Rhizodeposition under ambient and elevated CO2 levels

As global CO₂ levels rise, can soils store more carbon and so buffer atmospheric CO₂ levels? Answering this question requires a knowledge of the rates of C inputs to soil and of CO₂ outputs via decomposition. Below-ground inputs from roots are a major component of the C flow into soils but are still poorly understood. In this article, new techniques for measuring rhizodeposition are reviewed and discussed and the need for cross-comparisons between methods is identified. One component of rhizodeposition, root exudation, is examined in more detail and evidence is presented which suggests that current estimates of exudate flow into soils are incorrect. A mechanistic mathematical model is used to explore how exudate flows might change under elevated CO₂.     

Cloning of the amphibolic Calvin cycle/OPPP enzyme d-ribulose-5-phosphate 3-epimerase (EC 5.1.3.1) from spinach chloroplasts: functional and evolutionary aspects

Exploiting the differential expression of genes for Calvin cycle enzymes in bundle-sheath and mesophyll cells of the C4 plant Sorghum bicolor L., we isolated via subtractive hybridization a molecular probe for the Calvin cycle enzyme d-ribulose-5-phosphate 3-epimerase (R5P3E) (EC 5.1.3.1), with the help of which several full-size cDNAs were isolated from spinach. Functional identity of the encoded mature subunit was shown by R5P3E activity found in affinity-purified glutatione S-transferase fusions expressed in Escherichia coli and by three-fold increase of R5P3E activity upon induction of E. coli overexpressing the spinach subunit under the control of the bacteriophage T₇ promoter, demonstrating that we have cloned the first functional ribulose-5-phosphate 3-epimerase from any eukaryotic source. The chloroplast enzyme from spinach shares about 50% amino acid identity with its homologues from the Calvin cycle operons of the autotrophic purple bacteria Alcaligenes eutrophus and Rhodospirillum rubrum. A R5P3E-related eubacterial gene family was identified which arose through ancient duplications in prokaryotic chromosomes, three R5P3E-related genes of yet unknown function have persisted to the present within the E. coli genome. A gene phylogeny reveals that spinach R5P3E is more similar to eubacterial homologues than to the yeast sequence, suggesting a eubacterial origin for this plant nuclear gene.Abbreviations R5P3E d-ribulose-5-phosphate 3-epimerase - RPI ribose-5-phosphate isomerase - TKL transketolase - PRK phosphoribulokinase - GAPDH glyceraldehyde-3-phosphate dehydrogenase - FBP fructose-1,6-bisphophatase - FBP fructose 1,6-bisphosphate - G6PDH glucose-6-phosphate dehydrogenase - 6PGDH 6-phosphogluconate dehydrogenase - OPPP oxidative pentose phosphate pathway - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - FBA fructose-1,6-bisphophate aldolase - IPTG isopropyl -d-thiogalactoside - GST glutathione S-tranferase - PBS phosphate-buffered saline - TPI triosephosphate isomerase     

An assessment of the annual mass balance of carbon,nitrogen, and phosphorus in Narragansett Bay

Narragansett Bay is a relatively well-mixed, high salinity coastal embayment and estuary complex in southern New England (USA). Much of the shoreline is urban and the watershed is densely developed. We have combined our data on C, N, and P inputs to this system, on C, N, and P accumulation in the sediments, and on denitrification with extensive work by others to develop approximate annual mass balances for these elements. The results show that primary production within the bay is the major source of organic carbon (4 times greater than other sources), that land drainage and upstream sewage and fertilizer are the major sources of N, and that landward flowing bottom water from offshore may be a major source of dissolved inorganic phosphorus. Most of the nutrients entering the bay arrive in dissolved inorganic form, though DON is a significant component of the N carried by the rivers. About 40% of the DIN in the rivers is in the form of ammonia. Sedimentation rates are low in most of Narragansett Bay, and it appears that less than 20% of the total annual input of each of these elements is retained within the system. A very small amount of C, N, and P is removed in fisheries landings, denitrification in the sediments removes perhaps 10–25% of the N input, and most of the carbon fixed in the system is respired within it. Stoichiometric calculations suggest that some 10–20% of the organic matter formed in the bay is exported to offshore and that Narragansett Bay is an autotrophic system. Most of the N and P that enters the bay is, however, exported to offshore waters in dissolved inorganic form. This assessment of the overall biogeochemical behavior of C, N, and P in the bay is consistent with more rigorously constrained mass balances obtained using large living models or mesocosms of the bay at the Marine Ecosystem Research Laboratory (MERL).     

Effects of pasture introduction on soil CO2 emissions during the dry season in the state of Rondônia,Brazil

Soil CO₂ evolution rates, soil temperatures and moisture were measured during the dry season in two forest-to-pasture chronosequences in Rondônia, Brazil. The study included pastures ranging from 3 to 80 years-old. Mean dry-season CO₂ evolution from the forest in chronosequence 1, 88.8 mg CO₂-C m^–2h^–1 was lower than from the pastures which ranged from 111 to 158 mg CO₂-C m^–2h^–1. We found that temperature was not a good predictor of CO₂ emissions from pasture but that there was a significant relationship (r = 0.72,p < 0.05) between soil moisture and pasture emissions. The ¹³C of the soil CO₂ emissions also was measured on chronosequence I; ¹³C of the CO₂ emitted from the C3 forest was –29.43%. Pasture¹³CO₂ values increased from –17.91%. in the 3 year-old pasture to –12.86% in the 80 year-old, reflecting the increasing C₄ inputs with pasture age. Even in the youngest (3 year-old) pasture, 70 percent of the CO₂ evolved originated from C₄ pasture-derived carbon.     

Carbon flows in the Westerschelde estuary (The Netherlands) evaluated by means of an ecosystem model (MOSES)

The autotrophic production and heterotrophic consumption of organic matter in the Westerschelde, a highly turbid and eutrophic estuary in the Southwest Netherlands is examined by means of a dynamic simulation model. The model describes the ecologically relevant processes in thirteen spatial compartments and adequately fits most observed data.Three autotrophic processes are included in the model. Net pelagic photosynthetic production is relatively low (average 41 gC m^–2 yr^–1) and three spatial compartments near the turbidity maximum zone are respiratory sinks of phytoplankton biomass. According to the model, net phytobenthic primary production is more important than pelagic primary production in the upstream half of the Westerschelde. On the scale of the entire estuary, benthic primary production amounts to about 60% of pelagic primary production. Water-column nitrification, which is very important in the nitrogen cycle, is most pronounced near the turbidity zone where it accounts for the major autotrophic fixation of carbon (up to 27 g C m^–2 yr^–1). Viewed on the scale of the total estuary, however, the process is not very important.Less than 20% of total organic carbon input to the estuary is primary produced, the remainder is imported from waste discharges and from the river.The degree of heterotrophy of the Westerschelde estuary proved to be one of the highest yet reported. On average 380 g carbon per square metre is net lost per year (range 200–1200 gC m^–2 yr^–1). The yearly community respiration (bacterial mineralization, respiration of higher trophic levels and sedimentation) is 4 to 35 times (estuarine mean of 6) higher than the net production. This degree of heterotrophy is highest near the turbidity maximum and generally decreases from the freshwater to the seaward boundary. About 75% of all carbon losses can be ascribed to pelagic heterotrophic processes; the sediment is only locally important.Mineralisation rates are highest in the turbidity region, but as only a fraction of total carbon resides here, less than 20% of all organic carbon is lost in this part of the estuary. This result is in contradiction with a previous budget of the estuary, based on data of the early seventies, where more than 80% of all carbon was estimated to be lost in the turbidity zone. Part of this discrepancy is probably caused by changes that have occurred in the estuary since that time.Due to the high heterotrophic activity, nearly all imported and in situ produced carbon is lost in the estuary itself and the Westerschelde is an insignificant source of organic matter to the coastal zone.The model estuary acts as a trap for reactive organic matter, both from the land, from the sea or in situ produced. Internal cycling, mainly in the water column, results in the removal of most of the carbon while the more refractory part is exported to the sea.     

Gas phase composition effects on suspension cultures of Taxus cuspidata

The effect of different concentrations and combinations of oxygen, carbon dioxide, and ethylene on cell growth and taxol production in suspension cultures of Taxus cuspidata was investigated using several factorial design experiments. Low head space oxygen concentration (10% v/v) promoted early production oftaxol. High carbon dioxide concentration (10% v/v) inhibited taxol production. The most effective gas mixture composition in terms of taxol production was 10% (v/v) oxygen, 0.5% (v/v) carbon dioxide, and 5 ppm ethylene. Cultures grown underambient concentration of oxygen had a delayed uptake of glucose and fructose compared to cultures grown under 10% (v/v) oxygen. Average calcium uptake rates into the cultured cells decreased and average phosphate uptake rates increased as ethylene was increased from 0 to 10 ppm. These results may indicate that gas composition alters partitioning of nutrients, which in turn affects secondary metabolite production. (c) 1995 John Wiley & Sons, Inc.     

Carbon and nitrogen cycling in intertidal sediments near Doel,Scheldt Estuary

Carbon and nitrogen cycling in intertidal mud flat sediments in the Scheldt Estuary was studied using measurements of carbon dioxide, methane and nitrous oxide emission rates and pore-water profiles of CO₂, ammonium and nitrate. A comparison between chamber measured carbon dioxide fluxes and those based on CO₂ pore-water gradients using Fick's First law indicates that apparent diffusion coefficients are 2 to 28 times higher than bulk sediment diffusion coefficients based on molecular diffusion. Seasonal changes in gaseous carbon fluxes or CO₂ pore water concentrations cannot be used directly, or in a simple way, to determine seasonal rates of mineralization, because of marked seasonal changes in pore-water storage and exchange parameters.The annual amount of carbon delivered to the sediment is 42 mol m^–2, of which about 42% becomes buried, the remaining being emitted as methane (7%) or carbon dioxide (50%). Each year about 2.6 mol N m^–2 of particulate nitrogen reaches the sediment; 1.1 mol m^–2 is buried and 1.6 mol m^–2 is mineralized to ammonium. Only 0.42 mol m^–2 yr^–1 of the ammonium produced escapes from the sediments, the remaining being first nitrified (1.2 mol m^–2 yr^–1) and then denitrified (1.7 mol m^–2 yr^–1). Simple calculations indicate that intertidal sediments may account for about 14% and 30% of the total estuarine retention of nitrogen and carbon, respectively.     

Stable carbon isotopes in tree rings of beech: climatic versus site-related influences

 Stable carbon isotopes in tree rings are a promising tool in palaeoclimate research, provided attempts are made to disentangle climatic from local effects (e.g. soil properties, competition, light). The ¹³C/¹²C variations in cellulose of tree rings of beech (Fagus sylvatica) were determined at several sites in the Swiss Central Plateau covering the last 50 years. We chose sites which differ in moisture conditions and sampled cores from four to six trees per site. The mean ¹³C/¹²C series from the different dry sites (distant by up to 40 km) are closely interrelated suggesting a common external cause. Correlation analysis with climate data proved the total precipitation in the months May, June and July to have the strongest effect on the carbon isotopes (r =  – 0.73). This result is in agreement with the commonly used model which relates the isotope discrimination to the water use efficiency. On the other hand, the isotope series of the wet sites are not as well correlated to the climate. At two of the sites (a dry and a humid) tree ring width suddenly increased. We used this effect as a test-case to study the influence of local growth conditions on the climate-isotope relationship. Received: 17 April 1996 / Accepted: 2 September 1996     

SOURCES OF INORGANIC CARBON FOR PHOTOSYNTHESIS BY THREE SPECIES OF MARINE DIATOM1

The utilization of inorganic carbon by three species of marine diatom, Skeletonema costatum (Grev.) Cleve. Ditylum brightwellii (West) Grun., and Chaetoceros calcitrans Paulsen was investigated using an inorganic carbon isotopic disequilibnum technique and inorganic carbon dose-response curves. Stable carbon isotope data of the diatoms are also presented. Observed rates of photosynthetic oxygen evolution were greater than could be accounted for by the theoretical rate of CO₂ supply from the uncatalyzed dehydration of HCO₃^? in the external medium, suggesting use of HCO₃^? as an inorganic carbon source. Data from the isotopic disequilibrium experiment demonstrate the use of both HCO₃^? and CO₂ for photosynthesis. Carbon isotope discrimination values support the use of HCO₃^? by the diatoms.     

KINETICS OF CELL DEATH IN THE CYANOBACTERIUM ANABAENA FLOS-AQUAE AND THE PRODUCTION OF DISSOLVED ORGANIC CARBON1

Kinetics of cell death and the production of dissolved organic carbon (DOC) were investigated in Anabaena flos-aquae (Lyngb.) Bréb grown on three different N sources (N₂nitrate, and ammonium) in a phosphorus (P)-limited chemostat. The fraction of live cells in the total population increased as growth rate increased with decreasing P limitation. Cell death was less in nitrate and ammonium media than in N₂. The specific death rate (γ), when calculated as the slope ofv^?1_x vs. D^?1, where v_xand D are live cell fraction (or cell viability) and dilution rate, respectively, was 0. 0082 day^?1 in N₂and 0.0042 day^?1 in nitrate. The slope of the plot in ammonium culture was not significant; however, the value of the live cell fraction was within the range for the NO^?₃culture. The fraction of live vegetative cells in N₂ culture was constant at all growth rates and the increase in the overall live cell fraction with growth rate was due entirely to an increase in live heterocysts. Live heterocysts comprised 3.5% of the total cells at a growth rate of 0.25 day^?1 and increased to 6.3% at 0.75 day^?1 with the ratio of live heterocysts to live vegetative cells linearly increasing with growth rate. The fraction of live vegetative cells was invariant in nitrate cultures us in N₂cultures. The live heterocysts fraction also increased with growth rate in nitrate cultures, along with the live heterocysts : live vegetative cells ratio, but the level was lower than in N₂cultures. DOC released from dead cells increased inversely with growth rate in N₂from 36.4% of the total DOC at a growth rate of 0.75 day^?1 to 54.15% at 0.25 day^?1. The contribution of cell death to the total DOC production in nitrate and ammonium media was significantly less than that under N₂DOC from dead cells consisted mainly of high-molecular-weight compounds, whereas DOC excreted from live cells was largely of low molecular weight.