Plant responses to the projected future levels of CO2 were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO2 were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO2 Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO2 on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO2 under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO2 stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO2 improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO2 stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO2 does not directly stimulate C4 photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO2 in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.

Elevated CO2 effects on plant carbon, nitrogen, and water relations: six important lessons from FACE

https://www.ukm.my/ipi/wp-content/uploads/2013/07/33_2011_Impacts-and-adaptation-of-European-crop-production-systems-to-climate-change.pdf

Impacts and adaptation of European crop production systems to climate change

https://www.ukm.my/ipi/wp-content/uploads/2013/07/25.2010Climate-impacts-on-European-agriculture-and-water-management-in-the-context-of-adaptation-and-mitigation.pdf

Climate impacts on European agriculture and water management in the context of adaptation and mitigation—The importance of an integrated approach

Soils contain the largest pool of terrestrial organic carbon (C) and are a major source of atmospheric carbon dioxide (CO2). Thus, they may play a key role in modulating climate change. Rising atmospheric CO2 is expected to stimulate plant growth and soil C input but may also alter microbial decomposition. The combined effect of these responses on long-term C storage is unclear. Combining meta-analysis with data assimilation, we show that atmospheric CO2 enrichment stimulates both the input (+19.8%) and the turnover of C in soil (+16.5%). The increase in soil C turnover with rising CO2 leads to lower equilibrium soil C stocks than expected from the rise in soil C input alone, indicating that it is a general mechanism limiting C accumulation in soil.

Faster Decomposition Under Increased Atmospheric CO2 Limits Soil Carbon Storage

Rhizodeposit-carbon provides a major energy source for microbial growth in the rhizosphere of grassland soils. However, little is known about the microbial communities that mediate the rhizosphere carbon dynamics, especially how their activity is influenced by changes in soil management. We combined a 13CO2 pulse-labeling experiment with phospholipid fatty acid (PLFA) analysis in differently managed Belgian grasslands to identify the active rhizodeposit-C assimilating microbial communities in these grasslands and to evaluate their response to management practices. Experimental treatments consisted of three mineral N fertilization levels (0, 225 and 450 kg N ha−1 y−1) and two mowing frequencies (3 and 5 times y−1). Phospholipid fatty acids were extracted from surface (0–5 cm) bulk (BU) and root-adhering (RA) soil samples prior to and 24 h after pulse-labeling and were analyzed by gas chromatography–combustion-isotope ratio mass spectrometry (GC–c-IRMS). Soil habitats significantly differed in microbial community structure (as revealed by multivariate analysis of mol% biomarker PLFAs) as well as in gram-positive bacterial rhizodeposit-C uptake (as revealed by greater 13C-PLFA enrichment following pulse-labeling in RA compared to BU soil in the 450N/5M treatment). Mowing frequency did not significantly alter the relative abundance (mol%) or activity (13C enrichment) of microbial communities. In the non-fertilized treatment, the greatest 13C enrichment was seen in all fungal biomarker PLFAs (C16:1ω5, C18:1ω9, C18:2ω6,9 and C18:3ω3,6,9), which demonstrates a prominent contribution of fungi in the processing of new photosynthate-C in non-fertilized grassland soils. In all treatments, the lowest 13C enrichment was found in gram-positive bacterial and actinomycetes biomarker PLFAs. Fungal biomarker PLFAs had significantly lower 13C enrichment in the fertilized compared to non-fertilized treatments in BU soil (C16:1ω5, C18:1ω9) as well as RA soil (all fungal biomarkers). While these observations clearly indicated a negative effect of N fertilization on fungal assimilation of plant-derived C, the effect of N fertilization on fungal abundance could only be detected for the arbuscular mycorrhizal fungal (AMF) PLFA (C16:1ω5). On the other hand, increases in the relative abundance of gram-positive bacterial PLFAs with N fertilization were found without concomitant increases in 13C enrichment following pulse-labeling. We conclude that in situ 13C pulse-labeling of PLFAs is an effective tool to detect functional changes of those microbial communities that are dominantly involved in the immediate processing of new rhizosphere-C.

Microbial community composition and rhizodeposit-carbon assimilation in differently managed temperate grassland soils

Response of aboveground grassland biomass and soil moisture to moderate long-term CO2 enrichment

In this paper, a general method for an a posteriori plant functional type (PFT) analysis of global change effects on community composition is developed. We apply the method to a case study, specifically the Giessen-FACE experiment. This experiment involves a Central European meadow that has been exposed to moderate CO 2 -enrichment since May 1998. The method for an a posteriori PFT-analysis: The method consists of four working steps and uses a combination of standard gradient analysis and Random Forests (RF). (1) The trait composition of the species is studied using Principal Components Analysis. Species trait information is gathered from databases. Natural PFT, i.e. groups of species with similar trait-sets, are identified specifically for the community under study. (2) A ranking of the species according to standardized/absolute CO 2 abundance response is obtained from Redundancy Analysis. Initially, species with a response above or below the median are grouped into three response groups (RG) each having similar behaviour, i.e. positive/negative or no-response. (3) The outlyingness measure of RF is used to shift RG boundaries until satisfactory RG homogeneity is achieved. RF is utilized to find the best traits for the RG classification. The behaviour of species representative of the RG is derived from RF class centers. (4) From knowledge gained in steps 1-3, hypotheses about the causes underlying the community alteration are built. Strengths/weaknesses of the method are discussed. Application of the method to the case study: The community consists of three natural PFT. Five species are summer-green forbs of varying competitiveness. Four species are evergreen ruderal forbs characterized as (semi-) basal rosette plants. The third natural PFT contains evergreen, more or less competitive species, mostly grasses, but also a few forbs. Negative standardized CO 2 -response was practically restricted to two natural PFT, i.e. the summer-greens, irrespective of their competitiveness, and the evergreen ruderals. Standard positive response covered part of the evergreen competitive natural PFT. Among them was Glechoma hederacea, one of the forbs with the greatest similarity to grasses. Two hypotheses were formulated to explain the response pattern: (1) Summer-greens lost in competition with evergreens, because the annual time-integral they can use for enhanced growth was more limited with year-round CO 2 -enrichment. (2) As rosette plants, ruderal evergreens lagged behind evergreen competitors because periods with full sunlight, which enabled them to gain additional carbon, were shorter for them. Absolute responses were additionally dependent on dominance patterns. The most striking difference to standard responses was the restriction of positive response to (sub-)dominant grasses.

/pdf/plant-functional-types-and-elevated-co-2-a-method-of-3a1acuqa91.pdf

S. Janze

Papers

Response of aboveground grassland biomass and soil moisture to moderate long-term CO2 enrichment

Plant functional types and elevated CO 2 : A method of scanning for causes of community alteration