Influences of temperature and precipitation before the growing season on spring phenology in grasslands of the central and eastern Qinghai-Tibetan Plateau

Spatial variations in phenological responses to temperature have not been reported for grasslands of the Qinghai-Tibetan Plateau. Using satellite-derived normalized difference vegetation index and meteorological records from 1982 to 2006, we characterized the spatial patterns of grassland green-up onset in relation to air temperature and precipitation before the growing season (“preseason” henceforth) in the central and eastern plateau by combining linear programming with correlation analysis. Green-up onset near half of the meteorological stations was significantly correlated (p < 0.10) with precipitation and thermal spring onset (TSO) date based on the cumulative temperature less than 6 weeks before the onset. The green-up onset paralleled the advance in TSO in the southwestern, southeastern, eastern, and northeastern parts of the plateau. The TSO and preseason precipitation (PPT) explained part of the inter-annual phenological variations, with r² varying between 0.05 and 0.55 and averaging 0.28, and did not explain delay of green-up onset in some areas. Increasing preseason temperature tended to advance green-up onset in relatively moist areas. PPT exerted a stronger influence on green-up onset in drier areas. These results indicate spatial differences in the key environmental influences on spring phenology. To improve the ability to predict onset, ground-based community-level phenological studies and spatial scaling-up of the phenology-climate relationship will be necessary.     

Integrated diurnal soil respiration model during growing season of a typical temperate steppe: Effects of temperature, soil water content and biomass production

Soil respiration was measured with the enclosed chamber method in an ungrazed Leymus chinensis steppe during the growing seasons of 2001 and 2002. Soil respiration rate (R_S) was significantly influenced by air temperature (T) at the diurnal scale, and could be described by Van't Hoff's equation (R_S = R₁₀ exp(β(T − 10))). At the seasonal scale, the normalized soil respiration rate at 10 °C (R₁₀) was mainly controlled by soil water content (R² = 0.717, P < 0.001), while the sensitivity of soil respiration to temperature (Q₁₀) was partially affected by absolute growth rate (R² = 0.482, P = 0.004). Thus, soil respiration could be described as R_S = (20.015W − 84.085) (0.103AGR + 1.786)^(T−10)/10 during the growing seasons, integrating soil water content (W) and absolute growth rate (AGR) into the temperature-dependent soil respiration equation. It was validated by the observed soil respiration rates in this study (R² = 0.890, P < 0.001) and observations from near-field experiment (R² = 0.687, P = 0.011). It implied that accurately evaluating annual soil respiration should include the effects of plant biomass production and other abiotic factors besides air temperature.     

Soil CO2 flux in six monospecific forest plantations in Southern Rwanda

Forest soils contain the largest carbon stock of all terrestrial biomes and are probably the most important source of carbon dioxide (CO₂) to atmosphere. Soil CO₂ fluxes from 54 to 72-year-old monospecific stands in Rwanda were quantified from March 2006 to December 2007. The influences of soil temperature, soil water content, soil carbon (C) and nitrogen (N) stocks, soil pH, and stand characteristics on soil CO₂ flux were investigated. The mean annual soil CO₂ flux was highest under Eucalyptus saligna (3.92 μmol m⁻² s⁻¹) and lowest under Entandrophragma excelsum (3.13 μmol m⁻² s⁻¹). The seasonal variation in soil CO₂ flux from all stands followed the same trend and was highest in rainy seasons and lowest in dry seasons. Soil CO₂ flux was mainly correlated to soil water content (R² = 0.36-0.77), stand age (R² = 0.45), soil C stock (R² = 0.33), basal area (R² = 0.21), and soil temperature (R² = 0.06-0.17). The results contribute to the understanding of factors that influence soil CO₂ flux in monocultural plantations grown under the same microclimatic and soil conditions. The results can be used to construct models that predict soil CO₂ emissions in the tropics.     

Effects of vegetation restoration on soil organic carbon sequestration at multiple scales in semi-arid Loess Plateau,China

Soil organic carbon (SOC) sequestration by vegetation restoration is the theme of much current research. Since 1999, the program of “Grain for Green”has been implemented in the semi-arid Loess Plateau, China. Its scope represents the largest vegetation restoration activity in China. However, it is still unclear for the SOC sequestration effects of vegetation cover change or natural succession promoted by the revegetation efforts at different scales under the semi-arid conditions. In this study, the changes in SOC stocks due to the vegetation restoration in the middle of Loess Plateau were estimated at patch, hill slope transect and small watershed scale from 1998 to 2006. Soil samples were taken from field for the determination of cesium-137 (¹³⁷Cs) and SOC contents. Vegetation cover change from 1998 to 2006 at the small watershed scale was assessed using Geographic Information System. The results showed that cropland transforming to grassland or shrubland significantly increased SOC at patch scale. Immature woodland, however, has no significant effect. When vegetation cover has no transformation for mature woodland (25 years old), SOC has no significant increase implying that SOC has come to a stable level. At hill slope scale, three typical vegetation cover patterns showed different SOC sequestration effects of 8.6%, 24.6%, and 21.4% from 1998 to 2006, and these SOC increases mainly resulted from revegetation. At the small watershed scale, SOC stocks increased by 19% in the surface soil layer at 0–20 cm soil depth from 1998 to 2006, which was equivalent to an average SOC sequestration rate of 19.92 t C y^− 1 km^− 2. Meanwhile, SOC contents showed a significant positive correlation (P < 0.001) with the ¹³⁷Cs inventory at every soil depth interval. This implied significant negative impacts of soil erosion on SOC sequestration. The results have demonstrated general positive effects of vegetation restoration on SOC sequestration at multiple scales. However, soil erosion under rugged topography modified the spatial distribution of the SOC sequestration effects. Therefore, vegetation restoration was proved to be a significant carbon sink, whereas, erosion could be a carbon source in high erosion sensitive regions. This research can contribute to the performance assessment of ecological rehabilitation projects such as “Grain to Green” and the scientific understanding of the impacts of vegetation restoration and soil erosion on soil carbon dynamics in semi-arid environments.     

Effects of woody debris, microtopography, and organic matter amendments on the biotic community of constructed depressional wetlands

To increase wetland acreage and biodiversity, Delaware agencies constructed >220 depressional wetlands. During construction, agencies included amendments thought to increase biodiversity. Because the efficacy of amendments is unknown, we investigated their effects on macroinvertebrate and vegetative communities. We selected 20 standardized wetlands (five contained coarse woody debris (CWD) and microtopography amendments (land surface ridges and furrows), five had neither, five had CWD only, and five had microtopography only). Additionally, 12 wetlands had received organic matter amendments (i.e., straw). Insect richness (P = 0.010; r² = 0.16), insect biomass (P = 0.023; r² = 0.13), intolerant insect biomass (P = 0.033, r² = 0.03), Ephemeroptera biomass (P = 0.027; r² = 0.12), and Odonata biomass (P = 0.046; r² = 0.10) increased with CWD volume. Obligate plant percent cover increased with microtopographic variation (P = 0.029; r² = 0.120). Although organic matter amendments did not increase percent soil organic matter (t_13.7 = −1.16, P = 0.264), total (P = 0.027; r² = 0.12), native (P = 0.036; r² = 0.11), and facultative (P = 0.001; r² = 0.24) plant richness increased with percent soil organic matter. To enhance biodiversity, constructed wetlands should contain CWD, but additional research is needed to understand the benefits of microtopography and organic matter amendments.     

The relative effects of road traffic and forest cover on anuran populations

Road traffic and the loss of forests are both known to have negative effects on anurans. However, the relative importance of these two predictors is poorly understood because forest cover in the landscape is usually negatively correlated with the density of roads and traffic. To evaluate the independent effects of traffic and forest cover, we selected 36 ponds near Ottawa, Canada, at the center of four landscape types: low forest/low traffic; low forest/high traffic; high forest/low traffic; and high forest/high traffic, where traffic and forest cover were measured within 100-2000 m of the edge of each pond. We surveyed all ponds in 2005 and re-surveyed a 23-pond subset in 2006. The negative association between species richness and traffic density was stronger (partial R² = 0.34; P < .001) than the positive association of species richness with forest cover (partial R² = 0.10; P > .05) in the landscape. Three of six common species showed stronger associations with traffic density than with forest cover - Bufo americanus, Rana pipiens, and Hyla versicolor; two species - Pseudacris crucifer and Rana sylvatica - showed stronger associations with forest cover than with traffic; while Rana clamitans showed similar associations with traffic and forest cover. Our results show that the overall negative effect of traffic on anuran populations in northeastern North America is at least as great as the negative effect of deforestation, and also that the relative effects of these two predictors on anuran abundance vary between species.     

Field characterization of olive (Olea europaea L.) tree crown architecture using terrestrial laser scanning data

Since the introduction of Terrestrial Laser Scanning (TLS) instruments, there now exists a means of rapidly digitizing intricate structural details of vegetation canopies using Light Detection and Ranging (LiDAR) technology. In this investigation, Intelligent Laser Ranging and Imaging System (ILRIS-3D) data was acquired of individual tree crowns at olive (Olea europaea L.) plantations in Córdoba, Spain. In addition to conventional tripod-mounted ILRIS-3D scans, the unit was mounted on a platform (12 m above ground) to provide nadir (top-down) observations of the olive crowns. 24 structurally variable olive trees were selected for in-depth analysis. From the observed 3D laser pulse returns, quantitative retrievals of tree crown structure and foliage assemblage were obtained. Robust methodologies were developed to characterize diagnostic architectural parameters, such as tree height (r² = 0.97, rmse = 0.21 m), crown width (r² = 0.97, rmse = 0.13 m), crown height (r² = 0.86, rmse = 0.14 m), crown volume (r² = 0.99, rmse = 2.6 m³), and Plant Area Index (PAI) (r² = 0.76, rmse = 0.26 m²/m²). With the development of such LiDAR-based methodologies to describe vegetation architecture, the forestry, agriculture, and remote sensing communities are now faced with the possibility of replacing current labour-intensive inventory practices with, modern TLS systems. This research demonstrates that TLS systems can potentially be the new observational tool and benchmark for precise characterization of vegetation architecture for improved agricultural monitoring and management.     

Feasibility of using FGD gypsum to conserve water and reduce erosion from an agricultural soil in Georgia

Crop production in Georgia and the Southeastern U.S. can be limited by water. Highly-weathered, drought-prone soils are susceptible to runoff and erosion. Rainfall patterns generate runoff producing storms followed by extended periods of drought during the crop growing season. Thus, supplemental irrigation is often needed to sustain profitable crop production. Increased water retention and soil conservation would efficiently improve water use and reduce irrigation amounts/costs and sedimentation, and sustain productive farm land, thus improving producer's profit margin. Soil amendments, such as flue gas desulfurization (FGD) gypsum, have been shown to retain rainfall and/or irrigation water through increased infiltration while decreasing runoff (R) and sediment (E). Objectives were to quantify rainfall partitioning and sediment delivery improvements with surface applied FGD gypsum from an Ultisol managed to conventional till (CT) and to assess the feasibility of using FGD gypsum on agricultural land in southern Georgia. A field study (Faceville loamy sand, Typic Kandiudult) was established (2006, 2007) near Dawson, GA managed to CT, irrigated cotton (Gossypium hirsutum L.). FGD gypsum application rates evaluated were 0, 1.1, 2.2, 4.5, and 9 Mg ha^− 1. Gypsum treatments and simulated rainfall (50 mm h^− 1 for 1 h) were applied to 2-m wide × 3-m long field plots (n = 3). Runoff and E were measured from each 6-m² plot (slope = 1%). FGD gypsum plots averaged 26% more infiltration (INF), 40% less R, 58% less E, 27% lower maximum R rates (R_max), and 2 times lower maximum E rates (E_max) than control plots. Values of INF and water for crop use increased, and R, E, R_max, and E_max decreased as FGD gypsum application rate increased. Values of INF, R, E, R_max, and E_max for 9 Mg ha^− 1 plots were as much as 17% greater, 35% less, 1.9 times less, 35% less, and 1.9 times less than those from other FGD gypsum plots, respectively; and 40% greater, 40% less, 2.2 times less, 52% less, and 2.9 times less than those from control plots, respectively. Applying FGD gypsum to agricultural lands is a cost-effective management practice for producers in Georgia that beneficially impacts natural resource conservation, producer profit margins, and environmental quality. Agriculture in the Southeast provides a viable market for the electric power industry to convert disposal costs of FGD gypsum into a profitable commodity.     

Sulphur immobilization and arylsulphatase activity in two calcareous arable and fallow soils as affected by glucose additions

In this study we examined the effects of glucose-C on the activities of fungi and bacteria determined by the method of substrate-induced respiration (SIR) in combination with the selective inhibition technique, the immobilized-S and the arylsulphatase (ARS) activity in two calcareous arable and fallow soils. The amounts of glucose-C were added at six doses: 0, 125, 250, 500, 750 and 1000 mg kg^− 1 soil to the soils and then incubated for one week with a Na₂³⁵SO₄ solution (518.9 kBq kg^− 1 dry soil and 20 mg S kg^− 1 dry soil) prior to analysis. At the highest dose of 1000 mg kg^− 1 soil, fungal activity increased by 59.1% (of the dose 0) versus 45.5% for bacterial activity in the arable soil, while in the fallow soil the increases were more marked and corresponded to 69.9% and 71.1%, respectively. Largest increase in immobilized-S was observed in the arable soil (300.7%) compared with the fallow soil (153.1%). In contrast, the ARS activity increased by 16.4% in the arable soil versus 32.1% in the fallow soil. These results indicate that glucose proportionately affected more the intensities of immobilized-S than those of ARS. Strong positive correlation coefficients were found between fungal activities and immobilized-S in the arable soil (r = 0.96, P < 0.01) and in the fallow soil (r = 0.98, P < 0.001). However, non-significant correlations were observed between fungal activities and ARS in both studied soils. As to bacterial activities, positive significant correlation coefficients were found with immobilized-S in the arable soil (r = 0.95, P < 0.01) and in the fallow soil (r = 0.90, P < 0.05) as well as with ARS activities in the arable soil (r = 0.83, P < 0.05) and in the fallow soil (r = 0.97, P < 0.01). Overall, we also found positive and significant correlation coefficients of immobilized-S with ARS activities in the arable soil (r = 0.86, P < 0.05) and in the fallow soil (r = 0.83, P < 0.05). Accordingly, the results showed a presence of extracellular arylsulphatase activity of 38.7 mg p-nitrophenol kg^− 1 soil h^− 1 in the arable soil and of 63.5 mg p-nitrophenol kg^− 1 soil h^− 1 in the fallow soil. It was concluded that fallowing maintained larger activities of fungi, bacteria and arylsulphatase compared with the arable soil.     

Time and burial depth influencing the viability and bacterial colonization of sclerotia of Sclerotinia sclerotiorum

Sclerotia are the primary over wintering inoculum of Sclerotinia sclerotiorum (Lib.) de Bary. The effects of tillage on the primary inoculum are not well understood. The purpose of this research was to study sclerotial viability over time and between burial depths in soil, to identify bacteria colonizing and degrading the sclerotia, and determine whether these bacteria may be utilized as biological control agents. Correlation analysis indicated that a significant negative relationship existed between sclerotial viability and elapsed temporal factors (R²=−0.68, P<0.0001), and depth of burial (R²=−0.58, P<0.0001). After twelve months, sclerotia on the soil surface had the highest viability (57.5%), followed by those at the 5 cm depth (12.5%), and only 2.5% of those placed at the 10 cm depth remained viable. A significant negative relationship between sclerotial viability and bacterial populations also existed (R²=−0.60, P<0.0001). Two hundred and sixty-eight bacteria were isolated from sclerotia, 29 of which showed strong in vitro antagonism to the mycelial growth of S. sclerotiorum. Biodiversity of the inhibitory bacterial isolates was minimal on sclerotia from the soil surface and within all depths sampled at three months (i.e. in January). All burial depths within the April and July sampling dates produced bacterial diversities that were distinct from each other.     

Unusually high levels of bio-available phosphate in the soils of Ogasawara Islands, Japan: Putative influence of seabirds

Ogasawara Islands are important ecosystems sustaining many indigenous spices. To clarify the indigenous soil environments of Ogasawara Islands, we studied the chemistry of the soils. Many surface soils were low in bio-available P (0 to 0.55 g P₂O₅ kg⁻¹, average: 0.04 g P₂O₅ kg⁻¹ as Bray II P, n = 22), but several soils were found to contain extremely large amounts of bio-available P (1.36 to 6.98 g P₂O₅ kg⁻¹, average: 2.93 g P₂O₅ kg⁻¹, n = 5). From soil profile analyses, the authors concluded that the extremely large amount of bio-available P could not be explained by the effects of parent materials with high P contents nor the effect of fertilizations by human activity, but the effects of natural seabird activities in the past could be the cause. The soil profiles with large amounts of bio-available P indicate deep migration of soil materials from A horizons, which could be a result of intensive mixing of upper horizons by seabird activities. The intensive mixing was supported by the low mechanical impedance of the horizons for the P-accumulating soils (8.17 ± 2.54 kg cm⁻², n = 8) than those for the non-P-accumulating soils (17.46 ± 3.52 kg cm⁻², n = 36). It is likely that in the past seabirds, such as shearwaters, made burrows in the soils for nesting and propagating and inadvertently transported a large amount of P from the sea to the soils, resulting in the extremely large amounts of bio-available P in the present soils.     

Hot-water extractable carbon in soils: a sensitive measurement for determining impacts of fertilisation, grazing and cultivation

Using pre-established trial sites on allophanic soils, we investigated the impacts of long to medium-term pastoral management practices, such as fertilisation and grazing intensity, on a range of soil biological and biochemical properties; hot water-extractable C (HWC), water-soluble C (WSC), hot-water extractable total carbohydrates, microbial biomass-C and N and mineralisable N. These properties were examined for their usefulness as soil quality indicators responding to changes in the rhizosphere caused by management practices. Adjacent cropping, market garden and native bush sites located on similar soil types were included to determine the changes in soil biological and biochemical properties resulting from changes in land use. The seasonal variability of HWC and its relationship with other labile fractions of soil organic matter was also examined.Microbial biomass-C, mineralisable N and extractable total carbohydrates showed promise in differentiating treatment and land use effects. However, HWC was one of the most sensitive and consistent indicators examined at 52 different sites. The impact of different land uses on the amounts of HWC in the same soil type was far greater than that was observed for the soil organic carbon. The average values of HWC for soil under different land use were: native (4000 μg C g⁻¹ soil), sheep/beef pastures (3400), dairy pastures (3000), cropping (1000) and market gardening soils (850). HWC was also sensitive to differences within an ecosystem, e.g. effects of grazing intensities and effects of N or P fertilisers on pastures. The sheep and beef/cattle grazed pastures always had higher amounts of HWC than the intensively grazed dairy pastures. Nitrogen fertiliser application (200 and 400 kg N ha⁻¹ yr⁻¹) over the previous 5 yr had significant (P<0.001) negative impacts on HWC and other soil microbial properties. In contrast, long-term application of P fertilisers had a significant (P<0.001) positive effect on the HWC levels in pastoral soils. In the case of long-term P trials, HWC increased even though no increase in the total soil carbon pool was detected.HWC was positively correlated with soil microbial biomass-C (R²=0.84), microbial nitrogen (R²=0.72), mineralisable N (R²=0.86), and total carbohydrates (R²=0.88). All these correlations were significant at P<0.001 level of significance. The HWC was also positively correlated with WSC and total organic C. However, these correlations were poorer than those found for other soil parameters. Most of these measurements have been actively promoted as key indicators of soil quality. Given the strong correlations between HWC and other biochemical measurements, HWC could be used as an integrated measure of soil quality. When HWC is extracted, other pools of labile nutrients are also extracted along with C. Therefore it is suggested that decline in HWC would also indicate a decline in other labile organic pools of nutrients such as nitrogen, sulphur and phosphorus. About 40-50% of the C in the HWC extract was present as carbohydrates.     

Ozone deposition onto bare soil: A new parameterisation

The variables controlling ozone deposition onto bare soil are still unknown and it is necessary to understand this pathway well, as it represents a significant sink for ozone. Eddy-covariance measurements of ozone (O₃) fluxes were performed over bare soils in agricultural land. Three datasets with contrasted meteorological conditions and soil nitric oxide (NO) emissions were used to study the factors controlling soil deposition. It is considered that ozone deposition can be represented with an aerodynamic resistance (R_a), a quasi-laminar boundary layer resistance (Rb O₃), and an additional resistance, named soil resistance (R_soil). Although it is assumed in previous studies that soil resistance is a function of soil water content (SWC) and could be considered constant as variation of SWC at monthly scale are generally weak, the results of this study indicate that SWC is not the main factor controlling R_soil which shows daily and hourly variations. The main factor controlling soil resistance is the surface relative humidity which is positively correlated with R_soil, contrary to non stomatal resistance onto canopies which show a negative correlation with relative humidity. The relationship between R_soil and the surface relative humidity is probably due to a decrease in the surface available for ozone deposition, due to an increasing adsorption of water molecules onto the ground with relative humidity. A new parameterisation of R_soil was established, where R_soil is a function of the surface relative humidity only (R_soil = R_{soil min} × e^(k×RH_surf), and R_{soil min} = 21 ± 1.01 s m⁻¹ and k = 0.024 ± 0.001, mean ± SD). The measured and parameterised ozone deposition velocities agree well when soil NO emissions are negligible. However, when there are large soil NO emissions, the parameterised ozone deposition strongly underestimates the measured deposition velocity even if the chemical destruction of ozone by reaction with NO in the air column was evaluated to be negligible. This suggests that soil NO emissions enhance soil ozone deposition by chemical reaction at or near the soil surface. The new parameterisation allows a better estimation of soil deposition, especially during daytime when R_soil is overestimated using previously published parameterisations. It is an important step towards a better parameterisation of the non-stomatal uptake of ozone.     

Fire exclusion and nitrogen mineralization in low elevation forests of western Montana

Little is known regarding how fire exclusion influences nitrogen (N) cycling in low elevation forests of western Montana. Nor is it clear how the change in fire frequency that has resulted from forest management has influenced ecosystem function in terms of plant-soil-microbe interactions. A fire chronosequence approach was used to examine the influence of forest succession on soil biochemical properties and microbimal activity at 10 sites with varying time since fire (2-130 years). The rate of decomposition of buried tongue depressors and cotton strips, was found to decrease significantly (R²=0.410, P=0.087 and R²=0.761, P=0.003, respectively) with time since fire (TSF). Net N mineralization and nitrification, as estimated by resin sorbed and concentrations, both exhibited significant non-linear decreases (R²=0.870, P=0.000 and R²=0.620, P=0.007, respectively) with TSF. Nitrification potential measured using an aerated soil slurry method, also decreased significantly (R²=0.595, P=0.009) with TSF. These decreases in N availability along with an increase in the metabolic quotient and a decrease in labile C pools with TSF indicated a decline in substrate quality and microbial activity with secondary forest succession. The concentration of total phenols in mineral soil showed no significant trend with TSF, but was negatively correlated (R²=0.486, P=0.025) with resin sorbed concentration indicating either enhanced immobilization or perhaps chemical inhibition. These results imply that biochemical processes (decomposition and N transformations) may be limited by the lack of available substrate and potentially as a result of rapid immobilization, chemical inhibition or a combination of both at least partially induced by changes in vegetation with TSF. Our results suggest that N availability in ponderosa pine ecosystems of the inland Northwest are directly dependent upon fire history and secondary successional stage.     

Modeling vertical movement of organic matter in a soil incubated for 41 years with C labeled straw

The distribution of organic matter (OM) in the soil profile reflects the balance between inputs and decomposition at different depths as well as transport of OM within the profile. In this study we modeled movement of OM in the soil profile as a result of mechanisms resulting in dispersive and advective movement. The model was used to interpret the distribution of ¹⁴C in the soil profile 41 years after the labeling event. The model fitted the observed distribution of ¹⁴C well (R²=0.988, AIC_c=−82.6), with a dispersion constant of D=0.71 cm² yr⁻¹ and an advection constant of v=0.0081 cm yr⁻¹. However, the model consistently underestimated the amount of OM in the soil layers from 27 to 37 cm depth. A possible explanation for this is that different fractions of OM are transported by different mechanisms. For example, particulate OM, organomineral colloids and dissolved OM are not likely to be transported by the same mechanisms. A model with two OM fractions, one moving exclusively by dispersive processes (D=0.26 cm² yr⁻¹) and another moving by both dispersive (D=0.99 cm² yr⁻¹) and advective (v=0.23 cm yr⁻¹) processes provided a slightly better fit to the data (R²=0.995, AIC_c=−83.6). More importantly, however, this model did not show the consistent underestimation from 27 to 37 cm soil depth. This corroborates the assumption that differing movement mechanisms for different OM fractions are responsible for the observed distribution of ¹⁴C in the profile. However, varying dispersion, advection, and decay of OM with depth are also possible explanations.     

鄂尔多斯高原1982-2006年植被变化及其驱动因子

植被变化对全球气候变化的响应是近年来生态学研究的热点问题。鄂尔多斯高原地处中国西部生态敏感地带,研究该区域植被变化及其对气候变化的响应具有重要意义。利用1982—2006年的NO-AA/AVHRR NDVI数据和当地同期气象资料,研究了鄂尔多斯高原地区植被覆盖变化及其与气象因子的关系。结果表明:(1)1982—2006年鄂尔多斯高原全年平均NDVI显著增加(p=0.001 7),增加率为每年0.04%,平均温度也在显著增加(p<0.001),平均降水波动大但没有显著趋势(p>0.05);(2)鄂尔多斯高原植被NDVI增加的主要区域是典型草原,其春、夏、秋3季的季节平均NDVI都显著增加(p<0.001);(3)鄂尔多斯高原植被NDVI在生长阶段增加的主要驱动因子是降水,非生长阶段的变化主要是由温度引起的。     

Effects of vegetation and slope aspect on water budget in the hill and gully region of the Loess Plateau of China

Precipitation, throughfall, stemflow and soil water content were measured, and interception, transpiration, evaporation, runoff, deep percolation and soil water recharge were estimated in the natural Liaotung Oak (Quercus liaotungensis) and regrown Black Locust (Robinia pseudoacacia) forestlands in the hill and gully region of the China Loess Plateau. Four stands (south- and north-facing slopes) of two forests were studied between May 27, 2006 and October 31, 2007. Hydrological fluxes were calculated using a coupled water and heat flow model called CoupModel. Throughfall, stemflow and soil water content were used to calibrate the model. The simulations indicated that, interception, vegetation transpiration and soil water evaporation were the main components of water consumption in the 4 stands, accounting for about 90% of the precipitation. The simulated interception and vegetation transpiration in the south-facing slope (154 and 327 mm in regrown forestland and 173 and 338 mm in natural forestland) were lower than those in the north-facing slope (219 and 344 mm in regrown forestland and 203 and 342 mm in natural forestland). Soil water evaporation in the south-facing slope (416 mm in regrown forestland and 373 mm in natural forestland) was larger than that in the north-facing slope (325 mm in regrown forestland and 330 mm in natural forestland) in the same vegetation stands. This was mainly due to greater vegetation density in the north-facing slope than in the south-facing slope. For the regrown forestlands, the simulated soil water recharge was larger under north-facing slope stands (90 mm) than under south-facing slope stands (76 mm), and the natural forestland in the north-facing slope had the largest soil water recharge (104 mm). The results indicated that vegetation species and slope aspects significantly influenced the water balance budget in the soil–vegetation–atmosphere system. The water budget differences among the 4 stands indicate that care is required for properly selecting regrown tree-species. Soil and water conservation measures must be applied scientifically when converting farmland to forest in the Loess Plateau of China, especially on the south-facing slopes.     

Performance of para-nitrophenyl phosphate and 4-methylumbelliferyl phosphate as substrate analogues for phosphomonoesterase in soils with different organic matter content

Phosphomonoesterase (PMEase) activity plays a key role in nutrient cycling and is a potential indicator of soil condition and ecosystem stress. We compared para-nitrophenyl phosphate (pNPP) and 4-methylumbelliferyl phosphate (MUP) as substrate analogues for PMEase in 7 natural ecosystem soils and 8 agricultural top soils with contrasting C contents (8.0-414 g kg⁻¹ C) and pH (3.0-7.5). PMEase activities obtained with pNPP (0.05-5 μmol g⁻¹ h⁻¹) were significantly less than activities obtained with MUP (0.9-13 μmol g⁻¹ h⁻¹), especially in soils with a high organic matter content (>130 g kg⁻¹). Only PMEase activities assayed with MUP correlated significantly with total C and total N (r=0.7, P<0.01 all), and pH (r=−0.71, P<0.01). PMEase activities obtained with the two substrate analogues were correlated when expressed on a C-content basis (r=0.8, P<0.001), but not when expressed on an oven-dry soil weight basis. This indicated that interference by organic matter is related to the quantity rather than to the quality of organic matter. Overall, assaying with MUP was more sensitive compared to assaying with pNPP, particularly in the case of high organic and acid soils.