G. B. Runion

Free-air CO2 enrichment effects on soil carbon and nitrogen

Since the onset of the industrial revolution, atmospheric CO2 concentration has increased exponentially to the current 370 #mol mo1-1 level, and continued increases are expected. Previous research has demonstrated that elevated atmospheric CO2 results in larger plants returning greater amounts of C to the soil. However, the effects of elevated CO 2 on C and N cycling and long-term storage of C in soil have not been examined. Soil samples (in 0-50, 50100, and 100-200 mm depth increments) were collected after 3 years of cotton (Gossypium hirsutum L.) production under free-air CO 2 enrichment (FACE, at 550 #tool CO 2 mol-l), in combination with 2 years of different soil moisture regimes (wet, 100% of evapotranspiration replaced, or dry, 75% and 67% of evapotranspiration replaced in 1990 and 1991, respectively) on a Trix clay loam (fine, loamy, mixed (calcareous), hyperthermic Typic Torrifluvent) at Maricopa, Arizona. Ambient plots (370 #mol CO2 mol -I (control)), in combination with the wet and dry soil moisture regimes, were also included in the study. Soil organic C and N concentrations, potential C and N mineralization, and C turnover were measured. Increased input of cotton plant residues under FACE resulted in treatment differences and trends toward increased organic C in all three soil depths. During the first 30 days of laboratory incubation, available N apparently limited potential C mineralization and C turnover in all treatments. Between 30 and 60 days of incubation, soils from FACE plots had greater potential C mineralization with both water regimes, but C turnover increased in soils from the dry treatment and decreased in soils where cotton was not water stressed. These data indicate that in high-CO 2 environments without water stress, increased C storage in soil is likely, but it is less likely where water stress is a factor. More research is needed before the ability of soil to store additional C in a high-CO 2 world can be determined.

Influence of CO2 enrichment and nitrogen fertilization on tissue chemistry and carbon allocation in longleaf pine seedlings.

One-year old, nursery-grown longleaf pine (Pinus palustris Mill.) seedlings were grown in 45-L pots containing a coarse sandy medium and were exposed to two concentrations of atmospheric CO2 (365 or 720 μmol-1) and two levels of nitrogen (N) fertility (40 or 400 kg N ha-1 yr-1) within open top chambers for 20 months. At harvest, needles, stems, coarse roots, and fine roots were separated and weighed. Subsamples of each tissue were frozen in liquid N, lyophilized at -50°C, and ground to pass a 0.2 mm sieve. Tissue samples were analyzed for carbon (C), N, nonpolar extractives (fats, waxes, and oils = FWO), nonstructural carbohydrates (total sugars and starch), and structural carbohydrates (cellulose, lignin, and tannins). Increased dry weights of each tissue were observed under elevated CO2 and with high N; however, main effects of CO2 were significant only on belowground tissues. The high N fertility tended to result in increased partitioning of biomass aboveground, resulting in significantly lower root to shoot ratios. Elevated CO2 did not affect biomass allocation among tissues. Both atmospheric CO2 and N fertility tended to affect concentration of C compounds in belowground, more than aboveground, tissues. Elevated CO2 resulted in lower concentrations of starch, cellulose, and lignin, but increased concentrations of FWO in root tissues. High N fertility increased the concentration of starch, cellulose, and tannins, but resulted in lower concentrations of lignin and FWO in roots. Differences between CO2 concentrations tended to occur only with high N fertility. Atmospheric CO2 did not affect allocation patterns for any compound; however the high N treatment tended to result in a lower percentage of sugars, cellulose, and lignin belowground.

Effects of Elevated Atmospheric CO2 on Invasive Plants : Comparison of Purple and Yellow Nutsedge (Cyperus rotundus L. and C esculentus L.)

The rise in atmospheric CO(2) concentration coupled with its direct, often positive, effect on the growth of plants raises the question of the response of invasive plants to elevated atmospheric CO(2) levels. Response of two invasive weeds [purple nutsedge (Cyperus rotundus L.) and yellow nutsedge (Cyperus esculentus L.)] to CO(2) enrichment was tested. Plants were exposed to ambient (375 micromol mol(-1)) or elevated CO(2) (ambient + 200 micromol mol(-1)) for 71 d in open top chambers. Photosynthetic rate did not differ between CO(2) treatments for either species. Conductance was lower in purple nutsedge and tended to be lower in yellow nutsedge. Purple nutsedge had higher instantaneous water use efficiency; a similar trend was noted for yellow nutsedge. Purple nutsedge had greater leaf area, root length and numbers of tubers and tended to have more tillers under high CO(2). In yellow nutsedge, only tuber number increased under CO(2) enrichment. Leaf dry weight was greater for both species when grown under elevated CO(2). Only purple nutsedge made seed heads; CO(2) level did not change seed head dry weight. Root dry weight increased under the high CO(2) treatment for purple nutsedge only, but tuber dry weight increased for both. Total dry weight of both species increased at elevated CO(2). Purple nutsedge (under elevated CO(2)) tended to increase allocation belowground, which led to greater root-to-shoot ratio (R:S); R:S of yellow nutsedge was unaffected by CO(2) enrichment. Findings suggest both species, purple more than yellow nutsedge, may be more invasive in a future high-CO(2) world.

Effects of elevated atmospheric CO2 on two southern forest diseases

Research into the effects of rising atmospheric carbon dioxide (CO2) on plant diseases remains limited despite the economic importance of this subject. Loblolly pine (Pinus taeda) seedlings were exposed to ambient and twice ambient levels of atmospheric CO2 prior to inoculation with the fusiform rust fungus (the obligate pathogen Cronartium quercuum f.sp. fusiforme, CQF) or the pitch canker fungus (the facultative pathogen Fusarium circinatum, FC). Additionally, northern red oak seedlings (Quercus rubra; an alternate host of CQF) were exposed to ambient or elevated levels of atmospheric CO2 prior to inoculation with CQF. In all cases, disease incidence (percent of plants infected) and disease severity (proportion of each plant affected) were determined; with the oak seedlings, the latent period (time to sporulation) was also monitored. In general, disease incidence was decreased by exposure to elevated CO2. This exposure also increased the latent period for CQF on oak seedlings. In no instance did exposure to elevated CO2 affect disease severity. This research demonstrated that plants may benefit from exposure to the increasing concentration of CO2 in the atmosphere through decreases in fungal disease incidence.

Elevated atmospheric carbon dioxide effects on soybean and sorghum gas exchange in conventional and no-tillage systems.

Increasing atmospheric CO(2) concentration has led to concerns about potential effects on production agriculture. In the fall of 1997, a study was initiated to compare the response of two crop management systems (conventional tillage and no-tillage) to elevated CO(2). The study used a split-plot design replicated three times with two management systems as main plots and two atmospheric CO(2) levels (ambient and twice ambient) as split plots using open-top chambers on a Decatur silt loam soil (clayey, kaolinitic, thermic Rhodic Paleudults). The conventional system was a grain sorghum [Sorghum bicolor (L.) Moench.] and soybean [Glycine max (L.) Merr.] rotation with winter fallow and spring tillage practices. In the no-tillage system, sorghum and soybean were rotated, and three cover crops were used [crimson clover (Trifolium incarnatum L.), sunn hemp (Crotalaria juncea L.), and wheat (Triticum aestivum L.)]. Over multiple growing seasons, the effect of management and CO(2) concentration on leaf-level gas exchange during row crop (soybean in 1999, 2001, and 2003; sorghum in 2000, 2002, and 2004) reproductive growth were evaluated. Treatment effects were fairly consistent across years. In general, higher photosynthetic rates were observed under CO(2) enrichment (more so with soybean) regardless of residue management practice. Elevated CO(2) led to decreases in stomatal conductance and transpiration, which resulted in increased water use efficiency. The effects of management system on gas exchange measurements were infrequently significant, as were interactions of CO(2) and management. These results suggest that better soil moisture conservation and high rates of photosynthesis can occur in both tillage systems in CO(2)-enriched environments during reproductive growth.

Free-air CO2 enrichment of sorghum: soil carbon and nitrogen dynamics.

The positive impact of elevated atmospheric CO(2) concentration on crop biomass production suggests more carbon inputs to soil. Further study on the effect of elevated CO(2) on soil carbon and nitrogen dynamics is key to understanding the potential for long-term carbon storage in soil. Soil samples (0- to 5-, 5- to 10-, and 10- to 20-cm depths) were collected after 2 yr of grain sorghum [Sorghum bicolor (L.) Moench.] production under two atmospheric CO(2) levels: (370 [ambient] and 550 muL L(-1) [free-air CO(2) enrichment; FACE]) and two water treatments (ample water and limited water) on a Trix clay loam (fine, loamy, mixed [calcareous], hyperthermic Typic Torrifluvents) at Maricopa, AZ. In addition to assessing treatment effects on soil organic C and total N, potential C and N mineralization and C turnover were determined in a 60-d laboratory incubation study. After 2 yr of FACE, soil C and N were significantly increased at all soil depths. Water regime had no effect on these measures. Increased total N in the soil was associated with reduced N mineralization under FACE. Results indicated that potential C turnover was reduced under water deficit conditions at the top soil depth. Carbon turnover was not affected under FACE, implying that the observed increase in soil C with elevated CO(2) may be stable relative to ambient CO(2) conditions. Results suggest that, over the short-term, a small increase in soil C storage could occur under elevated atmospheric CO(2) conditions in sorghum production systems with differing water regimes.

Impacts of Enhanced-Efficiency Nitrogen Fertilizers on Greenhouse Gas Emissions in a Coastal Plain Soil under Cotton.

Enhanced-efficiency N fertilizers (EENFs) have the potential to increase crop yield while decreasing soil N loss. However, the effect of EENFs on greenhouse gas (GHG) emissions from different agricultural systems is not well understood. Thus, studies from a variety of locations and cropping systems are needed to evaluate their impact. An experiment was initiated on a Coastal Plain soil under cotton ( L.) production for comparing EENFs to traditional sources. Nitrogen sources included urea, ammonia sulfate (AS), urea-ammonia sulfate (UAS), controlled-release, polymer-coated urea (Environmental Smart Nitrogen [ESN]), stabilized granular urea (SuperU), poultry litter (PL), poultry litter plus AgrotainPlus (PLA), and an unfertilized control. Carbon dioxide (CO), nitrous oxide (NO), and methane (CH) fluxes were monitored regularly after fertilization through harvest from 2009 to 2011 using a closed-chamber method. Poultry litter and PLA had higher CO flux than other N treatments, while ESN and SU were generally lowest following fertilization. Nitrous oxide fluxes were highly variable and rarely affected by N treatments; PL and PLA were higher but only during the few samplings in 2010 and 2011. Methane fluxes were higher in 2009 (wet year) than 2010 or 2011, and N treatments had minimal impact. Global warming potential (GWP), calculated from cumulative GHG fluxes, was highest with PL and PLA and lowest for control, UAS, ESN, and SU. Results suggest that PL application to cotton increases GHG flux, but GHG flux reductions from EENFs were infrequently different from standard inorganic fertilizers, suggesting their higher cost may render them presently impractical.