Kathleen Savage

Minimizing artifacts and biases in chamber-based measurements of soil respiration

Soil respiration is one of the largest and most important fluxes of carbon in terrestrial ecosystems. While eddy covariance methods are becoming widely used to measure nighttime total ecosystem respiration, the use of chambers placed over the soil is the most direct way of measuring respiration occurring within the soil and litter layers. Several decades of experience with chamber-based measurements have revealed most of the potential sources of error with this methodology. The objectives of this paper are to review several recently expressed concerns about uncertainties of chamber-based measurements of CO2 emissions from soils, to evaluate the direction and magnitude of these potential errors, and explain procedures that minimize these errors and biases. Disturbance of diffusion gradients cause underestimate of fluxes by less than 15% in most cases, and can be partially corrected for with curve fitting and/or can be minimized by using brief measurement periods. Underpressurization or overpressurization of the chamber caused by flow restrictions in air circulating designs can cause large errors, but can also be avoided with properly sized chamber vents and unrestricted flows. We found very small pressure differentials (±0.1 Pa) and modest (∼15%), inconsistent errors in flux estimates using our chamber design. Somewhat larger pressure differentials (±0.9 Pa) were observed under windy conditions, and the accuracy of chamber-based measurements made under such conditions needs more research. Spatial and temporal heterogeneity can be addressed with appropriate chamber sizes and numbers and frequency of sampling. For example, means of eight randomly chosen flux measurements from a population of 36 measurements made with 300 cm 2 diameter chambers in tropical forests and pastures were within 25% of the full population mean 98% of the time and were within 10% of the full population mean 70% of the time. Finally, comparisons of chamber-based measurements with tower-based measurements require analysis of the scale of variation within the purported tower footprint. In a forest at Howland, ME, soil respiration rates differed by a factor of 2 between very poorly drained and well drained soils, but these differences were mostly fortuitously cancelled when spatially extrapolated over purported footprints of 600–2100 m length. While all of these potential sources of measurement error and sampling biases must be carefully considered, properly designed and deployed chambers provide a reliable means of accurately measuring soil respiration in terrestrial ecosystems.

Belowground carbon allocation in forests estimated from litterfall and IRGA-based soil respiration measurements

Allocation of C to belowground plant structures is one of the most important, yet least well quantified fluxes of C in terrestrial ecosystems. In a literature review of mature forests worldwide, Raich and Nadelhoffer (1989) suggested that total belowground carbon allocation (TBCA) could be estimated from the difference between annual rates of soil respiration and aboveground litterfall. Here we analyze new measurements of soil respiration and litterfall, including data from the Ameriflux network. Our results generally agree with Raich and Nadelhoffer’s previous work. A regression analysis of data from mature forests produced the following relationship: annual soil respiration = 287 + 2.80 × annual litterfall. This regression slope indicates that, on average, soil respiration is roughly three times ab oveground litterfall-C, which further implies that TBCA is roughly twice annual aboveground litterfall-C. These inferences are based on the uncertain assumption of soil C stocks being at steady state. Nevertheless, changes in soil C would have to be very large to modify the conclusion that TBCA is generally much larger than litterfall. Among only mature temperate hardwood forests, however, the correlation between litterfall and soil respiration was poor, and the correlation among years for a single site was also poor. Therefore, the regression cannot be relied upon to provide accurate estimates of soil respiration or TBCA for individual sites. Moreover, interannual variation in TBCA, short-term changes in C stocks, or different temporal scales controlling leaf litter production and soil respiration may cause important deviations from the global average. The regression slope for data from young forests is steeper, possibly indicating proportionally greater TBCA, but the steady-state assumption is more problematic for young forests. This method

Interannual variation of soil respiration in two New England forests

Soil respiration is an important component of the annual carbon balance of forests, but few studies have addressed interannual variation in soil respiration. The objectives of this study were to investigate the seasonal and interannual variation in soil respiration, temperature, precipitation, and soil water content in two New England forest soils and to develop and evaluate empirical models for predicting variations in soil respiration using temperature and soil moisture content. We have been measuring soil respiration, using dynamic chambers in well-drained upland sites and poorly drained wetland sites since 1995 at the Harvard Forest, Massachusetts, and since 1996 at the Howland Forest, Maine. The upland sites had consistently greater rates of respiration than wetlands. Prolonged drought periods in 1995, 1998, and 1999 at the Harvard Forest resulted in decreased soil respiration rates in the uplands, particularly once soil moisture contents decreased below about −150 kPa. In contrast, wetland respiration increased upon drying. The interannual variation in soil respiration at the Harvard Forest, 0.23 kg C m−2 yr−1, exceeds the interannual variation in net ecosystem exchange (NEE), 0.14 kg C m−2 yr−1 previously measured for this forest, indicating that interannual variation in soil respiration can have an important influence on NEE. Interannual variation was lower at the Howland Forest, and the effects of low soil moisture content on respiration rates were more subtle. The onset of spring was variable among years at both forests, owing to variation in both temperature and precipitation, and contributed to 33–59% of the annual variability in total carbon release. At the upland sites, parameterization of empirical regression models for respiration as a function of soil temperature was inconsistent among years, indicating an important effect of interannual variation in soil water content. The negative residuals of the Harvard Forest temperature regression model were best explained by drought conditions (soil matric potentials ≤−150 kPa). This function was only applicable during severe drought and did not account for less severe dry periods that also reduced soil moisture and soil respiration. An empirical regression model for the wetlands as a function of temperature was significantly improved with the addition of a soil moisture function, which increased respiration rates under dry conditions and decreased it under wet conditions. Climatic changes resulting in drier conditions will likely decrease soil respiration rates in uplands and increase soil respiration in wetlands.

Seasonal patterns and controls on net ecosystem CO2 exchange in a boreal peatland complex

We measured seasonal patterns of net ecosystem exchange (NEE) of CO2 in a diverse peatland complex underlain by discontinuous permafrost in northern Manitoba, Canada, as part of the Boreal Ecosystems Atmosphere Study (BOREAS). Study sites spanned the full range of peatland trophic and moisture gradients found in boreal environments from bog (pH 3.9) to rich fen (pH 7.2). During midseason (July-August, 1996), highest rates of NEE and respiration followed the trophic sequence of bog (5.4 to −3.9 μmol CO2 m−2 s−1) < poor fen (6.3 to −6.5 μmol CO2 m−2 s−1) < intermediate fen (10.5 to −7.8 μmol CO2 m−2 s−1) < rich fen (14.9 to −8.7 μmol CO2m−2 s−1). The sequence changed during spring (May-June) and fall (September-October) when ericaceous shrub (e.g., Chamaedaphne calyculata) bogs and sedge (Carex spp.) communities in poor to intermediate fens had higher maximum CO2 fixation rates than deciduous shrub-dominated (Salix spp. and Betula spp.) rich fens. Timing of snowmelt and differential rates of peat surface thaw in microtopographic hummocks and hollows controlled the onset of carbon uptake in spring. Maximum photosynthesis and respiration were closely correlated throughout the growing season with a ratio of approximately 1/3 ecosystem respiration to maximum carbon uptake at all sites across the trophic gradient. Soil temperatures above the water table and timing of surface thaw and freeze-up in the spring and fall were more important to net CO2 exchange than deep soil warming. This close coupling of maximum CO2 uptake and respiration to easily measurable variables, such as trophic status, peat temperature, and water table, will improve models of wetland carbon exchange. Although trophic status, aboveground net primary productivity, and surface temperatures were more important than water level in predicting respiration on a daily basis, the mean position of the water table was a good predictor (r2 = 0.63) of mean respiration rates across the range of plant community and moisture gradients. Q10 values ranged from 3.0 to 4.1 from bog to rich fen, but when normalized by above ground vascular plant biomass, the Q10 for all sites was 3.3.

Estimating parameters of a forest ecosystem C model with measurements of stocks and fluxes as joint constraints.

We conducted an inverse modeling analysis, using a variety of data streams (tower-based eddy covariance measurements of net ecosystem exchange, NEE, of CO2, chamber-based measurements of soil respiration, and ancillary ecological measurements of leaf area index, litterfall, and woody biomass increment) to estimate parameters and initial carbon (C) stocks of a simple forest C-cycle model, DALEC, using Monte Carlo procedures. Our study site is the spruce-dominated Howland Forest AmeriFlux site, in central Maine, USA. Our analysis focuses on: (1) full characterization of data uncertainties, and treatment of these uncertainties in the parameter estimation; (2) evaluation of how combinations of different data streams influence posterior parameter distributions and model uncertainties; and (3) comparison of model performance (in terms of both predicted fluxes and pool dynamics) during a 4-year calibration period (1997–2000) and a 4-year validation period (“forward run”, 2001–2004). We find that woody biomass increment, and, to a lesser degree, soil respiration, measurements contribute to marked reductions in uncertainties in parameter estimates and model predictions as these provide orthogonal constraints to the tower NEE measurements. However, none of the data are effective at constraining fine root or soil C pool dynamics, suggesting that these should be targets for future measurement efforts. A key finding is that adding additional constraints not only reduces uncertainties (i.e., narrower confidence intervals) on model predictions, but at the same time also results in improved model predictions by greatly reducing bias associated with predictions during the forward run.

Methane and carbon dioxide exchanges between the atmosphere and northern boreal forest soils

CH4 and CO2 fluxes were measured in upland boreal forest soils near Thompson, Manitoba, from May 16 to September 16, 1994. Most sites consumed atmospheric CH4, fluxes ranging from +0.6 to −2.6 mg CH4 m−2 d−1, and emitted CO2 at rates between 0.2 and 26.8 g CO2 m−2 d−1. There was some evidence of episodic CH4 emissions after heavy rainfall from soils which normally consumed CH4. There were two distinct groups: sites in which both CO2 and CH4 exchange was strong (mean 5.2 g CO2 m−2 d−1, −1.0 mg CH4 m−2 d−1); and those which had a weak exchange of both gases (mean 2.5 g CO2 m−2 d−1, −0.2 mg CH4 m−2 d−1). The presence of black spruce trees, a Sphagnum spp. ground cover and a thick organic layer (20–50 cm) characterized the weak exchange group. These characteristics were indicative of colder, wetter conditions with slower N cycling and longer path lengths to the zone of CH4 oxidation. The strong exchange group had either aspen, jack pine, or birch trees; a vascular plant cover; and a thin organic layer (1–5 cm). These characteristics were indicative of warmer, drier conditions with faster N cycling and shorter path lengths to the zone of CH4 oxidation. The seasonal average of both CO2 and CH4 flux from 11 sites could be predicted by regressions involving the amount of soil organic matter (0–5 cm depth) and seasonal temperature at a depth of 20 cm (r2=0.69 and 0.82, respectively). Using these vegetation characteristics and a Landsat image of the Boreal Ecosystem-Atmosphere Study (BOREAS) northern study area for areal weighting, net gas fluxes were calculated for the upland soils, estimated to be −0.4 mg CH4 m−2 d−1 and 4 g CO2 m−2 d−1.

Diel patterns of autotrophic and heterotrophic respiration among phenological stages

Improved understanding of the links between aboveground production and allocation of photosynthate to belowground processes and the temporal variation in those links is needed to interpret observations of belowground carbon cycling processes. Here, we show that combining a trenching manipulation with high-frequency soil respiration measurements in a temperate hardwood forest permitted identification of the temporally variable influence of roots on diel and seasonal patterns of soil respiration. The presence of roots in an untrenched plot caused larger daily amplitude and a 2-3 h delay in peak soil CO2 efflux relative to a root-free trenched plot. These effects cannot be explained by differences in soil temperature, and they were significant only when a canopy was present during the growing season. This experiment demonstrated that canopy processes affect soil CO2 efflux rates and patterns at hourly and seasonal time scales, and it provides evidence that root and microbial processes respond differently to environmental factors.

Soil respiration in a northeastern US temperate forest: a 22‐year synthesis

To better understand how forest management, phenology, vegetation type, and actual and simulated climatic change affect seasonal and inter-annual variations in soil respiration (R-s), we analyzed m ...

Foundation species loss affects vegetation structure more than ecosystem function in a northeastern USA forest

Loss of foundation tree species rapidly alters ecological processes in forested ecosystems. Tsuga canadensis, an hypothesized foundation species of eastern North American forests, is declining throughout much of its range due to infestation by the nonnative insect Adelges tsugae and by removal through pre-emptive salvage logging. In replicate 0.81-ha plots, T. canadensis was cut and removed, or killed in place by girdling to simulate adelgid damage. Control plots included undisturbed hemlock and mid-successional hardwood stands that represent expected forest composition in 50–100 years. Vegetation richness, understory vegetation cover, soil carbon flux, and nitrogen cycling were measured for two years prior to, and five years following, application of experimental treatments. Litterfall and coarse woody debris (CWD), including snags, stumps, and fallen logs and branches, have been measured since treatments were applied. Overstory basal area was reduced 60%–70% in girdled and logged plots. Mean cover and richness did not change in hardwood or hemlock control plots but increased rapidly in girdled and logged plots. Following logging, litterfall immediately decreased then slowly increased, whereas in girdled plots, there was a short pulse of hemlock litterfall as trees died. CWD volume remained relatively constant throughout but was 3–4× higher in logged plots. Logging and girdling resulted in small, short-term changes in ecosystem dynamics due to rapid regrowth of vegetation but in general, interannual variability exceeded differences among treatments. Soil carbon flux in girdled plots showed the strongest response: 35% lower than controls after three years and slowly increasing thereafter. Ammonium availability increased immediately after logging and two years after girdling, due to increased light and soil temperatures and nutrient pulses from leaf-fall and reduced uptake following tree death. The results from this study illuminate ecological processes underlying patterns observed consistently in region-wide studies of adelgid-infested hemlock stands. Mechanisms of T. canadensis loss determine rates, magnitudes, and trajectories of ecological changes in hemlock forests. Logging causes abrupt, large changes in vegetation structure whereas girdling (and by inference, A. tsugae) causes sustained, smaller changes. Ecosystem processes depend more on vegetation cover per se than on species composition. We conclude that the loss of this late-successional foundation species will have long-lasting impacts on forest structure but subtle impacts on ecosystem function.

Contribution of soil respiration in tropical, temperate, and boreal forests to the 18O enrichment of atmospheric O2

[1] The 18 O content of atmospheric O2 is an important tracer for past changes in the biosphere. Its quantitative use depends on knowledge of the discrimination against 18 O associated with the various O2 consumption processes. Here we evaluated, for the first time, the in situ 18 O discrimination associated with soil respiration in natural ecosystems. The discrimination was estimated from the measured [O2] and d 18 Oo f O2 in the soilair. The discriminations that were found are 10.1 ± 1.5%, 17.8 ± 1.0%, and 22.5 ± 3.6%, for tropical, temperate, and boreal forests, respectively, 17.9 ± 2.5% for Mediterranean woodland, and 15.4 ± 1.6% for tropical shrub land. Current understanding of the isotopic composition of atmospheric O2 is based on the assumption that the magnitude of the fractionation in soil respiration is identical to that of dark respiration through the cytochrome pathway alone (� 18%). The discrimination we found in the tropical sites is significantly lower, and is explained by slow diffusion in soil aggregates and root tissues that limits the O2 concentration in the consumption sites. The high discrimination in the boreal sites may be the result of high engagement of the alternative oxidase pathway (AOX), which has high discrimination associated with it (� 27%). The intermediate discrimination (� 18%) in the temperate and Mediterranean sites can be explained by the opposing effects of AOX and diffusion limitation that cancel out. Since soil respiration is a major component of the global oxygen uptake, the contribution of large variations in the discrimination, observed here, to the global Dole Effect should be considered in global scale studies. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/ atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 1040 Geochemistry: Isotopic composition/chemistry; 1615 Global Change: Biogeochemical processes (4805); KEYWORDS: Dole Effect, oxygen isotopes, soil respiration Citation: Angert, A., et al., Contribution of soil respiration in tropical, temperate, and boreal forests to the 18 O enrichment of