Abundance, production and mortality of fine roots under elevated atmospheric CO 2 in an oak-scrub ecosystem

Elevated atmospheric carbon dioxide (CO2) often stimulates the growth of fine roots, yet there are few reports of responses of intact root systems to long‐term CO2 exposure. We investigated the effects of elevated CO2 on fine root growth using open top chambers in a scrub oak ecosystem at Kennedy Space Center, Florida for more than 7 years. CO2 enrichment began immediately after a controlled burn, which simulated the natural disturbance that occurs in this system every 10–15 years. We hypothesized that (1) root abundance would increase in both treatments as the system recovered from fire; (2) elevated CO2 would stimulate root growth; and (3) elevated CO2 would alter root distribution. Minirhizotron tubes were used to measure fine root length density (mm cm−2) every three months. During the first 2 years after fire recovery, fine root abundance increased in all treatments and elevated CO2 significantly enhanced root abundance, causing a maximum stimulation of 181% after 20 months. The CO2 stimulation was initially more pronounced in the top 10 cm and 38–49 cm below the soil surface. However, these responses completely disappeared during the third year of experimental treatment: elevated CO2 had no effect on root abundance or on the depth distribution of fine roots during years 3–7. The results suggest that, within a few years following fire, fine roots in this scrub oak ecosystem reach closure, defined here as a dynamic equilibrium between production and mortality. These results further suggest that elevated CO2 hastens root closure but does not affect maximum root abundance. Limitation of fine root growth by belowground resources – particularly nutrients in this nutrient‐poor soil – may explain the transient response to elevated CO2.

The direct and indirect effects of increasing levels of atmospheric carbon dioxide (CO2) on plant nitrogen (N) content were studied in a shortgrass steppe ecosystem in northeastern Colorado, USA. Beginning in 1997 nine experimental plots were established: three open-top chambers with ambient CO2 levels (approximately 365 μmol mol−1), three open-top chambers with twice-ambient CO2 levels (approximately 720 μmol mol−1), and three unchambered control plots. After 3 years of growing-season CO2 treatment, the aboveground N concentration of plants grown under elevated atmospheric CO2 decreased, and the carbon–nitrogen (C:N) ratio increased. At the same time, increased aboveground biomass production under elevated atmospheric CO2 conditions increased the net transfer of N out of the soil of elevated-CO2 plots. Aboveground biomass production after simulated herbivory was also greater under elevated CO2 compared to ambient CO2. Surprisingly, no significant changes in belowground plant tissue N content were detected in response to elevated CO2. Measurements of individual species at peak standing phytomass showed significant effects of CO2 treatment on aboveground plant tissue N concentration and significant differences between species in N concentration, suggesting that changes in species composition under elevated CO2 will contribute to overall changes in nutrient cycling. Changes in plant N content, driven by changes in aboveground plant N concentration, could have important consequences for biogeochemical cycling rates and the long-term productivity of the shortgrass steppe as atmospheric CO2 concentrations increase.

The effects of elevated atmospheric CO2on fine root decomposition over a 828‐day period were investigated using open top chambers with both ambient and elevated (700 ppm) CO2treatments in an oak–palmetto scrub ecosystem at Kennedy Space Center, Florida. Carbon dioxide enrichment of the chambers began 15 May 1996. The experiment included roots grown in ambient and elevated carbon dioxide. Vertical litterbags installed in September 1996 in each elevated and ambient chamber incubated from December 1996 to December 1998 showed no significant treatment effect on fine root or rhizome mass loss. Initial fine root percentage mass loss varied from 10.3% to 13.5% after three months; 55.5% to 38.3% of original mass had been lost after 828 days. A period of nitrogen immobilization occurred in both fine roots and rhizomes in the elevated CO2incubation, which is a potential mechanism for nitrogen conservation for this system in an elevated CO2world.

Rising atmospheric carbon dioxide (CO2) is expected to increase forest productivity, resulting in greater carbon (C) storage in forest ecosystems. Because elevated atmospheric CO2 does not increase nitrogen (N) use efficiency in many forest tree species, additional N inputs will be required to sustain increased net primary productivity (NPP) under elevated atmospheric CO2. We investigated the importance of free amino acids (AAs) as a source for forest N uptake at the Duke Forest Free Air CO2 Enrichment (FACE) site, comparing its importance with that of better‐studied inorganic N sources. Potential proteolytic enzyme activity was monitored seasonally, and individual AA concentrations were measured in organic horizon extracts. Potential free AA production in soils ranged from 190 to 690 nmol N g−1 h−1 and was greater than potential rates of soil NH4+ production. Because of this high potential rate of organic N production, we determined (1) whether intact AA uptake occurs by Pinus taeda L., the dominant tree species at the FACE site, (2) if the rate of cycling of AAs is comparable with that of ammonium (NH4+), and (3) if atmospheric CO2 concentration alters the aforementioned N cycling processes. A field experiment using universally labeled ammonium (15NH4+) and alanine (13C3H715NO2) demonstrated that 15N is more readily taken up by plants and heterotrophic microorganisms as NH4+. Pine roots and microbes take up on average 2.4 and two times as much NH4+ 15N compared with alanine 15N 1 week after tracer application. N cycling through soil pools was similar for alanine and NH4+, with the greatest 15N tracer recovery in soil organic matter, followed by microbial biomass, dissolved organic N, extractable NH4+, and fine roots. Stoichiometric analyses of 13C and 15N uptake demonstrated that both plants and soil microorganisms take up alanine directly, with a 13C : 15N ratio of 3.3 : 1 in fine roots and 1.5 : 1 in microbial biomass. Our results suggest that intact AA (alanine) uptake contributes substantially to plant N uptake in loblolly pine forests. However, we found no evidence supporting increased recovery of free AAs in fine roots under elevated CO2, suggesting plants will need to acquire additional N via other mechanisms, such as increased root exploration or increased N use efficiency.

Corn, with C4 photosynthetic metabolism, often has no photosynthetic or yield response to elevated carbon dioxide concentrations. In C3 species, the yield stimulation at elevated carbon dioxide concentrations often decreases with nitrogen limitation. I tested whether such a nitrogen interaction occurred in corn, by growing sweet corn in field plots in open top chambers at ambient and elevated (ambient + 180 mmol·mol-1) carbon dioxide concentrations for four seasons, with six nitrogen application rates, ranging from half to twice the locally recommended rate. At the recommended rate of nitrogen application, no carbon dioxide effect on production occurred. However, both ear and leaf plus stem biomass were lower for the elevated carbon dioxide treatment than for the ambient treatment at less than the recommended rate of nitrogen application, and higher at the highest rates of nitrogen application. There were no significant responses of mid-day leaf gas exchange rates to nitrogen application rate for either carbon dioxide treatment, and elevated carbon dioxide did not significantly increase leaf carbon dioxide assimilation rates at any nitrogen level. Leaf area index during vegetative growth increased more with nitrogen application rate at elevated than at ambient carbon dioxide. It is concluded that elevated carbon dioxide increased the responsiveness of corn growth to nitrogen application by increasing the response of leaf area to nitrogen application rate, and that elevated carbon dioxide increased the amount of nitrogen required to achieve maximum yields.

Active carbon-pools in rhizosphere of wheat ( Triticum aestivum L.) grown under elevated atmospheric carbon dioxide concentration in a Typic Haplustept in sub-tropical India

Growth performance and antioxidative response in bread and durum wheat plants grown with varied potassium treatments under ambient and elevated carbon dioxide

Maize is an important C4 crop and how it will respond to elevated atmospheric carbon dioxide and ozone levels is not well documented. To understand how the growth and nutritional quality of maize will be affected under elevated carbon dioxide (CO2) and tropospheric ozone (O3) interaction, a field experiment was conducted under free air O3 and CO2 enrichment rings (FAOCE) growing HQPM-1 and PMH-1 maize cultivars at New Delhi, India. Each cultivar was exposed to ambient and elevated CO2 (559 ppm) alone and along with ambient and elevated O3 (71.8 ppb) throughout the growing period. Elevated CO2 (EC) significantly increased the leaf area index (10.8–22.5%), chlorophyll (11.2–17.3%) and photosynthetic rate (12.1–16.5%) in the two cultivars over the ambient. O3 exposure of 27 ppm hr (AOT4O) under elevated O3 (EO) treatment led to a significant decline in yield (p < 0.01) by 9.2% in HQPM-1 and 9.8% in PMH-1. Under elevated CO2 the increase in grain yield was higher under HQPM-1 (25.4%) as compared to PMH-1 (9.04%). The protein content increased under EO (8.1–12.5%) and decreased under EC (13.4–13.6%) in the two maize cultivars due to yield dilution effect. Lysine, phosphorus and potassium content of the grain significantly decreased in both the cultivars under elevated CO2. Carbohydrate and amylose concentrations in grains increased (9.9–15.5%) under EC and decreased (10.8–16.7%) under EO, however, no significant change in yield, protein, amylase, carbohydrate, lysine, potassium and phosphorus was observed under the interaction treatment ECO as compared to the ambient. After two years of study we could conclude that elevated CO2 (559 ppm) was able to offset the negative effect of elevated O3 (71 ppb) on grain yield by 11.2% in PMH-1 and by 18.8% in HQPM-1 without significantly affecting the grain quality in both the maize cultivars.

Stands of spring wheat grown in open-top chambers (OTCs) were used to assess the individual and interactive effects of season-long exposure to elevated atmospheric carbon dioxide (CO 2 ) and ozone (O 3 ) on the photosynthetic and gas exchange properties of leaves of differing age and position within the canopy. The observed effects were related to estimated ozone fluxes to individual leaves. Foliar chlorophyll content was unaffected by elevated CO 2 , but photosynthesis under saturating irradiances was increased by up to 100% at 680 μmol mol -1 CO 2 relative to the ambient CO 2 control; instantaneous water use efficiency was improved by a combination of increased photosynthesis and reduced transpiration. Exposure to a seasonal mean O 3 concentration (7 h d -1 ) of 84 nmol mol - - 1 under ambient CO 2 accelerated leaf senescence following full expansion, at which time chlorophyll content was unaffected. Stomatal regulation of pollutant uptake was limited since estimated O 3 fluxes to individual leaves were not reduced by elevated atmospheric CO 2 . A common feature of 03-treated leaves under ambient CO 2 was an initial stimulation of photosynthesis and stomatal conductance for up to 4 d and 10 d, respectively, after full leaf expansion, but thereafter both variables declined rapidly. The 03-induced decline in chlorophyll content was less rapid under elevated CO 2 and photosynthesis was increased relative to the ambient CO 2 treatment. A/C i analyses suggested that an increase in the amount of in vivo active RuBisCO may be involved in mitigating O 3 -induced damage to leaves. The results obtained suggest that elevated atmospheric CO 2 has an important role in restricting the damaging effects of O 3 on photosynthetic activity during the vegetative growth of spring wheat, and that additional direct effects on reproductive development were responsible for the substantial reductions in grain yield obtained at final harvest, against which elevated CO 2 provided little or no protection.

Impact of Elevated Atmospheric Carbon Dioxide and Water Deficit on Flower Development and Pyrethrin Accumulation in Pyrethrum

Atmospheric carbon dioxide (CO2) levels have reached an all-time high, contributing to increased average temperature, more frequent and severe weather patterns, increasing sea levels, and the acidification of the ocean. Increasing levels of atmospheric CO2 are also showing a negative impact on staple food crops, such as rice. In addition to added heat stresses decreasing growth and productivity of rice crops, micronutrient concentration, specifically iron (Fe) and zinc (Zn), shows a significant decline when grown in elevated atmospheric CO2. This is likely to most significantly impact those who are already impoverished and living in a state of chronic food insecurity, thus further contributing to micronutrient deficiency diseases, also known as hidden hunger. A systematic review of the literature regarding micronutrient concentration, specifically Fe and Zn, in rice grains grown under elevated CO2 was conducted according to PRISMA guidelines for systematic reviews. Eight of the studies found met the required guidelines and were included in this review. While early studies were contradictory, later studies show a clear correlation between elevated atmospheric CO2 and decreased Fe and Zn concentrations. The mechanism of action remains unclear at this time, and more research is needed into ways to mitigate these effects moving forward.

Temporal responses of community fine root populations to long-term elevated atmospheric CO 2 and soil nutrient patches in model tropicalecosystems

Elevated atmospheric CO(2) concentrations and warming may affect the quality of litters of forest plants and their subsequent decomposition in ecosystems, thereby potentially affecting the global carbon cycle. However, few data on root tissues are available to test this feedback to the atmosphere. In this study, we used fine (diameter < or = 2 mm) and small (2-10 mm) roots of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings that were grown for 4 yr in a 2 x 2 factorial experiment: ambient or elevated (+ 180 ppm) atmospheric CO(2) concentrations, and ambient or elevated (+3.8 degrees C) atmospheric temperature. Exposure to elevated CO(2) significantly increased water-soluble extractives concentration (%WSE), but had little effect on the concentration of N, cellulose, and lignin of roots. Elevated temperature had no effect on substrate quality except for increasing %WSE and decreasing the %lignin content of fine roots. No significant interaction was found between CO(2) and temperature treatments on substrate quality, except for %WSE of the fine roots. Short-term (< or = 9 mo) root decomposition in the field indicated that the roots from the ambient CO(2) and ambient temperature treatment had the slowest rate. However, over a longer period of incubation (9-36 mo) the influence of initial substrate quality on root decomposition diminished. Instead, the location of the field incubation sites exhibited significant control on decomposition. Roots at the warmer, low elevation site decomposed significantly faster than the ones at the cooler, high elevation site. This study indicates that short-term decomposition and long-term responses are not similar. It also suggests that increasing atmospheric CO(2) had little effect on the carbon storage of Douglas-fir old-growth forests of the Pacific Northwest.

The long-term effects of CO2 enrichment on fine root productivity, mortality, and survivorship in a scrub-oak ecosystem at Kennedy Space Center, Florida, USA

Effects of elevated atmospheric carbon dioxide on insect–plant interactions