植被砍伐对土壤养分的影响

研究了不同砍伐频率下,香港、深圳和鹤山研究样地山坡地Ｎ、Ｐ、Ｋ含量的差异．不同采样地之间,养分全量和有效量差异显著．在尚未砍伐的山坡地,土壤养分含量显著高于经常砍伐的山坡地．未伐土壤的Ｎ含量顺序为香港样地＞深圳样地＞鹤山样地．下层土壤Ｐ的含量高于上层土壤．深圳样地常代土壤养分含量高于鹤山．结果建议采用最适砍伐频率和在最适季节砍伐,以便减少砍伐产生的不利影响．     

Long-term effects of agriculture on soil carbon pools and carbon chemistry along a Hawaiian environmental gradient

Intensive agriculture has the potential to reduce soil carbon stocks in the years following initial cultivation, although the magnitude and direction of the effect can vary with ecosystem and management factors. Agriculture can also shift the carbon chemistry of soils via changes in crop plant chemistry, decomposition, and/or soil amendments [e.g. black carbon (i.e. charcoal)]. It is possible that soil carbon levels can recover if intensive cultivation ends, but the factors driving the extent and quality of this recovery are not well understood. Here, we examined soil carbon pool sizes and carbon chemistry >200 years after intensive cultivation by early Hawaiians. We compared soils from an extensive pre-European-contact agricultural field system with reference sites under similar modern management. Sites were selected along a climate and soil weathering gradient to investigate interactions between historic land use and ecosystem properties, such as soil mineralogy, in driving soil carbon recovery. Soil carbon content was measured from 0 to 30 cm depth, and carbon chemistry was assessed using ¹³C nuclear magnetic resonance spectroscopy. Overall, we found significantly lower soil carbon stocks in pre-contact agricultural sites compared to reference sites. Radiocarbon dating of bulk soil carbon showed a trend toward older carbon in agricultural versus reference soils, suggesting decreased retention of newer C in agricultural sites. Radiocarbon dating of macroscopic charcoal particles from under agricultural field walls indicated that there were black carbon inputs concurrent with pre-contact agricultural activity. Nonetheless, black carbon and carbonyl carbon levels were lower in agricultural versus reference soils, suggesting decreased retention of specific carbon groups in cultivated sites. Proteins were the only biomolecule higher in abundance in agricultural versus reference sites. Finally, there was an interacting effect of soil mineralogy and historic land use on soil carbon stocks. Whereas short range order (SRO) minerals were positively associated with total soil carbon overall, differences in soil carbon between agricultural and reference soils were largest in soils with high concentrations of SRO minerals. Our results indicate that the negative effect of agriculture on soil carbon stocks can be long-lived, may be associated with persistent changes in soil carbon chemistry, and can vary with soil mineralogical properties.     

Elemental stoichiometry of basal resources and benthic macroinvertebrates along a land use gradient in a Great Basin watershed

The effects of land use on the elemental stoichiometry of aquatic organisms have rarely been studied in semi-arid watersheds. In eight semi-arid sub-watersheds differing in land use, we determined which predictor variable(s) best explains the elemental variability in two basal food resources and benthic macroinvertebrates (BMI). The elemental composition of periphyton and seston was best explained by percentage of urban and agricultural areas, forested land and associated differences in SRP, DOC, and stream water N:P ratios. In contrast, consumer elemental stoichiometry was related to taxonomic identity and feeding mode. Elemental imbalances were higher for collector-gatherer than for scraper and collector-filterer. However, high spatial and temporal variability in the elemental composition of basal food resources obscured clear spatial patterns of imbalances between nutrient-poor upstream and nutrient-rich downstream sites. Results from this study suggest that land use can affect BMI due to alteration in stoichiometry of their food resources. However, taxonomy and allometry must be taken into account to better understand spatial and temporal changes in the elemental composition of BMI. Our results indicate the importance of considering multiple effects to accurately assess land use effects on producer and consumer stoichiometry, particularly the in highly variable Great Basin watersheds.     

Transplants along the vegetation gradient on central Australian sandridges

To assess the importance of static catenary factors in controlling the floristic gradient on central Australian sandridges, plant species characteristic of particular topographic zones were transplanted to zones outside their normal topographic range. Both upper-slope species transplanted downslope, and lower-slope species transplanted upslope suffered higher mortality than control plants transplanted within the same topographic zone.     

Foliar and wood chemistry of sugar maple along a gradient of soil acidity and stand health

The decline of sugar maple (Acer saccharum Marsh.) in forest of north-eastern North America is an important environmental issue. In this study, relationships between, soil, wood and foliar chemistry were assessed for 17 stands distributed within a large area of the Quebec sugar maple forest and that were growing on soils with a strong gradient of acidity and base saturation. There were many significant relationships between variables describing the acid-base status of the top-B soil (Ca and Mg concentrations, exchangeable acidity and base saturation) and Ca and Mn concentrations and Ca/Mn and Mg/Mn in tree tissues. Manganese was the element that showed the strongest inverse non-linear relationships with top-B soil base saturation with variance explanation of 71 and 65%, for wood and foliage, respectively. The 17 sites were divided in two groups according to their level of decline. The declining stands had significantly higher wood Mn and Mg concentrations and lower Ca/Mn ratios and significantly higher foliar Mn and lower Ca and Al concentrations. It was impossible to determine if these differences were a cause or a symptom of sugar maple health. However, the increase in Mn concentrations in tree tissues with increasing soil acidity, as well as the higher Mn concentrations in declining as compared to healthy stands suggest that Mn, as well as low Ca availability, could be an important contributing factor in the sugar maple decline.     

Multiscale soil and vegetation patchiness along a gradient of herbivore impact in a semi-arid grazing system in West Africa

We studied the degree and scale of patchiness of vegetation and selected soil variables along a gradient of herbivore impact. The gradient consisted of a radial pattern of `high', `intermediate' and `low' herbivore impact around a watering point in a semi-arid environment in Burkina Faso (West Africa). We hypothesised that, at a certain range of herbivore impact, vegetated patches alternating with patches of bare soil would occur as a consequence of plant-soil feedbacks and run-off-run-on patterns. Indeed, our transect data collected along the gradient showed that vegetated patches with a scale of about 5–10 m, alternating with bare soil, occurred at intermediate herbivore impact. When analysing the data from the experimental sites along the gradient, however, we also found a high degree of patchiness of vegetation and soil variables in case of low and high herbivore impact. For low herbivore impact, most variation was spatially explained, up to 100% for vegetation biomass and soil temperature, with a patch scale of about 0.50 m. This was due to the presence of perennial grass tufts of Cymbopogon schoenanthus. Patterns of soil organic matter and NH₄-N were highly correlated with these patterns of biomass and soil temperature, up to r=0.7 (P<0.05) for the in situ correlation between biomass and NH₄-N. For high herbivore impact, we also found that most variation was spatially explained, up to 100% for biomass and soil temperature, and 84% for soil moisture, with three distinct scales of patchiness (about 0.50 m, 1.80 m and 2.80 m). Here, microrelief had a corresponding patchy structure. For intermediate herbivore impact, again most variation was spatially explained, up to 100% for biomass and soil temperature, and 84% for soil moisture, with a patch scale of about 0.95 m. Here, we found evidence that vegetated patches positively affected soil moisture through less run-off and higher infiltration of rainwater that could not infiltrate into the bare soil elsewhere, which was not due to microrelief. Thus, we conclude that our findings are in line with our initial hypothesis that, at intermediate herbivore impact, vegetated patches alternating with patches of bare soil persist in time due to positive plant-soil feedbacks.     

Patterns of species diversity and soil nutrients along a chronosequence of vegetation recovery in Hainan Island, South China

Understanding the pattern of species diversity and soil factors can enhance our knowledge of the mechanism of vegetation recovery, however, there is still a gap in the knowledge of succession rate and trend for species diversity in relation to soil nutrients during the vegetation recovery process. Patterns of species diversity and soil nutrients during the tropical vegetation recovery as well as the correlation between species diversity and soil nutrients were explored in Hainan Island, located in southern China. Plots assigned as grassland stage (GS), shrub stage (SS), secondary forest stage (SFS), and primary forest stage (PFS) were established using a chronosequence approach. Results showed that species richness and evenness increased from GS to PFS. Species dominance/diversity curves were fitted using the lognormal distribution model (r ² ?=?0.891?C0.972). Species richness for the herb layer was maximal at SFS, whereas species richness for both the shrub layer and tree layer reached their maximum at PFS. Species turnover and soil total phosphorus decreased, whereas organic matter and total nitrogen increased from GS to PFS. Organic matter and total nitrogen were both positively correlated with species richness and total coverage, and total phosphorus was positively correlated with species turnover. The results clearly demonstrate that diversity asymptotically increases and positively correlates with increasing soil fertility, and the total phosphorus value is predicted to be an important soil factor that affects successional rate during tropical vegetation recovery processes.     

Temperature and vegetation effects on soil organic carbon quality along a forested mean annual temperature gradient in North America

Both climate and plant species are hypothesized to influence soil organic carbon (SOC) quality, but accurate prediction of how SOC process rates respond to global change will require an improved understanding of how SOC quality varies with mean annual temperature (MAT) and forest type. We investigated SOC quality in paired hardwood and pine stands growing in coarse textured soils located along a 22 °C gradient in MAT. To do this, we conducted 80‐day incubation experiments at 10 and 30 °C to quantify SOC decomposition rates, which we used to kinetically define SOC quality. We used these experiments to test the hypotheses that SOC quality decreases with MAT, and that SOC quality is higher under pine than hardwood tree species. We found that both SOC quantity and quality decreased with increasing MAT. During the 30 °C incubation, temperature sensitivity (Q₁₀) values were strongly and positively related to SOC decomposition rates, indicating that substrate supply can influence temperature responsiveness of SOC decomposition rates. For a limited number of dates, Q₁₀ was negatively related to MAT. Soil chemical properties could not explain observed patterns in soil quality. Soil pH and cation exchange capacity (CEC) both declined with increasing MAT, and soil C quality was positively related to pH but negatively related to CEC. Clay mineralogy of soils also could not explain patterns of SOC quality as complex (2 : 1), high CEC clay minerals occurred in cold climate soils while warm climate soils were dominated by simpler (1 : 1), low CEC clay minerals. While hardwood sites contained more SOC than pine sites, with differences declining with MAT, clay content was also higher in hardwood soils. In contrast, there was no difference in SOC quality between pine and hardwood soils. Overall, these findings indicate that SOC quantity and quality may both decrease in response to global warming, despite long‐term changes in soil chemistry and mineralogy that favor decomposition.     

Porcupine disturbances and vegetation pattern along a resource gradient in a desert

Summary In the Negev Desert, Israel, the Indian crested porcupine, Hystrix indica, digs similar sized, discrete, elongated pits (257±21.3 cm³; n=144) while foraging for below-ground plant storage organs. In these digs, soil moisture content is higher than in the surrounding soil matrix. The digs disturb population and community structure due to porcupine consumption or damage of 18 species of plants, and repopulation by 55 plant species. Over the past 14 years we have studied dig dynamics on a rocky hillslope with three distinct habitats as regards soil moisture content. Midslope soil moisture is the highest, decreasing towards upper and lower slope. We have counted a total of 6,609 digs in the area: 2141 on the upper, 3211 in the middle and 1257 on the lower part of the slope. The number of digs at midslope is significantly higher than on the other parts of the slope (ANOVA; P<0.0001). There is a significant (P<0.05) correlation between the mean number of porcupine digs and the cumulative rainfall amount for the 2 years prior to dig formation. To study plant repopulation in digs, all plants in 144 digs along the slope and from equal sized plots in the undistarbed soil matrix were collected. In all, 288 samples with 20 584 plants were collected, 2042 from the matrix and 18,542 from digs. Of the 55 species, we focused on the abundance patterns of Filago desertorum, Picris cyanocarpa and Bromus rubens, which made up 69.5% of all the individuals in the digs and 68.3% in the matrix. Our results showed that all three species increased in abundance in the digs as compared to the matrix. F. desertorum density increased by a factor of 2.9, P. cyanocarpa by 9.5 and B. rubens by 12.0. There were species-specific responses in abundance to the location of the digs along the moisture gradient. The only species whose abundance responded to the moisture gradient was F. desertorum. P. cyanocarpa demonstrated peak abundance in the location with the poorest moisture regime, while B. rubens showed peak abundance at the intermediate part of the moisture gradient. We suggest a scheme for integrating the increase in density and the species-specific responses to the digs along a water gradient based on R.H. Whittaker's view of individual species abundances along an environmental gradient.     

In search of a hydrological explanation for vegetation changes along a fen gradient in the Biebrza Upper Basin (Poland)

The understanding of succession from rich fen to poorer fen types requires knowledge of changes in hydrology, water composition, peat chemistry and peat accumulation in the successional process. Water flow patterns, water levels and water chemistry, mineralisation rates and nutrient concentrations in above-ground vegetation were studied along a extreme-rich fen-moderate-rich fen gradient at Biebrza (Poland). The extreme-rich fen was a temporary groundwater discharge area, while in the moderate-rich fen groundwater flows laterally towards the river. The moderate-rich fen has a rainwater lens in spring and significant lower concentrations of calcium and higher concentrations of phosphate in the surface water. Mineralisation rates for N, P and K were higher in the moderate-rich fen. Phosphorus concentrations in plant material of the moderate-rich fen were higher than in the extreme-rich fen, but concentrations of N and K in plant material did not differ between both fen types. Water level dynamics and macro-remains of superficial peat deposits were similar in both fen types.We concluded that the differences observed in the moderate-rich and the extreme-rich fens were caused by subtile differences in the proportion of water sources at the peat surface (rainwater and calcareous groundwater, respectively). Development of an extreme-rich fen into a moderate-rich fen was ascribed to recent changes in river hydrology possibly associated with a change in management practices. The observed differences in P-availability between the fen types did not result in significantly different biomass. Moreover, biomass production in both fen types was primarily N-limited although P-availability was restricted too in the extreme-rich fen. Aulacomnium palustre, the dominant moss in the moderate-rich fen, might be favoured in competition because of its broad nutrient tolerance and its quick establishment after disturbance. It might outcompete low productive rich fen species which were shown to be N-limited in both fens. We present a conceptual model of successional pathways of rich fen vegetation in the Biebrza region.     

A comparison of soil climate and biological activity along an elevation gradient in the eastern Mojave Desert

Summary Soil temperature, moisture, and CO₂ were monitored at four sites along an elevation transect in the eastern Mojave Desert from January to October, 1987. Climate appeared to be the major factor controlling CO₂ partial pressures, primarily through its influence of rates of biological reactions, vegetation densities, and organic matter production. With increasing elevation, and increasing actual evapotranspiration, the organic C, plant density, and the CO₂ content of the soils increased. Between January and May, soil CO₂ concentrations at a given site were closely related to variations in soil temperature. In July and October, temperatures had little effect on CO₂, presumably due to low soil moisture levels. Up to 75% of litter placed in the field in March was lost by October whereas, for the 3 lower elevations, less than 10% of the litter placed in the field in April was lost through decomposition processes.     

The role of soil mobility in controlling a vegetation gradient

Mature plants of species which characteristically occupy the mobile crests and upper slopes of the central Australian sandridges are more tolerant of sand burial or undercutting than those of the lower slopes and swales. Seedlings growing on the mobile crest areas are susceptible to sand burial, even during wet years. The young shoots of vegetatively spreading species are less susceptible than individual seedlings of the same size.     

Temperature adaptation of soil bacterial communities along an Antarctic climate gradient: predicting responses to climate warming

Soil microorganisms, the central drivers of terrestrial Antarctic ecosystems, are being confronted with increasing temperatures as parts of the continent experience considerable warming. Here we determined short‐term temperature dependencies of Antarctic soil bacterial community growth rates, using the leucine incorporation technique, in order to predict future changes in temperature sensitivity of resident soil bacterial communities. Soil samples were collected along a climate gradient consisting of locations on the Antarctic Peninsula (Anchorage Island, 67 °34′S, 68 °08′W), Signy Island (60 °43′S, 45 °38′W) and the Falkland Islands (51 °76′S 59 °03′W). At each location, experimental plots were subjected to warming by open top chambers (OTCs) and paired with control plots on vegetated and fell‐field habitats. The bacterial communities were adapted to the mean annual temperature of their environment, as shown by a significant correlation between the mean annual soil temperature and the minimum temperature for bacterial growth (T_min). Every 1 °C rise in soil temperature was estimated to increase T_min by 0.24–0.38 °C. The optimum temperature for bacterial growth varied less and did not have as clear a relationship with soil temperature. Temperature sensitivity, indicated by Q₁₀ values, increased with mean annual soil temperature, suggesting that bacterial communities from colder regions were less temperature sensitive than those from the warmer regions. The OTC warming (generally <1 °C temperature increases) over 3 years had no effects on temperature relationship of the soil bacterial community. We estimate that the predicted temperature increase of 2.6 °C for the Antarctic Peninsula would increase T_min by 0.6–1 °C and Q₁₀ (0–10 °C) by 0.5 units.     

Changes in soil heterotrophic respiration,carbon availability,and microbial function in seven forests along a climate gradient

Soil microbial communities play an essential role in soil carbon (C) emission and C sequestration in forest ecosystems. However, little information is available regarding the relationship between soil C dynamics and microbial substrate utilization at large scales. Along the North–South Transect of Eastern China (NSTEC), seven forests representative of boreal, temperate and tropical biomes were examined. Soil heterotrophic respiration (R_h), soil dissolved organic C (DOC), microbial biomass C (MBC), and microbial community-level physiological profiles (CLPPs) were investigated using biochemical measurements, static chamber-gas chromatography analysis, and Biolog-Eco microplates, respectively. We found that soil R_h rates were significantly higher in subtropical and boreal forests than in temperate forests. Conversely, the concentrations of soil DOC and MBC, as well as microbial metabolic activity and functional diversity, were consistently higher in temperate forests than in subtropical forests. There were considerably different substrate utilization profiles among the boreal, temperate, and subtropical forests. Soil microorganisms from the temperate and boreal forests mainly metabolized high-energy substrates, while those from the subtropical forests used all substrates equally. In addition, soil R_h rates were significantly and negatively related to soil labile C concentrations, total metabolic activity, and the intensity of individual substrate utilization, indicating that soil microbes assimilated more soil substrates, thereby reducing CO₂ emissions. Overall, our study suggests that climate factors, as well as substrate availability, dominate the activities and functions of soil microbes at large scales.     

Separating the effects of climate and vegetation on evapotranspiration along a successional chronosequence in the southeastern US

We combined Eddy‐covariance measurements with a linear perturbation analysis to isolate the relative contribution of physical and biological drivers on evapotranspiration (ET) in three ecosystems representing two end‐members and an intermediate stage of a successional gradient in the southeastern US (SE). The study ecosystems, an abandoned agricultural field [old field (OF)], an early successional planted pine forest (PP), and a late‐successional hardwood forest (HW), exhibited differential sensitivity to the wide range of climatic and hydrologic conditions encountered over the 4‐year measurement period, which included mild and severe droughts and an ice storm. ET and modeled transpiration differed by as much as 190 and 270 mm yr^?1, respectively, between years for a given ecosystem. Soil water supply, rather than atmospheric demand, was the principal external driver of interannual ET differences. ET at OF was sensitive to climatic variability, and results showed that decreased leaf area index (L) under mild and severe drought conditions reduced growing season (GS) ET (ET_GS) by ca. 80 mm compared with a year with normal precipitation. Under wet conditions, higher intrinsic stomatal conductance (g_s) increased ET_GS by 50 mm. ET at PP was generally larger than the other ecosystems and was highly sensitive to climate; a 50 mm decrease in ET_GS due to the loss of L from an ice storm equaled the increase in ET from high precipitation during a wet year. In contrast, ET at HW was relatively insensitive to climatic variability. Results suggest that recent management trends toward increasing the land‐cover area of PP‐type ecosystems in the SE may increase the sensitivity of ET to climatic variability.     

Fine-scale spatial organization of tallgrass prairie vegetation along a topographic gradient

Fine-scale spatial patterns of native tallgrass prairie vegetation were studied on Konza Prairie, Kansas, USA. Three sites, upland, slope, and lowland, were sampled in an ungrazed watershed. Presence of vascular plant species was recorded in two 25.6 m long transects of contiguous 5×5 cm micro-quadrats on each topographic position. Spatial patterns of species and functional groups were analyzed by information theory models ofJuhász-Nagy. Within-community variability of coexistence was expressed by the diversity and spatial dependence of local species combinations. Considerable diversity in the local coexistence of species was found on each site. Upland and hillside communities were richer and more diverse in species combinations than lowland. Spatial scale effected coexistence relationships. The maxima of information theory estimates varied between 15 and 30 cm. There was no trend in the variation of characteristic scales along the topographical gradient. Above 10 m, all sites tended to be homogeneous. The analysis of spatial associations revealed that variability in the local coexistence of species was strongly constrained in all topographic positions. Overall spatial association of species was the lowest on lowland. The characteristic scales of maximum association were between 1.2 m and 3 m at all sites. The maxima of information theory estimates for the functional group-based data appeared at smaller plot sizes than for the species based analyses. Only weak spatial associations were detected among the functional groups indicating that individuals of functional groups coexist well at small scales, and form combinations close to random expectations. The length of transects did not effect the relative associations. Strong positive correlations were found between the number of components (species or functional groups) and the maxima, of information theory models suggesting that richness is a good predictor of within-community coexistence relations. However, there was no relationship between richness and the characteristic scales of community patterns.     

Savanna tree influence on understory vegetation and soil nutrients in northwestern Kenya

Abstract. Contrary to observations and models in which trees and herbaceous plants are viewed as competitors, we found that trees in an African savanna have positive impacts on herbaceous biomass production and composition, and on soil nutrient status. In the Turkana District of northwestern Kenya, we investigated vegetation and soil gradients along equi-angular transects radiating from the boles of individual Acacia tortilis trees. Total herbaceous biomass averaged 260 ± 17(se) g/m² at the bole and declined to 95 ± 8 g/m² in the tree interspaces. Soil organic carbon and total nitrogen concentrations were greatest (0.72 % and 0.083 %, respectively) in shallow soils near the bole and declined rapidly toward the interspaces and with increasing depth. Transects were also established between tree pairs to assess effects of differential canopy proximities. Grass production averaged 220 ± 21 g / m² below overlapping canopies, 150 ± 15 g / m² under individual canopies, and 95 ± 8 g / m² in interstitial areas. Detrended correspondence analysis revealed that shifts in species composition were correlated with distance from tree bole out to the edge of the canopy. Species response, in terms of relative cover, to increasing distance from the bole, seemed to fall into five general classes: 1) greatest at the bole, 2) increasing with distance from the bole, 3) greatest in the mid canopy zone, 4) least at the bole and 5) no response. Trees did not influence herbaceous compositionbeyondtree canopies. It is assumed that shade cast by the tree canopy with subsequent reductions of understory water stress and temperature and increased nutrient concentrations may be the most important factors affecting understory soil and vegetation.     

Soil N chemistry in oak forests along a nitrogen deposition gradient

Anthropogenic N deposition may change soil conditions in forest ecosystems as demonstrated in many studies of coniferous forests, whereas results from deciduous forests are relatively scarce. Therefore the influence of N deposition on several variables was studied in situ in 45 oak-dominated deciduous forests along a N deposition gradient in southern Sweden, where the deposition ranged from 10 to 20 kg N ha⁻¹ year⁻¹. Locally estimated NO ₋ ³ deposition, as measured with ion-exchange resins (IER) on the soil surface, and grass N concentration (%) were positively correlated with earlier modelled regional N deposition. Furthermore, the δ¹⁵N values of grass and uppermost soil layers were negatively correlated with earlier modelled N deposition. The data on soil NO ₋ ³ , measured with IER in the soil, and grass N concentration suggest increased soil N availability as a result of N deposition. The δ¹⁵N values of grass and uppermost soil layers indicate increased nitrification rates in high N deposition sites, but no large downward movements of NO ₋ ³ in these soils. Only a few sites had NO ₋ ³ concentrations exceeding 1 mg N l⁻¹ in soil solution at 50 cm depth, which showed that N deposition to these acid oak-dominated forests has not yet resulted in extensive leaching of N. The δ¹⁵N enrichment factor was the variable best correlated with NO ₋ ³ concentrations at 50 cm and is thus a variable that potentially may be used to predict leaching of NO ₋ ³ from forest soils.