Formation, transformation and transport of black carbon (charcoal) in terrestrial and aquatic ecosystems.

Black carbon (BC) is ubiquitous in terrestrial environments and its unique physical and chemical properties suggest that it may play an important role in the global carbon budget (GCB). A critical issue is whether the global production of BC results in significant amounts of carbon (C) being removed from the short-term bio-atmospheric carbon cycle and transferred to the long-term geological carbon cycle. Several dozen field and laboratory based studies of BC formation during the burning of biomass have been documented. Findings are difficult to interpret because they have been expressed in an inconsistent manner, and because different physical and chemical methods have been used to derive them. High error terms documented in many of these studies also highlight the problems associated with the quantification of the amount of biomass C consumed in fire, the amount of residue produced and the constituents of that residue. To be able to estimate the potential for BC as a carbon sink, issues regarding its definition, the methods used in its identification and measurement, and the way it is expressed in relation to other components of the carbon cycle need to be addressed. This paper presents BC data in a standard way; BC production as a percentage of the amount of C consumed by fire (BC/CC), which can be readily integrated into a larger carbon budget. Results from previous studies and new data from Australian ecosystems were recalculated in this way. As part of this process, several BC estimates derived solely from physical methods were discarded, based on their inability to accurately identify and quantify the BC component of the post-fire residue. Instead, more focus was placed on BC estimates obtained by chemical methods. This recalculated data lowered the estimate for BC formation in forest fires from 4% to 5% to <3% BC/CC. For savannah and grassland fires a value of <3% is consistent with reported data, but considerable variation among estimates remains. An updated flow-chart linking the sources, fluxes and pools of BC formed in the terrestrial environment with the aquatic and marine environments, and estimates of mean residence times for BC are also presented.

Foliage, fine-root, woody-tissue and stand respiration in Pinus radiata in relation to nitrogen status.

We measured respiration of 20-year-old Pinus radiata D. Don trees growing in control (C), irrigated (I), and irrigated + fertilized (IL) stands in the Biology of Forest Growth experimental plantation near Canberra, Australia. Respiration was measured on fully expanded foliage, live branches, boles, and fine and coarse roots to determine the relationship between CO(2) efflux, tissue temperature, and biomass or nitrogen (N) content of individual tissues. Efflux of CO(2) from foliage (dark respiration at night) and fine roots was linearly related to biomass and N content, but N was a better predictor of CO(2) efflux than biomass. Respiration (assumed to be maintenance) per unit N at 15 degrees C and a CO(2) concentration of 400 micro mol mol(-1) was 1.71 micro mol s(-1) mol(-1) N for foliage and 11.2 micro mol s(-1) mol(-1) N for fine roots. Efflux of CO(2) from stems, coarse roots and branches was linearly related to sapwood volume (stems) or total volume (branches + coarse roots) and growth, with rates for maintenance respiration at 15 degrees C ranging from 18 to 104 micro mol m(-3) s(-1). Among woody components, branches in the upper canopy and small diameter coarse roots had the highest respiration rates. Stem maintenance respiration per unit sapwood volume did not differ among treatments. Annual C flux was estimated by summing (1) dry matter production and respiration of aboveground components, (2) annual soil CO(2) efflux minus aboveground litterfall, and (3) the annual increment in coarse root biomass. Annual C flux was 24.4, 25.3 and 34.4 Mg ha(-1) year(-1) for the C, I and IL treatments, respectively. Total belowground C allocation, estimated as the sum of (2) and (3) above, was equal to the sum of root respiration and estimated root production in the IL treatment, whereas in the nutrient-limited C and I treatments, total belowground C allocation was greater than the sum of root respiration and estimated root production, suggesting higher fine root turnover or increased allocation to mycorrhizae and root exudation. Carbon use efficiency, the ratio of net primary production to assimilation, was similar among treatments for aboveground tissues (0.43-0.50). Therefore, the proportion of assimilation used for construction and maintenance respiration on an annual basis was also similar among treatments.

Considerations in modeling change in temperate forest nitrogen cycles.

The general features of nitrogen (N) cycles in temperate forests and the important processes to consider when modeling change in these cycles include atmospheric inputs, N fixation, litter (root and aboveground) transfers and decomposition, soil processes, N uptake and effects on productivity and litter quality, and N outputs. Nitrogen cycling is closely linked with the carbon (C) and water cycles. Thus models of N cycling must include aspects of these other cycles. Although much is known about individual processes, development of a generic model of forest N cycling is not possible at present because the links and interactions among the individual processes are not well understood. The weakest links with respect to the N cycle are: quantification of atmospheric (especially dry) deposition rates in polluted environments, controls on C and N allocation in vegetation, controls on N turnover in fine roots, controls on decomposition of the older components of soil organic matter, and feedbacks among N availability, litter "quality" and subsequent N mineralization rates. To examine possible effects of long-term change in climate or atmospheric chemistry on the storage of C and other elements in forest ecosystems, we need to model in detail the effects of these factors on complex soil processes such as organic matter decomposition. Some promising models have been developed, but they need to be validated across a range of forest types before they can be used with confidence for long-term prediction. Mean annual leaf litter N concentration offers potential as a simple index of annual N uptake in forests.