OVERVIEW OF BELOWGROUND ECOLOGICAL RESEARCH AND FERTILIZATION EXPERIMENTS ON THE HOG ISLAND CHRONOSEQUENCE Frank P. Day Several intensive studies of primary production, belowground processes, and the controlling influences of groundwater hydrology and nitrogen availability along the Hog Island chronosequence were completed during the past year and summaries of the results are included in the following reports by Day, Lakshmi and Day, Conn and Day, Stevenson and Day, Weber and Day, and Dilustro and Day. Particularly noteworthy is the successful utilization of minirhizotrons in a fertilization experiment on the 36-year old dune. The fine scale observation of root dynamics on the same root cohorts through time allows a refined look at belowground effects, and this technique will be continued and expanded in use on the VCR-LTER site in the future. Several long-term monitoring efforts and experiments are continuing along the Hog Island chronosequence. Groundwater monitoring is in its 4th year and a synthesis of those results will be generated this summer. Video recordings in the minirhizotrons are currently being obtained early and late in the growing season for the 2nd year following the initial year of frequent, more intensive observations. Digitization and interpretation of the current recordings await the establishment of the RA position proposed for the site. The long-term nitrogen fertilization experiment is in its 3rd year, and initial results will be analyzed and compiled this summer. Plant cover and species composition are being monitored in nitrogen ammended and control plots across the chronosequence. ----------------------------------------------------------------- PLANT RESPONSE TO NITROGEN FERTILIZATION ACROSS A VIRGINIA COAST RESERVE DUNE CHRONOSEQUENCE. Frank P. Day Experimental and control plots (1 m2) were established on three different age dunes (24, 36, and 120 yr old) on Hog Island, part of the Virginia Coast Reserve LTER site. Nitrogen (15 g m-2 yr-1) was added to the treatment plots in the form of urea. At the end of the 1991 growing season, plant biomass was harvested from the plots and weighed. Biomass decreased from young to old dune (174 g m-2 to 108 g m-2 in controls), but root/shoot ratios increased in the controls (0.35 to 0.50). Biomass increased in response to fertilization on all three sites; however, the response was muted on the oldest dune (547 g m-2 to 338 g m-2 from young to old). Root/shoot ratios decreased in response to fertilization, but were the same across sites (0.21). The damping of the response to N additions from younger to older dunes may reflect the higher natural levels of N in the older dune soils or other limiting factors such as soil moisture. -------------------------------------------------------------- NITROGEN AVAILABILITY AND N MINERALIZATION RATES ALONG A COMMUNITY CHRONOSEQUENCE ON HOG ISLAND, VIRGINIA COAST RESERVE Bharatha Lakshmi and Frank P. Day Available soil nitrogen and N-mineralization rates on a dynamic nutrient-poor island are important in understanding the succession of coastal island systems. On a 6, 24, 36 and 120 year-old chronosequence on Hog Island, the nitrogen availability in the dunes increased with age. But in the associated swales the nitrogen concentrations were higher with the dominance of Myrica cerifera, a nitrogen fixing species. In general, the swales had higher soil nitrogen levels (0.016-0.052 g m2) than dunes (0.015-0.038 g m2) and the concentrations of ammonium-N were higher than the nitrate+nitrite-N. Application of urea to the dunes resulted in a 10-13 fold increase in nitrogen with highest accumulation in the oldest dune. Net N-mineralization was highest in the younger dune (0.053 mg kg-1 day-1), and with fertilization this rate increased 15-fold. Fertilization had only a minimal effect on mineralization in the oldest dune. These results indicated that the younger dunes were N limited and the limitation was minimized with age. Higher nitrogen levels in the older dunes might be due to an input of N-rich litter from the adjacent Myrica dominated swales. ---------------------------------------------------------------- THE INFLUENCES OF MICROENVIRONMENT AND NITROGEN AVAILABILITY ON ROOT DECOMPOSITION AND NITROGEN DYNAMICS ON A BARRIER ISLAND CHRONOSEQUENCE Christine E. Conn and Frank P. Day Hog Island (LTER-VCR) is a barrier island lying alongside the Virginia coastline of the Delmarva Peninsula. Barrier island ecosystems are typically nutrient poor environments. Nutrient cycles are strongly influenced by leaching potentials in this sandy system. The turnover of organic matter and the concomitant nutrient dynamics during decomposition are essential to the understanding of nutrient cycling in this potentially leaky system. This work focused upon the dynamics of root decomposition. The influence of nitrogen availability on root decomposition dynamics, both directly and indirectly through controls on litter quality, was investigated. In addition, a chronosequence of dune and swale landscape units was used to evaluate the role of environmental factors and how belowground processes differed along the chronosequence. In aboveground litter decay, nitrogen immobilization occurs often and may be enhanced in nitrogen limited environments. It was hypothesized that similar patterns would occur during belowground decay in a nitrogen limited ecosystem. Litter bags containing native roots were buried along the dune chronosequence. Nitrogen availability increased with dune age. In addition, each dune site was fertilized with 15 g N/m2/year. After 111 days, nitrogen was immobilized in the oldest 120 year old dune (108% of initial N). A net loss in absolute amounts of nitrogen occurred in the younger 24 and 36 year old dune sites (65-79% of initial N). In addition, rates of decay were highest in the oldest dune and lowest in the younger 24 year old dune. Absolute amounts of nitrogen stabilized in all sites after 218 days (66-75% initial N remaining) while mass loss continued. In the younger sites, rates of nitrogen release were greater in control plots than in fertilized plots. In addition, decay rates were increased by nitrogen addition. No differences in percent initial nitrogen remaining or decay rates occurred between fertilized and control plots in the 120 year old site during early stages of decomposition. Both exhibited net nitrogen immobilization (107-110% initial N after 150 days). Beyond this point, the 120 year old fertilized plots continued to retain greater amounts of nitrogen than the control plots. Differences in litter quality between the three dune sites were correlated with differences observed in decay rates and nitrogen immobilization potentials. Greater decay rates in the oldest dune may be related, in part, to the significantly greater nitrogen content in roots from the oldest dune (0.80% vs. 0.59% in youngest dune) producing an inherently more decomposable litter. Nitrogen immobilization was expected in the poorest quality litter, however, this did not occur. Nitrogen immobilized in native litter incubated in the oldest dune may be a result of the greater lignin content found in these roots (20%) in comparison to roots from younger dunes (9-13%). Lignin content is known to be positively correlated with nitrogen immobilization, presumably due to increased physicochemical complexing between various nitrogen species and lignin components. Contrary to nitrogen limited aboveground systems, overall nitrogen immobilization potential was low to non- existent. The interaction between landform age, topographic position and environmental regime along the chronosequence was used to study environmental controls on belowground decomposition rates. Notable differences in hydrology and soil redox potential were evident between dune and swale sites. Mean water table position dropped from younger to older sites and was higher in swales (4.8 cm aboveground to 14.7 cm belowground) than in dunes (91.2 cm to 116.5 cm belowground). Mean soil redox levels exhibited no differences between dunes (423 to 573 Mv) and were lower in swales (-35 to 239 Mv). Older swales had higher soil redox levels. Roots collected from one site were incubated in all dune and swale sites to factor out the influences of litter quality. Decomposition of the standard root type was greater in dunes (40.8 - 57.5 % mass remaining) than in swales (74.2 - 86.3 % mass remaining). Multiple regression analysis demonstrated hydrology and soil redox potential were strongly correlated with belowground decomposition rates. Hydrology appears to strongly influence belowground decomposition rates, both directly and indirectly through modification of other environmental parameters. For example, soil temperatures were cooler in the swales and soil redox potentials reflected depth and duration of saturation. Hydrologic influences in swale regions contribute to lower turnover time of organic matter and its subsequent accumulation. This is in contrast to the more rapid turnover of organic matter observed in the dry, well-drained dune sites. In addition, more nitrogen and phosphorus was released from decaying litter in swales, possibly due to leaching processes mediated by a high, fluctuating water table. ---------------------------------------------------------------- FINE ROOT PRODUCTION ALONG A CHRONOSEQUENCE OF BARRIER ISLAND COMMUNITIES. Mark J. Stevenson and Frank P. Day. Fine root production was quantified by an ingrowth core method along a chronosequence of dune communities on Hog Island, a Virginia Coast Reserve LTER site. The dune communities are dominated by Ammophila breviligulata, Spartina patens, and Aristida tuberculosa. Production estimates for fine roots (<= 2 mm), were estimated using biomass ingrowth into root-free soil volumes for one growing season. Fine root production was greater in N-fertilized plots than unfertilized plots. The most substantial level of fine root production for unfertilized plots occurred in the upper 0-10 cm depth in R120. The unfertilized plots showed no real differences in production between communities at 10-20 cm, 20-30 cm and 30-40 cm depth. R24 and R36 produced similar root production measurements in their N-fertilized plots. There was no substantial increase in total phosphorous concentrations in any of the dune communities. There was an increase in total nitrogen concentrations in fine roots from all dune communities in N-fertilized plots. ---------------------------------------------------------------- THE EFFECT OF NITROGEN FERTILIZATION ON THE PHENOLOGY OF ROOTS IN A BARRIER ISLAND DUNE ECOSYSTEM: A MINIRHIZOTRON ANALYSIS. EVERETT P. WEBER and FRANK P. DAY. Little work has been done on the phenology of root growth and senescence largely due to methodological difficulties. The application of minirhizotron technology has enabled the tracking of individual roots through an entire growing season. As a result, direct measures of turnover, root growth, and senescence are possible. Small plots on a 36 year old dune on Hog Island, a barrier island in the Virginia Coast Reserve Long Term Ecological Research Site, were fertilized with nitrogen. Minirhizotron tubes were installed in each fertilized and control plot. Each tube was sampled monthly for nine months, March through November. Preliminary results showed an increase in root density from March to June with the fertilized plots showing a higher root density than the unfertilized plots for both March (256% greater) and April (140% greater). Only 4% of the roots sampled in April were present in the March sampling. The minirhizotron method allows a high resolution perspective of the belowground environment and direct monitoring of phenomena which previously were obtainable only through indirect measures. -------------------------------------------------------------- ABOVEGROUND BIOMASS AND NET PRIMARY PRODUCTION ALONG A BARRIER ISLAND DUNE CHRONOSEQUENCE. John J.Dilustro and Frank P. Day. Aboveground biomass was examined along a chronosequence of dune communities on Hog Island, a Virginia Coast Reserve LTER site. The dominant species are 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. 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/m2 to 5 g/m2. The same month showed less variation in Ammophila breviligulata; it increased from 17g/m2 to 39g/m2 across increasing dune age. Ammophila breviligulata had greater biomass for only the 120 year old dune. There appeared to be a midsummer decline in biomass due to drought conditions. This variation in production of aboveground biomass across dune age may be controlled by moisture, microclimatic conditions and soil nitrogen levels.