Effects of Elevated Atmospheric CO2 on Root Growth, Turnover and Decomposition in a Scrub Oak Ecosystem

Atmospheric carbon dioxide levels are increasing and predicted to double this century. The implications of this rise on vegetation structure and function are not well understood. Measurement of root growth response to elevated atmospheric carbon dioxide is critical to understanding soil carbon input. I investigated the effects of elevated carbon dioxide on fine root growth and decomposition using open top chambers with both ambient and elevated (700 PPM) CO2 treatments in an oak-palmetto scrub ecosystem at Kennedy Space Center, Florida. Minirhizotron tubes were installed in each elevated and control chamber to allow observation of roots. Each tube was sampled for root length density (mm cm−2) every three months. Carbon dioxide enrichment of the chambers began May 15, 1996. By December 1998 root length density (RLD) increased to 19.1 mm cm−2 for the control chambers and 37.7 mm cm−2 for the enriched chambers in the top 101-cm of soil. Root distribution was unchanged under elevated carbon dioxide. Fine root production increased with elevated carbon dioxide and mortality was unaffected over 33 months. Root length elongation increased significantly over a one-month period in June 1997. I also measured the effects of elevated carbon dioxide on the decomposition rates of roots grown in ambient and elevated carbon dioxide. Fine root decomposition rates were obtained from root litterbags incubated from December 1996 to December 1998 and showed no significant treatment effect. Initial percent 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 CO2 treatment, which is potentially a mechanism for nitrogen conservation for this system in an elevated CO2 world. Significant fine root length-mass relationships were applied to minirhizotron measurements and a 180% increases in root biomass was calculated at the end of the study. The increased rates of fine root growth coupled with no change in decomposition rate suggest a potential increased rate of carbon input into the soil

Aboveground Biomass and Net Primary Production Along a Virginia Barrier Island Dune Chronosequence

Aboveground biomass was examined along a chronosequence of dune communities on Hog Island, a Virginia Coast Reserve LTER site. The dominant species were Ammophila breviligulata and Spartina patens. Aboveground biomass was harvested monthly from ten quadrats on dunes 6, 24, 36, and 120 years old. Sampling was conducted from April to November 1993. Biomass values were greater for younger dunes. Total aboveground biomass decreased with increasing site age and ranged from 152 g m-2 on the 120 year old dune to 205 g m-2 on the 6 year old dune in October 1993. Spartina patens biomass was greater than Ammophila breviligulata for the 6, 24, and 36 year old dune ridges. It also showed a pattern of decreasing biomass with increasing dune age; in July it ranged from 72 g m-2 to 5 g m-2. The same month showed less variation in Ammophila breviligulata; it increased from 17 g m-2 to 39 g m-2 across increasing dune age. Ammophila breviligulata had greater biomass for only the 120 year old dune. Net aboveground primary productivity did not vary greatly among different age dunes. There appeared to be a midsummer decline in biomass due to drought conditions. The aboveground net primary production (ANPP) from the sum of species peaks was 259 g m-2 yr-1 for the 6 year old dune, 226 g m-2 yr-1 for the 24 year old dune, 256 g m-2 yr-1 for the 36 year old dune and 274 g m-2 yr-1 for the 120 year old dune. Nitrogen and phosphorus concentrations in plant tissue were low. Biomass and nutrient values are reflective of production in a stressed environment. The variation in production of aboveground biomass across dune age may be controlled by moisture, microclimatic conditions and soil nitrogen levels

Avian Response to Forest Management and Military Training Activities at Fort Benning, Georgia

Evaluating intensity and effects of land use disturbance is difficult, espe­cially in sites with multiple land use. We conducted point counts to deter­mine if abundance of bird species could be used to assess military train­ing and forestry management practices at Fort Benning, Georgia. We evaluated heavy and light use sites in the 1st growing season after pre­scribed fire and in the 3rd growing season postfire. Results focus on species common to early successional habitats and pine-grasslands and on forest species and habitat generalists. In the 3rd growing season post­fire, Indigo buntings (Passerino cyanea) and northern bobwhites (Colinus virginianus) were more abundant in recently burned heavy use sites than in light use sites. Conversely, red-eyed vireos (Vireo olivaceus) were more abundant in light use sites in the 3rd growing season postfire than in recently burned, heavy use sites. Further study could help determine if these species are indicators of disturbance

Carbon dioxide fluxes across the Sierra de Guadarrama, Spain

Understanding the spatial and temporal variation in soil respiration within small geographic areas is essential to accurately assess the carbon budget on a global scale. In this study, we investigated the factors controlling soil respiration in an altitudinal gradient in a southern Mediterranean mixed pine–oak forest ecosystem in the north face of the Sierra de Guadarrama in Spain. Soil respiration was measured in five Pinus sylvestris L. plots over a period of 1 year by means of a closed dynamic system (LI-COR 6400). Soil temperature and water content were measured at the same time as soil respiration. Other soil physico-chemical and microbiological properties were measured during the study. Measured soil respiration ranged from 6.8 to 1.4 lmol m-2 s-1, showing the highest values at plots situated at higher elevation. Q10 values ranged between 1.30 and 2.04, while R10 values ranged between 2.0 and 3.6. The results indicate that the seasonal variation of soil respiration was mainly controlled by soil temperature and moisture. Among sites, soil carbon and nitrogen stocks regulate soil respiration in addition to soil temperature and moisture. Our results suggest that application of standard models to estimate soil respiration for small geographic areas may not be adequate unless other factors are considered in addition to soil temperature

The Effects of 11 Yr of CO2 Enrichment on Roots in a Florida Scrub-Oak Ecosystem

Uncertainty surrounds belowground plant responses to rising atmospheric CO2 because roots are difficult to measure, requiring frequent monitoring as a result of fine root dynamics and long-term monitoring as a result of sensitivity to resource availability. We report belowground plant responses of a scrub-oak ecosystem in Florida exposed to 11yr of elevated atmospheric CO2 using open-top chambers. We measured fine root production, turnover and biomass using minirhizotrons, coarse root biomass using ground-penetrating radar and total root biomass using soil cores. Total root biomass was greater in elevated than in ambient plots, and the absolute difference was larger than the difference aboveground. Fine root biomass fluctuated by more than a factor of two, with no unidirectional temporal trend, whereas leaf biomass accumulated monotonically. Strong increases in fine root biomass with elevated CO2 occurred after fire and hurricane disturbance. Leaf biomass also exhibited stronger responses following hurricanes. Responses after fire and hurricanes suggest that disturbance promotes the growth responses of plants to elevated CO2. Increased resource availability associated with disturbance (nutrients, water, space) may facilitate greater responses of roots to elevated CO2. The disappearance of responses in fine roots suggests limits on the capacity of root systems to respond to CO2 enrichment

Soil respiration in cucumber field under crop rotation in solar greenhouse

Crop residues are the primary source of carbon input in the soil carbon pool. Crop rotation can impact the plant biomass returned to the soil, and influence soil respiration. To study the effect of previous crops on soil respiration in cucumber (Cucumis statirus L.) fields in solar greenhouses, soil respiration, plant height, leaf area and yield were measured during the growing season (from the end of Sept to the beginning of Jun the following year) from 2007 to 2010. The cucumber was grown following fallow (CK), kidney bean (KB), cowpea (CP), maize for green manure (MGM), black bean for green manure (BGM), tomato (TM), bok choy (BC). As compared with CK, KB, CP, MGM and BGM may increase soil respiration, while TM and BC may decrease soil respiration at full fruit stage in cucumber fields. Thus attention to the previous crop arrangement is a possible way of mitigating soil respiration in vegetable fields. Plant height, leaf area and yield had similar variation trends under seven previous crop treatments. The ratio of yield to soil respiration revealed that MGM is the crop of choice previous to cucumber when compared with CK, KB, CP, BGM, TM and BC