Frequency and Extent of Water Limitation to Primary Production in a Mesic Temperate Grassland

The frequency and extent of water limitation to aboveground net primary production (ANPP) in a mesic grassland in NE Kansas (Konza Prairie, USA) was assessed with an 8-year irrigation experiment. Since 1991, transects spanning upland and lowland sites in annually burned, ungrazed tallgrass prairie were provided with supplemental water to satisfy evapotranspirational demands. This protocol minimized water limitations during the growing season, as well as interannual variability in water stress. Irrigation of this mesic grassland increased ANPP in 6 of 8 years by an average of 26% when compared to control transects. Although interannual variation in ANPP was greater in uplands than lowlands at nominal levels of precipitation, reducing interannual variability in water availability via irrigation eliminated topographic differences; the irrigation protocol also reduced interannual variability in ANPP by as much as 40%. The addition of supplemental water enabled us to extend the relationship between annual precipitation and ANPP in grasslands to precipitation levels (average, 1153 mm; maximum, 1346 mm) similar to those experienced by more mesic grasslands that today exist only as remnants several hundred kilometers east of Kansas. This relationship was linear (r ²= 0.81), with maximum ANPP (738 g/m²) similar to values reported for sites in Illinois and Wisconsin. After 8 years of irrigation, production of the C₃ forb component was twice that in control sites. These results indicate that water limitations in grasslands at the western edge of the presettlement extent of tallgrass prairie affect ANPP in most years and that this high frequency of water limitation may lead to greater dominance of the C₄ grasses than is seen in more eastern grassland sites. Received 18 January 2000; accepted 19 July 2000.     

Inter-Annual Precipitation Variability Decreases Switchgrass Productivity from Arid to Mesic Environments

Cellulosic biofuels are an important source of renewable biomass within the alternative energy portfolio. Switchgrass (Panicum virgatum L.), a perennial C₄ grass native to North America, is widely studied as a biofuel feedstock for its consistently high yields and minimal input requirements. The influences of precipitation amount and temporal variability on the fertilizer response of switchgrass productivity are not fully understood. Moreover, global climate models predict changes in rainfall patterns towards lower and increasingly variable soil water availability in several productive areas worldwide, which may impact net primary production of biofuel crops. We conducted a meta-analysis of aboveground net primary production of switchgrass from 48 publications encompassing 82 different locations, 11 soil types, 52 switchgrass cultivars, fertilizer inputs between 0 to 896 kg N ha^?1 year^?1, and 1 to 6 years of annual productivity measures repeated on the same stand. Productivity of the lowland ecotype doubled with N rates >?131 kg N ha^?1 year^?1, but upland ecotype productivity increased only by 50%. Results showed an optimum N rate of 30 to 60 kg N ha^?1 year^?1 for both ecotypes, after which biomass gain per unit of N added decreased. Growing season precipitation (GSPPT) and inter-annual precipitation variability (inter-PPTvar) affected both ecotypes similarly. Long-term mean annual precipitation (MAP) differentially affected lowland and upland productivity, depending on the N level. Productivity responses to MAP and GSPPT were similar for both upland and lowland ecotypes at none or low N rates. When N increased beyond 60 kg N ha^?1 year^?1, lowland cultivars had a greater growth response to MAP than uplands. Productivity increased with increasing GSPPT and MAP and had a positive linear response to MAP ranging from 600 to 1200 mm year^?1. One third of the variability in switchgrass production was accounted for by inter-PPTvar. After accounting for MAP, sites with higher inter-PPTvar had lower switchgrass productivity than sites with lower inter-PPTvar. Increased inter-annual variation in precipitation reduced production of both ecotypes. Predicted changes in the amount and timing of precipitation thus likely will exert greater influence on production of upland than lowland ecotypes of switchgrass.     

Additive Effects of Warming and Increased Nitrogen Deposition in a Temperate Old Field: Plant Productivity and the Importance of Winter

Both climate warming and atmospheric nitrogen (N) deposition are predicted to alter plant productivity and species composition over the next century. However, the extent to which their effects may interact is unclear. For example, over winter, the effects of warming on soil freezing dynamics may promote ecosystem N losses, thereby limiting increases in productivity in response to warming, yet these losses may be compensated for by increased N deposition. We measured plant production and species composition in response to warming (winter-only or year-round) and N addition in a temperate old field. We used shoot allometric relationships to estimate aboveground production non-destructively and sampled root biomass destructively throughout two growing seasons. We also used spectral data (normalized difference vegetation index—NDVI) to examine the treatment effects on the timing of plant green-up and senescence. In 2007, which featured an exceptionally dry summer, there were no treatment effects on plant growth. However, in 2008, warming (both winter-only and year-round) and N addition combined approximately doubled aboveground productivity, and these effects were additive. Warming increased root biomass, but no N effect was evident. Conversely, N addition increased NDVI, but NDVI was unresponsive to warming. Overall, our results do not support the hypothesis that warming-induced changes to soil freezing dynamics limit plant productivity in our system. On the contrary, they demonstrate that winter warming alone can increase primary productivity to the same extent as year-round warming, and that this effect may interact very strongly with interannual variation in precipitation.     

Warming and Elevated CO<Subscript>2</Subscript> Interact to Alter Seasonality and Reduce Variability of Soil Water in a Semiarid Grassland

Global changes that alter soil water availability may have profound effects on semiarid ecosystems. Although both elevated CO₂ (eCO₂) and warming can alter water availability, often in opposite ways, few studies have measured their combined influence on the amount, timing, and temporal variability of soil water. Here, we ask how free air CO₂ enrichment (to 600 ppmv) and infrared warming (+?1.5 °C day, +?3 °C night) effects on soil water vary within years and across wet-dry periods in North American mixed-grass prairie. We found that eCO₂ and warming interacted to influence soil water and that those interactions varied by season. In the spring, negative effects of warming on soil water largely offset positive effects of eCO₂. As the growing season progressed, however, warming reduced soil water primarily (summer) or only (autumn) in plots treated with eCO₂. These interactions constrained the combined effect of eCO₂ and warming on soil water, which ranged from neutral in spring to positive in autumn. Within seasons, eCO₂ increased soil water under drier conditions, and warming decreased soil water under wetter conditions. By increasing soil water under dry conditions, eCO₂ also reduced temporal variability in soil water. These temporal patterns explain previously observed plant responses, including reduced leaf area with warming in summer, and delayed senescence with eCO₂ plus warming in autumn. They also suggest that eCO₂ and warming may favor plant species that grow in autumn, including winter annuals and C₃ graminoids, and species able to remain active under the dry conditions moderated by eCO₂.     

Acclimation of Foliar Respiration and Photosynthesis in Response to Experimental Warming in a Temperate Steppe in Northern China

Background

Thermal acclimation of foliar respiration and photosynthesis is critical for projection of changes in carbon exchange of terrestrial ecosystems under global warming.

Methodology/Principal Findings

A field manipulative experiment was conducted to elevate foliar temperature (T _leaf) by 2.07°C in a temperate steppe in northern China. R _d/T _leaf curves (responses of dark respiration to T _leaf), A _n/T _leaf curves (responses of light-saturated net CO₂ assimilation rates to T _leaf), responses of biochemical limitations and diffusion limitations in gross CO₂ assimilation rates (A _g) to T _leaf, and foliar nitrogen (N) concentration in Stipa krylovii Roshev. were measured in 2010 (a dry year) and 2011 (a wet year). Significant thermal acclimation of R _d to 6-year experimental warming was found. However, A _n had a limited ability to acclimate to a warmer climate regime. Thermal acclimation of R _d was associated with not only the direct effects of warming, but also the changes in foliar N concentration induced by warming.

Conclusions/Significance

Warming decreased the temperature sensitivity (Q ₁₀) of the response of R _d/A _g ratio to T _leaf. Our findings may have important implications for improving ecosystem models in simulating carbon cycles and advancing understanding on the interactions between climate change and ecosystem functions.     

Seasonality of a Floodplain Subsidy to the Fish Community of a Large Temperate River

Ecosystems - This study examines the temporal and spatial variability of the floodplain trophic contribution to the fish community of Lake Saint-Pierre, a large (≈ 300&nbsp;km2),...     

Soil Microbial Community Response to Drought and Precipitation Variability in the Chihuahuan Desert

Increases in the magnitude and variability of precipitation events have been predicted for the Chihuahuan Desert region of West Texas. As patterns of moisture inputs and amounts change, soil microbial communities will respond to these alterations in soil moisture windows. In this study, we examined the soil microbial community structure within three vegetation zones along the Pine Canyon Watershed, an elevation and vegetation gradient in Big Bend National Park, Chihuahuan Desert. Soil samples at each site were obtained in mid-winter (January) and in mid-summer (August) for 2 years to capture a component of the variability in soil temperature and moisture that can occur seasonally and between years along this watershed. Precipitation patterns and amounts differed substantially between years with a drought characterizing most of the second year. Soils were collected during the drought period and following a large rainfall event and compared to soil samples collected during a relatively average season. Structural changes within microbial community in response to site, season, and precipitation patterns were evaluated using fatty acid methyl ester (FAME) and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analyses. Fungal FAME amounts differed significantly across seasons and sites and greatly outweighed the quantity of bacterial and actinomycete FAME levels for all sites and seasons. The highest fungal FAME levels were obtained in the low desert scrub site and not from the high elevation oak–pine forests. Total bacterial and actinomycete FAME levels did not differ significantly across season and year within any of the three locations along the watershed. Total bacterial and actinomycete FAME levels in the low elevation desert-shrub and grassland sites were slightly higher in the winter than in the summer. Microbial community structure at the high elevation oak–pine forest site was strongly correlated with levels of NH₄ ⁺–N, % soil moisture, and amounts of soil organic matter irrespective of season. Microbial community structure at the low elevation desert scrub and sotol grasslands sites was most strongly related to soil pH with bacterial and actinobacterial FAME levels accounting for site differences along the gradient. DGGE band counts of amplified soil bacterial DNA were found to differ significantly across sites and season with the highest band counts found in the mid-elevation grassland site. The least number of bands was observed in the high elevation oak–pine forest following the large summer-rain event that occurred after a prolonged drought. Microbial responses to changes in precipitation frequency and amount due to climate change will differ among vegetation zones along this Chihuahuan Desert watershed gradient. Soil bacterial communities at the mid-elevation grasslands site are the most vulnerable to changes in precipitation frequency and timing, while fungal community structure is most vulnerable in the low desert scrub site. The differential susceptibility of the microbial communities to changes in precipitation amounts along the elevation gradient reflects the interactive effects of the soil moisture window duration following a precipitation event and differences in soil heat loads. Amounts and types of carbon inputs may not be as important in regulating microbial structure among vegetation zones within in an arid environment as is the seasonal pattern of soil moisture and the soil heat load profile that characterizes the location.     

Response of Two Dominant Boreal Freshwater Wetland Plants to Manipulated Warming and Altered Precipitation

This study characterized the morphological and photosynthetic responses of two wetland plant species when they were subject to 2–6°C fluctuations in growth temperature and ±50% of precipitation, in order to predict the evolution of natural wetlands in Sanjiang Plain of North-eastern China. We investigated the morphological and photosynthetic responses of two dominant and competitive boreal freshwater wetland plants in Northeastern China to manipulation of warming (ambient, +2.0°C, +4.0°C, +6.0°C) and altered precipitation (−50%, ambient, +50%) simultaneously by incubating the plants from seedling to senescence within climate-controlled environmental chambers. Post-harvest, secondary growth of C. angustifolia was observed to explore intergenerational effects. The results indicated that C. angustifolia demonstrated a greater acclimated capacity than G. spiculosa to respond to climate change due to higher resistance to temperature and precipitation manipulations. The accumulated effect on aboveground biomass of post-harvest secondary growth of C. angustifolia was significant. These results explain the expansion of C. angustifolia during last 40 years and indicate the further expansion in natural boreal wetlands under a warmer and wetter future. Stability of the natural surface water table is critical for the conservation and restoration of G. spiculosa populations reacting to encroachment stress from C. angustifolia expansion.     

Nitrogen Fixation and Leaching of Biological Soil Crust Communities in Mesic Temperate Soils

Biological soil crust is composed of lichens, cyanobacteria, green algae, mosses, and fungi. Although crusts are a dominant source of nitrogen (N) in arid ecosystems, this study is among the first to demonstrate their contribution to N availability in xeric temperate habitats. The study site is located in Lucas County of Northwest Ohio. Using an acetylene reduction technique, we demonstrated potential N fixation for these crusts covering sandy, acidic, low N soil. Similar fixation rates were observed for crust whether dominated by moss, lichen, or bare soil. N inputs from biological crusts in northwestern Ohio are comparable to those in arid regions, but contribute substantially less N than by atmospheric deposition. Nitrate and ammonium leaching from the crust layer were quantified using ion exchange resin bags inserted within intact soil cores at 4 cm depth. Leaching of ammonium was greater and nitrate less in lichen than moss crusts or bare soil, and was less than that deposited from atmospheric sources. Therefore, biological crusts in these mesic, temperate soils may be immobilizing excess ammonium and nitrate that would otherwise be leached through the sandy soil. Moreover, automated monitoring of microclimate in the surface 7 cm of soil suggests that moisture and temperature fluctuations in soil are moderated under crust compared to bare soil without crust. We conclude that biological crusts in northwestern Ohio contribute potential N fixation, reduce N leaching, and moderate soil microclimate.     

Relationship of Productivity to Species Richness in the Xinjiang Temperate Grassland

The relationship between species richness (SR) and aboveground net primary productivity (ANPP) is still a central and debated issue in community ecology. Previous studies have often emphasized the relationship of alpha diversity (number of species identity) to the mean ANPP with respect to the SR-ANPP relationship while neglecting the contribution of beta diversity (dissimilarity in species composition) to the mean ANPP and to the stability of ANPP (coefficient of ANPP: CV of ANPP). In this study, we used alpha and beta diversity, mean ANPP and the CV of ANPP collected from 159 sites and belonging to three vegetation types in the Xinjiang temperate grassland to first examine their trends along climatic factors and among different vegetation types and then test the relationship among alpha (beta) diversity and mean ANPP and the CV of ANPP. Our results showed that in the Xinjiang temperate grasslands, alpha diversity was positively and linearly correlated with MAP but unimodally correlated with MAT. Meanwhile, beta diversity was unimodally correlated with MAP but linearly correlated with MAT. Relative to desert steppe, meadow steppe and typical steppe had the highest alpha and beta diversity, respectively. Except for ANPP exhibiting a quadratic relationship with MAP, no significant relationship was found among ANPP, the CV of ANPP and climatic factors. ANPP and the CV of ANPP also exhibited no apparent patterns in variation among different vegetation types. Our results further showed that mean ANPP was closely associated with alpha diversity. Both linear and unimodal relationships were detected between alpha diversity and mean ANPP, but their particular form was texture-dependent. Meanwhile, the CV of ANPP was positively correlated with beta diversity. Our results indicated that in addition to incorporating alpha diversity and mean ANPP, incorporating beta diversity and the CV of ANPP could expand our understanding of the SR-ANPP relationship.     

Minor Changes in Soil Bacterial and Fungal Community Composition Occur in Response to Monsoon Precipitation in a Semiarid Grassland

Arizona and New Mexico receive half of their annual precipitation during the summer monsoon season, making this large-scale rain event critical for ecosystem productivity. We used the monsoon rains to explore the responses of soil bacterial and fungal communities to natural moisture pulses in a semiarid grassland. Through 454 pyrosequencing of the 16S rRNA gene and ITS region, we phylogenetically characterized these communities at 22 time points during a summer season. Relative humidity increased before the rains arrived, creating conditions in soil that allowed for the growth of microorganisms. During the course of the study, the relative abundances of most bacterial phyla showed little variation, though some bacterial populations responded immediately to an increase in soil moisture once the monsoon rains arrived. The Firmicutes phylum experienced over a sixfold increase in relative abundance with increasing water availability. Conversely, Actinobacteria, the dominant taxa at our site, were negatively affected by the increase in water availability. No relationship was found between bacterial diversity and soil water potential. Bacterial community structure was unrelated to all environmental variables that we measured, with the exception of a significant relationship with atmospheric relative humidity. Relative abundances of fungal phyla fluctuated more throughout the season than bacterial abundances did. Variation in fungal community structure was unrelated to soil water potential and to most environmental variables. However, ordination analysis showed a distinct fungal community structure late in the season, probably due to plant senescence.     

Warming and Nitrogen Addition Increase Litter Decomposition in a Temperate Meadow Ecosystem

Background

Litter decomposition greatly influences soil structure, nutrient content and carbon sequestration, but how litter decomposition is affected by climate change is still not well understood.

Methodology/Principal Findings

A field experiment with increased temperature and nitrogen (N) addition was established in April 2007 to examine the effects of experimental warming, N addition and their interaction on litter decomposition in a temperate meadow steppe in northeastern China. Warming, N addition and warming plus N addition reduced the residual mass of L. chinensis litter by 3.78%, 7.51% and 4.53%, respectively, in 2008 and 2009, and by 4.73%, 24.08% and 16.1%, respectively, in 2010. Warming, N addition and warming plus N addition had no effect on the decomposition of P. communis litter in 2008 or 2009, but reduced the residual litter mass by 5.58%, 15.53% and 5.17%, respectively, in 2010. Warming and N addition reduced the cellulose percentage of L. chinensis and P. communis, specifically in 2010. The lignin percentage of L. chinensis and P. communis was reduced by warming but increased by N addition. The C, N and P contents of L. chinensis and P. communis litter increased with time. Warming and N addition reduced the C content and C:N ratios of L. chinensisand P. communis litter, but increased the N and P contents. Significant interactive effects of warming and N addition on litter decomposition were observed (P<0.01).

Conclusion/Significance

The litter decomposition rate was highly correlated with soil temperature, soil water content and litter quality. Warming and N addition significantly impacted the litter decomposition rate in the Songnen meadow ecosystem, and the effects of warming and N addition on litter decomposition were also influenced by the quality of litter. These results highlight how climate change could alter grassland ecosystem carbon, nitrogen and phosphorus contents in soil by influencing litter decomposition.     

Response of Leaf Spectral Reflectance in Loblolly Pine to Increased Atmospheric Ozone and Precipitation Acidity

Leaf reflectance responses to increased ozone and precipitationacidity were determined at visible (400–720 nm) and infra-red(720–2500 nm) wavelengths for field-grown seedlings ofloblolly pine (Pinus taeda L.). Treatments were maintained continuouslyfrom 3 April to 3 November. In a relatively ozone-sensitivevariety of pine, increased ozone decreased visible and infra-redreflectance by late May. By late August, continued exposureto ozone increased visible and infra-red reflectance. As oflate November, ozone at 17ambient or greater concentrationshad increased visible reflectance substantially in an ozone-insensitiveas well as the ozone-sensitive variety. Acidic precipitationdecreased visible and infra-red reflectances in both varietiesin May and decreased infra-red reflectances in the ozone-sensitivevariety in August, but did not produce significant reflectanceresponses in November. Visible reflectance responses to ozoneand acidic precipitation, particularly those near 700 nm, werespectrally similar to reflectance responses reported previouslyfor other deleterious environmental influences. Key words: Pinus taeda, reflectance, ozone, acidic precipitation     

Response of the Shortgrass Steppe to Changes in Rainfall Seasonality

Studies in temperate grassland ecosystems have shown that differences in composition of C₃ and C₄ plant functional types can have important influences on ecosystem pools and processes. We used a plant community dynamics model (STEPPE) linked to a biogeochemical cycling model (CENTURY) to determine how ecosystem properties in shortgrass steppe are influenced by plant functional type composition. Because of phenological differences between C₃ and C₄ plants, we additionally simulated the effects of precipitation seasonality on plant communities and examined how C₃ and C₄ composition interacts with precipitation to affect ecosystems. The model output suggests that differences in C₃ and C₄ composition can lead to differences in soil organic carbon (C) and nitrogen (N) within 1000 simulation years. Soil organic C and N (g C and N m^{− 2} to 0.2-m depth) were least in a 100% C₄ community compared with a 100% C₃ community and a mixed C₃–C₄ community. A change in the time of maximum precipitation from summer to spring in a simulated shortgrass steppe slightly favored C₃ plants over C₄ plants. The proportion of total net primary production accounted for by C₃ plants increased from 21% to 25% after 200 years, when 90 mm of precipitation was switched from summer to spring. Soil organic matter (SOM) was relatively stable in the C₄-dominated communities with respect to changes in precipitation seasonality, whereasSOM in the C₃ community was sensitive to precipitation seasonality changes. These results suggest an important interaction between plant community composition and precipitation seasonality on SOM, with phenology playing a key role. Received 9 June 1998; accepted 6 January 1999     

Altering Rainfall Timing and Quantity in a Mesic Grassland Ecosystem: Design and Performance of Rainfall Manipulation Shelters

Global climate change is predicted to alter growing season rainfall patterns, potentially reducing total amounts of growing season precipitation and redistributing rainfall into fewer but larger individual events. Such changes may affect numerous soil, plant, and ecosystem properties in grasslands and ultimately impact their productivity and biological diversity. Rainout shelters are useful tools for experimental manipulations of rainfall patterns, and permanent fixed-location shelters were established in 1997 to conduct the Rainfall Manipulation Plot study in a mesic tallgrass prairie ecosystem in northeastern Kansas. Twelve 9 x 14–m fixed-location rainfall manipulation shelters were constructed to impose factorial combinations of 30% reduced rainfall quantity and 50% greater interrainfall dry periods on 6 x 6–m plots, to examine how altered rainfall regimes may affect plant species composition, nutrient cycling, and above- and belowground plant growth dynamics. The shelters provided complete control of growing season rainfall patterns, whereas effects on photosynthetic photon flux density, nighttime net radiation, and soil temperature generally were comparable to other similar shelter designs. Soil and plant responses to the first growing season of rainfall manipulations (1998) suggested that the interval between rainfall events may be a primary driver in grassland ecosystem responses to altered rainfall patterns. Aboveground net primary productivity, soil CO₂ flux, and flowering duration were reduced by the increased interrainfall intervals and were mostly unaffected by reduced rainfall quantity. The timing of rainfall events and resulting temporal patterns of soil moisture relative to critical times for microbial activity, biomass accumulation, plant life histories, and other ecological properties may regulate longer-term responses to altered rainfall patterns.     

Responses of Plant Community Composition and Biomass Production to Warming and Nitrogen Deposition in a Temperate Meadow Ecosystem

Climate change has profound influences on plant community composition and ecosystem functions. However, its effects on plant community composition and biomass production are not well understood. A four-year field experiment was conducted to examine the effects of warming, nitrogen (N) addition, and their interactions on plant community composition and biomass production in a temperate meadow ecosystem in northeast China. Experimental warming had no significant effect on plant species richness, evenness, and diversity, while N addition highly reduced the species richness and diversity. Warming tended to reduce the importance value of graminoid species but increased the value of forbs, while N addition had the opposite effect. Warming tended to increase the belowground biomass, but had an opposite tendency to decrease the aboveground biomass. The influences of warming on aboveground production were dependent upon precipitation. Experimental warming had little effect on aboveground biomass in the years with higher precipitation, but significantly suppressed aboveground biomass in dry years. Our results suggest that warming had indirect effects on plant production via its effect on the water availability. Nitrogen addition significantly increased above- and below-ground production, suggesting that N is one of the most important limiting factors determining plant productivity in the studied meadow steppe. Significant interactive effects of warming plus N addition on belowground biomass were also detected. Our observations revealed that environmental changes (warming and N deposition) play significant roles in regulating plant community composition and biomass production in temperate meadow steppe ecosystem in northeast China.     

The Consequences of Precipitation Seasonality for Mediterranean-Ecosystem Vegetation of South Africa

Globally, mediterranean-climate ecosystem vegetation has converged on an evergreen, sclerophyllous and shrubby growth form. The particular aspects of mediterranean-climate regions that contribute to this convergence include summer droughts and relatively nutrient-poor soils. We hypothesised that winter-precipitation implies stressful summer droughts and leaches soils due to greater water availability (i.e. balance between precipitation and potential evapotranspiration; P–PET) during cold periods. We conducted a comparative analysis of normalised difference vegetation indices (NDVI) and edaphic and climate properties across the biomes of South Africa. NDVI was strongly correlated with both precipitation and P–PET (r² = 0.8). There was no evidence, however, that winter-precipitation reduces NDVI in comparison to similar amounts of summer-precipitation. Base saturation (BS), a measure of soil leaching was, however, negatively related to P–PET (r² = 0.64). This led to an interaction between P–PET and BS in determining NDVI, indicating the existence of a trade-off between water availability and soil nutrients that enables NDVI to increase with precipitation, despite negative consequences for soil nutrient availability. The mechanism of this trade-off is suggested to be that water increases nutrient accessibility. This implies that along with nutrient-depauperate geologies and long periods of time since glaciation, the winter-precipitation may have contributed to the highly leached status of the soils. Since many of the ecophysiological characteristics of mediterranean-ecosystem flora are associated with low nutrient availabilities (e.g. evergreen foliage, sclerophylly, cluster roots), we conclude that mediterranean-climates promote convergence of growth-forms in these regions through high leaching capacity.     

Empirical Leucine-to-Carbon Conversion Factors for Estimating Heterotrophic Bacterial Production: Seasonality and Predictability in a Temperate Coastal Ecosystem

Leucine-to-carbon conversion factors (CFs) are needed for converting substrate incorporation into biomass production of heterotrophic bacteria. During 2006 we performed 20 dilution experiments for determining the spatiotemporal variability of empirical CFs in temperate Atlantic coastal waters. Values (0.49 to 1.92 kg C mol Leu⁻¹) showed maxima in autumn to early winter and minima in summer. Spatially averaged CFs were significantly negatively correlated with in situ leucine incorporation rates (r = −0.91) and positively correlated with phosphate concentrations (r = 0.76). These relationships, together with a strong positive covariation between cell-specific leucine incorporation rates and carbon contents (r = 0.85), were interpreted as a strategy to maximize survival through protein synthesis and low growth rates under nutrient limitation (low CFs) until favorable conditions stimulate cell division relative to protein synthesis (high CFs). A multiple regression with in situ leucine incorporation rates and cellular carbon contents explained 96% of CF variance in our ecosystem, suggesting their potential prediction from more easily measurable routine variables. The use of the theoretical CF of 1.55 kg C mol Leu⁻¹ would have resulted in a serious overestimation (73%) of annual bacterial production rates. Our results emphasize the need for considering the temporal scale in CFs for bacterial production studies.Bacterial production (BP) is a key parameter for evaluating the role of heterotrophic bacterioplankton in ocean carbon cycling. However, BP cannot be directly measured and is rather estimated from related metabolic processes. Incorporation of radioactively labeled substrates such as thymidine (TdR) and leucine (Leu) are by far the most widespread approaches. Both methods are based on measuring some aspect of cellular macromolecular synthesis (DNA in the case of TdR and protein in the case of Leu). Substrate incorporation rates are then converted into rates of macromolecular synthesis and eventually into rates of biomass production (i.e., cells or cellular carbon or nitrogen) (17). This final step requires some conversion factor (CF). Since CFs are not easy to measure routinely and since CF determination usually involves the incubation of natural samples for several days, literature values are still often used in spite of strong evidence of their variability (11). The values of these constant CFs are 3.1 or 1.55 kg C mol Leu⁻¹ (assuming an isotope dilution of 2 or no isotope dilution, respectively) (26) and 2 × 10¹⁸ cells mol TdR⁻¹ (5).Given the reported high variability in empirically determined CFs in many ecosystems (16), it should always be preferred to estimate them rather than using a fixed theoretical value, especially in low-productivity environments (23), where empirical CFs are usually much lower than the theoretical ones (2). Sources of empirical CF variability include the design of dilution culture incubations and the choice of calculation methods (11), in addition to ecologically relevant characteristics, such as the physiological state of bacteria and the amount and quality of organic and inorganic substrates (24). Recent studies tend to include empirical CFs, but seldom has the seasonal component been taken into account. If this component is significant, there would be uncertainty in quantifications of the role of the bacterioplankton in global carbon cycling.With the aim of determining the spatial and temporal variability of leucine-to-carbon (Leu-to-C) empirical CFs in temperate coastal waters, we conducted an annual cycle of dilution culture experiments at three stations located in the south Bay of Biscay continental shelf. On the one hand, we wanted to assess the ecological implications of this variability for quantifying carbon fluxes through the ecosystem. On the other hand, we also wanted to explore the predictability of the empirical Leu-to-C CFs in this temperate ecosystem from easily and routinely measurable environmental variables such as inorganic nutrient concentrations and bacterial activity and cellular properties.     

Precipitation Pattern Regulates Soil Carbon Flux Responses to Nitrogen Addition in a Temperate Forest

Ecosystems - Changes in precipitation frequency and intensity are predicted to be more intense and frequent accompanying climate change and may have immediate or potentially prolonged effects on...     

Temperate Grassland Responses to Climate Change: an Analysis using the Hurley Pasture Model

The Hurley Pasture Model is process-based and couples the carbon,nitrogen and water cycles in the soil-grass-animal system. Itwas used to examine the responses of grasslands in southern,lowland and northern, upland climates in Britain. Short-termresponse to step-wise increases in CO₂concentration (350 to700 µmol mol^-1) and temperature (5 °C) were contrastedwith long-term equilibrium (the term ‘equilibrium’is equivalent to ‘steady state’ throughout thispaper) responses and with responses to gradually increasing[CO₂] and temperature. Equilibrium responses to a range of climatevariables were also examined. Three conclusions were drawn regarding the interpretation ofexperiments: (1) initial ecosystem responses to step-wise changescan be different in both magnitude and sign to equilibrium responses,and this can continue for many years; (2) grazing can drasticallyalter the magnitude and sign of the response of grasslands toclimate change, especially rising temperatures; and (3) effectsof changes in climate, especially temperature and rainfall,are likely to be highly site-specific. It was concluded thatexperiments should try to lessen uncertainty about processeswithin models rather than try to predict ecosystem responsesdirectly. Three conclusions were also drawn about the operation of grasslandsas carbon sinks: (1) increasing [CO₂] alone will produce a carbonsink, as long as it continues to accelerate photosynthesis andincrease net primary productivity; (2) by contrast, increasingtemperatures alone are likely to produce a carbon source, becausesoil respiration is accelerated more than net primary productivity,even when assuming the same temperature function for most soiland plant biochemical processes; and (3) the net effect of projectedincreases in [CO₂] and temperature is likely to be a carbonsink of 5–15 g C m^-2yr^-1in humid, temperate grasslandsfor several decades, which is consistent with the magnitudeof the hypothesized current global terrestrial carbon sink. Grassland; climate change; carbon dioxide; temperature; ecosystem; model; carbon sink