vegetation type. The savanna had the highest total P and MBP concentrations and the P. tabulaeformis plantation had the highest BPi and resin-P among all vegetation types. Among forest plantations, P. simonii plantation had the highest total P and MBP. These results suggest that U. macrocarpa savanna is the best system conserving soil nutrient (particularly P) stocks and microbial activity, followed by the grassland and P. simonii plantation, while the pine plantations are the worst.     

深圳丘陵地带不同植被类型森林土壤有机碳组分和团聚体分布特征研究

研究森林土壤有机碳组分和团聚体分布特征,对摸清森林土壤结构形成及其碳稳定机制有重要科学意义。本研究以深圳市丘陵地带针叶林、阔叶人工林和次生阔叶林等3种不同植被类型、70个调查点森林土壤为研究对象,各调查点按0~10 cm和＞10~30 cm剖面进行采样,对土壤有机碳组分和团聚体含量进行分析。结果表明,3种植被类型表层土壤（0~10 cm）的有机质（OM）和全氮（TN）含量存在显著差异（P＜0.05）,而亚表层土壤（＞10~30 cm）养分间的差异均未达到显著水平。不同植被类型土壤各有机碳组分均存在差异,表层土壤有机碳组分均高于亚表层,且以活性有机碳含量最高。此外,不同植被类型表层土壤间的差异主要体现在微团聚体上（＜0.25 mm）,亚表层土壤则主要体现在微团聚体和1~2 mm团聚体上。除表层土壤电导率（EC）与惰性有机碳间的相关性外,两层土壤的EC、OM和TN含量与4种有机碳组分均呈极显著正相关（P＜0.001）;＞10 mm团聚体对有机碳矿化有显著正向调控,＞2~5 mm团聚体则表现为显著负影响。由此认为,3种植被类型间土壤养分和有机碳组分含量存在差异,且表层土壤的含量总是高于亚表层;不同土壤团聚体间的差异主要体现在微团聚体上;土壤养分含量是调节有机碳矿化的关键因子。     

Chemical properties of forest soils along a fly-ash deposition gradient in eastern Germany

Chemical characteristics of forest soils subjected to long-term deposition of alkaline and acid air pollutants were analysed in spruce (Picea abies (L.) Karst.) stands in eastern Germany. Three forest sites along an emission gradient of 3, 6, and 15 km downwind of a coal-fired power plant were selected, representing high, intermediate, and low fly-ash input rates. Past emissions caused an accumulation of mineral fly-ash constituents in the organic layer, resulting in an atypically high mass of organic horizons of forest soils, especially in the F and H horizons. Total mass of organic layers at the site with heavy deposition loads was as high as 128 t ha^–1, compared to 58 t ha^–1 at the low input site. Fly-ash deposition significantly increased the pH values in the L, F and H horizons and mineral topsoil (0–10 cm). Significantly higher concentrations of NH₄Cl-extractable cations (i.e. effective cation exchange capacities) and base saturations of >66% were found in the humic horizons at sites where the pH was increased due to the direct and indirect (i.e. higher proportions of deciduous trees) effects of fly-ash emissions. Stocks of basic cations were dominated by Ca²⁺ and decreased significantly along the fly-ash deposition gradient from 33.6 to 5.3 kmol_c ha^–1. Proportions of water-soluble basic cations out of the total potentially exchangeable (i.e. NH₄Cl-extractable) basic cations generally increased in the forest soil with decreasing deposition loads following the cation exchange capacity and base saturation along the fly-ash gradient. Higher proportions of monovalent cations, such as K⁺ and Na⁺, were observed in the water extracts from fly-ash-affected forest soils, while the NH₄Cl-extracts were dominated by bivalent cations, such as Ca²⁺ and Mg²⁺. These results suggest a greater leaching tendency for monovalent cations in these soils. Stocks of organic C and total N in the humus layer decreased from sites with high fly-ash deposition levels to sites with low levels, from 57.4 to 46.4 t C ha^–1 and from 2.43 to 1.99 t N ha^–1. The C/N ratios of the organic horizons varied from 22 to 25, revealing no distinct pattern along the fly-ash gradient. Measurements of hot-water-extractable and water-soluble organic C suggested a reduced availability or a faster decomposition of soil organic matter in soils with historically high fly-ash loads.     

Canopy cover and groundlayer vegetation dynamics in a fire managed eastern sand savanna

Savanna vegetation is characterized by high and variable ground layer species richness regulated by functional group interactions with fire regimes and canopy cover. Frequent fire selects for C4 grasses and prairie forbs in canopy openings and C3 graminoid species and shade-adapted forbs and shrubs in canopy shade. Frequent fire also maximizes heterogeneity in partial canopy cover and species richness across the full canopy gradient. However, few studies have linked fire induced change in tree canopy cover with groundlayer vegetation dynamics in relation to this model. In 1986 and in 2007, we measured canopy cover and sampled groundlayer vegetation in 1 m² plots along 53 transects at the Tefft Savanna, a fire managed 197 ha eastern sand savanna with strong canopy cover and elevation gradients. We analyzed temporal change in canopy cover and groundlayer vegetation, correlating percent change in canopy cover with change in ground layer functional groups. After 20 years of burning at 3 fires/decade, elevation accounted for 62% of the variation in an NMS ordination of groundlayer vegetation. However, canopy cover, which averaged 24-86% in 2007, had a significant secondary effect on the ordination. Five vegetation types classified by TWINSPAN varied significantly in elevation and canopy cover. Woody vegetation comprised 8 of the 12 species with greatest niche breadths, and tended to predominant in woodland or forest, where tree cover averaged 50% or more. Forbs had greater richness in savanna, which averaged less than 30% canopy cover. The C3 sedge Carex pensylvanica was the dominant graminoid species under woodland canopy cover, and was co-dominant with the C4 grasses Andropogon scoparius and Sorghastrum nutans under savanna canopy cover. As in other savannas, N-fixing species sorted across shade and canopy openings, and heterogeneity among transects was maximized at mid-canopy cover. Over time, canopy cover decreased up to 50%, creating more open savanna conditions at mid to high elevations. This decrease was associated with a 20-100 % increase in species richness and was significantly correlated with increasing richness and cover of C4 grasses and summer flowering prairie and woodland forbs. These results support a canopy cover model of fire-maintained savanna vegetation, with greater abundance of C4 grasses and prairie forb species associated with lower canopy cover, greater heterogeneity at mid-canopy cover, and species richness maximized across the light gradient. They also indicate that decreasing canopy cover caused by repeated burning increases species richness and abundance of C4 and prairie forb species.     

The influence of trees on soil fertility on two contrasting semi-arid soil types at Matopos,Zimbabwe

An investigation into the influence of indigenous trees on soil fertility was conducted in an area of semi-arid tropical savanna in Zimbabwe on two contrasting soil types: dystrophic savanna soils (sandy soils) and eutrophic savanna soils (fine-textured soils). The study adds further support to the growing literature showing that trees have a positive influence on soil fertility. The study suggests that tree clearance, as advocated in these agropastoral systems, may not necessarily result in long-term benefits. It is argued that the primary mechanism by which soil fertility is improved is through increased litter and soil organic matter compartments under trees. The influence of trees on cation levels is greater on sandy soils than fine-textured soils because the exchange capacity of fine-textured soils is determined largely by soil texture whereas organic matter is the prime determinant of exchange capacity in sandy soils. The present study demonstrates that fertility improvement under trees is not at the expense of fertility decline in the surface soils of the zone around the tree. Leaf quality, as reflected simply in leaf C:N ratio, influences decomposition rates but the activities of termites probably confound any simple relationship. Litter quality of tree species is probably important in determining levels of soil organic matter under canopies, with higher levels under species with lower leaf quality.     

南方林区生物防火林带建设初探

经生物防火林带工程建设的实践和探索。根据生物防火林带的作用机理,模拟湿润亚热带地区顶极植被常绿阔叶林的生物多样性、生活型组成类型和结构的复杂性,结合建设地不同林地的实际情况,提出针叶林地、针阔混交林地、毛竹林地、果园、灌木林地及乔木树种疏林地的生物防火林带建设技术。旨在较短时期(4～6年)内产生阻隔林火效能,减免森林火灾所造成的损失。为人类的生存、经济社会的发展提供资源保障。     

The role of fire and nutrient loss in the genesis of the forest soils of Tasmania and southern New Zealand

The dominant soil patterns in forested or previously forested landscapes in southern New Zealand and Tasmania are described. Soil properties on adjacent sunny and shady aspects in hill country of the South Island of New Zealand are compared to soil properties under adjacent ‘dry’ and ‘wet’ eucalypt forest in Tasmania.

A soil contrast index or SCI is defined for comparing soil contrasts on parent materials of different absolute nutrient contents. Three soil groups are defined using the SCI. Group 1 soil pairs are stable New Zealand soils in which exchangeable Ca + Mg + K values are higher on drier sunny aspects than on moister shady aspects. Group 2 soil pairs are New Zealand soils in which soils on sunny aspects display evidence of topsoil erosion by wind; consequently some soil pairs on dry (sunny) aspects have lower levels of exchangeable Ca + Mg + K than soils on moister (shady) aspects. Group 3 soil pairs are Tasmanian. Soils on drier sites (under dry eucalypt forest) invariably have lower exchangeable Ca + Mg + K values than soils on moister sites (under wet eucalypt forest), which is the reverse of the pattern in SCI Group 1 soils in New Zealand.

Except on clay-rich parent materials, Tasmanian soils under dry forest generally have texture-contrast profiles and a mean C/N ratio in topsoils (A1 horizons) of 29. Soils under wet forest generally have uniform or gradational texture profiles and a mean topsoil C/N ratio of 15. The texture-contrast soils show strong clay eluviation with sand or sandy loam textures in upper horizons and clayey textures in lower horizons. However, in New Zealand texture-contrast soils are all but absent, and do not occur in the previously forested areas described in this paper. Topsoils (Ah horizons and soils sampled to 7.5 cm depth) in New Zealand areas sampled in this study have a mean C/N ratio of 15, regardless of whether they occur on sunny or shady aspects.

We propose that the frequency and spatial occurrence of fire are the dominant processes causing: (1) the marked difference in levels of nutrients and different topsoil C/N ratios in soils of Tasmania; (2) the development of texture-contrast soils under dry forests in Tasmania; and (3) the difference between soil patterns in New Zealand and Tasmania. Fire depletes nutrients in forests by causing losses to the atmosphere, losses by runoff, and losses by leaching. Nutrient loss by fire encourages fire-tolerant vegetation adapted to lower soil nutrient status, so frequent fire is a feedback mechanism that causes progressive soil nutrient depletion. By destroying organic matter and diminishing organic matter supply to the soil surface fire inhibits clay–organic matter linkages and soil faunal mixing and promotes clay eluviation. Fire frequency is likely to have increased markedly with the arrival of humans at ca. 34 000 years B.P. in Tasmania and ca. 800 years B.P. in New Zealand. We argue that texture-contrast soils have not formed in New Zealand because of the short history of frequent fires in that country. A corollary of this conclusion is that texture-contrast soils in Tasmania are, at least in part, anthropogenic in origin.     

Soil carbon distribution and quality in a montane rangeland-forest mosaic in northern Utah

Relatively little is known about soil organic carbon (SOC) dynamics in montane ecosystems of the semi-arid western U.S. or the stability of current SOC pools under future climate change scenarios. We measured the distribution and quality of SOC in a mosaic of rangeland-forest vegetation types that occurs under similar climatic conditions on non-calcareous soils at Utah State University's T.W. Daniel Experimental Forest in northern Utah: the forest types were aspen [Populus tremuloides] and conifer (mixture of fir [Abies lasiocarpa] and spruce [Picea engelmannii]); the rangeland types were sagebrush steppe [Artemisia tridentata], grass-forb meadow, and a meadow-conifer ecotone. Total SOC was calculated from OC concentrations, estimates of bulk density by texture and rock-free soil volume in five pedons. The SOC quality was expressed in terms of leaching potential and decomposability. Amount and aromaticity of water-soluble organic carbon (DOC) was determined by water extraction and specific ultra violet absorbance at 254 nm (SUVA) of leached DOC. Decomposability of SOC and DOC was derived from laboratory incubation of soil samples and water extracts, respectively.

Although there was little difference in total SOC between soils sampled under different vegetation types, vertical distribution, and quality of SOC appeared to be influenced by vegetation. Forest soils had a distinct O horizon and higher SOC concentration in near-surface mineral horizons that declined sharply with depth. Rangeland soils lacked O horizons and SOC concentration declined more gradually. Quality of SOC under rangelands was more uniform with depth and SOC was less soluble and less decomposable (i.e., more stable) than under forests. However, DOC in grass-forb meadow soils was less aromatic and more bioavailable, likely promoting C retention through cycling. The SOC in forest soils was notably more leachable and decomposable, especially near the soil surface, with stability increasing with soil depth. Across the entire dataset, there was a weak inverse relationship between the decomposability and the aromaticity of DOC. Our data indicate that despite similar SOC pools, vegetation type may affect SOC retention capacity under future climate projections by influencing potential SOC losses via leaching and decomposition.     

Microbial and Plant Community Structure Across a Primary Succession Gradient

The formation of the organic layer within Scandinavian forest soil started about 10000 yr ago, following the retreat of the continental ice sheet. Since then the land has been slowly rising in northern Europe and uplift still occurs on the coast of the Bothnian Bay at a rate of about 0.6-0.9 m per 100 yr. Four, 300 m long, successional gradients were studied from the shoreline to a Scots pine (Pinus sylvestris) stand with a fully developed humus layer of a few centimetres' thickness. The plantless shoreline was followed by small foredunes and dunes, characterized by Agrostis stolonifera and Leymus arenarius, respectively, and the deflation basin characterized by lichens and sparse Festuca ovina and Deschampsia flexuosa. The study sites situated in Scots pine stands of about 25 and 40 yr age were characterized by sparse dwarf shrubs, lichens and bryophytes. The amount of organic matter in soil increased along the gradient. When the microbial biomass, estimated as indicative phospholipid fatty acids (PLFAs), was calculated on the organic matter basis, the total microbial biomass as well the amount of bacterial PLFAs decreased from the earlier stages of succession towards the pine forest. The ratio of fungal to bacterial PLFAs increased along the succession. The bacterial community structure in the shore soil was different to that in the dune soil or in forested zones. On the plantless shore the microbial community was almost completely described by PLFAs indicative of Gram-negative bacteria. In addition to these bacterial PLFAs, dunes were characterized by PLFAs indicative of actinomycetes. Thus, the fungal part of the microbial community seemed to respond most to the accumulation of organic matter and increasing C/N ratio, while the bacterial biomass and bacterial community structure seemed also to reflect the composition of the vegetation and the varying quality of the organic matter.