Examining the Physical Controls on Soil Organic Matter Decomposition Through Depth: Results of a Temperate Forest Soil Mesocosm Experiment
The physical environment in soils exerts a primary control upon rates of soil organic matter (SOM) decomposition, however quantifying the relationships between temperature, moisture and SOM decomposition is methodologically challenging. In situ it is difficult to separate the effects of temperature and moisture on decomposition, while in the lab, disturbance effects may inflate or alter apparent relationships. To overcome these methodological challenges, we investigate the depth dependence of SOM decomposition measured as soil surface carbon dioxide flux in a managed red spruce temperate forest soil by subjecting minimally disturbed shallow (0-25 cm) and deep (25-50 cm) soil cores to varying thermal and moisture conditions in a climate controlled phytotron facility. Parallel incubations with constant and diurnal temperature cycling were carried out upon soils representing a range of moisture contents. The CO2 flux from shallow soils were up to 10 times greater than corresponding deep soils, with clear moisture driven changes in decomposition rates at fixed temperatures observed. The temperature sensitivity of decomposition decreased with increasing moisture for shallow soils, but displayed no clear trend in corresponding deep soils. The results allow insight into how the physical controls that govern SOM decomposition in shallow and deep layers differ, highlighting the critical role of moisture in determining the relationship between decomposition and temperature, and providing a model of soil moisture-decomposition relationships that can be extended to other similar forests within the region.
The Effect Of Soil Moisture Content And Nitrogen And Phosphorous Addition On The Production Of CO2 And N2O In A Mature Red Spruce (Picea rubens Sarg.) Forest Soil
Temperature, moisture and nutrient availability are key controls on the production of CO2 and N2O in the soil and all are altered when a system is perturbed by land management activities. Though the effect of temperature on production has been well characterized, the effects of moisture and nutrients on the production of CO2 and N2O are lesser known. This study examines the effects of soil moisture content and nutrient availability on the production of CO2 and N2O in the top 50 cm of mineral soil collected from a mature red spruce forest. To evaluate the effects of moisture and nutrients on the production of CO2 and N2O, we measured production of CO2 and N2O in an aerobic laboratory incubation conducted at 21 C. In a full factorial design, soils collected from four depth intervals (0-5, 5-20, 20-35, and 35- 50 cm) were adjusted to four water contents (30, 50, 75 and 100% water holding capacity) and amended with nitrogen, phosphorous or both. We compare the effects of the treatments on rates of production of CO2 and N2O to determine if the relative control of these factors on production differs with depth. The findings of this study suggest that the relative control of moisture and nutrient availability on production of CO2 and N2O differ within the soil profile, indicating the need for taking a multi-factor approach to understanding changes in greenhouse gas production from the soil in a managed system.
Does Forest Management Affect The Structure Of The Soil Carbon Pool? Insights From A Long-Term Incubation Of Soils From A Managed Red Spruce Forest Chronosequence
Changes in the rate of microbial respiration throughout a long-term incubation of a soil have been used to empirically partition the soil carbon (C) pool into fractions. Using the rates at which the microbial community consumes each fraction, we can estimate turnover times and use these estimates to comment on a fraction's functional significance in the soil. An assumption of this approach is that the microbial community will consume the most labile fraction of the pool first. Since several studies have demonstrated that this pool of C is sensitive to management-induced changes in soil carbon cycling, long-term incubation of soils may be a useful technique for examining management-induced changes in soil C cycling. Using rates of respiration measured over a 526 d aerobic incubation conducted at 16 C, we present estimates of C fraction sizes and their turnover times for soils collected at six depths in the mineral soil from five managed red spruce forests representing ecologically significant stages of post-harvest regeneration. Our findings indicate that the structure of the C pool changes significantly on the timescale of ecological succession.
Hydrogen coupled CO2 fixation in legume cropping systems
Electron flow from oxidation of excess H2 released by root nodules was shown to contribute to microbial CO2 fixation in soybean crops. This discovery has important implications for carbon storage in soils used to grow legumes; however, further research is needed to understand the fate and turnover time of this H2-coupled CO2 fixation. Isotopic labeling of soil through incubation with 13CO2 was used to elucidate movement of sequestered carbon into soil carbon pools. Measurement of isotopic shifts was determined using Isotope Ratio Mass Spectrometry. Preliminary experiments have confirmed CO2 uptake through an isotopic shift (Δ13C -20.4 to -14.5 ‰) in 24 hour incubated soils labeled with 13CO2 (1% v/v, 99.5 Atom%) under elevated H2 concentration (6000 ppm). Other incubation experiments have confirmed the biotic nature of observed CO2 uptake by comparing isotopic shifts in oven dried and autoclaved soils to moist soil. Under an elevated H2 atmosphere, no significant isotopic shift was observed in dry and autoclaved soils whereas moist soil showed an isotopic shift of Δ13C -21.9 to 11.4 ‰ over 48 hours. Future experiments will involve longer incubations (7 days) and will be aimed at determining isotopic shifts within soil carbon pools. Samples will be incubated and fractionated into microbial biomass, light fraction carbon, and acid stable carbon and subsequent isotopic analysis will be carried out. This will help determine the distribution of H2- coupled fixed CO2 within soil carbon pools and the turnover time of sequestered carbon. This and further research may lead to modification of greenhouse gas coefficients for leguminous crops that includes a CO2 fixation component.
Simulating the effects of forest managements on carbon sequestration: TREPLEX- Management model development
With common concern surrounding the impact of increased atmospheric CO2 on global climate change, the role of forest management (i.e. thinning) on carbon sequestration is growing as a hotspot in the post Kyoto period. However, the combination strategies between forest management and carbon management are less established. Jack pine is one of the most important commercial and reforestation species in lake states of the United States and Canada, and the specie was reported to show stronger response to forest management like thinning. Obviously, there is an urgent need for understanding how harvesting intensity (i.e., thinning) affects C sequestration in jack pine stands. The aim of this study is to quantify and predict the biomass and carbon sequestration in thinned jack pine stands in eastern Canada. TRIPLEX is a generic hybrid model for predicting forest growth and carbon and nitrogen dynamics. The TRIPLEX-Management concept model was developed. The following carbon components were considered: above ground live biomass carbon, standing dead biomass carbon, harvested wood product carbon and soil organic carbon. Thinning was linked with LAI (Leaf Area Index), stand density and soil conditions and included in NPP and biomass production and allocation models. The model was also integrated with DBH distribution models, biomass allometric models, and wood products C models as well as the established height-diameter models. It is expected to optimize thinning regimes for carbon and forest management in order to mitigate climate change impacts.
Investigating the Relative Importance of Nitrification and Denitrification in Generating Nitrous Oxide Emissions From N-amended Soils of a Managed Northern Temperate Forest Chronosequence Using δ15N and δ18O
Nitrous oxide (N2O) is emitted from forest soils, however, our understanding of the processes controlling these emissions is insufficient to narrow down current flux estimates in northern temperate forests. Here, we investigate the relative importance of nitrification and denitrification in generating potential N2O emissions in a managed orthic hummo-ferric podzol forest soil of Nova Scotia, Canada using nitrogen amendments, natural δ15N and δ18O in N2O and a controlled laboratory incubation approach. Shallow soil samples were collected at sites characterized by similar soil type, topography, and climate, and represented 125 ± yr old growth along with previously harvested 80 yr, 45 yr, 15 yr forests, and a recently clear-cut (3 month) forest soil. Experimental conditions subjected soil samples along this age sequence of harvested red spruce forests to conditions favorable for either nitrification or denitrification. The temporal change in N2O concentration and δ15N and δ18O of emitted N2O was monitored in order to investigate N2O emission patterns and how the isotopic composition of N2O changed through time. A linear increase in N2O concentration with time was observed in the headspace of sample bottles under conditions favourable for denitrification, whereas no significant change in N2O concentrations, with the exception of the recent 3-month old clear-cut, was observed in samples under conditions favourable for nitrification. In general, the δ15N and δ18O of emitted N2O during denitrification conditions were around -40 and 15 ‰, respectively, an isotopic signature of denitrification. The δ15N and δ18O during nitrification favorable conditions were 10 and 30 ‰, respectively, except at the 3-month old clear-cut where it was -40 and 10 ‰, respectively. The depleted isotopic signature and higher N2O emissions from the recently clear-cut soils demonstrates the importance of denitrification as a N2O source even under conditions favoring nitrification.