Rachhpal S. Jassal

Northern regions are experiencing considerable climate change affecting the state of permafrost, peat accumulation rates, and the large pool of carbon (C) stored in soil, thereby emphasizing the importance of monitoring surface C fluxes in different landform sites along a climate gradient. We studied surface net C exchange (NCE) and ecosystem respiration (ER) across different landforms (upland, peat plateau, collapse scar) in mid-boreal to high subarctic ecoregions in the Mackenzie Valley of northwestern Canada for three years. NCE and ER were measured using automatic CO₂ chambers (ADC, Bioscientific LTD., Herts, England), and soil respiration (SR) was measured with solid state infrared CO₂ sensors (Carbocaps, Vaisala, Vantaa, Finland) using the concentration gradient technique. Both NCE and ER were primarily controlled by soil temperature in the upper horizons. In upland forest locations, ER varied from 583 to 214 g C·m⁻²·year⁻¹ from mid-boreal to high subarctic zones, respectively. For the bog and peat plateau areas, ER was less than half that at the upland locations. Of SR, nearly 75% was generated in the upper 5 cm layer composed of live bryophytes and actively decomposing fibric material. Our results suggest that for the upland and bog locations, ER significantly exceeded NCE. Bryophyte NCE was greatest in continuously waterlogged collapsed areas and was negligible in other locations. Overall, upland forest sites were sources of CO₂ (from 64 g·C·m⁻²·year⁻¹ in the high subarctic to 588 g C·m⁻²·year⁻¹ in mid-boreal zone); collapsed areas were sinks of C, especially in high subarctic (from 27 g·C·m⁻² year⁻¹ in mid-boreal to 86 g·C·m⁻²·year⁻¹ in high subarctic) and peat plateaus were minor sources (from 153 g·C·m⁻²·year⁻¹ in mid-boreal to 6 g·C·m⁻²·year⁻¹ in high subarctic). The results are important in understanding how different landforms are responding to climate change and would be useful in modeling the effect of future climate change on the soil C balance in the northern regions.

Core Ideas: A method for simultaneous measurements of soil CO₂ and CH₄ fluxes is presented. A laser‐based cavity ring‐down spectrometer is coupled to automated chambers. A differential equation is solved for the small flow that is exhausted. The system allowed using linear fit to mixing ratio versus chamber closure time. V_eff increased by 7% due to GHG adsorption and 4% due to soil porosity. We present a method of simultaneously measuring soil CO₂ and CH₄ fluxes using a laser‐based cavity ring‐down spectrometer (CRDS) coupled to an automated non‐steady‐state chamber system. The differential equation describing the change in the greenhouse gas (GHG) mixing ratio in the chamber headspace following lid closure is solved for the condition when a small flow rate of chamber headspace air is pulled through the CRDS by an external pump and exhausted to the atmosphere. The small flow rate allows calculation of fluxes assuming linear relationships between the GHG mixing ratios and chamber lid closure times of a few minutes. We also calibrated the chambers for effective volume (V_eff) and show that adsorption of the GHGs on the walls of the chamber caused V_eff to be 7% higher than the geometric volume, with the near‐surface soil porosity causing another 4% increase in V_eff.

To examine the effect of nitrogen (N) fertilization on forest carbon (C) sequestration and water loss, we used an artificial neural network model to estimate C fluxes and evapotranspiration (ET) in response to N fertilization during four post-fertilization years in a Pacific Northwest chronosequence of three Douglas-fir stands aged 61, 22 and 10 years old in 2010 (DF49, HDF88 and HDF00, respectively). Results showed that N fertilization increased gross primary productivity (GPP) for all three sites in all four years with the largest absolute increase at HDF00 followed by HDF88. Ecosystem respiration increased in all four years at HDF00, but decreased over the last three years at HDF88 and over all four years at DF49. As a result, fertilization increased the net ecosystem productivity of all three stands with the largest increase at HDF88, followed by DF49. Fertilization had no discernible effect on ET in any of the stands. Consequently, fertilization increased water use efficiency (WUE) in all four post-fertilization years at all three sites and also increased light use efficiency (LUE) of all the stands, especially HDF00. Our results suggest that the effects of fertilization on forest C sequestration and water loss may be associated with stand age and fertilization; the two younger stands appeared to be more efficient than the older stand with respect to GPP, WUE and LUE.

This study investigated how nitrogen (N) fertilization with 200 kg N ha⁻¹ of a 58‐year‐old West Coast Douglas fir stand influenced its net greenhouse gas (GHG) global warming potential (GWP) in the first year after fertilization. Effects of fertilization on GHG GWP were calculated considering changes in soil N₂O emissions, measured using the static chamber technique and the soil N₂O gradient technique; eddy covariance (EC) measured net ecosystem productivity (NEP); and energy requirements of fertilizer production, transport, and its aerial spreading. We found significant N₂O losses in fertilized plots compared to a small uptake in nonfertilized plots. Chamber‐measured N loss in the fertilized plots was about 16 kg N₂O ha⁻¹ in the first year, which is equivalent to 10 kg N ha⁻¹ or 5% of the applied fertilizer N. Soil N₂O emissions measured using the gradient technique, however, exceeded the chamber measurements by about 50%. We also compared a polymer‐coated slow‐release urea with regular urea and found that the former delayed N₂O emissions but the year‐end total loss was about the same as that from regular urea. Change in NEP due to fertilization was determined by relating annual NEP for the nonfertilized stand to environmental controls using an empirical and a process‐based model. Annual NEP increased by 64%, from 326 g C m⁻², calculated assuming that the stand was not fertilized, to the measured value of 535 g C m⁻² with fertilization. At the end of the year, net change in GHG GWP was −2.28 t CO₂ ha⁻¹ compared to what it would have been without fertilization, thereby indicating favorable effect of fertilization even in the first year after fertilization with significant emissions of N₂O.

We analyzed 17 months (August 2005 to December 2006) of continuous measurements of soil CO₂ efflux or soil respiration (R_S) in an 18‐year‐old west‐coast temperate Douglas‐fir stand that experienced somewhat greater than normal summertime water deficit. For soil water content at the 4 cm depth (θ) > 0.11 m³ m⁻³ (corresponding to a soil water matric potential of −2 MPa), R_S was positively correlated to soil temperature at the 2 cm depth (T_S). Below this value of θ, however, R_S was largely decoupled from T_S, and evapotranspiration, ecosystem respiration and gross primary productivity (GPP) began to decrease, dropping to about half of their maximum values when θ reached 0.07 m³ m⁻³. Soil water deficit substantially reduced R_S sensitivity to temperature resulting in a Q₁₀ significantly < 2. The absolute temperature sensitivity of R_S (i.e. dR_S/dT_S) increased with θ up to 0.15 m³ m⁻³, above which it slowly declined. The value of dR_S/dT_S was nearly 0 for θ < 0.08 m³ m⁻³, thereby confirming that R_S was largely unaffected by temperature under soil water stress conditions. Despite the possible effects of seasonality of photosynthesis, root activity and litterfall on R_S, the observed decrease in its temperature sensitivity at low θ was consistent with the reduction in substrate availability due to a decrease in (a) microbial mobility, and diffusion of substrates and extracellular enzymes, and (b) the fraction of substrate that can react at high T_S, which is associated with low θ. We found that an exponential (van't Hoff type) model with Q₁₀ and R₁₀ dependent on only θ explained 92% of the variance in half‐hourly values of R_S, including the period with soil water stress conditions. We hypothesize that relating Q₁₀ and R₁₀ to θ not only accounted for the effects of T_S on R_S and its temperature sensitivity but also accounted for the seasonality of biotic (photosynthesis, root activity, and litterfall) and abiotic (soil moisture and temperature) controls and their interactions.

A physically‐based multi‐layer numerical model is developed to determine the coupled transport of heat and water in the soil and in the soil‐atmosphere boundary layer. Using inputs of standard weat... [traduit par la rédaction] On a mis au point un modèle à couches multiples fondé sur des principes physiques pour determiner le transport couplé de la chaleur et de l'eau dans le sol et dans la cou...