Direct and Indirect Effects of Invasive Plants on Soil Chemistry and Ecosystem Function

Invasive plants have a multitude of impacts on plant communities through their direct and indirect effects on soil chemistry and ecosystem function. For example, plants modify the soil environment through root exudates that affect soil structure, and mobilize and/or chelate nutrients. The long-term impact of litter and root exudates can modify soil nutrient pools, and there is evidence that invasive plant species may alter nutrient cycles differently from native species. The effects of plants on ecosystem biogeochemistry may be caused by differences in leaf tissue nutrient stoichiometry or secondary metabolites, although evidence for the importance of allelochemicals in driving these processes is lacking. Some invasive species may gain a competitive advantage through the release of compounds or combinations of compounds that are unique to the invaded community—the “novel weapons hypothesis.” Invasive plants also can exert profound impact on plant communities indirectly through the herbicides used to control them. Glyphosate, the most widely used herbicide in the world, often is used to help control invasive weeds, and generally is considered to have minimal environmental impacts. Most studies show little to no effect of glyphosate and other herbicides on soil microbial communities. However, herbicide applications can reduce or promote rhizobium nodulation and mycorrhiza formation. Herbicide drift can affect the growth of non-target plants, and glyphosate and other herbicides can impact significantly the secondary chemistry of plants at sublethal doses. In summary, the literature indicates that invasive species can alter the biogeochemistry of ecosystems, that secondary metabolites released by invasive species may play important roles in soil chemistry as well as plant-plant and plant-microbe interactions, and that the herbicides used to control invasive species can impact plant chemistry and ecosystems in ways that have yet to be fully explored.     

Effect of conversion of natural grassland to cropland on nitric oxide emissions from Venezuelan savanna soils. A four-year monitoring study

Conversion of “natural” savanna to agricultural soil is proceeding at a very fast rate. In this work, nitric oxide (NO) emissions were measured immediately after conversion of a “natural” grassland to fertilized agricultural fields (corn, sorghum and cultivated pasture), and also during the second and fourth year after conversion. Large fluxes were observed after plowing grassland soil, and almost no difference was observed between ammonium-fertilized and unfertilized plowed grassland soils. Soil water content and pH related negatively to NO emissions. NO emissions were positively correlated with soil nitrate concentrations and carbon content (corn only) whereas little or no correlation was found with ammonium concentrations. This suggests that in these savanna soils, the NO emitted from soil is mainly denitrification-derived. The disturbed soils emitted on average ∼7 times more NO than the original grassland. During the second and fourth year of cultivation the emissions from corn and sorghum fields (plowed every year) were around 10 times higher than from the control grassland soils. The fertilizer-induced emission (FEI) values for NO emission estimated in this work show that similar agricultural practices could lead to dramatically different FEI values depending on the water content of the soils. These FEI results corroborates that modeling approaches to determine the global agriculture-derived NO emissions should take into consideration the main factors that regulate the NO emission at the scale of functional units with similar climate, soil and management conditions instead of expressing it as a percentage of the fertilizer applied. In our sandy loam savanna soils these factors are inorganic nitrogen content, pH and WFPS.     

土壤微生物多样性研究的新进展

土壤微生物是土壤生态系统的一个重要组成部分,在维持整个土壤生态系统能量流动和物质循环中发挥着关键作用,同时,土壤微生物多样性影响土壤生态系统的结构、功能和过程。因此,对土壤微生物多样性的研究有着重大意义。本文从土壤微生物多样性的影响因素以及目前该领域研究所采用的研究方法等方面阐述了国内外土壤微生物多样性的研究现状,同时对各种研究方法的适用范围、优缺点等作了较为详细的分析、比较,并对该领域未来研究工作的重点方面作了展望。     

Bulk soil and particle size-associated C and N under grazed and ungrazed regimes in Mountainous arid and semi-arid rangelands

Sheep grazing is known to influence soil organic carbon (SOC) storage and dynamics in rangelands. However, very little is known of grazing impacts on measurable SOC pools associated with primary particles, particulate organic matter (POM) and carbohydrates in the heavily grazed rangelands of Zagros Mountains, Iran. Bulk SOC, total nitrogen (N), POM and hot-water extractable carbohydrates (HWC) as well as different SOC and N fractions in particle-size separates were studied in natural mountainous rangelands of arid (Boroujen with 255?mm annual rainfall) and semi-arid (Sabzkouh with 860?mm annual rainfall) sites, Central Zagros. Two sheep grazing regimes including grazed and ungrazed (for 20?C25?years) rangelands with four replicates were identified as the grazing treatments at each site. Soil samples (0?C15?cm) were taken and analyzed for bulk SOC, total N, POM, HWC, and SOC and N associated with physical fractions. Bulk SOC contents were similar for both ungrazed and grazed regimes, while total N contents significantly decreased under grazed conditions. Bulk soil POM and HWC contents decreased considerably and clearly by sheep grazing, indicating that these fractions of total soil organic matter (SOM) pool may be suitable indicators for detecting the grazing effects on bulk SOC changes and storage in these arid and semi-arid ecosystems. Semi-arid rangeland sites contained more bulk SOC, total N and POM contents than arid rangeland sites. These differences were primarily due to the large differences in vegetation composition, annual rainfall and soil conditions between the two rangelands. After 20?C25?years of grazing over 10?C33?% of SOC and total N losses occurred in the sand- and clay-size fractions with 10?% increases in the silt fraction. This means sheep grazing increases the contribution of the silt fraction to bulk soil N. We found evidence that sheep grazing decreases soil POM and HWC pools, and the sand fraction C, suggesting a lower recent annual input of decomposable organic C in heavily grazed rangelands. Sheep grazing had no influence on the potential C mineralization of the bulk soil at the semi-arid site (Sabzkouh), but reduced C mineralization at the arid-site (Boroujen), indicating that sheep grazing may affect SOC dynamics by changes in substrate quality at the former, but by substrate quantity at the later. In brief, long-term sheep grazing can potentially lead to losses of both labile and no-labile SOM in these arid and semi-arid rangelands.     

Grazing intensity effects on soil nitrogen mineralization in semi-arid grassland on the Loess Plateau of northern China

Soil nitrogen transformation has been the subject of growing attention in many semi-arid grassland ecosystems. In our study, we employed an intact soil core in situ incubation technique and measured seasonal changes in soil net nitrogen mineralization and nitrification rates. The measurements were taken from the upper 0–10 cm soil layer of a permanent grassland during a growing season in a 8.5-year field experiment on the Loess Plateau, China that had four grazing intensities (0, 2.7, 5.3 and 8.7 sheep ha⁻¹). Our results demonstrate marked seasonal variations in inorganic nitrogen pools, net nitrogen mineralization and net nitrification. The rates of mineralization and nitrification were highest in August and lowest in September. No consistent differences in monthly net nitrogen mineralization and monthly nitrification rates were observed among the different grazing intensities. Sheep grazing stimulated nitrogen transformation, and the most stimulation occurred at a heavy grazing intensity of 8.7 sheep ha⁻¹. The mean soil net nitrification rate was positively correlated with the soil C/N ratio and pH. The mean N mineralization rate was negatively correlated with soil organic carbon, but was positively correlated with the soil C/N ratio. Our study demonstrated net nitrogen mineralization and nitrification rates were strongly linked to grazing intensity, soil temperature and moisture content.     

磷脂脂肪酸分析在湖泊沉积物微生物生态学研究中的应用

磷脂脂肪酸分析(PLFA)技术是近年来出现的一种新的微生物生态学研究方法,它是利用微生物细胞内磷脂脂肪酸的成分和含量相对稳定的原理,通过对环境样品中的磷脂脂肪酸进行提取和分析,以获得微生物生物量和群落结构变化的信息,已被应用于土壤、水体和植物根系等环境中微生物生态学的研究.综述了PLFA技术在湖泊沉积物微生物生态学研究中的应用,同时指出了PLFA技术存在的问题及其发展前景.     

不同侵蚀部位微生物变异性研究

土壤是一个由生物和非生物组成的复杂综合体,肥力是土壤的本质和属性。土壤养分含量和土壤微生物特征是表示土壤肥力高低的重要指标,水土流失不仅导致土壤养分含量下降,而且对土壤微生物特征产生明显的影响,进而影响到土壤养分的循环和转化。本研究对华南赤红壤丘陵坡地不同侵蚀部位微生物特性的变异性进行了对比分析。结果表明：随着土壤侵蚀程度的加剧和植被的破坏,土壤微生物总数逐渐减少,土壤基础呼吸和土壤诱导呼吸显著下降。在没有草本植物的情况下,仅有人工荷木并不能起到明显的水土保持作用,近地表覆盖的草本植物应是水土保持植被建设必不可少植物类群。以此为华南赤红壤丘陵区土壤生态环境修复和水土保持提供科学依据。     

Phenolic Responses of Mountain Crowberry (Empetrum nigrum ssp. hermaphroditum) to Global Climate Change are Compound Specific and Depend on Grazing by Reindeer (Rangifer tarandus)

Mountain crowberry (Empetrum nigrum ssp. hermaphroditum) is a keystone species in northern ecosystems and exerts important ecosystem-level effects through high concentrations of phenolic metabolites. It has not been investigated how crowberry phenolics will respond to global climate change. In the tundra, grazing by reindeer (Rangifer tarandus) affects vegetation and soil nutrient availability, but almost nothing is known about the interactions between grazing and global climate change on plant phenolics. We performed a factorial warming and fertilization experiment in a tundra ecosystem under light grazing and heavy grazing and analyzed individual foliar phenolics and crowberry abundance. Crowberry was more abundant under light grazing than heavy grazing. Although phenolic concentrations did not differ between grazing intensities, responses of crowberry abundance and phenolic concentrations to warming varied significantly depending on grazing intensity. Under light grazing, warming increased crowberry abundance and the concentration of stilbenes, but decreased e.g., the concentrations of flavonols, condensed tannins, and batatasin-III, resulting in no change in total phenolics. Under heavy grazing, warming did not affect crowberry abundance, and induced a weak but consistent decrease among the different phenolic compound groups, resulting in a net decrease in total phenolics. Our results show that the different phenolic compound groups may show varying or even opposing responses to warming in the tundra at different levels of grazing intensity. Even when plant phenolic concentrations do not directly respond to grazing, grazers may have a key control over plant responses to changes in the abiotic environment, reflecting multiple adaptive purposes of plant phenolics and complex interactions between the biotic and the abiotic factors.     

Bioactive Molecules in Soil Ecosystems: Masters of the Underground

Complex biological and ecological processes occur in the rhizosphere through ecosystem-level interactions between roots, microorganisms and soil fauna. Over the past decade, studies of the rhizosphere have revealed that when roots, microorganisms and soil fauna physically contact one another, bioactive molecular exchanges often mediate these interactions as intercellular signal, which prepare the partners for successful interactions. Despite the importance of bioactive molecules in sustainable agriculture, little is known of their numerous functions, and improving plant health and productivity by altering ecological processes remains difficult. In this review, we describe the major bioactive molecules present in below-ground ecosystems (i.e., flavonoids, exopolysaccharides, antibiotics and quorum-sensing signals), and we discuss how these molecules affect microbial communities, nutrient availability and plant defense responses.     

Nitrogen pools and fluxes in grassland soils sequestering carbon

Carbon sequestration in agricultural, forest, and grassland soils has been promoted as a means by which substantial amounts of CO₂ may be removed from the atmosphere, but few studies have evaluated the associated impacts on changes in soil N or net global warming potential (GWP). The purpose of this research was to (1) review the literature to examine how changes in grassland management that affect soil C also impact soil N, (2) assess the impact of different types of grassland management on changes in soil N and rates of change, and (3) evaluate changes in N₂O fluxes from differently managed grassland ecosystems to assess net impacts on GWP. Soil C and N stocks either both increased or both decreased for most studies. Soil C and N sequestration were tightly linked, resulting in little change in C:N ratios with changes in management. Within grazing treatments N₂O made a minor contribution to GWP (0.1–4%), but increases in N₂O fluxes offset significant portions of C sequestration gains due to fertilization (10–125%) and conversion (average = 27%). Results from this work demonstrate that even when improved management practices result in considerable rates of C and N sequestration, changes in N₂O fluxes can offset a substantial portion of gains by C sequestration. Even for cases in which C sequestration rates are not entirely offset by increases in N₂O fluxes, small increases in N₂O fluxes can substantially reduce C sequestration benefits. Conversely, reduction of N₂O fluxes in grassland soils brought about by changes in management represents an opportunity to reduce the contribution of grasslands to net greenhouse gas forcing.     

Multiple benefits of manure: The key to maintenance of soil fertility and restoration of depleted sandy soils on African smallholder farms

Manure is a key nutrient resource on smallholder farms in the tropics, especially on poorly buffered sandy soils, due to its multiple benefits for soil fertility. Farmers preferentially apply manure to fields closest to homesteads (homefields), which are more fertile than fields further away (outfields). A three-year experiment was established on homefields and outfields on sandy and clayey soils to assess the effects of mineral nitrogen (N) fertilizer application in combination with manure or mineral phosphorus (P) on maize yields and soil chemical properties. Significant maize responses to application of N and manure were observed on all fields except the depleted sandy outfield. Large amounts of manure (17 t ha⁻¹ year⁻¹) were required to significantly increase soil organic carbon (SOC), pH, available P, and base saturation, and restore productivity of the depleted sandy outfield. Sole N as ammonium nitrate (100 kg N ha⁻¹) or in combination with single superphosphate led to acidification of the sandy soils, with a decrease of up to 0.8 pH units after three seasons. In a greenhouse experiment, N and calcium (Ca) were identified as deficient in the sandy homefield, while N, P, Ca, and zinc (Zn) were deficient or low on the sandy outfield. The deficiencies of Ca and Zn were alleviated by the addition of manure. This study highlights the essential role of manure in sustaining and replenishing soil fertility on smallholder farms through its multiple effects, although it should be used in combination with N mineral fertilizers due to its low capacity to supply N.     

Phosphorus and potassium cycling in a long-term no-till integrated soybean-beef cattle production system under different grazing intensities insubtropics

Long-term integrated crop-livestock system enables constant and more efficient nutrient cycling because animal, pasture and crop residues release nutrients at different rates. Therefore, appropriate management of these systems is needed to maximize benefits from nutrient cycling. The objective of this study was to evaluate how grazing intensity affected the release rates of phosphorus (P) and potassium (K) in pasture, dung and soybean residues in a no-till long-term integrated crop-livestock system. The experiment was established in 2001 on a clayey Oxisol after soybean harvest. Treatments consisted of pasture with sward heights maintained at 1020, 30 and 40 cm by different cattle stocking rates and a non-grazed (NG) treatment. Decomposition and release rates of nutrients in the pasture and dung were determined using litter bags, which were installed at soybean seeding and pasture seeding during two pasture-crop cycles (2009–2011). Lighter grazing intensities resulted in greater P release rate from pasture and dung residues. Pasture and dung residues released K at a very high rate and were not influenced by grazing intensity. The P and K released from soybean residue were not affected by grazing intensity; however, decomposition of soybean leaves was greater than of stems. Greatest rates of total P and K released were from pasture and dung residues under lighter grazing intensities and in the NG areas. Large amounts of P (~25 kg ha^?1) and K (~130–180 kg ha^?1) were cycled in a complete soybean-beef cattle integrated system and must be considered in the fertilization management.     

Lipid Composition Differences of Periphyton,Crustaceans, and Small Fishes in Response to Eutrophication and Management in the Florida Everglades,USA

Eutrophication of the Florida Everglades, USA, has altered the characteristics of the ecosystem, but management strategies are being implemented to accelerate recovery. In this study, we described lipid compositional similarities and differences between periphyton, fish, and crustaceans, and explored if eutrophication and creation of new open-water sloughs in phosphorus (P)-impacted regions of a Northern Everglades impoundment resulted in changes in periphyton biomass and lipid composition, and the lipid composition of a ubiquitous omnivore, Gambusia holbrooki. Lipid biomarker analysis provided insight into microbial community composition, quality of basal resources, and potential resources utilized by consumers. Periphyton biomass and phospholipid fatty acid (PLFA) composition differed in response to eutrophication, but not between P-impacted control and treatment plots. Shifts in relative abundances of lipids indicative of diatoms and green algae mirrored known taxonomic shifts due to eutrophication. For fauna, PLFA were a small and relatively distinct component of the overall total lipid make-up, and profiles were similar between control and treatment plots. However, the PLFA profile of G. holbrooki differed between oligotrophic and eutrophic regions. Fish and crustacean lipids contained significantly greater relative abundances of polyunsaturated fatty acids than were found in periphyton, and profiles differed between fish and crustaceans, suggesting organisms were selectively accumulating or elongating and desaturating lipids de novo, to meet physiological needs. This study builds on findings of microbial responses to eutrophication and recent observations that consumer PLFA profiles can also shift with P-enrichment.     

Comparison of net ecosystem CO2 exchange in cropland and grassland with an automated closed chamber system

Field measurements of net ecosystem CO₂ exchange (NEE) with high temporal resolution are essential to construct a meaningful ecosystem C balance. The objectives of this study were to monitor NEE in high temporal resolution in cropland and grassland between middle August and middle November (2006) at Kleinhohenheim, Germany and to evaluate NEE in autumn. A fully automated temperature controlled closed chamber system with an infrared CO₂ analyzer was used to measure NEE. The measured NEE varied between the two ecosystems depending on changes in above-ground vegetation and environmental factors. The diurnal NEE pattern of daytime CO₂ uptake and night time CO₂ release was evident in the grassland, but not in the cropland as the crops were harvested at the beginning of the measurement period. The grassland generally showed higher night time NEE, but lower daytime NEE than the cropland. Night time NEE showed exponential dependence on air and soil temperature, resulting in Q₁₀ of 1.8 and 1.9 (for air temperature), 2.3 and 2.4 (for soil temperature) in the grassland and cropland, respectively. The average daily NEE was 2.77 and 1.86 g CO₂-C m^?2 day^?1 in the cropland and grassland, respectively. Both ecosystems were sources of CO₂, during 3 months in autumn, but the grassland emitted less CO₂ by 87.9 g CO₂-C m^?2 than the cropland.     

Fertilizers and eutrophication in southwestern Australia: Setting the scene

An excess of plant nutrients has caused serious eutrophication in aquatic ecosystems of southwestern Australia manifested by excessive growth and accumulation of green and bluegreen algae. Phosphorus is generally the limiting nutrient for algal growth and phosphatic fertilizers applied to nutrient-deficient, leaching, sandy soils are the main source of P, supplemented by rural industry point sources. Nitrogen is the limiting nutrient in marine embayments with little drainage from the land. Measures to reduce the load of P delivered to drainage include basing fertilizer application rates on soil testing for P and the use of less soluble P fertilizers. Catchment management plans are being implemented with community involvement to reduce P loads and maintain agricultural production. This introductory paper reviews the history of eutrophication in southwestern Australia and of studies into its causes, principally in the large Peel-Harvey estuary. It briefly summarises other papers in this special issue concerned with different aspects of the problem: how to fertilize the land without causing eutrophication.     

Does grassland vegetation drive soil microbial diversity?

Does plant diversity drive soil microbial diversity in temperate, upland grasslands? Plants influence microbial activity around their roots by release of carbon and pot studies have shown an impact of different grass species on soil microbial community structure. Therefore it is tempting to answer yes. However, evidence from field studies is more complex. This evidence is reviewed at three different scales. First, studies from the plant community scale are considered that have compared soil microbial community structure in pastures of different vegetation composition, as a consequence of pasture improvement. These show fungi dominating the biomass in unimproved pastures and bacteria when lime and fertilizers have been applied. Secondly, evidence for interactions between individual grass species and soil microbes is discussed at the level of the rhizosphere, by considering both pot experiments and field studies. These have produced contrasting and inconclusive results, often due to spatial heterogeneity of soil properties and microbial communities. In particular, increased soil pH and fertility in urine patches and other nutrient cycling processes interact to increase the spatially complexity of soil microbial communities. Finally three studies which have measured microbial community structure in the rhizoplane are considered. These show that bacterial diversity is not directly related to plant diversity, although fungal diversity is. In addition, the soil fungal community has been demonstrated to have an effect upon the composition of the bacterial community. We suggest that while current vegetation influences fungal communities (particularly mycorrhizae) and litter inputs fungal saprotrophs, bacterial community structure is influenced more by the quality or composition of soil organic matter, thereby reflecting carbon inputs to the soil over decades.     

Microbial nitrogen dynamics in south central Chilean agricultural and forest ecosystems located on an Andisol

The natural soil N supply in volcanic soils (Andisols) can be a significant source of plant-available N for agro-ecosystems. Nevertheless, intensive farming systems in south Chile apply high fertilization rates, which lead to high production costs and involve a risk for adverse ecosystem effects. In order to achieve sustainable land management, a better understanding of the processes that govern soil N availability and loss, and their external drivers, is required. In this study, we selected a winter-cropland, a summer crop-winter fallow rotation, and a forest, used as a reference ecosystem. Gross N transformations (¹⁵N isotope dilution) and microbial community structure (phospho-lipid fatty acid analysis) in the topsoil were determined. Gross N mineralization was about ten times lower in the agro-ecosystems than in the forest, while gross nitrification was low in all sites. Gross N immobilization equalized or exceeded the gross inorganic N production in all sites. Microbial biomass was 3–5 times more abundant in the forest than in the agro-ecosystems. A positive relationship between the ratio fungi/bacteria and total microbial biomass was observed in these Andisols. We suggest that the reduction in fungal biomass induced a lower extracellular enzyme production and limited soil organic matter depolymerisation in the agro-ecosystems. We conclude that soil N cycling was unable to provide a significant N input for the croplands, but also the risk for ecosystem N losses was low, even under fallow soil conditions. Current fertilization practices appropriately anticipated the soil N cycling processes, but further research should indicate the potential of alternative land management to reduce fertilizer cost.     

Effect of human activities on forest ecosystems: N cycle and soil fertility

Forests are important terrestrial ecosystems, with particular nutrient cycling mechanisms to maintain structure and functions. Nitrogen is essential for forest growth and development, and commonly limited for the forest productivity. N cycles in forest ecosystems are frequently disturbed by intensive human activities. Based on a variety of research results, some potentially important human disturbances are discussed and their effects on forest ecosystems are reviewed. Precipitation is a considerable N input to forest ecosystems. However acid precipitation is detrimental to the ecosystems in the long run. Acidification causes remarkable reduction in forest productivity in the world, due to the harmful effect of acid on plant physiology and more importantly to the reduction in soil fertility by lowering mineralization and increasing N loss by runoff and leaching. The most important nutrient cycling mechanism in forest ecosystems is litterfall. Removal of trunks only for commercial use will not affect N cycle in forest ecosystems significantly, but attention on the intensity and rotation times of harvest should be paid. Clear-cutting should be prevented in forest harvesting. It deserves more attention that the change of environment after clear-cutting will affect the N cycling processes in forest ecosystems, which substantially influence soil fertility and forest productivity. Ammonification and nitrification processes are stimulated after harvesting, by which N is becoming more moveable. Unfortunately in the situation of no assimilation after clear-cutting, much of N will be lost out of the ecosystems and soil fertility will be diminished. The N pool in forest floor and underlying mineral soil is big, but forest productivity is generally low in natural conditions. Forest management is needed to meet the increasing demand for forest products. Optimization of stands structure is the most economic way to increase soil fertility and forest productivity. Mixed coniferous-broad leaved forest is recommended for plantation practice. Addition of fertilizer N effectively promotes forest productivity and may compensate for the N loss from the systems by harvesting.     

How does plant leaf senescence of grassland species influence decomposition kinetics and litter compounds dynamics?

The influence of litter quality on plant litter decomposition rates is a crucial aspect of the soils C cycle. In grassland ecosystems, leaf litter, which is not removed either by herbivores or by mowing, returns to soil after the senescence process (brown litter). In grassland managed by mowing, another significant proportion of litter returns to the soil before senescence through harvesting losses (green litter). We hypothesized that changes in leaf tissue quality due to the senescence process would lead to contrasting decomposition dynamics of brown litter compared to green litter. Our conceptual approach included the monitoring of decomposition of green (fresh leaves) and brown litter (dead leaves, still attached to the plant) of three different grassland species (Lolium perenne, Festuca arundinacea and Dactylis glomerata) during a 1 year field incubation. After 0, 2, 4, 20 and 44 weeks, we retrieved the litterbags and analysed the remaining material for carbon and nitrogen content and stable isotope composition. Additionally, we determined the lignin content and composition by CuO oxidation and the non-cellulosic neutral carbohydrate content and composition after TFA hydrolysis. As expected, green litter, being higher in N and soluble compounds, while showing a lower C:N ratio and lower lignin contents compared to brown litter, was degraded at a higher rate. Carbon decomposition kinetics suggests that both leaf litter types consist of two pools with contrasting turnover times. The size of the active pool was related to the initial content of soluble plant litter compounds and the size of the recalcitrant pool was related to the lignin to N ratio of initial plant material. More lignin was lost from green litter compared to brown litter. P-coumaryl-type lignin units were decomposed at a higher rate than vanillyl and syringyl units. Total non cellulosic polysaccharide content showed little changes for both litter types. In contrast, the ratios of hexoses/pentoses (C6/C5) and desoxy sugars/pentoses (desoxy/C5) increased during decomposition of green litter only. This is an indication for an increasing contribution of microbial derived compounds being consistant with the higher decomposition rate of this material. Our results showed that grassland management (grazing versus mowing) could influence soil carbon sequestration through different proportions of green and brown litter returned to soil.     

Influence of different land-cover types on the changes of selected soil properties in the mountain region of Rawalakot Azad Jammu and Kashmir

The study focused on the impact of change in land-cover types on soil quality inferred by measuring the relative changes in chemical and physical properties of non-disturbed and disturbed soil system. Soil samples were collected from major land-cover types in the mountain region: natural forest, grassland and cultivated land (arable). The natural forest served as a control against which to assess changes in soil properties resulting from the removal of natural vegetation or cultivation of soil. Soil samples were collected from 0–15 and 15–30 cm depth six times during the year and examined for their nutrient status, i.e. soil organic matter (SOM); total N (TN); available P (AP); available K (AK); cation exchange capacity (CEC), pH and physical properties like particle size distribution, bulk density (BD), and porosity. Significant differences among land-cover types were found for SOM, TN, AP; AK, CEC and pH. Soil collected from the forest had the highest levels of all nutrients followed by grassland while soil from the arable site had very low nutrient status indicated an extractive effect of cultivation and agricultural practices on soil. With significantly lower clay contents (20%), texturally the soil of arable site was quite different from that of the natural forest and grassland. Similarly, a 13% more BD and 12% lower porosity showed structural deterioration of arable soil. The changes in clay contents, BD and porosity due to cultivation suggest adverse effects on environmental protection functions of soil. The correlation coefficient between OM to TN, AP, AK and CEC suggesting that within a narrow range of soil, OM may serves as a suitable indicator of soil quality. Natural vegetation appeared to be a main contributor of soil quality as it maintained the organic carbon stock and increased the nutrient status of soil and is therefore, important to sustain high-altitude ecosystems and reinstate the degraded lands in the mountain region.