Respiration, Rather Than Photosynthesis, Determines Rice Yield Loss Under Moderate High-Temperature Conditions

Photosynthesis is an important biophysical and biochemical reaction that provides food and oxygen to maintain aerobic life on earth. Recently, increasing photosynthesis has been revisited as an approach for reducing rice yield losses caused by high temperatures. We found that moderate high temperature causes less damage to photosynthesis but significantly increases respiration. In this case, the energy production efficiency is enhanced, but most of this energy is allocated to maintenance respiration, resulting in an overall decrease in the energy utilization efficiency. In this perspective, respiration, rather than photosynthesis, may be the primary contributor to yield losses in a high-temperature climate. Indeed, the dry matter weight and yield could be enhanced if the energy was mainly allocated to the growth respiration. Therefore, we proposed that engineering smart rice cultivars with a highly efficient system of energy production, allocation, and utilization could effectively solve the world food crisis under high-temperature conditions.

Shifts in soil microbial stoichiometry and metabolic quotient provide evidence for a critical tipping point at 1% soil organic carbon in an agricultural post-mining chronosequence

Soil microbial C:N:P stoichiometry and microbial maintenance respiration (i.e. metabolic quotient, qCO2) were monitored along a nutrient gradient in soils from a 52-year space-for-time chronosequence of reclaimed agricultural land after brown-coal mining. Land reclamation produced loess soils of initially low (0.2%) SOC. Consecutive agricultural land management led to a gradual recovery of SOC contents. Our data revealed sudden shifts in microbial stoichiometry and metabolic quotient with increasing SOC at a critical value of 1% SOC. As SOC increased, accrual rate of C into microbial biomass decreased, whereas microbial N increased. Simultaneously, metabolic quotient strongly decreased with increasing SOC until the same critical value of 1% SOC and remained at a constant low thereafter. The microbial fractions of the soil in samples containing < 1% SOC were out of stoichiometric equilibrium and were inefficient at immobilising C due to high maintenance respiration. Increasing SOC above the threshold value shifted the soil microbes towards a new equilibrium where N became growth limiting, leading to a more efficient acquisition of C. The shift in microbial N accrual was preluded by high variation in microbial biomass N in soils containing 0.5–0.9% SOC indicative of a regime shift between microbial stoichiometric equilibria. Our data may help in establishing a quantitative framework for SOC targets that, along with agricultural intensification, may better support feedback mechanisms for a sustainable accrual of C in soils.

Sapwood proportion and nitrogen content in boreal and temperate tree species

&lt;p&gt;The spatial and temporal variation in plant respiration is one of the largest unknowns in the global land carbon budget. While respiration rates are directly related to temperature, plant respiration of trees is also determined by their stem sapwood proportion, tissue nitrogen (N) contents and other factors. The sapwood proportion is related to the biomass fraction of respiring living cells in the tree stem. The respiratory costs that plants have to invest to maintain basic functions (maintenance respiration) are related to the vegetation N content, since maintenance respiration supports protein repair and replacement, and most plant organic N is in proteins.&lt;/p&gt;&lt;p&gt;Here we explore the variation and underlying drivers in these two plant traits (stem sapwood proportion, tissue N contents) and derive novel estimates of their spatial distribution in northern hemisphere boreal and temperate forests. For the first task, we make use of measurements of sapwood and total cross-sectional area in tree stems and of N contents per dry matter in stems, roots and leaves. Such data are collected from plant trait databases like TRY, the biomass and allometry database (BAAD) and extensive literature reviews covering the most common boreal and temperate tree species. For the second task, we apply the derived tree level relationships between these traits and the underlying drivers (species, climate, soil variables) in combination with satellite radar remote sensing based products of compartment (stem, branch, root and leaf) biomass and tree species distribution maps covering the entire northern boreal and temperate forests.&lt;/p&gt;&lt;p&gt;We find that both the proportion of sapwood to total stem biomass and the response of the N content to environmental conditions are fundamentally different among tree genera. For instance, the sapwood proportions are spanning from 20&amp;#8211;30% in larch to &gt; 70% in pine and birch forests. These findings highlight the need to consider genera-specific differences when estimating the response of plant respiration to changes in climate and forest management.&lt;/p&gt;

HETEROFOR 1.0: a spatially explicit model for exploring the response of structurally complex forests to uncertain future conditions – Part 1: Carbon fluxes and tree dimensional growth

Given the multiple abiotic and biotic stressors resulting from global changes, management systems and practices must be adapted in order to maintain and reinforce the resilience of forests. Among others, the transformation of monocultures into uneven-aged and mixed stands is an avenue to improve forest resilience. To explore the forest response to these new silvicultural practices under a changing environment, one needs models combining a process-based approach with a detailed spatial representation, which is quite rare. We therefore decided to develop our own model (HETEROFOR for HETEROgeneous FORest) according to a spatially explicit approach, describing individual tree growth based on resource sharing (light, water and nutrients). HETEROFOR was progressively elaborated within Capsis (Computer-Aided Projection for Strategies in Silviculture), a collaborative modelling platform devoted to tree growth and stand dynamics. This paper describes the carbon-related processes of HETEROFOR (photosynthesis, respiration, carbon allocation and tree dimensional growth) and evaluates the model performances for three broadleaved stands with different species compositions (Wallonia, Belgium). This first evaluation showed that HETEROFOR predicts well individual radial growth (Pearson's correlation of 0.83 and 0.63 for the European beech and sessile oak, respectively) and is able to reproduce size–growth relationships. We also noticed that the net to gross primary production (npp to gpp) ratio option for describing maintenance respiration provides better results than the temperature-dependent routine, while the process-based (Farquhar model) and empirical (radiation use efficiency) approaches perform similarly for photosynthesis. To illustrate how the model can be used to predict climate change impacts on forest ecosystems, we simulated the growth dynamics of the mixed stand driven by three IPCC climate scenarios. According to these simulations, the tree growth trends will be governed by the CO2 fertilization effect, with the increase in vegetation period length and the increase in water stress also playing a role but offsetting each other.

Vertical Strata and Stem Carbon Dioxide Efflux in Cycas Trees

Stem respiration is influenced by the vertical location of tree stems, but the influence of vertical location on stem respiration in a representative cycad species has not been determined. We quantified the influence of vertical strata on stem carbon dioxide efflux (Es) for six arborescent Cycas L. species to characterize this component of stem respiration and ecosystem carbon cycling. The influence of strata on Es was remarkably consistent among the species, with a stable baseline flux characterizing the full mid-strata of the pachycaulous stems and an increase in Es at the lowest and highest strata. The mid-strata flux ranged from 1.8 μmol·m−2·s−1 for Cycas micronesica K.D. Hill to 3.5 μmol·m−2·s−1 for Cycas revoluta Thunb. For all species, Es increased about 30% at the lowest stratum and about 80% at the highest stratum. A significant quadratic model adequately described the Es patterns for all six species. The increase of Es at the lowest stratum was consistent with the influence of root-respired carbon dioxide entering the stem via sap flow, then contributing to Es via radial conductance to the stem surface. The substantial increase in Es at the highest stratum is likely a result of the growth and maintenance respiration of the massive cycad primary thickening meristem that constructs the unique pachycaulous cycad stem.

Dependence of microalgae culture specific endogenous loss rate and gross productivity on irradiance

Microalgal growth rate is determined by the difference between gross productivity and endogenous biomass loss rate. Gross productivity is a function of surface irradiance and reaches the maximal value under saturating light intensity. Endogenous biomass consumption of microalgal cells is mainly due to the respiration which can be sub-divided into growth and maintenance respiration. The paper proposes equations for calculating microalgae culture gross productivity and specific loss rate based on production characteristics. Verification of the obtained equations was performed for Arthrospira platensis and Dunaliella salina cultivated under constant illumination with different irradiance. It was demonstrated that gross productivity and specific loss rate increase linearly with increasing irradiance. Specific loss rate varied from 0,01 to 0,58 h-1 in A. platensis and from 0,04 to 0,35 day-1 in D. salina. Specific maintenance respiration rate was determined to be 0,01 h-1 in A. platensis and 0,01–0,04 day-1 in D. salina. The proposed calculation method enables a sufficiently accurate determination of productivity and loss rates based on culture density dynamics measures.

HETEROFOR 1.0: a spatially explicit model for exploring the response of structurally complex forests to uncertain future conditions. I. Carbon fluxes and tree dimensional growth

Given the multiple abiotic and biotic stressors resulting from global changes, management systems and practices must be adapted in order to maintain and reinforce the resilience of forests. Among others, the transformation of monocultures into uneven-aged and mixed stands is an avenue to improve forest resilience. To explore the forest response to these new silvicultural practices under a changing environment, one need models combining a process-based approach with a detailed spatial representation, which is very rare. We therefore decided to develop our own model (HETEROFOR) according to a spatially explicit approach describing individual tree growth based on resource sharing (light, water and nutrients). HETEROFOR was progressively elaborated through the integration of various modules (light interception, phenology, water cycling, photosynthesis and respiration, carbon allocation, mineral nutrition and nutrient cycling) within CAPSIS, a collaborative modelling platform devoted to tree growth and stand dynamics. The advantage of using such a platform is to use common development environment, model execution system, user- interface and visualization tools and to share data structures, objects, methods and libraries. This paper describes the carbon-related processes of HETEROFOR (photosynthesis, respiration, carbon allocation and tree dimensional growth) and evaluates the model performances for a mixed oak and beech stand in Wallonia (Belgium). This first evaluation showed that HETEROFOR predicts well individual radial growth and is able to reproduce size-growth relationships. We also noticed that the more empirical options for describing maintenance respiration and crown extension provide the best results while the process-based approach best performs for photosynthesis. To illustrate how the model can be used to predict climate change impacts on forest ecosystems, the growth dynamics in this stand was simulated according to four IPCC climate scenarios. According to these simulations, the tree growth trends will be governed by the CO2 fertilization effect with the increase in vegetation period length and in water stress also playing a role but offsetting each other.

Modelling Carbon Fluxes as an Aid to Understanding Perennial Ryegrass (Lolium perenne) Root Dynamics

Despite the importance of roots in determining plant performance, the factors controlling their development and longevity remain poorly understood. Grass morphology is based on repeating units called phytomers, with each capable of producing one leaf, one daughter tiller, and one or more roots. We developed a phytomer-based understanding of root birth, growth and senescence in Lolium perenne, using a modeling approach to explore seasonal effects on root turnover dynamics, and to explore cultivar differences in these processes. Similar to leaves, roots exhibit a clear progression from initiation, growing for approximately seven phyllochrons, with growth rates strongly influenced by environmental conditions. In spring, the phyllochron decreased over the experiment, while it increased in autumn. In spring, C availability exceeding maintenance respiratory requirements allowed root growth at each phytomer position, with a 70/30 split between maintenance and growth. Under C-deficient conditions in autumn, this split was approximately 80/20, with growth limited to younger phytomer positions, while older roots were more susceptible to starvation-induced senescence due to their high C requirements for maintenance respiration.

Implementing the nitrogen cycle into the dynamic global vegetation, hydrology, and crop growth model LPJmL (version 5.0)

The well-established dynamical global vegetation, hydrology, and crop growth model LPJmL is extended with a terrestrial nitrogen cycle to account for nutrient limitations. In particular, processes of soil nitrogen dynamics, plant uptake, nitrogen allocation, response of photosynthesis and maintenance respiration to varying nitrogen concentrations in plant organs, and agricultural nitrogen management are included in the model. All new model features are described in full detail and the results of a global simulation of the historic past (1901–2009) are presented for evaluation of the model performance. We find that the implementation of nitrogen limitation significantly improves the simulation of global patterns of crop productivity. Regional differences in crop productivity, which had to be calibrated via a scaling of the maximum leaf area index, can now largely be reproduced by the model, except for regions where fertilizer inputs and climate conditions are not the yield-limiting factors. Furthermore, it can be shown that land use has a strong influence on nitrogen losses, increasing leaching by 93 %.