Rate Of Carbon Loss Research Articles

Assessing carbon and water dynamics of no-till and conventional tillage cropping systems in the inland Pacific Northwest US using the eddy covariance method

Analysis of carbon and water budgets in cropping systems is important for understanding the impacts of different management practices and meteorological conditions, in the context of climate change, on agriculture. We have established a pair of long-term eddy covariance flux towers at the R. J. Cook Agronomy Farm (CAF) near Pullman, Washington, US. The tower sites have similar crop rotation, weather conditions, and management practices except tillage. One site has been under no-till management (CAF-NT) since 1998, while the other site has used conventional tillage practice (CAF-CT) over the same time period. Measurements conducted above a garbanzo bean crop (Cicer arietinum) between October 2012 and September 2013 showed the CAF-NT site was close to CO2 neutral with an annual cumulative net ecosystem exchange of CO2 (NEE) of −20±38gCm−2. By contrast, the CAF-CT site was a CO2 source with an annual NEE of 117±39gCm−2 during the same time period. The annual NEE values at each site were significantly different (p<0.05). When carbon losses via harvest export were taken into account, the net rates of carbon loss from each ecosystem to the atmosphere were 32±50 and 178±48gCm−2 for CAF-NT and CAF-CT, respectively, indicating CAF-NT was close to carbon neutral and CAF-CT was a net carbon source during the measurement period. Partitioning of NEE into gross primary productivity (GPP) and total ecosystem respiration (Reco) shows that the annual cumulative GPP of each site did not differ much, but CAF-NT had lower annual cumulative Reco compared to CAF-CT during the period of October 2012–September 2013. Water budget analyses presented similar magnitudes of annual sums of evapotranspiration (ET), 425 and 416mm for CAF-NT and CAF-CT, respectively. ET was partitioned into evaporation (E) and transpiration (T) for both sites, and CAF-CT had a larger fraction of annual E compared to CAF-NT during the measurement period. Relationships between carbon and water fluxes were investigated, and for the period of October 2012–September 2013, high correlations were found in GPP vs. T and NEE vs. ET at both sites. In summary, the site deploying no-till practices had lower total ecosystem respiration and evaporation throughout the measurement period, more net carbon uptake and a greater ratio of transpiration over evapotranspiration (T/ET) during the growing season.

Heterogeneous Oxidation of Atmospheric Organic Aerosol: Kinetics of Changes to the Amount and Oxidation State of Particle-Phase Organic Carbon.

Atmospheric oxidation reactions are known to affect the chemical composition of organic aerosol (OA) particles over timescales of several days, but the details of such oxidative aging reactions are poorly understood. In this study we examine the rates and products of a key class of aging reaction, the heterogeneous oxidation of particle-phase organic species by the gas-phase hydroxyl radical (OH). We compile and reanalyze a number of previous studies from our laboratories involving the oxidation of single-component organic particles. All kinetic and product data are described on a common basis, enabling a straightforward comparison among different chemical systems and experimental conditions. Oxidation chemistry is described in terms of changes to key ensemble properties of the OA, rather than to its detailed molecular composition, focusing on two quantities in particular, the amount and the oxidation state of the particle-phase carbon. Heterogeneous oxidation increases the oxidation state of particulate carbon, with the rate of increase determined by the detailed chemical mechanism. At the same time, the amount of particle-phase carbon decreases with oxidation, due to fragmentation (C-C scission) reactions that form small, volatile products that escape to the gas phase. In contrast to the oxidation state increase, the rate of carbon loss is nearly uniform among most systems studied. Extrapolation of these results to atmospheric conditions indicates that heterogeneous oxidation can have a substantial effect on the amount and composition of atmospheric OA over timescales of several days, a prediction that is broadly in line with available measurements of OA evolution over such long timescales. In particular, 3-13% of particle-phase carbon is lost to the gas phase after one week of heterogeneous oxidation. Our results indicate that oxidative aging represents an important sink for particulate organic carbon, and more generally that fragmentation reactions play a major role in the lifecycle of atmospheric OA.

Modeling relationships between water table depth and peat soil carbon loss in Southeast Asian plantations

Plantation-associated drainage of Southeast Asian peatlands has accelerated in recent years. Draining exposes the upper peat layer to oxygen, leading to elevated decomposition rates and net soil carbon losses. Empirical studies indicate positive relationships between long-term water table (WT) depth and soil carbon loss rate in peatlands. These correlations potentially enable using WT depth as a proxy for soil carbon losses from peatland plantations. Here, we compile data from published research assessing WT depth and carbon balance in tropical plantations on peat. We model net carbon loss from subsidence studies, as well as soil respiration (heterotrophic and total) from closed chamber studies, as a function of WT depth. WT depth across all 12 studies and 59 sites is 67 ± 20 cm (mean ± standard deviation). Mean WT depth is positively related to net carbon loss, as well as soil respiration rate. Our models explain 45% of net carbon loss variation and 45–63% of soil respiration variation. At a 70 cm WT depth, the subsidence model suggests net carbon loss of 20 tC ha−1 yr−1 (95% confidence interval (CI) 18–22 tC ha−1 yr−1) for plantations drained for >2 yr. Closed chamber-measured total soil respiration at this depth is 20 tC-CO2 ha−1 yr−1 (CI 17–24 tC-CO2 ha−1 yr−1) while heterotrophic respiration is 17 tC-CO2 ha−1 yr−1 (CI 14–20 tC-CO2 ha−1 yr−1), ∼82% of total respiration. While land use is not a significant predictor of soil respiration, WT depths are greater at acacia (75 ± 16 cm) than oil palm (59 ± 15 cm) sample sites. Improved spatio-temporal sampling of the full suite of peat soil carbon fluxes—including fluvial carbon export and organic fertilizer inputs—will clarify multiple mechanisms leading to carbon loss and gain, supporting refined assessments of the global warming potential of peatland drainage.

Terrestrial ecosystems response to future changes in climate and atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; concentration

Abstract. The response of the terrestrial carbon cycle to future changes in climate and atmospheric CO2 is assessed by analysing simulation results for the 2006–2100 period made with the second generation Canadian Earth system model (CanESM2) for the RCP 2.6, RCP 4.5 and RCP 8.5 climate change scenarios. Our interest is in the extent to which global terrestrial carbon pools and sinks, in particular those of the Amazonian region, are vulnerable to the adverse effects of climate change. CanESM2 results indicate that land remains an overall sink of atmospheric carbon for the 2006–2100 period. The net carbon uptake by land in response to changes in climate and atmospheric CO2 is close to 20, 80 and 140 Pg C for the RCP 2.6, 4.5 and 8.5 scenarios, respectively. The latitudinal structure of future atmosphere–land CO2 flux remains similar to that observed for the historical period with northern mid- to high-latitude regions gaining carbon from the atmosphere while the tropics remain either carbon neutral or a modest source of atmospheric carbon depending on scenario. These changes occur in conjunction with simulated precipitation and soil moisture increases over northern mid- and high-latitude land regions and precipitation and soil moisture decreases over the South American continent in all scenarios. Compared to other regions of the globe, which are either carbon sinks or near neutral, the Amazonian region is simulated to be a net source of carbon during the 21st century. Moreover, and unexpectedly, the rate of carbon loss to the atmosphere from the Amazonian region is largely independent of the differences between the three scenarios considered.

Ar ions and oxygen plasma interactions of amine terminated organosilicate glass: A combined experimental and ab initio simulations study

Ar+ ion bombardment (900 eV) of organosilicate glass (OSG) in the presence of NH3 (1 × 10−6 Torr) yields an overlayer containing Si-NHx bonds. The NHx layer decreases the rate of carbon loss from the remaining film upon subsequent oxygen plasma exposure, due to preferential removal of N from the surface region. Ab initio density functional theory calculations have been performed to investigate the stability of the bonds present in low-k dielectrics utilizing a trimethyltrisiloxane model system. Calculated bond energies are 6.30 eV (Si-NH2), 6.27 eV (Si-OH), 5.69 eV (Si-CH3), and 5.54 eV (Si-H). The slightly higher calculated Si-NH2 bond energy is consistent with experiment and indicates that the nitrided OSG surface layer inhibits carbon loss in part by inhibition of O2 diffusion into the bulk.

Determination of tropical deforestation rates and related carbon losses from 1990 to 2010.

We estimate changes in forest cover (deforestation and forest regrowth) in the tropics for the two last decades (1990–2000 and 2000–2010) based on a sample of 4000 units of 10 ×10 km size. Forest cover is interpreted from satellite imagery at 30 × 30 m resolution. Forest cover changes are then combined with pan-tropical biomass maps to estimate carbon losses. We show that there was a gross loss of tropical forests of 8.0 million ha yr−1 in the 1990s and 7.6 million ha yr−1 in the 2000s (0.49% annual rate), with no statistically significant difference. Humid forests account for 64% of the total forest cover in 2010 and 54% of the net forest loss during second study decade. Losses of forest cover and Other Wooded Land (OWL) cover result in estimates of carbon losses which are similar for 1990s and 2000s at 887 MtC yr−1 (range: 646–1238) and 880 MtC yr−1 (range: 602–1237) respectively, with humid regions contributing two-thirds. The estimates of forest area changes have small statistical standard errors due to large sample size. We also reduce uncertainties of previous estimates of carbon losses and removals. Our estimates of forest area change are significantly lower as compared to national survey data. We reconcile recent low estimates of carbon emissions from tropical deforestation for early 2000s and show that carbon loss rates did not change between the two last decades. Carbon losses from deforestation represent circa 10% of Carbon emissions from fossil fuel combustion and cement production during the last decade (2000–2010). Our estimates of annual removals of carbon from forest regrowth at 115 MtC yr−1 (range: 61–168) and 97 MtC yr−1 (53–141) for the 1990s and 2000s respectively are five to fifteen times lower than earlier published estimates.

Priming of soil organic carbon decomposition induced by corn compared to soybean crops

The rate of soil organic carbon (CS) loss via microbial respiration (decomposition rate k, y−1), and the rate of stabilization of vegetation inputs (CV) into CS (humification rate h, y−1) are usually considered independent of CV. However, short-term laboratory studies suggest that the quality and quantity of CV controls k, which is often referred to as a priming effect. We investigated how the chemical composition of different residues, (corn and soybean) controls k and h under field conditions in a no-till ecosystem. Using CV-driven shifts in δ13C, we estimated changes in carbon (C) stocks, k and h of both the labile particulate organic matter fraction (CPOM) and the stabilized mineral associated organic matter fraction (CMAOM). After two years of high C inputs (corn: 4.4 Mg ha−1 y−1 aboveground and C:N = 78; soybean: 3.5 Mg ha−1 y−1, C:N = 17), we found no changes in CPOM and CMAOM stocks in the top 5-cm of soil or in deeper layers. However, CMAOM in corn had higher k (0.06 y−1) and C output fluxes (0.67 Mg ha−1 y−1) than in soybean (0.03 y−1 and 0.32 Mg ha−1 y−1), but similar rates and fluxes in CPOM in the top 5-cm of soil. In addition, while C inputs to CPOM were also similar for both crops, C inputs from CV to CMAOM were higher in corn (0.51 Mg ha−1 y−1) than in soybean (0.19 Mg ha−1 y−1). Overall, corn plots had higher k and C inputs into CMAOM and therefore higher C cycling in this fraction. Our data suggests that the type of crop residue strongly influences C cycling in the topsoil of no-till cropping systems by affecting both the stabilization and the decomposition of soil organic matter.

Short period of oxygenation releases latch on peat decomposition

Extreme summer droughts are expected to occur more often in the future in NW Europe due to climate change. These droughts might accelerate the rate of peat oxidation in drained peat areas, with impacts on soil subsidence, GHG emission and water quality. This study aimed at providing more insight in the oxidation of deep peat layers that had not previously been exposed to air, the so-called secondary decomposition.We incubated two types of peat (eutrophic and oligotrophic), sampled from permanently anoxic peat layers from nature reserves and agricultural peat meadows. Peat samples were incubated for thirteen weeks under anoxic conditions, but were exposed to air for one to eight weeks. The production of CO2 and CH4 was quantified as a proxy for decomposition; concentrations of soluble nutrients and phenolic compounds were also measured.The results showed that oxygenation led to a steep increase in the rate of decomposition, indicated by higher carbon loss rates during and after oxygenation compared to non-oxygenated samples. Carbon loss rates increased more for eutrophic peat (agricultural area: 352%, nature reserve: 182%) than for oligotrophic peat (83% and 159% respectively). Most peat samples investigated showed higher post-oxygenation CO2 and/or CH4 production compared to the anoxic pre-oxygenation period. This indicates that oxygenation stimulates decomposition, even after anoxic conditions have returned. Contrary to the enzymic latch theory, no effects of oxygenation on the concentrations of soluble or condensed phenolic compounds were detected. Soluble nutrient concentrations did not change due to oxygenation either. Noteworthy is the occurrence of pyrite mineralization and associated acidification in eutrophic peat. Thus, low summer water levels, for example due to climate change, should be avoided in order to limit exceptionally high decomposition rates and associated problems such as increasing subsidence rates, greenhouse gas emission, sulfate release and acidification.

Hot oxygen and carbon escape from the martian atmosphere

The escape of hot O and C atoms from the present martian atmosphere during low and high solar activity conditions has been studied with a Monte-Carlo model. The model includes the initial energy distribution of hot atoms, elastic, inelastic, and quenching collisions between the suprathermal atoms and the ambient cooler neutral atmosphere, and applies energy dependent total and differential cross sections for the determination of the collision probability and the scattering angles. The results yield a total loss rate of hot oxygen of 2.3–2.9×1025s−1 during low and high solar activity conditions and is mainly due to dissociative recombination of O2+ and CO2+. The total loss rates of carbon are found to be 0.8 and 3.2×1024s−1 for low and high solar activity, respectively, with photodissociation of CO being the main source. Depending on solar activity, the obtained carbon loss rates are up to ~40 times higher than the CO2+ ion loss rate inferred from Mars Express ASPERA-3 observations. Finally, collisional effects above the exobase reduce the escape rates by about 20–30% with respect to a collionless exophere.

Plasma damage mechanisms in low k organosilicate glass and their inhibition by Ar ion bombardment

In-situ x-ray photoelectron spectroscopy and ex-situ Fourier transform infrared spectroscopy studies of vacuum ultraviolet (VUV) photons with or without O2, and O radicals point to distinct mechanisms of carbon abstraction in nanoporous organosilicate glass (OSG) films. VUV alone in the absence of O2 results in Si-CH3 bond scission and recombination preferentially at silicon monomethyl sites, obeying diffusion kinetics. In contrast, the presence of O2 interferes with recombination, resulting in diffusion-dominated carbon loss kinetics, enhanced Si oxidation, and greatly accelerating the rate of carbon loss in both the near surface and bulk regions of the OSG, at both monomethyl and dimethyl sites. Carbon abstraction due to exposure to (O(3P)) does not follow diffusion kinetics, and such interactions yield a SiO2-like surface layer inhibiting further O diffusion. Results indicate that diffusion-dominated carbon abstraction kinetics previously observed for OSG exposure to O2 plasma damage is primarily attributable to the diffusion of O2 down OSG nanopores, reacting at photoactivated sites, rather than the diffusion of O radicals. OSG pretreatment by 900 eV Ar+ bombardment effectively inhibits both VUV + O2 and O damage mechanisms by formation of ∼1 nm thick SiO2-like surface region that inhibits both O and O2 diffusion.

Nutrient and carbon dynamics in peat from rich fens and Sphagnum-fens during different gradations of drought

Drought has major impacts on microbial decomposition and net N- and P-release in peat. The separate effects of aeration (oxygen intrusion) during moderate drought and desiccation (oxygen intrusion plus water deficiency) during severe drought are, however, poorly understood. This information is vital to understand the biogeochemical and ecological effects of different gradations of drought in peatlands. In addition, effects may differ between rich fen peat and Sphagnum-dominated poor fen peat. We therefore conducted a controlled incubation experiment involving both soil types to quantify the rates of decomposition, net N-mineralization, net P-release, denitrification, and the partitioning of C, N and P in soils and microbial biomass under three different incubation conditions. Soils were incubated under (1) anaerobic, waterlogged conditions, (2) aerobic, moist conditions, characteristic for moderate drought in which oxygen intrusion takes place, and (3) aerobic, desiccated conditions to simulate severe drought.Our results show that under anaerobic, waterlogged conditions, net N-mineralization rates per mass dry peat soil and per microbial C mass were much higher (on average 10 times) in the Sphagnum-peat than in peat from rich fens, probably caused by higher microbial N-demand and N-immobilization in rich fens. The response upon aeration differed greatly between rich fen peat and Sphagnum-peat. Whereas aeration led to increased carbon loss and net N-mineralization rates in the rich fen peat, these rates did not change for Sphagnum-peat. The absence of aeration effects in Sphagnum-dominated fens suggests that decomposition rates are more strongly determined by litter quality than by oxygen intrusion. Upon further desiccation, both net P-release and DOC production, which remained unchanged upon aeration, increased significantly in both fen types. This may be due to microbial mortality and/or a change in microbial composition. The low anaerobic net N-mineralization rates and the strong response to aeration in rich fens compared to Sphagnum-fens, as well as the strong increase in P-availability upon further desiccation in both fen types, have important implications for peatland management in relation to drought.

Leaf level emissions of volatile organic compounds (VOC) from some Amazonian and Mediterranean plants

Abstract. Emission inventories defining regional and global biogenic volatile organic compounds (VOC) emission strengths are needed to determine the impact of VOC on atmospheric chemistry (oxidative capacity) and physics (secondary organic aerosol formation and effects). The aim of this work was to contribute with measurements of tree species from the poorly described tropical vegetation in direct comparison with the quite well-investigated, highly heterogeneous emissions from Mediterranean vegetation. VOC emission from sixteen plant species from the Mediterranean area were compared with twelve plant species from different environments of the Amazon basin by an emission screening at leaf level using branch enclosures. Analysis of the volatile organics was performed online by a proton-transfer-reaction mass spectrometer (PTR-MS) and offline by collection on adsorbent tubes and subsequent gas chromatographic analysis. Isoprene was the most dominant compound emitted followed by monoterpenes, methanol and acetone. The average loss rates of VOC carbon in relation to the net CO2 assimilation were found below 4% and indicating normal unstressed plant behavior. Most of the Mediterranean species emitted a large variety of monoterpenes, whereas only five tropical species were identified as monoterpene emitters exhibiting a quite conservative emission pattern (α-pinene &lt; limonene &lt; sabinene &lt; ß-pinene). Mediterranean plants showed additional emissions of sesquiterpenes. In the case of Amazonian plants no sesquiterpenes were detected. However, missing of sesquiterpenes may also be due to a lack of sensitivity of the measuring systems. Furthermore, our screening activities cover only 1% of tree species of such tropical areas as estimated based on recent biodiversity reports. Methanol emissions, an indicator of growth, were found to be common in most of the tropical and Mediterranean species. A few species from both ecosystems showed acetone emissions. The observed heterogeneous emissions, including reactive VOC species which are not easily detected by flux measurements, give reason to perform more screening at leaf level and, whenever possible, within the forests under ambient conditions.

Overview of the Mount Tai Experiment (MTX2006) in central East China in June 2006: studies of significant regional air pollution

Abstract. We conducted an intensive field campaign at the summit of Mt. Tai (36.26° N, 117.11° E, 1534 m above sea level), Shandong Province, located at the center of central East China, during the period 28 May to 30 June 2006, to study seasonal maxima of regional air pollution with respect to ozone (O3) and aerosols. The specific objectives, campaign design, and major findings are summarized. High concentrations of O3 and its precursors, and aerosols, were detected and studied in the context of annual variations. Most importantly, we identified that emissions from regional-scale open crop residue burning after the harvesting of winter wheat, together with photochemical aging, strongly increased the concentrations of O3, aerosols, and primary pollutants in this month of year. Studies of in situ photochemical activity, regional source attribution of O3, O3–aerosol interactions, validation of satellite observations of tropospheric NO2, behaviors of volatile organic compounds and organic/inorganic aerosol species, loss rates of black carbon (BC), and instrument inter-comparisons are also summarized. The observed BC levels must have a strong impact on the regional climate.

Allochthonous and autochthonous contributions to carbon accumulation and carbon store in southeastern Australian coastal wetlands

Estimates of carbon store and carbon accumulation rate in mangrove and saltmarsh are beset by issues of scale and provenance. Estimates at a site do not allow scaling to regional estimates if the drivers of variability are not known. Also, carbon accumulation within soils provides a net offset only if carbon is derived in-situ, or would not otherwise be sequestered. We use a network of observation sites extending across 2000 km of southeastern Australian coastline to determine the influence of geomorphic setting and coastal wetland vegetation type on rates of carbon accumulation, carbon store and probable sources. Carbon accumulation above feldspar marker horizons over a 10-year period was driven primarily by tidal range and position in the tidal frame, and was higher for mangrove and saltmarsh dominated by Juncus kraussii than for other saltmarsh communities. The rate of carbon loss with depth varied between geomorphic settings and was the primary determinant of carbon store. A down-core enrichment in δ13C was consistent with an increased relative contribution of mangrove root material to soil carbon, as mangrove roots were found to be consistently enriched compared to leaves. We conclude that while surface carbon accumulation is driven primarily by tidal transport of allocthonous sediment, in-situ carbon sequestration is the dominant source of recalcitrant carbon, and that mangrove and saltmarsh carbon accumulation and store is high in temperate settings, particularly in mesotidal and fluvial geomorphic settings.

Effect of salinity-altering pulsing events on soil organic carbon loss along an intertidal wetland gradient: a laboratory experiment

Salinity changes resulting from storm surge, tides, precipitation, and stormwater run-off are common in coastal wetlands. Soil microbial communities respond quickly to salinity changes, altering the rate of soil organic carbon (SOC) loss and associated biogeochemical processes. This study quantified the impact of salinity-altering pulses on SOC loss, defined as microbial respiration (CO2 flux) at high and low tide, CH4 flux, and dissolved OC (DOC) release, in 3 intertidal wetlands (Jacksonville, FL, USA). Intact soil cores from a freshwater tidal, brackish, and salt marsh were exposed to simulated tides and 3 salinity pulsing events during a 53-day laboratory experiment. Soil and water physio-chemical properties, nutrient release, and microbial indicators were measured. Microbial respiration was the dominate pathway of SOC loss (>97 %). Soil hydraulic conductivity was greater in brackish and salt marshes and was critical to overall soil respiration. High tide CO2 flux was greatest in the freshwater marsh (58 % of SOC loss) and positively correlated with DOC concentration; low tide CO2 flux was greatest in brackish and salt marshes (62 and 70 % of SOC loss, respectively) and correlated with NH4 + and microbial biomass. The freshwater marsh was sensitive to brackish pulses, causing a 112 % increase in respiration, presumably from accelerated sulfate reduction and N-cycling. SOC loss increased in the salt marsh pulsed with freshwater, suggesting freshwater run-off may reduce a salt marsh’s ability to keep-pace with sea level rise. Increased inundation from storm surges could accelerate SOC loss in freshwater marshes, while decreasing SOC loss in brackish and salt marshes.

Quantifying the climate-change consequences of shifting land use between forest and agriculture

Land-use change between forestry and agriculture can cause large net emissions of carbon dioxide (CO2), and the respective land uses associated with forest and pasture lead to different on-going emission rates of methane (CH4) and nitrous oxide (N2O) and different surface albedo. Here, we quantify the overall net radiative forcing and consequent temperature change from specified land-use changes.These different radiative agents cause radiative forcing of different magnitudes and with different time profiles. Carbon emission can be very high when forests are cleared. Upon reforestation, the former carbon stocks can be regained, but the rate of carbon sequestration is much slower than the rate of carbon loss from deforestation. A production forest may undergo repeated harvest and regrowth cycles, each involving periods of C emission and release.Agricultural land, especially grazed pastures, have much higher N2O emissions than forests because of their generally higher nitrogen status that can be further enhanced through intensification of the nitrogen cycle by animal excreta. Because of its longevity in the atmosphere, N2O concentrations build up nearly linearly over many decades. CH4 emissions can be very high from ruminant animals grazing on pastures. Because of its short atmospheric longevity, the CH4 concentration from a converted pasture accumulates for only a few decades before reaching a new equilibrium when emission of newly produced CH4 is balanced by the oxidation of previously emitted CH4. Albedo changes generally have the opposite radiative forcing from those of the GHGs and partly negate their radiative forcing. Overall and averaged over 100years, CO2 is typically responsible for 50% of radiative forcing and CH4 and N2O for 25% each. Albedo changes can negate the radiative forcing by the three greenhouse gases by 20–25%.

How useful are MIR predictions of total, particulate, humus, and resistant organic carbon for examining changes in soil carbon stocks in response to different crop management? A case study

Measures of particulate organic carbon (POC), humus organic carbon (HOC), and resistant organic carbon (ROC) (primarily char) are often used to represent the active, slow, and inert carbon pools used in soil carbon models. However, these fractions are difficult to measure directly, and mid infrared (MIR) spectroscopic techniques are increasingly being investigated to quantify these fractions and total organic carbon (TOC). This study examined the change in MIR-predicted pools of TOC, POC, HOC, and ROC in response to different crop management between two time periods (1981 and 2008) in a long-term wheat cropping trial in Queensland, Australia. The aims were (i) to assess the ability of MIR to detect changes in carbon stocks compared with direct measurements of TOC (LECO-TOC); and (ii) to assess how well the behaviour of POC, HOC, and ROC corresponded with the active, slow, and inert conceptual carbon pools. Significant declines in carbon stocks were observed over time using both LECO-TOC and MIR-predicted stocks of TOC, POC, HOC, and ROC, although MIR-TOC under-estimated loss by 27–30% compared with LECO-TOC. The decline in MIR-POC and MIR-HOC was consistent with the expected behaviour of the active and slow conceptual pools; however, the decline in ROC was not consistent with that of the inert pool. In addition, MIR measurements did not accurately detect differences in the rate of carbon loss under different crop management practices.

Coarse Woody Debris Dynamics Following Biomass Harvesting: Tracking Carbon and Nitrogen Patterns During Early Stand Development in Upland Black Spruce Ecosystems

Coarse woody debris (CWD) in the boreal ecosystem has been hypothesized to play an important nutritional role following stand-replacing disturbances such as fire or harvest. Sites with shallow soil over bedrock, or those with coarse-textured soils, can be especially susceptible to overstory removal because low carbon and nutrient pools may limit stand productivity in subsequent rotations. On these site types, CWD can provide essential nutrition to the developing second growth stand, prior to internal cycling processes stabilizing at crown closure (15 years to 20 years after stand initiation) through slow and steady decomposition. The current study sites were established in 1994 and in 2008 (14 years following harvesting) and were approaching crown closure. The experimental harvest areas were designed to document carbon loss and nutrient fluxes after the application of four levels of biomass removal from mature black spruce forested stands in northwestern Ontario, Canada. Two soil types (fresh, loamy : dry, sandy), with stand replicates (blocks), were selected to test whether residual CWD represents a source or sink for nutrients, and if the decay pattern varied depending on soil type. Measurement/sampling of CWD was done immediately after the harvest treatments were applied, and again in year 4 and year 14. The biomass removal treatment with the greatest carbon loss and fastest CWD decay rate had the highest initial mass of CWD, indicating possible synergistic decay dynamics. Nitrogen concentration in the CWD continued to increase from the initial measurements to year 14 (from 900 ppm to 2400 ppm), but was largely a function of increasing carbon loss. When converted to N content in CWD (kg ha-1), however, nitrogen exhibited an initial upward trend (i.e., immobilization) through years 1to 4 (from 50 kg ha-1 up to 80 kg ha-1) and a subsequent release in years 5 to 14 (from 80 kg ha-1down to 27 kg ha-1). This trend was more apparent on the dry, sandy sites where N content peaked at almost 100 kg ha-1 at year 4, but then reduced to 26 kg ha-1 by year 14. We compared the average loss of N from CWD in years 4 to 14 (5.3 kg ha-1 yr-1) to the total soil inorganic N pool (based on a fresh K2SO4 extraction), and found that the N loss from CWD represented a substantive portion (80%) of the available N pool, particularly on the dry, sandy sites. After an initial peak in year 4, black spruce foliar N decreased significantly (p<.0001) through to year 10 but began to rebound by year 15. This increase, presumably, was in part the result of the documented release of N from CWD. These results suggest that CWD, although a small contributor to the total N pool, makes a substantial contribution to the relatively small available N pool, especially on dry, sandy soils. The trend of initial N immobilization and subsequent release shows CWD may also serve to buffer the initial leaching of nutrients from the site following harvesting and provide an available source of N to the regenerating stand prior to crown closure.

Sensitivity of peatland carbon loss to organic matter quality

Peatland soils store substantial amounts of organic matter (OM). During peat formation, easily decomposable OM is preferentially lost and more recalcitrant moieties accumulate. In a peat profile, OM quality thus scales with depth. Drainage and ongoing climate change poses the risk of rapid OM loss when formerly anoxic peat layers oxidize. During peat decomposition, deeper, more recalcitrant peat is exposed to the oxygen‐rich surface, which may influence the decomposition rate. We show that the soil respiration rate of a disturbed temperate peatland is strongly controlled by the peat's quality and especially its polysaccharides content. The polysaccharide content of soil profiles in a wider range of peatland sites with differing degrees of disturbance was inferred by means of solid‐state13C NMR and DRIFT spectroscopy. The data confirmed a strong decline in polysaccharide content with depth and a poor OM quality of surface peat in soils drained decades ago. We combined the evidence from respiration and spectroscopic measurements to deduce the sensitivity of peatland carbon loss with respect to OM quality by scaling measured quality to a 142‐years record of peatland subsidence and carbon loss at one of the sites. According to the functional relationship between quality and respiration, the measured average annual carbon loss rate of 2.5 t C ha−1 at that site was 20 t C ha−1 at the onset of peatland drainage and dropped to less than 1 t C ha−1 in recent times.

Variation in the photochemical lability of dissolved organic matter in a large boreal watershed

Boreal watersheds contain a vast quantity of terrestrially derived dissolved organic matter (DOM) originating from wetland and forest soils, yet variation in the potential for photochemical transformation of boreal aquatic DOM sources remains poorly understood. Laboratory solar radiation exposure experiments were conducted on DOM samples collected in three seasons, across nine sites, representing contrasting catchment composition and watershed position to assess variation in the photochemical lability of boreal DOM source and stable carbon isotopic signature (δ13C) of photomineralized DOM. Dissolved organic carbon (DOC) loss rates during laboratory exposure were lowest in summer, suggesting that DOM may have been more photo-degraded during summer. DOM from upstream portions of forested stream sites and wetland-influenced sites was more photolabile relative to downstream portions and the river DOM, suggesting potential losses in photolabile DOM downstream and in the lower reaches of the watershed. Increased a254:a350 and spectral slope following sample exposure suggest photoproduction of low molecular weight (LMW) CDOM and/or a higher photoreactivity of high molecular weight versus LMW compounds. Photomineralization of nitrogen was regulated by organic nitrogen concentration and resulted in NH4 +-photoproduction rates between 0.01 and 0.3 µM N h−1 and ecologically significant increases in NH4 + for these waters. The δ13C of the photomineralized DOM was positively correlated to initial DOC concentration and generally lower when initial DOC concentrations were lower, suggesting variation in photomineralized DOM δ13C may be a result of kinetic isotope fractionation. Results from this study demonstrate significant variation in the photochemical lability of boreal watershed sources of DOM. Such variation suggests landscape and environmental change has the potential to alter the biogeochemical role photochemical transformations play in downstream portions of boreal watersheds.