Economically Viable Forest Harvesting Practices That Increase Carbon Sequestration

Eric A. Davidson and Neal Scott The Woods Hole Research Center

Stewart M. Goltz, University of Maine

David Hollinger, USDA-Forest Service

Forests store carbon (C) as they accumulate biomass, but forests are also commercial sources of timber and wood fiber. In most carbon accounting budgets, forest harvesting is usually considered to cause a net release of carbon from the terrestrial biosphere to the atmosphere. As the debate about controlling or mitigating atmospheric CO₂ concentrations moves from study of the scientific issues to a search for practical solutions, a central question becomes whether commercial use of forests could be managed to contribute to terrestrial sequestration of C, rather than cause net release of C to the atmosphere. Can forest management practices be developed that will meet the multiple goals of providing wood and paper products, creating economic returns from natural resources, and also sequestering C from the atmosphere? The objective of the proposed project is to determine whether shelterwood cutting regimes now being adopted in the commercial forests of Maine and other areas can achieve these multiple goals.

At the Howland Integrated Forest Study Area in Maine where we work with International Paper (IP), shelterwood cuts involve removing about 30% of the basal area of the overstory trees. Much of the timber is then processed into wood products, which will persist for several decades. The shelterwood cut encourages growth of subcanopy trees by opening up the forest canopy to increase light penetration to the forest floor, thus stimulating growth and C sequestration. On the other hand, decomposition of slash left on the site from the harvest operation and decomposition of wastes created during timber processing will release some C to the atmosphere, thus offsetting some of the C sequestered in vegetation. Decomposition of soil organic matter and dead roots may also release CO₂ to the atmosphere.

We propose to evaluate the carbon sequestration consequences of these shelterwood cuts in a typical northeastern commercial forest through intensive field measurements and integrative modeling. We will measure whole-ecosystem C exchange in harvested and nearby unharvested mature spruce forests via micrometeorological and mensuration methodologies. We hypothesize that shelterwood management will increase the net sequestration of C (onsite plus offsite) compared to a stand that is not being optimally managed for timber productivity. Tower-based eddy covariance measurements can measure net exchange of C by a forested ecosystem over a half-hour with a sensitivity of less than 1 umol m^-2 s^-1 (about 1 gC m^-2 day^-1). We propose a novel test of this methodology for quantifying C exchange in paired harvested and nonharvested tracts of mature temperate coniferous forest. The degree of confidence in these estimates will be critically evaluated. Furthermore, additional measurements of tree growth, biomass inventories, decomposition of woody slash and natural coarse woody debris, and soil respiration will be measured to help explain the causes of changes in net ecosystem exchange of C measured by the eddy covariance method. This design will result in a sensitive evaluation of the C sequestration implications of what is becoming the dominant softwood harvest method in the northeastern United States.

Pasture Management Strategies for Sequestering Soil Carbon

Alan J. Franzluebbers, U.S. Department of Agriculture

The proposed project seeks to integrate the measurement of soil organic carbon (SOC) sequestration in pasture management systems with soil quality, water quality, and animal performance and productivity in a unique combination of replicated water catchments with diverse plant genetic resources. Sequestration of C in soils under pastures has great potential to offset a portion of the annual greenhouse gas emissions in the USA, because (1) pastures constitute a major land use, (2) fixation of atmospheric CO₂ by pasture plants occurs throughout a great portion of the year, (3) decomposition of organic materials in pastures is slowed by limited water due to rapid plant utilization, and (4) a large root biomass and return of feces to land provide continuous C inputs. There is very little quantitative information on SOC sequestration in pasturelands of the eastern USA. With this project, we will be determining the rate and magnitude of SOC accumulation under three important management variables that producers have control over: (1) plant genetic source, (2) poultry litter versus inorganic fertilizer application, and (3) grazing of cattle versus haying.

Mechanics of Soil Carbon Sequestration by Nitrogen Deposition

R.L. Sinsabaugh, University of Toledo, D.L. Moorhead, D.R. Zak

Forest ecosystems are particularly significant in global change scenarios because of their contribution to global productivity and their large reservoirs of C and N. Because N and CO₂ availability limit plant growth, elevated N deposition and higher atmospheric CO₂ concentration increase primary production creating a terrestrial sink for C and N. The magnitude of this sink is dependent on rates of decomposition as well as rates of production. N deposition affects the decomposition process through changes in litter chemistry and alterations in microbial community composition. In the early stages, rates of plant litter decomposition often increase, but repression is frequently observed for lignified or humifed material. Our previous studies have shown that this repression is associated with the loss of phenol oxidase activity. This effect was long predicted by extrapolation from culture studies, where it has been shown that basidiomycetes do not produce ligninolytic enzymes when mineral N is available. However, our most recent work shows that phenol oxidase activity is suppressed in both fungal-dominated litter and bacterial-dominated soil organic matter, so we believe that N deposition exerts broader effects on microbial activity. The purpose of this project is to resolve the mechanisms that link N deposition with soil organic matter production and to assess the potential of this approach for manipulating carbon storage. The field work will be conducted at nine well-characterized sites across Michigan that represent the major classes of northern temperate forest. The specific objectives are to measure the efficiency of decomposition (using enzyme turnover activities) in relation to carbon quality, N deposition rate, and microbial community composition (using phospholipid fatty acid analysis, PLFA). This information combined with tracer studies of the movement of ¹³C labelled substrates through the microbial community will support development of a model that can quantify the effect of N deposition on soil C storage from measurements of C quality, N deposition rate, and litter deposition rate.

Soil C Saturation: Determining Rates and Limits of Carbon Sequestration

Johan W. Six, Colorado State University

Increasing soil C through changes in land use and management is a low cost and environmentally beneficial method of sequestering substantial amounts of atmospheric CO₂. However, it is generally viewed that soils, like other biological sinks (e.g. vegetation stocks), have an inherent upper limit above which no additional C can be stored. The magnitude of this upper or 'saturation' limit is crucial to know as it will govern the ultimate significance of the soil sink and the time period over which it can be exploited for CO₂ sequestration. However, at present, we have little knowledge of the 'C carrying capacity' of soils and moreover we do not know how rates of C sequestration may differ for soils that are far from, versus close to, some saturation level. This proposal provides an experimental and theoretical framework to determine the saturation limits of different soils, the controls on those saturation limits, and their influence on the kinetics of soil C turnover and stabilization relative to C sequestration. We propose to investigate the role of physiochemical soil characteristics in determining and constraining C sequestration rates and to quantify soil C saturation levels. We hypothesize that there is a level at which soil C becomes saturated and this level is determined by the behavior of four different C pools: 1) a chemically protected C pool, 2) a silt- and clay-protected C pool, 3) a microaggregate-protected C pool, and 4) an unprotected C pool. We will integrate field sampling, laboratory analyses, and mathematical modelling to investigate how climate, soil texture, base saturation, input rates, input quality, and management interact to affect each of the four soil C pools. Each of the measurable pools are influenced by a different set of driving variables and our field and laboratory experiments will enable us to study the behavior of each pool independently and in combination. Our work will also provide the knowledge necessary to integrate each compartment into a cohesive model. Field sampling across broad climatic gradients and fine-scale textural and management gradients will enable us to assess the influence of driving variables on total soil C levels, on the four soil C pools, and consequently determine the potential for C sequestration. Field sampling will be structured to test a number of hypotheses relating to factors influencing soil C. Long-term incubations will be used to evaluate how litter quality, soil texture, and base saturation influence C sequestration and the distribution of sequestered C in the four soil C pools. An innovative approach will be used to assess how the C saturation deficit, or the difference between in situ soil C levels and the physiochemically determined saturation level, influences C sequestration amounts and rates.

The proposed research will help to determine the limits to which soils are likely to operate as C sinks, even with the advent of future technologies which may significantly boost C inputs. Equally important, the work will help provide answers to crucial questions regarding stability of C sequestered, duration of C sequestration, and response of sequestered soil C to disturbance. Our approach will result in a more thorough understanding of soil C dynamics which can be translated directly into a functional, mechanistic, verifiable mathematical model. Results from this work will be able to be extrapolated extensively, applied to many types of management change, and can be readily incorporated as improvements to soil C models.

Quantifying the importance of belowground plant allocation for sequestration of carbon in soils

Margaret S. Torn, Lawrence Berkeley National Laboratory

Todd Dawson, University of California-Berkeley

The recent DOE road map for Carbon Sequestration Science highlights the potential for sequestration by increasing plant allocation of C to belowground biomass and thus reducing decomposition losses. However, to design or evaluate such strategies, we must greatly improve measurements of the rates of C allocation belowground and the subsequent residence times of carbon in the root and soil system.

We propose to fill essential gaps in quantifying the efficacy of sequestration through belowground plant allocation by: (1) Quantifying the stocks and lifetime of live fine and coarse roots; (2) Determining the lower bound of NPP "pumped" into soil carbon through these roots; (3) Comparing leaf and root decomposition including rates, microbial communities and humification products; (4) Characterizing the turnover times of soil organic matter pools, and (5) Tracking the partitioning of recent plant photosynthate to rapidly lost root respiration and exudate mineralization, and more slowly lost root tissues and soil organic matter (SOM).

Our approach will take advantage of several new methods (radiocarbon analysis of roots and SOM, ¹³C tracking of decomposition products, and isotope-label PLFA analysis). The radiocarbon method in particular allows direct determinations of root age, a measure not currently possible with any other technique. At four northern latitude forest research stations, we will make comparisons of belowground allocation sequestration potential based on species and forest type, including deciduous vs. conifer and re-growing vs. mature managed forests.

Ultimately this work should allow us to develop a template for more rapid assessment of the best ecosystems and species to target for future carbon sequestration efforts.

Carbon Sequestration in Dryland & Irrigated Agroecosystems: Quantification at Different Scales for Improved Prediction

Shashi B. Verma, University of Nebraska

We propose a focused interdisciplinary research program to improve our understanding of biophysical controls on soil carbon (C) sequestration and to apply this knowledge towards development of improved methods to predict annual C sequestration. Recent studies have highlighted the potential of agroecosystems to offset a significant amount of anthropogenic C emission through soil C sequestration; therefore, our research will be conducted within the context of the major agroecosystems of the north-central USA. Nebraska is uniquely situated for this research because of its location at the intersection of major continental climate zones with both rainfed and irrigated cropping systems. Our over-arching hypothesis is two-fold: (1) through the use of innovative management practices, that increase plant primary production and minimize adverse environmental effects, the major agroecosystems in the north-central USA will substantially increase present rates of C sequestration and (2) by improving our understanding of biophysical controls on annual C balance we can predict the effects of various management practices on C sequestration in these agroecosystems.

We will investigate C sequestration within three major agroecosystems (a rainfed maize-soybean rotation, an irrigated maize-soybean rotation, and an irrigated continuous maize system). Our effort will include: (a) quantifying annual amounts of C sequestered and the associated interannual variability, at the landscape level, employing eddy covariance flux systems year-round, (b) quantifying soil C changes using georeferenced soil samples, and (c) developing reliable, cost-effective procedures for predicting annual C sequestration and changes in soil C stocks at the scale of a single production field using detailed crop yield mapping. We will also make detailed measurements of plant photosynthesis and respiration, and soil C respiration. We will examine interannual variability in C sequestration in terms of biophysical and physiological controlling factors. We will also quantify "C costs" of applied energy-dependent inputs (e.g., N fertilizer, irrigation, grain drying), and changes in N₂O and CH₄ emissions and integrate these results into net C sequestration values. With the information developed in these agroecosystems we will identify management systems that maximize net C sequestration.

In-Situ Non-Invasive Soil Carbon Measurement (SCM)

Lucian Wielopolski, Brookhaven National Laboratory

This project will develop a robust, flexible, non-invasive, and practical method for monitoring and verifying temporal changes in soil carbon in situ. The method is based on Inelastic Neutron Scattering (INS) of fast neutrons from the carbon nucleus and detection of the subsequently emitted 4.4 MeV gamma rays. We have demonstrated a proof-of-principle using a clinical facility that measures whole body carbon in patients, and 25 kg sand samples that were mixed with granular carbon. Results from our preliminary measurements strongly suggest that the requirement to measure changes of 100 gC/m² can be met with a precision of about 5%. The proposed system will allow multiple and sequential measurements in a static mode, covering area of about 2 m2 or a scan of large areas. The two major objectives of this project are: (1) to construct a prototype of a field deployable Soil Carbon Measurements (SCM) system, and (2) to characterize, calibrate and test the SCM system in the Free-Air CO₂ Enrichment (FACE) facility at the Duke Forest, NC, where laboratory carbon measurements of soil core samples are currently in progress.

The project will be done in close collaboration with Dr. William H. Schlesinger (Duke University), Soil Scientist at the FACE experiment, Dr. George Hendrey (BNL) leader of the FACE experiment, where the field measurements in soil will be performed, and with Dr. Hugo Rogers, Plant Physiologist, (National Soil Dynamics Laboratory, Auburn AL) where extensive calibrations will be carried out.

Genetic and Environmental Controls on Carbon Allocation and Partitioning in Woody Plants - Implications for Ecosystem Carbon Sequestration

Stan D. Wullschleger, G.A. Tuskan, A.W. King and T.J. Tschaplinski, Oak Ridge National Laboratory

Enhancing the natural capacity of terrestrial ecosystems to store carbon is fast becoming a popular carbon management strategy. Such an approach offers a viable and attractive option for stabilizing rising CO₂ concentrations in the atmosphere. Our research seeks to build upon the natural potential of plants and soils to sequester carbon by better understanding the genetic and molecular control of processes that determine sequestration success. These processes include not only the photosynthetic uptake of CO₂ from the atmosphere, but also aspects related to securely storing that carbon in chemical forms that are resistant to microbial degradation and allocating carbon preferentially to roots where it can better contribute to soil carbon sequestration. Our study will take advantage of a genetically well-characterized population of hybrid poplars growing in the Pacific Northwest. For every individual in this population, the chemical composition of leaves and roots, and the fraction of total carbon allocated to roots, will be determined. These traits will be compared against a genetic map that is being established for hybrid popular and genes important to carbon sequestration will be identified. Insights derived from this investigation will be applied to carbon sequestration research, first by assessing the implications of our findings to terrestrial ecosystems through a mechanistic model of carbon sequestration and secondly, through a long-term field experiment that will test specific hypotheses regarding the importance of chemical composition and allocation to carbon sequestration. Our study will uncover plant-based controls on ecosystem carbon sequestration and identify fundamental mechanisms that hopefully will lead to enhanced carbon storage in terrestrial ecosystems.