Litterfall 15N abundance indicates declining soil nitrogen availability in a free-air CO2 enrichment experiment.

Forest productivity increases in response to carbon dioxide (CO2) enrichment of the atmosphere. However, in nitrogen-limited ecosystems, increased productivity may cause a decline in soil nitrogen (N) availability and induce a negative feedback on further enhancement of forest production. In a free-air CO2 enrichment (FACE) experiment, the response of sweetgum (Liquidambar styraciflua L.) productivity to elevated CO2 concentrations [CO2] has declined over time, but documenting an associated change in soil N availability has been difficult. Here we assess the time history of soil N availability through analysis of natural 15N abundance in archived samples of freshly fallen leaf litterfall. Litterfall delta15N declined from 1998 to 2005, and the rate of decline was significantly faster in elevated [CO2]. Declining leaf litterfall delta15N is indicative of a tighter ecosystem N cycle and more limited soil N availability. By integrating N availability over time and throughout the soil profile, temporal dynamics in leaf litterfall delta15N provide a powerful tool for documenting changes in N availability and the critical feedbacks between C and N cycles that will control forest response to elevated atmospheric CO2 concentrations.

Characterization of biochars produced from cornstovers for soil amendment.

Through cation exchange capacity assay, nitrogen adsorption-desorption surface area measurements, scanning electron microscopic imaging, infrared spectra and elemental analyses, we characterized biochar materials produced from cornstover under two different pyrolysis conditions, fast pyrolysis at 450 °C and gasification at 700 °C. Our experimental results showed that the cation exchange capacity (CEC) of the fast-pyrolytic char is about twice as high as that of the gasification char as well as that of a standard soil sample. The CEC values correlate well with the increase in the ratios of the oxygen atoms to the carbon atoms (O:C ratios) in the biochar materials. The higher O:C ratio was consistent with the presence of more hydroxyl, carboxylate, and carbonyl groups in the fast pyrolysis char. These results show how control of biomass pyrolysis conditions can improve biochar properties for soil amendment and carbon sequestration. Since the CEC of the fast-pyrolytic cornstover char can be about double that of a standard soil sample, this type of biochar products would be suitable for improvement of soil properties such as CEC, and at the same time, can serve as a carbon sequestration agent.

Belowground fate of (15)N injected into sweetgum trees (Liquidambar styraciflua) at the ORNL FACE Experiment.

Nitrogen (N) cycling can be an important constraint on forest ecosystem response to elevated atmospheric CO(2). Our objective was to trace the movement of (15)N, injected into tree sap, to labile and stable forms of soil organic matter derived partly from the turnover of tree roots under elevated (545 ppm) and ambient (394 ppm) atmospheric CO(2) concentrations at the Oak Ridge National Laboratory (ORNL) FACE (Free-Air Carbon Dioxide Enrichment) Experiment. Twenty-four sweetgum trees, divided equally between CO(2) treatments, were injected with 3.2 g (15)N-ammonium sulfate (99 atom %), and soil samples were collected beneath the trees over a period of 89 weeks. For 16 cm deep soil samples collected beneath the study trees, there was 28% more fine root (less than or equal to 2 mm diameter) biomass under elevated CO(2) (P = 0.001), but no significant treatment effect on the amounts of necromass, coarse root biomass, or on the N concentrations in tree roots and necromass. Nitrogen-15 moved quickly into roots from the stem injection site and the (15)N content of roots, necromass, and labile organic matter (i.e. particulate organic matter, POM) increased over time. At 89 weeks post-injection, approximately 76% of the necromass (15)N originated from fine root turnover. Nitrogen-15 in POM had a relatively long turnover time (47 weeks) compared with (15)N in roots (16 to 22 weeks). Over the 1.7 year period of the study, (15)N moved from roots into slower cycling POM and the disparity in turnover times between root N and N in POM could impose progressive limitations on soil N availability with stand maturation irrespective of atmospheric CO(2), especially if the release of N through the decomposition of POM is essential to sustain forest net primary production.

Spatial scaling of functional gene diversity across various microbial taxa.

Understanding the spatial patterns of organisms and the underlying mechanisms shaping biotic communities is a central goal in community ecology. One of the most well documented spatial patterns in plant and animal communities is the positive-power law relationship between species (or taxa) richness and area. Such taxa-area relationships (TARs) are one of the principal generalizations in ecology, and are fundamental to our understanding of the distribution of global biodiversity. However, TARs remain elusive in microbial communities, especially in soil habitats, because of inadequate sampling methodologies. Here, we describe TARs as gene-area relationships (GARs), at a whole-community level, across various microbial functional and phylogenetic groups in a forest soil, using a comprehensive functional gene array with >24,000 probes. Our analysis indicated that the forest soil microbial community exhibited a relatively flat gene-area relationship (slope z = 0.0624), but the z values varied considerably across different functional and phylogenetic groups (z = 0.0475-0.0959). However, the z values are several times lower than those commonly observed in plants and animals. These results suggest that the turnover in space of microorganisms may be, in general, lower than that of plants and animals.

Predicted effects of prescribed burning and harvesting on forest recovery and sustainability in southwest Georgia, USA.

A model-based analysis of the effect of prescribed burning and forest thinning or clear-cutting on stand recovery and sustainability was conducted at Fort Benning, GA, in the southeastern USA. Two experiments were performed with the model. In the first experiment, forest recovery from degraded soils was predicted for 100 years with or without prescribed burning. In the second experiment simulations began with 100 years of predicted stand growth, then forest sustainability was predicted for an additional 100 years under different combinations of prescribed burning and forest harvesting. Three levels of fire intensity (low, medium, and high), that corresponded to 17%, 33%, and 50% consumption of the forest floor C stock by fire, were evaluated at 1-, 2-, and 3-year fire return intervals. Relative to the control (no fire), prescribed burning with a 2- or 3-year return interval caused only a small reduction in predicted steady state soil C stocks (< or =25%) and had no effect on steady state tree wood biomass, regardless of fire intensity. Annual high intensity burns did adversely impact forest recovery and sustainability (after harvesting) on less sandy soils, but not on more sandy soils that had greater N availability. Higher intensity and frequency of ground fires increased the chance that tree biomass would not return to pre-harvest levels. Soil N limitation was indicated as the cause of unsustainable forests when prescribed burns were too frequent or too intense to permit stand recovery.

Laser-induced breakdown spectroscopy for the environmental determination of total carbon and nitrogen in soils.

Soils from various sites have been analysed with the laser-induced breakdown spectroscopy (LIBS) technique for total elemental determination of carbon and nitrogen. Results from LIBS have been correlated to a standard laboratory-based technique (sample combustion), and strong linear correlations were obtained for determination of carbon concentrations. The LIBES technique was used on soils before and after acid washing, and the technique appears to be useful for the determination of both organic and inorganic soil carbon. The LIBS technique has the potential to be packaged into a field-deployable instrument.

Deposition of H15 NO3 vapour to white oak, red maple and loblolly pine foliage: experimental observations and a generalized model.

Nitric acid vapour enriched with 15 N (H15 NO3 ) was volatilized into the cuvette of an open-flow gas exchange system containing red maple (Acer rubrum L.), white oak (Quercus alba L.), or loblolly pine (Pinus taeda L.) seedling shoots to facilitate direct measurements of total foliar deposition, and subsequent assessments of the rate of HNO3 movement across the cuticle (transcuticular uptake). Total H15 NO3 vapour deposition to foliar surfaces ranged from <5 to 27 nmol m-2 s-1 the variability being largely accounted for by differences in HNO3 concentrations and leaf conductance. Mean whole-leaf conductance to HNO3 ranged between 0.9 and 3.4 mm s-1 for hardwoods and between 6 and 34 mm s-1 for loblolly pine. Of the total H15 NO3 vapour deposited to leaves, an average of 39 to 48 % was immediately'bound'into hardwood foliage whereas only 3 % was bound to loblolly pine needles. This implies that rain events might extract greater amounts of HNO3 -derived nitrate in throughfall from conifer canopies as compared to hardwood canopies. Post-exposure H15 NO3 uptake rates across the leaf cuticle increased with surface nitrate concentrations, but were 1 to 2 orders of magnitude lower (O06 to 0.24 nmol m-2 s-1 ) than total HNO3 , deposition during exposures. A generalized leaf-level model of HNO3 deposition to foliage capable of simulating deposition pathways to sorption sites on the leaf surface, and to the metabolically active leaf interior via transcuticular or stomatal pathways is formulated and suggested for use in planning future work on HNO3 deposition.