Microbial activities in soils of a Mediterranean ecosystem in different successional stages

This study reports a comparative analysis of soil enzyme activities (β-glucosidase, protease, urease, arylsulphatase, phosphatase and fluorescein diacetate hydrolase), ATP, total N and organic matter contents in three vegetal successional stages (meadow, low shrubland and high maquis) of a Mediterranean ecosystem in the Natural Reserve of Castel Volturno (Campania, Italy). Because water availability is a major limiting factor of soil microbial activity in Mediterranean ecosystems, the analysis was performed in late spring (May), after the rainy period, and in early autumn (October), after the long dry summer.A significant decrease in protease, arylsulphatase, urease and β-glucosidase activities was observed in meadow soil in the autumn sampling, probably due to the prolonged summer drought. Combining the values measured in the two sampling dates, the high maquis tended to have higher levels of enzymes activities than shrubland and meadow. Notably, high maquis had significantly higher phosphatase and arylsulphatase activities than shrubland and meadow and, in addition, a higher ATP content compared to meadow. Drastic changes were observed in EA/ATP ratios between the sampling periods in the meadow and shrubland, suggesting changes in the efficiency of microbial community more likely linked to climatic fluctuations than to the successional stage. The more stable EA/ATP ratio in the maquis probably reflects a constant contribution of microbial biomass to enzyme secretion.In conclusion, our results point to an increase in soil microbial activity accompanying the succession from meadow to high maquis that probably reflects a parallel increase in soil functions. Nevertheless, spatial heterogeneity and, more important, temporal variations in soil activities often may obscure differences related to the plant cover type.     

Experimental drought reduced acid and alkaline phosphatase activity and increased organic extractable P in soil in a Quercus ilex Mediterranean forest

A six-year (1999–2005) experiment of drought manipulation was conducted in a Quercus ilex Mediterranean forest (Southern Catalonia) to simulate predicted climatic conditions projected for the decades to come. The aim was to investigate the direct and indirect effects of drought conditions on acid and alkaline phosphatase activity in soil and on P concentrations in soil, leaves and litter throughout the year. Soil acid phosphatase activity was higher than soil alkaline phosphatase activity. Drought reduced acid phosphatase activity in soil in all seasons, including summer and winter, the seasons with less biological activity due to water and cold stress. Reductions of soil water content between 13 and 29% reduced soil acid phosphatase activity between 22 and 27% depending on the season. Drought reduced alkaline phosphatase activity (by 28%) only in winter. Soil acid and alkaline phosphatase activities were positively correlated with soil water content in all seasons. In contrast short-term available-P which increased under drought in several seasons was weakly correlated with soil phosphatase activities. As a result, immediately/short-term available-P concentration ratios decreased in all the seasons (between 10 and 71%). Drought increased foliar P concentration and reduced the C/P concentration ratio in litter fall of the dominant tree Q. ilex. Drought also decreased the ratio between organic C and short-term available-P in soil. The results show that soil phosphatase activity is more directly dependent on changes in water availability than on changes in its substrate, short-term available-P. These effects of drought have several implications: the accumulation in the soil of labile P not directly available to plants, the increase in potential P losses from leaching and erosion during the torrential rainfalls typical of the Mediterranean climate, and changes in plant, litter and soil C:P stoichiometry that may lead to changes in soil trophic chains.     

The use of polyethylene mulches to change soil microclimate as revealed by enzyme activity and biomass nitrogen,sulphur and phosphorus

Summary The effects of white, black and clear polyethylene mulches on temporal fluctuations in protease, sulphatase and phosphatase activities and levels of available and biomass N, S and P in raised beds of soil under a strawberry crop were investigated under field conditions. During spring, summer and early autumn, clear and, to a lesser extent black, polyethylene mulch increased both maximum and minimum mean monthly temperatures over those recorded with white mulch or no mulch. During summer and autumn, soil moisture content measured at monthly intervals was higher under mulched than unmulched conditions. Levels of extractable nitrate and sulphate in soils during autumn followed the order: clear mulch = black mulch > white mulch > no mulch. However, there were no significant differences in levels of enzyme activity or concentrations of biomass N, S or P between treatments. There were significant seasonal fluctuations in estimates of biomass N, S and P, but there was no close relationship between fluctuations in any estimate. This discrepancy was attributed to errors in the chloroform fumigation technique. Enzyme activities showed significant temporal fluctuations but the three enzymes did not show similar times of minimum and maximum activity. The complexity of interactions between soil microbial and biochemical properties and nutrient availability was exemplified by the absence of a discernible relationship between seasonal fluctuations in enzyme activities and levels of biomass N, S and P and/or levels of extractable mineral N, S and P in the soil. Strawberry fruit yields and vegetative growth followed the order : clear mulch = black mulch>white mulch>no mulch. Clear, and to a lesser extent black mulch increased the percentage of total yields produced in the first 2 months of cropping.     

Plant regeneration functional groups modulate the response to fire of soil enzyme activities in a Mediterranean shrubland

Soil enzymes are critical to soil nutrient cycling function but knowledge on the factors that control their response to major disturbances such as wildfires remains very limited. We evaluated the effect of fire-related plant functional traits (resprouting and seeding) on the resistance and resilience to fire of two soil enzyme activities involved in phosphorus and carbon cycling (acid phosphatase and β-glucosidase) in a Mediterranean shrublands in SE Spain. Using experimental fires, we compared four types of shrubland microsites: SS (vegetation patches dominated by seeder species), RR (patches dominated by resprouter species), SR (patches co-dominated by seeder and resprouter species), and IP (shrub interpatches). We assessed pre- and post-fire activities of the target soil enzymes, available P, soil organic C, and plant cover dynamics over three years after the fire. Post-fire regeneration functional groups (resprouter, seeder) modulated both pre- and post-fire activity of acid phosphatase and β-glucosidase, with higher activity in RR and SR patches than in SS patches and IP. However, we found no major differences in enzyme resistance and resilience between microsite types, except for a trend towards less resilience in SS patches. Fire similarly reduced the activity of both enzymes. However, acid phosphatase and β-glucosidase showed contrasting post-fire dynamics. While β-glucosidase proved to be rather resilient to fire, fully recovering three years after fire, acid phosphatase showed no signs of recovery in that period. Overall, the results indicate a positive influence of resprouter species on soil enzyme activity that is very resistant to fire. Long-lasting decrease in acid phosphatase activity probably resulted from the combined effect of P availability and post-fire drought. Our results provide insights on how plant functional traits modulate soil biochemical and microbiological response to fire in Mediterranean fire-prone shrublands.     

Drought decreases soil enzyme activity in a Mediterranean Quercus ilex L. forest

Longer and more severe drought periods are expected in the near future for Mediterranean ecosystems. Soil enzymes play an essential role in the nutrient mineralization and their activity is an exceptional sensor in predicting the capacity of nutrient supply to plants. We conducted an experiment of water availability manipulation in evergreen oak mountain stands with the aim to study the effects of enhanced drought on the activity of five soil enzymes. The drought treatment consisted of runoff exclusion by a ditch along the entire top edge of the upper part of treatment plots and partial rain exclusion by suspending PVC strips and funnels. The reduction of 10% of soil moisture produced by runoff exclusion decreased urease activity by 10-67%, protease activity by 15-66% and β-glucosidase activity by 10-80%, depending on annual period and soil depth. The reduction of 21% of soil moisture produced by runoff and rainfall exclusion together reduced urease activity by 42-60%, protease activity by 35-45%, β-glucosidase activity by 35-83% and acid phosphatase activity by 31-40%. No significant effects were observed on alkaline phosphatase activity. The activities of the enzymes involved in the nitrogen cycle, protease and urease, were the most affected by drought. In all cases, the activities of these enzymes strongly decreased with soil depth and they were greater in spring than in autumn. These results show the link between drought and a slower nutrient turn-over, which decreases the nutrient supply to plants.     

The counteracting effects of elevated atmospheric CO2 concentrations and drought episodes: Studies of enchytraeid communities in a dry heathland

The potential impacts of interactions of multiple climate change factors in soil ecosystems have received little attention. Most studies have addressed effects of single factors such as increased temperature or atmospheric CO₂ but little is known about how such environmental factors will interact. In the present study we investigate the effects of in situ exposure to elevated atmospheric CO₂ concentration, increased temperatures and prolonged drought episodes on field communities of Enchytraeidae (Oligochaeta) in a dry heathland (Brandbjerg, Denmark). Increased CO₂ had a positive effect on enchytraeid biomass, whereas drought significantly reduced it. Elevated temperature did not result in any detectable effects. No interactions between the three factors were observed. Interestingly, the positive effect of increased CO₂ and the negative effect of drought were cancelled out when applied in combination. Thus, in the combined drought and CO₂ treatment, and when additionally combined with increased temperature, the total biomass of enchytraeids was similar to those in the ambient plots. The positive effect of increased CO₂ seemed to be influenced by season, as the response was most pronounced during summer, and less so in autumn. The impact of the drought was more consistent, with enchytraeids reduced at all sampling occasions. Moreover, the negative effect of drought seemed to depend on the inter-annual variability of precipitation. The year with a dry summer and autumn (2006) showed a stronger impact of drought on the enchytraeids, compared to the year with a wet summer and autumn (2007). Our study emphasises the importance of multi-factorial experimental design as a means to investigate effects of climatic changes.     

Soil N dynamics in a natural calcareous grassland under a changing climate

 This paper reports the results from a medium-term field scale investigation into the effects of simulated climate change on soil N mineralisation in a semi-natural calcareous grassland in southern England. The experiment utilised soil warming cables, automatic rainshelters and a watering system to examine two climate change scenarios: warmer winters with summer drought and warmer winters with enhanced summer rainfall. Gross N mineralisation rates in treated plots were determined, using ¹⁵N pool dilution techniques, at 6-weekly intervals over a 3-year period. Results from control plots showed a strong seasonality of mineralisation with highest rates in autumn and winter and lowest rates in summer. They suggest that water availability is the main constraint on microbial processes and plant growth. Unexpectedly, additional summer rainfall had no direct effect on N mineralisation at the time of application (summer). The treatment did, however, significantly (<0.05%) reduce rates in subsequent autumn and winter months. In contrast, summer drought significantly increased N mineralisation rates in autumn and winter. Winter warming similarly had no direct effect on N mineralisation in winter but decreased rates in spring. We hypothesise that the observed treatment effects result from changes in organic C and N input, in plant litter, resulting from the direct impact of climatic manipulation on perennial plant growth, death and senescence. This paper compares and contrasts the response to climate manipulation in the grassland system with results from other ecosystem types such as northern forests. Received: 1 December 1997     

Pinus halepensis Mill. plantations did not restore organic carbon,microbial biomass and activity levels in a semi-arid Mediterranean soil

Maquis is a dense evergreen shrub layer which, in semi-arid Mediterranean lands, is commonly linked to the presence of well-conserved soils with large contents of mineralizable substrates. It was our aim to test whether: (i) maquis promotes soil microbial biomass and activity, and (ii) mature pine plantations without a shrubby understory support microbial biomass and activity levels comparable to those of stands with maquis. Surface soil samples were taken in four sites that sustain pine plantations (PP), maquis with pines (MP), maquis (MQ) and grasslands (GS). Microbial biomass was inferred from the C content in the soil microbiota. The ATP content in fresh samples and the CO₂–C production from incubated samples were used to assess microbial activity, as was the activity of β-glucosidase and alkaline phosphatase. Topsoils under maquis (MP and MQ) were the most fertile, both chemically (high organic carbon contents) and physically (low bulk density, high aggregate stability) and showed by far the largest levels of microbial biomass and activity. These levels in soils under PP, which sustained a successful plantation in terms of tree canopy density but lacked the shrubby understory, were significantly smaller than those of the adjacent shrubland with pines (MP). Redundancy analysis extracted a main axis explaining 67% of the variation of the microbiological soil properties, which was interpreted as an environmental gradient of soil fertility. Along this axis, the samples were separated according to the presence or absence of a maquis dominated by late-successional species; other factors such as soil type, slope position and aspect were less influential. The effects of afforestation practices on the detritus-based system should be considered in the design of future forest restoration strategies in desertification-threatened lands.     

Can field populations of the enchytraeid,Cognettia sphagnetorum,adapt to increased drought stress?

The ability to evolve increased drought tolerance in response to climate change was investigated in the enchytraeid, Cognettia sphagnetorum. Populations exposed to reduced precipitation or increased night time temperature for more than six years were collected in mixed Calluna/grass heathland at the Mols Laboratory, Denmark. The level of prolonged drought and increased temperature corresponded to a predicted climate change scenario and has been applied since 1999. In autumn 2005, enchytraeids were sampled in 3 cm intervals down to 9 cm depth and total number, biomass, diversity and soil organic matter were determined. The drought treatment resulted in a significant reduction of the density and biomass of enchytraeids, as well as changes in the species composition. In total, five different genera were found at the site in all three treatments (control, temperature and drought). C. sphagnetorum was the dominant species, especially in the upper 0–3 cm, and was clearly affected by the drought treatment. C. sphagnetorum from all plots were cultured in the laboratory to rear second or third generation adults. Results showed that populations of drought treated plots had not developed an increased drought resistance compared to populations of control or warming plots even after several years of a putative severe selection. Lack of adaptive potential in C. sphagnetorum suggests that more frequent periods with drought in the future will have a very strong negative influence on enchytraeid density, biomass and diversity.     

Dynamics and availability of phosphorus in the rhizosphere of a temperate silvopastoral system

A field rhizosphere study was carried out over a period of 12 months on a 6-year-old silvopastoral trial in New Zealand. The trial comprised radiata pine (Pinus radiata) with lucerne (Medicago sativa) and perennial ryegrass (Lolium perenne) understoreys. The study was initiated because of the unique interrelationships between roots in silvopastoral systems and a paucity of understanding about the processes involved in phosphorus (P) dynamics in temperate silvopastoral systems. Improving our understanding in this area has important implications for nutrient management in silvopastoral systems. Rhizosphere soils were analysed to determine inorganic (Pi) and organic (Po) P fractions, macroporous resin Pi and Po, phosphatase enzyme activity, microbial biomass carbon and pH. Concentrations of labile Pi were consistently greater and Po lower in tree rhizosphere soil compared to the companion understorey, indicating that radiata pine when grown with a productive understorey mineralised Po to a greater extent than either understorey species. Tree rhizosphere soil from under lucerne and lucerne rhizosphere soil contained the lowest concentrations of total Pi and Po compared with tree under ryegrass and ryegrass rhizosphere soils. This was partly attributed to higher levels of phosphatase enzyme activity in the lucerne rhizosphere soils. The results suggest the combination of lucerne with radiata pine may enhance greater utilisation of soil P, although this requires further investigation. Lower levels of labile Po, and higher levels of labile Pi and phosphatase enzyme activity, were determined in tree and understorey lucerne and ryegrass rhizosphere soils in spring compared with autumn. This data confirmed that overall rates of soil organic P mineralisation are greatest in spring.     

Effect of liming on some chemical,biochemical, and microbiological properties of acid soils under spruce (Picea abies L.)

Summary Soil pH, total organic C, total N, exchangeable Al, available P, CO₂ evolution, microbial biomass C and N, phosphatase and dehydrogenase activities were determined in acid soils sampled under spruce subjected to acid deposition, before and after liming. A slight decrease in pH values was observed from the edge of a tree canopy to the base of the trunk in acid soils. Liming drastically reduced exchangeable Al and increased CO₂ evolution, microbial biomass, and the metabolic quotient. The microbial biomass C to total organic C ratio increased after liming but did not reach 2%, the average value considered valid in soils where the C content is in equilibrium, that is when C inputs are equal to C outputs. The microbial biomass C:N ratio decreased after liming, thus indicating that bacteria became predominant over fungi when soil acidity decreased. Dehydrogenase activity but not phosphatase activity was increased by liming. The decrease in phosphatase activity was not completely related to the increase in available P, but was also dependent on microbial growth and the decrease in acid phosphatase, the predominant component of acid soils.     

Recovery of enchytraeid populations after severe drought events

The potential impact of changes in precipitation patterns associated with climate changes was investigated in Enchytraeidae (Oligochaeta) in a Danish heathland. The amount of precipitation was manipulated during spring and summer in an experimental field site in order to reveal effects of three different drought regimes: weak drought (WD), medium drought (MD) and high drought (HD). Enchytraeids were sampled every six to eight weeks (0–9 cm depth) for more than the eight months and soil water potential (SWP) and soil water content (SWC) was measured on a regular basis for five months.The enchytraeid communities were generally reduced due to a natural drought spell. The HD treatment significantly reduced the moisture level of the soil further with SWP below −15 bar (5 and 10 cm depth) and SWC around 5% (v/v) for more than two months. As a result almost no enchytraeids were found in HD plots after two months with high drought stress. Nevertheless, the HD-treated enchytraeid communities recovered within two months, as there was no significant difference in biomass and density of the different treatments at that time. During periods with extreme low SWP enchytraeids were practically absent in the top soil (0–3 cm), but a few animals were found in 3–6 cm. During this period SWP was around −15 bar even in 20 cm depth, indicating that active stages could not have survived. Thus, we suggest that the species present must be dependent on a drought tolerant stage, as vertical migration could not have supported the observed recovery.SWP and SWC were both significantly correlated with the total density and biomass of enchytraeids. However, density was better correlated with the SWP and SWC compared to biomass, which can be due to hatching of cocoons and increased fragmentation rate.     

Effects of seasonal grazing and precipitation regime on the soil macroinvertebrates of a Mediterranean old-field

Soil macroinvertebrate communities (SMC) are well known to influence major ecosystem processes, but relatively few investigations have examined the mechanisms and factors involved in SMC regulation. We conducted a factorial experiment with combinations of seasonal grazing by sheep and irrigation (simulating different precipitation regimes) to assess their effects on the SMC of a semiarid Mediterranean old-field. We also analyzed effects on plant species richness, total aboveground biomass, and litter. The data were collected in autumn and spring, the two favorable seasons for SMC and primary production in the region, and season was included as an additional random factor. Main results were: 1) Ungrazed plots accumulated more aboveground plant biomass and litter during spring, providing extra food for soil biota. However, grazing during autumn or spring did not affect SMC characteristics. 2) Reduction of inter-annual precipitation variability in autumn and spring increased the abundance of two decomposer taxa: Oligochaeta and Diplopoda. Additionally, if summer drought was reduced, plant species richness, litter and the abundance of Isopoda were increased. 3) Oligochaeta and Diplopoda increase their abundance in spring, particularly, the most abundant taxon (Oligochaeta). We conclude that inter- and intra-annual variability in precipitation is a key environmental factor for the decomposer soil fauna in Mediterranean ecosystems, modifying the physical characteristics of the soils (humidity, hardness, etc.), as well as affecting the amount or characteristics of plant biomass or litter. The respiration system of the macroinvertebrates (cutaneous, tracheal or branquial) and the capacity to migrate vertically into the soil may determine the decomposers' responses to precipitation.     

1981-2019年鄱阳湖流域潜在蒸散变化特征及气候成因分析

为探明鄱阳湖流域潜在蒸散的变化特征,揭示不同季节潜在蒸散趋势差异及其气候成因,该研究基于1981-2019年鄱阳湖流域74个气象站点的逐日气象资料,利用Penman-monteith公式计算各站点逐日潜在蒸散量(ET_0),结合敏感性-贡献率法,分析了1981-2019年鄱阳湖流域ET_0在年、季尺度上的变化趋势及其主导气候因子。结果表明:1)1981-2019年鄱阳湖流域年尺度ET_0具有先降后升的特点,21世纪以前呈下降趋势,进入21世纪后下降趋势减弱,并在21世纪头10年后半段转为上升趋势。整体上,鄱阳湖流域ET_0呈显著增加趋势(1.50mm/a,P0.05),其中春季增加速率最快(0.81mm/a,P0.05),秋季次之(0.64mm/a,P0.05),冬季和夏季分别呈不显著增加和下降趋势(P0.05);2)最高气温升高是年均ET_0和春、秋季ET_0增加的主导因子,冬季ET_0增加的主导因子为最低气温的升高,而辐射下降是夏季ET_0下降的主导因子;3)不同气象要素贡献率的年代际变化分析表明,鄱阳湖流域年均ET_0由降转升的原因是2000年以后气温增加的正贡献超过风速和辐射下降的负贡献。该研究对鄱阳湖流域农田水分管理和季节性干旱监测具有重要意义。     

Changes in soil microbial substrate utilization in response to altered litter diversity and precipitation in a Mediterranean shrubland

This study aimed at quantifying the consequences of reduced precipitation and plant diversity on soil microbial community functioning in a Mediterranean shrubland of southern France. Across a natural gradient of shrub species diversity, we established a total of 92 plots (4 × 4 m) with and without a moderate rain exclusion treatment of about 12 % of total precipitation. Shrub diversity included all possible combinations of the four dominant species (Cistus albidus, Quercus coccifera, Rosmarinus officinalis, and Ulex parviflorus). Respective leaf litter mixtures of these species combinations were exposed in all plots over 2 years. We quantified how litter species richness and the reduction in precipitation affected the soil microbial substrate utilization (measured by CO₂ evolution using the MicroResp method) on soil samples collected underneath each individual litter mixture after 1 and 2 years of decomposition. Moderate precipitation reduction had a minor impact, but litter species richness and the dissimilarity in phenolic concentrations (estimated using Rao’s quadratic entropy) showed a positive effect on the diversity of substrates metabolized by the microbial communities. Moreover, litter species richness increased soil microbial activity by increasing the catabolic diversity of the soil microbial community. These effects were mostly driven by the presence of Quercus and Ulex leaf litter, which at the same time reduced microbial metabolic dominance, while the presence of Rosmarinus had opposite effects. Our data suggest that plant species loss can have stronger effects on the functioning of soil microbial communities than moderate drought, with potentially important feedbacks on biogeochemical cycling in Mediterranean shrubland ecosystems.     

适宜的水氮处理提高稻基农田土壤酶活性和土壤微生物量碳氮

为探讨节水灌溉与氮肥施用对稻田土壤微生物特性的影响,该试验采用防雨棚池栽试验,研究2个灌溉模式（常规灌溉与控制灌溉）与3个水平施氮量（90、180和270 kg/hm2））对稻基农田土壤脲酶活性、土壤过氧化氢酶活性、土壤磷酸酶活性、土壤转化酶活性、土壤微生物量碳及土壤微生物量氮的影响。研究结果表明,随着施氮水平增加,土壤脲酶活性和土壤微生物量氮增加,土壤过氧化氢酶活性、土壤磷酸酶活性、土壤转化酶活性、土壤微生物量碳、土壤微生物量碳与土壤微生物量氮的比值、土壤微生物熵均呈先增加后降低趋势;与常规灌溉相比,控制灌溉显著提高稻基农田土壤脲酶活性、土壤过氧化氢酶活性、土壤磷酸酶活性、土壤转化酶活性、土壤中微生物量碳、土壤微生物量氮、土壤微生物熵,降低土壤微生物量碳与土壤微生物量氮的比值。在该试验条件下,以控制灌溉模式下施氮量180 kg/hm2可获得最优的生物环境,土壤脲酶活性、土壤过氧化氢酶活性、土壤磷酸酶活性、土壤转化酶活性、土壤中微生物量碳、土壤微生物量氮分别达到3.02×10-2 mg/g、0.93 mL/g、5.70 mg/g、10.08 mL/g、237.58 mg/kg、52.60 m/kg。该研究对认识稻基农田水氮耦合关系、指导江淮丘陵季节性干旱区水稻优质节水高产高效栽培实践提供理论依据。     

Phosphatase activity in the rhizosphere and its relation to the depletion of soil organic phosphorus

Summary The distribution of phosphatase activity and of phosphate fractions of the soil in the proximity of roots was studied in order to evaluate the significance of phosphatases in P nutrition of various plants (Brassica oleracea, Allium cepa, Triticum aestivum, Trifolium alexandrinum). A considerable increase in both acid and alkaline phosphatase activity in all the four soil-root interfaces was observed. Maximum distances from the root surface at which activity increases were observed ranged from 2.0 mm to 3.1 mm for acid phosphatase and from 1.2 mm to 1.6 mm for alkaline phosphatase. The increase in phosphatase activity depended upon plant age, plant species and soil type. A significant correlation was noticed between the depletion of organic P and phosphatase activity in the rhizosphere soil of wheat (r = 0.99^**) and clover (r = 0.97^**). The maximum organic P depletion was 65% in clover and 86% in wheat, which was observed within a distance from the root of 0.8 mm in clover and 1.5 mm in wheat. Both the phosphatases in combination appear to be responsible for the depletion of organic P.     

Microbial growth rate measurements reveal that land-use abandonment promotes a fungal dominance of SOM decomposition in grazed Mediterranean ecosystems

The present study investigated the effects of land-use abandonment on the soil decomposer community of two grazed Mediterranean ecosystems (an annual grassland with scattered holm oaks and a low-density shrubland). To test the influence of grazing abandonment, a set of plots within each site were fenced and kept undisturbed during 4–5 years, during which above-ground plant community structure was monitored. After that, soil samples were collected from grazed and abandoned plots corresponding to the three different soil conditions: away from (“grass”) and below tree canopies (“oak”) within the annual grassland, and from the shrubland (“shrub”). Soil samples were split into two different layers (0–5 and 5–15 cm) and then analyzed for saprotrophic fungal (acetate into ergosterol incorporation) and bacterial (leucine incorporation) growth rates. Ergosterol content (as a fungal biomass estimator) and a standard set of soil chemistry variables were also measured. After 5 years of grazing exclusion, saprotrophic fungal growth rate clearly increased in both grass and oak surface layers whereas bacterial growth rate was not altered. This translated into significantly higher fungal-to-bacterial (F/B) growth rate ratios within the ungrazed plots. Similar trends were observed for the shrub soils after 4 years of exclusion. On the contrary, abandonment of grazing had negligible effects on the ergosterol content, as well as on the soil chemical variables (soil organic carbon, total N, C/N ratio, and pH), in all the three soil conditions assessed. These results indicated a shift toward a more fungal-dominated decomposer activity in soils following cessation of grazing and highlighted the sensitivity of the microbial growth rate parameters to changes associated with land use. Moreover, there were evidences of a faster fungal biomass turnover in the ungrazed plots, which would reflect an accelerated, though not bigger, fungal channel in soil organic matter mineralization.     

Interannual and interseasonal soil CO2 efflux and VOC exchange rates in a Mediterranean holm oak forest in response to experimental drought

Climate models predict drier conditions in the next decades in the Mediterranean basin. Given the importance of soil CO₂ efflux in the global carbon balance and the important role of soil monoterpene and volatile organic compounds (VOCs) in soil ecology, we aimed to study the effects of the predicted drought on soil CO₂, monoterpenes and other VOC exchange rates and their seasonal and interannual variations. We decreased soil water availability in a Mediterranean holm oak forest soil by means of an experimental drought system performed since 1999 to the present. Measurements of soil gas exchange were carried out with IRGA, GC and PTR-MS techniques during two annual campaigns of contrasting precipitation. Soil respiration was twice higher the wet year than the dry year (2.27±0.26 and 1.05±0.15, respectively), and varied seasonally from 3.76±0.85 μmol m⁻² s⁻¹ in spring, to 0.13±0.01 μmol m⁻² s⁻¹ in summer. These results highlight the strong interannual and interseasonal variation in CO₂ efflux in Mediterranean ecosystems. The drought treatment produced a significant soil respiration reduction in drought plots in the wet sampling period. This reduction was even higher in wet springs (43% average reduction). These results show (1) that soil moisture is the main factor driving seasonal and interannual variations in soil respiration and (2) that the response of soil respiration to increased temperature is constrained by soil moisture. The results also show an additional control of soil CO₂ efflux by physiology and phenology of trees and animals. Soil monoterpene exchange rates ranged from −0.01 to 0.004 nmol m⁻² s⁻¹, thus the contribution of this Mediterranean holm oak forest soil to the total monoterpenes atmospheric budget seems to be very low. Responses of individual monoterpenes and VOCs to the drought treatment were different depending on the compound. This suggests that the effect of soil moisture reduction in the monoterpenes and VOC exchange rates seems to be dependent on monoterpene and VOC type. In general, soil monoterpene and other VOC exchange rates were not correlated with soil CO₂ efflux. In all cases, only a low proportion of variance was explained by the soil moisture changes, since almost all VOCs increased their emission rates in summer 2005, probably due to the effect of high soil temperature. Results indicate thus that physical and biological processes in soil are controlling soil VOC exchange but further research is needed on how these factors interact to produce the observed VOCs exchange responses.     

Soil fertility differences across a land-use intensification gradient in the highlands of Chiapas,Mexico

In the highlands of Chiapas, southern Mexico, soil texture and soil chemical properties were measured in 70 agricultural fields covering a range of slope positions and managements. Fields represented four corn cropping systems: long fallow, short fallow, pasture–cultivation rotation, and annual continuous cultivation, in addition to fallow at rest (forest, shrubland, and pastures). Fields were located in four slope positions (upper, middle, and lower slopes, and doline floor) in a karst landscape developed on limestone with additions of acid volcanic ashes. Distribution of clays and sands were related to the toposequence and the percentage of clay fraction increased from upper slope to doline floor. Sand presented a reverse pattern. Some soil chemical properties also vary in a characteristic way along the toposequence. Exchangeable Ca²⁺ and Mg²⁺ effective cation exchange capacity (ECEC) and pH were higher in the doline floor than in the other slope positions. Soil organic matter and total N contents were 30.4 and 35.2% higher under long fallow than under annual continuous cultivation. Soil Olsen P was greater under cultivation than under fallow (irrespective of the cropping system), with the highest values under annual continuous cultivation (16.2±8.3 mg kg⁻¹) and the lowest in the forest (5.3±2.5 mg kg⁻¹). Exchangeable K⁺ was 74 and 51% higher in cultivated plots of the long and the short fallow than under forest and shrubland, respectively. Exchangeable Ca²⁺ and Mg²⁺ and ECEC did not vary significantly among the cropping systems. Inputs of basic cations through burning of tree and shrub vegetation increased the soil pH. These results indicate that land use intensification is leading to a decline in soil organic matter and total N, whereas Olsen P increased with intensification due to the continuous application of P fertilizers and sheep manure as soil conditioners in intensively cultivated fields.