Acclimatization of soil respiration to warming in a tall grass prairie

Does declining carbon-use efficiency explain thermal acclimation of soil respiration with warming?

Enhanced soil respiration in response to global warming may substantially increase atmospheric CO2 concentrations above the anthropogenic contribution, depending on the mechanisms underlying the temperature sensitivity of soil respiration. Here, we compared short-term and seasonal responses of soil respiration to a shifting thermal environment and variable substrate availability via laboratory incubations. To analyze the data from incubations, we implemented a novel process-based model of soil respiration in a hierarchical Bayesian framework. Our process model combined a Michaelis-Menten-type equation of substrate availability and microbial biomass with an Arrhenius-type nonlinear temperature response function. We tested the competing hypotheses that apparent thermal acclimation of soil respiration can be explained by depletion of labile substrates in warmed soils, or that physiological acclimation reduces respiration rates. We demonstrated that short-term apparent acclimation can be induced by substrate depletion, but that decreasing microbial biomass carbon (MBC) is also important, and lower MBC at warmer temperatures is likely due to decreased carbon-use efficiency (CUE). Observed seasonal acclimation of soil respiration was associated with higher CUE and lower basal respiration for summer- vs. winter-collected soils. Whether the observed short-term decrease in CUE or the seasonal acclimation of CUE with increased temperatures dominates the response to long-term warming will have important consequences for soil organic carbon storage.

Wheat production in Bangladesh: its future in the light of global warming

BACKGROUND AND AIMS

The most fundamental activity of the people of Bangladesh is agriculture. Modelling projections for Bangladesh indicate that warmer temperatures linked to climate change will severely reduce the growth of various winter crops (wheat, boro rice, potato and winter vegetables) in the north and central parts. In summer, crops in south-eastern parts of the country are at risk from increased flooding as sea levels increase.

KEY FACTS

Wheat is one of the most important winter crops and is temperature sensitive and the second most important grain crop after rice. In this review, we provide an up-to-date and detailed account of wheat research of Bangladesh and the impact that global warming may have on agriculture, especially wheat production. Although flooding is not of major importance or consequence to the wheat crop at present, some perspectives are provided on this stress since wheat is flood sensitive and the incidence of flooding is likely to increase.

PROJECTIONS

This information and projections will allow wheat breeders to devise new breeding programmes to attempt to mitigate future global warming. We discuss what this implies for food security in the broader context of South Asia.

How interactions between microbial resource demands, soil organic matter stoichiometry, and substrate reactivity determine the direction and magnitude of soil respiratory responses to warming

Recent empirical and theoretical advances inform us about multiple drivers of soil organic matter (SOM) decomposition and microbial responses to warming. Absent from our conceptual framework of how soil respiration will respond to warming are adequate links between microbial resource demands, kinetic theory, and substrate stoichiometry. Here, we describe two important concepts either insufficiently explored in current investigations of SOM responses to temperature, or not yet addressed. First, we describe the complete range of responses for how warming may change microbial resource demands, physiology, community structure, and total biomass. Second, we describe how any relationship between SOM activation energy of decay and carbon (C) and nitrogen (N) stoichiometry can alter the relative availability of C and N as temperature changes. Changing availabilities of C and N liberated from their organic precursors can feedback to microbial resource demands, which in turn influence the aggregated respiratory response to temperature we observe. An unsuspecting biogeochemist focused primarily on temperature sensitivity of substrate decay thus cannot make accurate projections of heterotrophic CO2 losses from diverse organic matter reservoirs in a warming world. We establish the linkages between enzyme kinetics, SOM characteristics, and potential for microbial adaptation critical for making such projections. By examining how changing microbial needs interact with inherent SOM structure and composition, and thus reactivity, we demonstrate the means by which increasing temperature could result in increasing, unchanging, or even decreasing respiration rates observed in soils. We use this exercise to highlight ideas for future research that will develop our abilities to predict SOM feedbacks to climate.

Is the change of winter wheat yield under warming caused by shortened reproductive period?

Previous reports from laboratory-controlled experiments and models considered that a shorter reproductive period could be the main reason for wheat yield reduction in the warmer world. However, this conclusion needs to be proved carefully by field-scale experiments. In this study, a field-scale continuous open-warming experiment was conducted to quantify the adjustment of winter wheat growth and yield under conventional tillage (CT) and no-till (NT) systems in the North China Plain (NCP). Canopy temperatures were warmed using infrared heaters between 1.0 and 1.6°C (daytime and nighttime, respectively) above the control. Wheat yields under CT were not significantly reduced over the two seasons (2010 and 2011), but yields under NT were 3.3% and 6.1% lower, respectively. The growing seasons for both CT and NT were shortened 6 days in 2010 and 11 days in 2011; however, the reproductive periods were maintained. The shortened days were due to a significantly shorter springtime re-greening stage followed by minimal changes in other phenological stages (jointing, flag completed, heading, anthesis, and grain-filling). The temporal advance by warming resulted in lower growing-season mean air temperatures (MAT) for warmed plots than the control from 0.23 to 4.22°C for the same subsequent phenological stages. Warming increased the number of tillers m⁻² and kernel weight, but tended to decrease the number of spikes m⁻² in the two tillage systems. The heavier kernels offset the yield reduction from smaller number of spikes. Warming increased the wheat aboveground biomass from 10% to 20% suggesting the potential to sequester more CO₂. This study suggests that winter wheat might adjust its growth (shortened vegetative period to maintain reproductive period) to partly compensate for the negative effects from global warming in this temperate irrigated cropland.

Temperature adaptation of bacterial communities in experimentally warmed forest soils

A detailed understanding of the influence of temperature on soil microbial activity is critical to predict future atmospheric CO₂ concentrations and feedbacks to anthropogenic warming. We investigated soils exposed to 3-4 years of continuous 5 °C-warming in a field experiment in a temperate forest. We found that an index for the temperature adaptation of the microbial community, T_min for bacterial growth, increased by 0.19 °C per 1 °C rise in temperature, showing a community shift towards one adapted to higher temperature with a higher temperature sensitivity (Q_{10(5-15 °C)} increased by 0.08 units per 1 °C). Using continuously measured temperature data from the field experiment we modelled in situ bacterial growth. Assuming that warming did not affect resource availability, bacterial growth was modelled to become 60% higher in warmed compared to the control plots, with the effect of temperature adaptation of the community only having a small effect on overall bacterial growth (<5%). However, 3 years of warming decreased bacterial growth, most likely due to substrate depletion because of the initially higher growth in warmed plots. When this was factored in, the result was similar rates of modelled in situ bacterial growth in warmed and control plots after 3 years, despite the temperature difference. We conclude that although temperature adaptation for bacterial growth to higher temperatures was detectable, its influence on annual bacterial growth was minor, and overshadowed by the direct temperature effect on growth rates.

Simple additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature

In recent years, increased awareness of the potential interactions between rising atmospheric CO2 concentrations ([ CO2 ]) and temperature has illustrated the importance of multifactorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and [ CO2 ] manipulation, and compares it with those obtained in single factor [ CO2 ] and temperature manipulation experiments. Across all combined elevated [ CO2 ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [ CO2 ]-only treatment than to those in the warming-only treatment. In contrast to warming-only experiments, both the combined and the [ CO2 ]-only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [ CO2 ]-only treatment, possibly due to the warming-induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [ CO2 ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [ CO2 ] and warming, i.e. the response to the combined treatment was usually less-than-additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long-term) multifactor manipulation experiments. Because single factor CO2 responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.

Effects of precipitation increase on soil respiration: a three-year field experiment in subtropical forests in China

Background

The aim of this study was to determine response patterns and mechanisms of soil respiration to precipitation increases in subtropical regions.

Methodology/Principal Findings

Field plots in three typical forests [i.e. pine forest (PF), broadleaf forest (BF), and pine and broadleaf mixed forest (MF)] in subtropical China were exposed under either Double Precipitation (DP) treatment or Ambient Precipitation (AP). Soil respiration, soil temperature, soil moisture, soil microbial biomass and fine root biomass were measured over three years. We tested whether precipitation treatments influenced the relationship of soil respiration rate (R) with soil temperature (T) and soil moisture (M) using R = (a+cM)exp(bT), where a is a parameter related to basal soil respiration; b and c are parameters related to the soil temperature and moisture sensitivities of soil respiration, respectively. We found that the DP treatment only slightly increased mean annual soil respiration in the PF (15.4%) and did not significantly change soil respiration in the MF and the BF. In the BF, the increase in soil respiration was related to the enhancements of both soil fine root biomass and microbial biomass. The DP treatment did not change model parameters, but increased soil moisture, resulting in a slight increase in soil respiration. In the MF and the BF, the DP treatment decreased soil temperature sensitivity b but increased basal soil respiration a, resulting in no significant change in soil respiration.

Conclusion/Significance

Our results indicate that precipitation increasing in subtropical regions in China may have limited effects on soil respiration.

Soil respiration under climate change: prolonged summer drought offsets soil warming effects

Climate change may considerably impact the carbon (C) dynamics and C stocks of forest soils. To assess the combined effects of warming and reduced precipitation on soil CO₂ efflux, we conducted a two-way factorial manipulation experiment (4 °C soil warming + throughfall exclusion) in a temperate spruce forest from 2008 until 2010. Soil was warmed by heating cables throughout the growing seasons. Soil drought was simulated by throughfall exclusions with three 100 m² roofs during 25 days in July/August 2008 and 2009. Soil warming permanently increased the CO₂ efflux from soil, whereas throughfall exclusion led to a sharp decrease in soil CO₂ efflux (45% and 50% reduction during roof installation in 2008 and 2009, respectively). In 2008, CO₂ efflux did not recover after natural rewetting and remained lowered until autumn. In 2009, CO₂ efflux recovered shortly after rewetting, but relapsed again for several weeks. Drought offset the increase in soil CO₂ efflux by warming in 2008 (growing season CO₂ efflux in t C ha⁻¹: control: 7.1 ± 1.0; warmed: 9.5 ± 1.7; warmed + roof: 7.4 ± 0.3; roof: 5.9 ± 0.4) and in 2009 (control: 7.6 ± 0.8; warmed + roof: 8.3 ± 1.0). Throughfall exclusion mainly affected the organic layer and the top 5 cm of the mineral soil. Radiocarbon data suggest that heterotrophic and autotrophic respiration were affected to the same extent by soil warming and drying. Microbial biomass in the mineral soil (0–5 cm) was not affected by the treatments. Our results suggest that warming causes significant C losses from the soil as long as precipitation patterns remain steady at our site. If summer droughts become more severe in the future, warming induced C losses will likely be offset by reduced soil CO₂ efflux during and after summer drought.

Tropical forest carbon balance in a warmer world: a critical review spanning microbial- to ecosystem-scale processes

Tropical forests play a major role in regulating global carbon (C) fluxes and stocks, and even small changes to C cycling in this productive biome could dramatically affect atmospheric carbon dioxide (CO(2) ) concentrations. Temperature is expected to increase over all land surfaces in the future, yet we have a surprisingly poor understanding of how tropical forests will respond to this significant climatic change. Here we present a contemporary synthesis of the existing data and what they suggest about how tropical forests will respond to increasing temperatures. Our goals were to: (i) determine whether there is enough evidence to support the conclusion that increased temperature will affect tropical forest C balance; (ii) if there is sufficient evidence, determine what direction this effect will take; and, (iii) establish what steps should to be taken to resolve the uncertainties surrounding tropical forest responses to increasing temperatures. We approach these questions from a mass-balance perspective and therefore focus primarily on the effects of temperature on inputs and outputs of C, spanning microbial- to ecosystem-scale responses. We found that, while there is the strong potential for temperature to affect processes related to C cycling and storage in tropical forests, a notable lack of data combined with the physical, biological and chemical diversity of the forests themselves make it difficult to resolve this issue with certainty. We suggest a variety of experimental approaches that could help elucidate how tropical forests will respond to warming, including large-scale in situ manipulation experiments, longer term field experiments, the incorporation of a range of scales in the investigation of warming effects (both spatial and temporal), as well as the inclusion of a diversity of tropical forest sites. Finally, we highlight areas of tropical forest research where notably few data are available, including temperature effects on: nutrient cycling, heterotrophic versus autotrophic respiration, thermal acclimation versus substrate limitation of plant and microbial communities, below-ground C allocation, species composition (plant and microbial), and the hydraulic architecture of roots. Whether or not tropical forests will become a source or a sink of C in a warmer world remains highly uncertain. Given the importance of these ecosystems to the global C budget, resolving this uncertainty is a primary research priority.

Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms

• It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales. • Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms. • We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration. • Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystem-climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.

Root carbon and protein metabolism associated with heat tolerance

Extensive past efforts have been taken toward understanding heat tolerance mechanisms of the aboveground organs. Root systems play critical roles in whole-plant adaptation to heat stress, but are less studied. This review discusses recent research results revealing some critical physiological and metabolic factors underlying root thermotolerance, with a focus on temperate perennial grass species. Comparative analysis of differential root responses to supraoptimal temperatures by a heat-adapted temperate C3 species, Agrostis scabra, which can survive high soil temperatures up to 45 °C in geothermal areas in Yellow Stone National Park, and a heat-sensitive cogeneric species, Agrostis stolonifera, suggested that efficient carbon and protein metabolism is critical for root thermotolerance. Superior root thermotolerance in a perennial grass was associated with a greater capacity to control respiratory costs through respiratory acclimation, lowering carbon investment in maintenance for protein turnover, and efficiently partitioning carbon into different metabolic pools and alternative respiration pathways. Proteomic analysis demonstrated that root thermotolerance was associated with an increased maintenance of stability and less degradation of proteins, particularly those important for metabolism and energy production. In addition, thermotolerant roots are better able to maintain growth and activity during heat stress by activating stress defence proteins such as those participating in antioxidant defence (i.e. superoxide dismutase, peroxidase, glutathione S-transferase) and chaperoning protection (i.e. heat shock protein).

Estimation of soil respiration using automated chamber systems in an oak (Quercus mongolica) forest at the Nam-San site in Seoul, Korea

Soil respiration (R(soil)) is the largest component of ecosystem respiration produced by the autotrophic and heterotrophic respirations. Its variability on multiple time scales strongly depends on environmental variables such as temperature and moisture. To investigate the temporal variations of R(soil) in a cool-temperate oak (Quercus mongolica) forest at the Nam-San site in Seoul, Korea, continuous measurements of R(soil) using the automated chamber systems, air and soil temperatures and soil moisture are made for the period from April 2010 to March 2011. The observed data indicate that the R(soil) shows a remarkable seasonal variation in accordance with temperatures with high in summer and low in winter. The R(soil) is found to be strongly correlated with soil temperature (T(s)) at the 5cm depth throughout the year. However, the high fluctuation of R(soil) is found to be related with soil moisture content (M(s)) during the forest growing season. The estimated annual Q(10) value using the 1.5m-high air temperature is found to be 2.4 that is comparable with other studies in temperate forest ecosystems. The optimal regression equation of R(soil) with the T(s) at 5cm depth and the M(s) at 15cm depth is found to be R(soil)=124.3 exp (0.097T(s))-55.3 (M(s))(2)+2931.9 (M(s))-38516 for T(s)≥0°C and R(soil)=0 for T(s)<0°C with r(2)=0.97, P<0.01, suggesting the importance of T(s) and M(s) for R(soil). The annual total soil respiration estimated by the optimal regression equation is found to be 1264gCm(-2) with a maximum of 685gCm(-2) in the summer season and a minimum of 33gCm(-2) in the winter season. The present study can be implemented for the determination of the carbon balance of a cool-temperate Q. mongolica forest with the provision of photosynthesis.

Responses of soil CO(2) efflux to precipitation pulses in two subtropical forests in southern China

This study was designed to examine the responses of soil CO(2) efflux to precipitation pulses of varying intensities using precipitation simulations in two subtropical forests [i.e., mixed and broadleaf forests (MF and BF)] in southern China. The artificial precipitation event was achieved by spraying a known amount of water evenly in a plot (50 × 50 cm(2)) over a 30 min period, with intensities ranging from 10, 20, 50 and 100 mm within the 30 min. The various intensities were simulated in both dry season (in December 2007) and wet (in May 2008) season. We characterized the dynamic patterns of soil CO(2) efflux rate and environmental factors over the 5 h experimental period. Results showed that both soil moisture and soil CO(2) efflux rate increased to peak values for most of the simulated precipitation treatments, and gradually returned to the pre-irrigation levels after irrigation in two forests. The maximum peak of soil CO(2) efflux rate occurred at the 10 mm precipitation event in the dry season in BF and was about 3.5 times that of the pre-irrigation value. The change in cumulative soil CO(2) efflux following precipitation pulses ranged from -0.68 to 1.72 g CO(2) m(-2) over 5 h compared to the pre-irrigation levels and was generally larger in the dry season than in the wet season. The positive responses of soil CO(2) efflux to precipitation pulses declined with the increases in precipitation intensity, and surprisingly turned to negative when precipitation intensity reached 50 and 100 mm in the wet season. These findings indicated that soil CO(2) efflux could be changed via pulse-like fluxes in subtropical forests in southern China as fewer but extreme precipitation events occur in the future.

Estimated soil respiration rates decreased with long-term soil microclimate changes in successional forests in southern China

The response of soil respiration to short-term environmental factors changes has been well studied, whereas the influences of long-term soil microclimate changes on soil respiration are still highly unclear, especially in tropical ecosystems. We hypothesized that soil carbon accumulation in southern China, especially in mature forest during recent years, partly resulted from reducing soil respiration rates. To test this hypothesis, we analyzed the temporal trends and variations of air temperature, soil temperature and soil water content (hereafter referred to as SWC), and then estimated soil respiration rates in the 1980s and 2000s with soil temperature and SWC by regression model in three subtropical forests which are at early-, mid-, and advanced-successional stages, respectively, in Dinghushan Nature Reserve (hereafter referred to as DNR) in southern China. The annual mean ambient air temperature increased by 1.03 ± 0.15°C in the last 50 years (1954-2007) in DNR. Rainfall amount in the corresponding period did not change significantly, but rainfall pattern changed remarkably in the last three decades (1978-2007). Soil temperature is correlated with ambient air temperature. The average SWC was 36.8 ± 8.4%, 34.7 ± 8.1% and 29.6 ± 8.1% in the 1980s, and then dropped sharply to 23.6 ± 2.9%, 20.5 ± 4.2% and 17.6 ± 3.9% in the 2000s, for the advanced, mid- and early-successional forests, respectively. Concurrent changes of soil temperature and SWC may have a negative effect on soil respiration rates for all three forests, implicated that soil respiration may have a negative feedback to regional climate change and carbon could be sequestered in subtropical forests in southern China.

Soil warming alters nitrogen cycling in a New England forest: implications for ecosystem function and structure

Global climate change is expected to affect terrestrial ecosystems in a variety of ways. Some of the more well-studied effects include the biogeochemical feedbacks to the climate system that can either increase or decrease the atmospheric load of greenhouse gases such as carbon dioxide and nitrous oxide. Less well-studied are the effects of climate change on the linkages between soil and plant processes. Here, we report the effects of soil warming on these linkages observed in a large field manipulation of a deciduous forest in southern New England, USA, where soil was continuously warmed 5°C above ambient for 7 years. Over this period, we have observed significant changes to the nitrogen cycle that have the potential to affect tree species composition in the long term. Since the start of the experiment, we have documented a 45% average annual increase in net nitrogen mineralization and a three-fold increase in nitrification such that in years 5 through 7, 25% of the nitrogen mineralized is then nitrified. The warming-induced increase of available nitrogen resulted in increases in the foliar nitrogen content and the relative growth rate of trees in the warmed area. Acer rubrum (red maple) trees have responded the most after 7 years of warming, with the greatest increases in both foliar nitrogen content and relative growth rates. Our study suggests that considering species-specific responses to increases in nitrogen availability and changes in nitrogen form is important in predicting future forest composition and feedbacks to the climate system.

Root standing crop and chemistry after six years of soil warming in a temperate forest

Examining the responses of root standing crop (biomass and necromass) and chemistry to soil warming is crucial for understanding root dynamics and functioning in the face of global climate change. We assessed the standing crop, total nitrogen (N) and carbon (C) compounds in tree roots and soil net N mineralization over the growing season after 6 years of experimental soil warming in a temperate deciduous forest in 2008. Roots were sorted into four different categories: live and dead fine roots (≤1mm in diameter) and live and dead coarse roots (1-4 mm in diameter). Total root standing crop (live plus dead) in the top 10 cm of soil in the warmed area was 42.5% (378.4 vs. 658.5 g m(-2)) lower than in the control area, while live root standing crop in the warmed area was 62% lower than in the control area. Soil net N mineralization over the growing season increased by 79.4% in the warmed relative to the control area. Soil warming did not significantly change the concentrations of C and C compounds (sugar, starch, hemicellulose, cellulose and lignin) in the four root categories. However, total N concentration in the live fine roots in the warmed area was 10.5% (13.7 vs. 12.4 mg g(-1)) higher and C:N ratio was 8.6% (38.5 vs. 42.1) lower than in the control area. The increase in N concentration in the live fine roots could be attributed to the increase in soil N availability due to soil warming. Net N mineralization was negatively correlated with both live and dead fine roots in the mineral soil that is home to the majority of roots, suggesting that soil warming increases N mineralization, decreases fine root biomass and thus decreases C allocation belowground.

Relative information contributions of model vs. data to short- and long-term forecasts of forest carbon dynamics

Biogeochemical models have been used to evaluate long-term ecosystem responses to global change on decadal and century time scales. Recently, data assimilation has been applied to improve these models for ecological forecasting. It is not clear what the relative information contributions of model (structure and parameters) vs. data are to constraints of short- and long-term forecasting. In this study, we assimilated eight sets of 10-year data (foliage, woody, and fine root biomass, litter fall, forest floor carbon [C], microbial C, soil C, and soil respiration) collected from Duke Forest into a Terrestrial Ecosystem model (TECO). The relative information contribution was measured by Shannon information index calculated from probability density functions (PDFs) of carbon pool sizes. The null knowledge without a model or data was defined by the uniform PDF within a prior range. The relative model contribution was information content in the PDF of modeled carbon pools minus that in the uniform PDF, while the relative data contribution was the information content in the PDF of modeled carbon pools after data was assimilated minus that before data assimilation. Our results showed that the information contribution of the model to constrain carbon dynamics increased with time whereas the data contribution declined. The eight data sets contributed more than the model to constrain C dynamics in foliage and fine root pools over the 100-year forecasts. The model, however, contributed more than the data sets to constrain the litter, fast soil organic matter (SOM), and passive SOM pools. For the two major C pools, woody biomass and slow SOM, the model contributed less information in the first few decades and then more in the following decades than the data. Knowledge of relative information contributions of model vs. data is useful for model development, uncertainty analysis, future data collection, and evaluation of ecological forecasting.

Soil warming, carbon-nitrogen interactions, and forest carbon budgets

Soil warming has the potential to alter both soil and plant processes that affect carbon storage in forest ecosystems. We have quantified these effects in a large, long-term (7-y) soil-warming study in a deciduous forest in New England. Soil warming has resulted in carbon losses from the soil and stimulated carbon gains in the woody tissue of trees. The warming-enhanced decay of soil organic matter also released enough additional inorganic nitrogen into the soil solution to support the observed increases in plant carbon storage. Although soil warming has resulted in a cumulative net loss of carbon from a New England forest relative to a control area over the 7-y study, the annual net losses generally decreased over time as plant carbon storage increased. In the seventh year, warming-induced soil carbon losses were almost totally compensated for by plant carbon gains in response to warming. We attribute the plant gains primarily to warming-induced increases in nitrogen availability. This study underscores the importance of incorporating carbon-nitrogen interactions in atmosphere-ocean-land earth system models to accurately simulate land feedbacks to the climate system.

Effect of warming and drought on grassland microbial communities

The soil microbiome is responsible for mediating key ecological processes; however, little is known about its sensitivity to climate change. Observed increases in global temperatures and alteration to rainfall patterns, due to anthropogenic release of greenhouse gases, will likely have a strong influence on soil microbial communities and ultimately the ecosystem services they provide. Therefore, it is vital to understand how soil microbial communities will respond to future climate change scenarios. To this end, we surveyed the abundance, diversity and structure of microbial communities over a 2-year period from a long-term in situ warming experiment that experienced a moderate natural drought. We found the warming treatment and soil water budgets strongly influence bacterial population size and diversity. In normal precipitation years, the warming treatment significantly increased microbial population size 40-150% but decreased diversity and significantly changed the composition of the community when compared with the unwarmed controls. However during drought conditions, the warming treatment significantly reduced soil moisture thereby creating unfavorable growth conditions that led to a 50-80% reduction in the microbial population size when compared with the control. Warmed plots also saw an increase in species richness, diversity and evenness; however, community composition was unaffected suggesting that few phylotypes may be active under these stressful conditions. Our results indicate that under warmed conditions, ecosystem water budget regulates the abundance and diversity of microbial populations and that rainfall timing is critical at the onset of drought for sustaining microbial populations.

Determinants of soil respiration in a semi-arid savanna ecosystem, Kruger National Park, South Africa

Soil respiration, which is a combination of root respiration and microbial respiration, represents one of the main carbon fluxes in savannas. However, it is remarkable how little is known about these components – regarding either process-level mechanisms or quantitative estimates, especially in savanna ecosystems. Given the extensive area of savannas worldwide, this limits our ability to understand and predict the critical changes in the global carbon budget that underlie the phenomenon of global climate change. From May 2000 to April 2001, bi-weekly soil respiration measurements from two savanna types were made in 14 sampling collars (diameter = 100 mm), using a PP Systems EGM-2 respirometer. Results indicated that there was a difference in the rate of respiration between the more clayey Acacia and sandier Combretum savanna soils (p = 0.028). The mean (± s.d.) soil respiration in the Acacia savanna was 0.540 g/m2/h ± 0.419 g/m2/h, whilst it was 0.484 g/m2/h ± 0.383 g/m2/h in the Combretum savanna. We also found that soil respiration was sensitive to soil moisture and soil temperature. The rate of soil respiration at both sites rose to a maximum when soil temperature was at 28 °C and declined at higher temperatures, despite different temperature sensitivities. Soil respiration increased approximately linearly with an increase of soil moisture. In both savanna sites soil is subject to a combination of high temperature and water stress, which controls the fluxes of soil carbon dioxide. We found that the two sites differed significantly in their soil moisture characteristics (p < 0.0001) but not with regard to temperature (p = 0.141), which implies that soil moisture is the main factor responsible for the differences in respiration between Acacia and Combretum savannas.Conservation implications: It is argued for many protected areas that they perform a climate change buffering function. Knowing the soil respiration rate and determining its controlling factors contribute to improved understanding of whether protected areas will be net sources or sinks of carbon in the future.

Twelve testable hypotheses on the geobiology of weathering

Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth's surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical Zone. (8) Rising atmospheric P(CO2) will increase rates and extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

Reproducibility and quantitation of amplicon sequencing-based detection

To determine the reproducibility and quantitation of the amplicon sequencing-based detection approach for analyzing microbial community structure, a total of 24 microbial communities from a long-term global change experimental site were examined. Genomic DNA obtained from each community was used to amplify 16S rRNA genes with two or three barcode tags as technical replicates in the presence of a small quantity (0.1% wt/wt) of genomic DNA from Shewanella oneidensis MR-1 as the control. The technical reproducibility of the amplicon sequencing-based detection approach is quite low, with an average operational taxonomic unit (OTU) overlap of 17.2%±2.3% between two technical replicates, and 8.2%±2.3% among three technical replicates, which is most likely due to problems associated with random sampling processes. Such variations in technical replicates could have substantial effects on estimating β-diversity but less on α-diversity. A high variation was also observed in the control across different samples (for example, 66.7-fold for the forward primer), suggesting that the amplicon sequencing-based detection approach could not be quantitative. In addition, various strategies were examined to improve the comparability of amplicon sequencing data, such as increasing biological replicates, and removing singleton sequences and less-representative OTUs across biological replicates. Finally, as expected, various statistical analyses with preprocessed experimental data revealed clear differences in the composition and structure of microbial communities between warming and non-warming, or between clipping and non-clipping. Taken together, these results suggest that amplicon sequencing-based detection is useful in analyzing microbial community structure even though it is not reproducible and quantitative. However, great caution should be taken in experimental design and data interpretation when the amplicon sequencing-based detection approach is used for quantitative analysis of the β-diversity of microbial communities.

A meta-analysis of responses of soil biota to global change

Global environmental changes are expected to impact the abundance of plants and animals aboveground, but comparably little is known about the responses of belowground organisms. Using meta-analysis, we synthesized results from over 75 manipulative experiments in order to test for patterns in the effects of elevated CO(2), warming, and altered precipitation on the abundance of soil biota related to taxonomy, body size, feeding habits, ecosystem type, local climate, treatment magnitude and duration, and greenhouse CO(2) enrichment. We found that the positive effect size of elevated CO(2) on the abundance of soil biota diminished with time, whereas the negative effect size of warming and positive effect size of precipitation intensified with time. Trophic group, body size, and experimental approaches best explained the responses of soil biota to elevated CO(2), whereas local climate and ecosystem type best explained responses to warming and altered precipitation. The abundance of microflora and microfauna, and particularly detritivores, increased with elevated CO(2), indicative of microbial C limitation under ambient CO(2). However, the effects of CO(2) were smaller in field studies than in greenhouse studies and were not significant for higher trophic levels. Effects of warming did not depend on taxon or body size, but reduced abundances were more likely to occur at the colder and drier sites. Precipitation limited all taxa and trophic groups, particularly in forest ecosystems. Our meta-analysis suggests that the responses of soil biota to global change are predictable and unique for each global change factor.

Forest soil CO2 fluxes as a function of understory removal and N-fixing species addition

We report on the effects of forest management practices of understory removal and N-fixing species (Cassia alata) addition on soil CO2 fluxes in an Eucalyptus urophylla plantation (EUp), Acacia crassicarpa plantation (ACp), 10-species-mixed plantation (Tp), and 30-species-mixed plantation (THp) using the static chamber method in southern China. Four forest management treatments, including (1) understory removal (UR); (2) C. alata addition (CA); (3) understory removal and replacement with C. alata (UR+CA); and (4) control without any disturbances (CK), were applied in the above four forest plantations with three replications for each treatment. The results showed that soil CO2 fluxes rates remained at a high level during the rainy season (from April to September), followed by a rapid decrease after October reaching a minimum in February. Soil CO2 fluxes were significantly higher (P < 0.01) in EUp (132.6 mg/(m2 x hr)) and ACp (139.8 mg/(m2 x hr)) than in Tp (94.0 mg/(m2 x hr)) and THp (102.9 mg/(m2 x hr)). Soil CO2 fluxes in UR and CA were significantly higher (P < 0.01) among the four treatments, with values of 105.7, 120.4, 133.6 and 112.2 mg/(m2 x hr) for UR+CA, UR, CA and CK, respectively. Soil CO2 fluxes were positively correlated with soil temperature (P < 0.01), soil moisture (P < 0.01), NO3(-)-N (P < 0.05), and litterfall (P < 0.01), indicating that all these factors might be important controlling variables for soil CO2 fluxes. This study sheds some light on our understanding of soil CO2 flux dynamics in forest plantations under various management practices.

Soil temperature and moisture sensitivities of soil CO2 efflux before and after tillage in a wheat field of Loess Plateau, China

As a conventional farming practice, tillage has lasted for thousands of years in Loess Plateau, China. Although recent studies show that tillage is a prominent culprit to soil carbon loss in croplands, few studies have investigated the influences of tillage on the responses of soil CO2 efflux (SCE) to soil temperature and moisture. Using a multi-channel automated CO2 efflux chamber system, we measured SCE in situ continuously before and after the conventional tillage in a rain fed wheat field of Loess Plateau, China. The changes in soil temperature and moisture sensitivities of SCE, denoted by the Q10 value and linear regression slope respectively, were compared in the same range of soil temperature and moisture before and after the tillage. The results showed that, after the tillage, SCE increased by 1.2-2.2 times; the soil temperature sensitivity increased by 36.1%-37.5%; and the soil moisture sensitivity increased by 140%-166%. Thus, the tillage-induced increase in SCE might partially be attributed to the increases in temperature and moisture sensitivity of SCE.

Do biotic interactions modulate ecosystem functioning along stress gradients? Insights from semi-arid plant and biological soil crust communities

Climate change will exacerbate the degree of abiotic stress experienced by semi-arid ecosystems. While abiotic stress profoundly affects biotic interactions, their potential role as modulators of ecosystem responses to climate change is largely unknown. Using plants and biological soil crusts, we tested the relative importance of facilitative-competitive interactions and other community attributes (cover, species richness and species evenness) as drivers of ecosystem functioning along stress gradients in semi-arid Mediterranean ecosystems. Biotic interactions shifted from facilitation to competition along stress gradients driven by water availability and temperature. These changes were, however, dependent on the spatial scale and the community considered. We found little evidence to suggest that biotic interactions are a major direct influence upon indicators of ecosystem functioning (soil respiration, organic carbon, water-holding capacity, compaction and the activity of enzymes related to the carbon, nitrogen and phosphorus cycles) along stress gradients. However, attributes such as cover and species richness showed a direct effect on ecosystem functioning. Our results do not agree with predictions emphasizing that the importance of plant-plant interactions will be increased under climate change in dry environments, and indicate that reductions in the cover of plant and biological soil crust communities will negatively impact ecosystems under future climatic conditions.

Global convergence in the temperature sensitivity of respiration at ecosystem level

The respiratory release of carbon dioxide (CO(2)) from the land surface is a major flux in the global carbon cycle, antipodal to photosynthetic CO(2) uptake. Understanding the sensitivity of respiratory processes to temperature is central for quantifying the climate-carbon cycle feedback. We approximated the sensitivity of terrestrial ecosystem respiration to air temperature (Q(10)) across 60 FLUXNET sites with the use of a methodology that circumvents confounding effects. Contrary to previous findings, our results suggest that Q(10) is independent of mean annual temperature, does not differ among biomes, and is confined to values around 1.4 +/- 0.1. The strong relation between photosynthesis and respiration, by contrast, is highly variable among sites. The results may partly explain a less pronounced climate-carbon cycle feedback than suggested by current carbon cycle climate models.

Concurrent and lagged impacts of an anomalously warm year on autotrophic and heterotrophic components of soil respiration: a deconvolution analysis

*Partitioning soil respiration into autotrophic (R(A)) and heterotrophic (R(H)) components is critical for understanding their differential responses to climate warming. *Here, we used a deconvolution analysis to partition soil respiration in a pulse warming experiment. We first conducted a sensitivity analysis to determine which parameters can be identified by soil respiration data. A Markov chain Monte Carlo technique was then used to optimize those identifiable parameters in a terrestrial ecosystem model. Finally, the optimized parameters were employed to quantify R(A) and R(H) in a forward analysis. *Our results displayed that more than one-half of parameters were constrained by daily soil respiration data. The optimized model simulation showed that warming stimulated R(H) and had little effect on R(A) in the first 2 months, but decreased both R(H) and R(A) during the remainder of the treatment and post-treatment years. Clipping of above-ground biomass stimulated the warming effect on R(H) but not on R(A). Overall, warming decreased R(A) and R(H) significantly, by 28.9% and 24.9%, respectively, during the treatment year and by 27.3% and 33.3%, respectively, during the post-treatment year, largely as a result of decreased canopy greenness and biomass. *Lagged effects of climate anomalies on soil respiration and its components are important in assessing terrestrial carbon cycle feedbacks to climate warming.

Temperature-associated increases in the global soil respiration record

Soil respiration, R(S), the flux of microbially and plant-respired carbon dioxide (CO(2)) from the soil surface to the atmosphere, is the second-largest terrestrial carbon flux. However, the dynamics of R(S) are not well understood and the global flux remains poorly constrained. Ecosystem warming experiments, modelling analyses and fundamental biokinetics all suggest that R(S) should change with climate. This has been difficult to confirm observationally because of the high spatial variability of R(S), inaccessibility of the soil medium and the inability of remote-sensing instruments to measure R(S) on large scales. Despite these constraints, it may be possible to discern climate-driven changes in regional or global R(S) values in the extant four-decade record of R(S) chamber measurements. Here we construct a database of worldwide R(S) observations matched with high-resolution historical climate data and find a previously unknown temporal trend in the R(S) record after accounting for mean annual climate, leaf area, nitrogen deposition and changes in CO(2) measurement technique. We find that the air temperature anomaly (the deviation from the 1961-1990 mean) is significantly and positively correlated with changes in R(S). We estimate that the global R(S) in 2008 (that is, the flux integrated over the Earth's land surface over 2008) was 98 +/- 12 Pg C and that it increased by 0.1 Pg C yr(-1) between 1989 and 2008, implying a global R(S) response to air temperature (Q(10)) of 1.5. An increasing global R(S) value does not necessarily constitute a positive feedback to the atmosphere, as it could be driven by higher carbon inputs to soil rather than by mobilization of stored older carbon. The available data are, however, consistent with an acceleration of the terrestrial carbon cycle in response to global climate change.

Soil carbon dynamics and carbon budget of newly reconstructed tall-grass prairies in south central Iowa

In addition to their aesthetic and environmental qualities, reconstructed prairies can act as C sinks and potentially offset rising atmospheric CO(2) concentration. The objective of this study was to quantify C budget components of newly established prairies on previously cultivated land. Net ecosystem production (NEP) was estimated using a C budgeting approach that assessed SOC content, soil surface CO(2)-C emission, and above- and belowground plant biomass. Study was conducted in southern Iowa, in 2005 to 2007. Results show that differences between sites for potential total C input were primarily due to root biomass contributions, which ranged from 0.8 to 5.4 Mg C ha(-1). Average potential aboveground biomass C input was 2.7 Mg C ha(-1) in 2006 and 5.5 Mg C ha(-1) in 2007. Total soil CO(2)-C emissions from heterotrophic respiration increased as prairie age increased from 2.9 to 4.0 Mg C ha(-1) and 3.1 to 4.7 Mg C ha(-1) in 2006 and 2007, respectively. Determination of NEP showed that the 1998 and 2003 reconstructed prairie sites had the greatest potential for soil C sequestration at 4.1 and 4.4 Mg C ha(-1). Increases in SOC content were only observed in the youngest established prairie site (2003) and the no-till site in 2003 at 2.1 and 2.6 Mg C ha(-1) yr(-1), respectively. Declines of SOC sequestration rates occurred when potential C equilibrium was reached (R(h) = NPP) within 10 yr since prairie establishment.

Spatial patterns in temperature sensitivity of soil respiration in China: estimation with inverse modeling

Temperature sensitivity of soil respiration (Q(10)) is an important parameter in modeling the effects of global warming on ecosystem carbon release. Experimental studies of soil respiration have ubiquitously indicated that Q(10) has high spatial heterogeneity. However, most biogeochemical models still use a constant Q(10) in projecting future climate change and no spatial pattern of Q(10) values at large scales has been derived. In this study, we conducted an inverse modeling analysis to retrieve the spatial pattern of Q(10) in China at 8 km spatial resolution by assimilating data of soil organic carbon into a process-based terrestrial carbon model (CASA model). The results indicate that the optimized Q(10) values are spatially heterogeneous and consistent to the values derived from soil respiration observations. The mean Q(10) values of different soil types range from 1.09 to 2.38, with the highest value in volcanic soil, and the lowest value in cold brown calcic soil. The spatial pattern of Q (10) is related to environmental factors, especially precipitation and top soil organic carbon content. This study demonstrates that inverse modeling is a useful tool in deriving the spatial pattern of Q(10) at large scales, with which being incorporated into biogeochemical models, uncertainty in the projection of future carbon dynamics could be potentially reduced.