Xu C, Guo L, Ping C-L, White DM. 2009. Chemical and isotopic characterization of size-fractionated organic matter from cryoturbated tundra soils, northern Alaska. Journal of Geophysical research 114, G03002.
These authors examined the isotopic composition and organic matter distribution in soil horizons and particle size fractions from two soils in Alaska, a moist acidic tundra and a moist non-acidic tundra. The organic matter quality and quantity in the deeper part of the active layer and down into the permafrost, material estimated to be between 3000 and 7000 years old indicated high susceptibility to microbial activity, that is, decomposition to CO2 and subsequent release to the atmosphere.
I read this paper as part of my background reading to understand the potential uses of a Picarro field-portable carbon isotope analyser; this paper includes a description of the δ13C values of organic matter throughout the soil / permafrost profiles of these Alaskan soils. These values, and the associated discussion of signatures of microbial activity, suggest it is quite possible to distinguish the source of CO2, permafrost-SOM, deep-SOM, shallow/autotrophic, based on the 13C content of effluxing CO2 and / or CO2 at various depths within a soil profile. Table 5 is particularly valuable in this regard.
Wednesday, August 24, 2011
Thursday, August 11, 2011
Lin et al., 2009
Lin X, Wang S, Ma X, Xu G, Luo C, Li Y, Jiang G, Xie Z. 2009. Fluxes of CO2, CH4, and N2O in an alpine meadow affected by yak excreta on the Qinghai-Tibetan plateau during summer grazing periods. Soil Biology and Biochemistry 41: 718-725.
I read this paper in an attempt to gain a better understanding of the methods used to compare greenhouse gas fluxes between ecosystems or treatments, and between gases, particularly the use of CO2-equivalents when estimating total global warming potential contributions by ecosystems that may be simultaneously sources and sinks for the greenhouse gases CO2, CH4, and N2O. In addition, this is one of a small number of studies I have been able to find that draw conclusions about global warming potentials based only on growing season measurements, rather than whole-year or growing season plus “cold season” (often what the tourist industry might call the shoulder seasons, spring and fall, though sometimes including winter as well).
These authors studied the effects of yak (Bos grunniens) excreta, dung and urine, on soil emissions of the three greenhouse gases. Excreta were hypothesized to increase GHG emissions because both are rich in nitrogen, especially inorganic forms of nitrogen such as urea, ammonia, and nitrate, contain sufficient water to stimulate microbial activity in dry soils, and, in the case of dung, are rich sources of labile organic carbon compounds and large microbial populations already present in the material. Furthermore, because production of CH4 by grazing mammals is strongly linked to their digestive systems, fresh dung may contain considerable dissolved CH4 that will be emitted quickly upon excretion.
The main results of this study were that while fresh dung did significantly shift a patch of meadow from a weak sink for CH4 to a source, this difference was not sufficient to render the larger meadow area a net source because the spatial distribution of dung patches, as well as the duration of the CH4 emission from dung, were relatively small. Urine application did not significantly increase CH4 emission, which is surprising considering the high N concentration and rapid, large addition of water represented by urination by a yak; both factors are expected to increase methanogenesis.
Emissions of CO2 were increased by dung, but not by urine when considering a longer, cumulative set of emissions. Interestingly, urine produced a significant pulse of CO2 nearly immediately upon application to the soil, though whether this CO2 is the result of hydrolysis of urea ((NH2)2CO + H2O --- 2NH3 + CO2) or increased microbial respiration is not clear.
Emissions of N2O were increased by both dung and urine application relative to untreated controls. However, the magnitude of the increase was less than predicted by IPCC (2001) guidelines for calculating the effects of grazing mammals on grasslands; those guidelines were based primarily on temperate low-altitude grasslands, not the high-altitude alpine meadows studied here. In general, patches of yak excreta accounted for an increase in N2O emissions of up to about 10% compared to ungrazed and untreated control meadow, while total CO2-equivalents emissions increased by about 1%, largely due to the small total areal extent of excreta patches.
I read this paper in an attempt to gain a better understanding of the methods used to compare greenhouse gas fluxes between ecosystems or treatments, and between gases, particularly the use of CO2-equivalents when estimating total global warming potential contributions by ecosystems that may be simultaneously sources and sinks for the greenhouse gases CO2, CH4, and N2O. In addition, this is one of a small number of studies I have been able to find that draw conclusions about global warming potentials based only on growing season measurements, rather than whole-year or growing season plus “cold season” (often what the tourist industry might call the shoulder seasons, spring and fall, though sometimes including winter as well).
These authors studied the effects of yak (Bos grunniens) excreta, dung and urine, on soil emissions of the three greenhouse gases. Excreta were hypothesized to increase GHG emissions because both are rich in nitrogen, especially inorganic forms of nitrogen such as urea, ammonia, and nitrate, contain sufficient water to stimulate microbial activity in dry soils, and, in the case of dung, are rich sources of labile organic carbon compounds and large microbial populations already present in the material. Furthermore, because production of CH4 by grazing mammals is strongly linked to their digestive systems, fresh dung may contain considerable dissolved CH4 that will be emitted quickly upon excretion.
The main results of this study were that while fresh dung did significantly shift a patch of meadow from a weak sink for CH4 to a source, this difference was not sufficient to render the larger meadow area a net source because the spatial distribution of dung patches, as well as the duration of the CH4 emission from dung, were relatively small. Urine application did not significantly increase CH4 emission, which is surprising considering the high N concentration and rapid, large addition of water represented by urination by a yak; both factors are expected to increase methanogenesis.
Emissions of CO2 were increased by dung, but not by urine when considering a longer, cumulative set of emissions. Interestingly, urine produced a significant pulse of CO2 nearly immediately upon application to the soil, though whether this CO2 is the result of hydrolysis of urea ((NH2)2CO + H2O --- 2NH3 + CO2) or increased microbial respiration is not clear.
Emissions of N2O were increased by both dung and urine application relative to untreated controls. However, the magnitude of the increase was less than predicted by IPCC (2001) guidelines for calculating the effects of grazing mammals on grasslands; those guidelines were based primarily on temperate low-altitude grasslands, not the high-altitude alpine meadows studied here. In general, patches of yak excreta accounted for an increase in N2O emissions of up to about 10% compared to ungrazed and untreated control meadow, while total CO2-equivalents emissions increased by about 1%, largely due to the small total areal extent of excreta patches.
Tuesday, July 26, 2011
Van Groenigen et al., 2005
Van Groenigen JW, Zwart KB, Harris D, van Kessel C. 2005. Vertical gradients of δ15N and δ18O in soil atmospheric N2O – temporal dynamics in a sandy soil. Rapid Communications in Mass Spectrometry 19: 1289-1295.
These authors examined the production and consumption of N2O in soil profiles using stable-isotope analysis. Enzymatic processes in the microbial nitrification and denitrification pathways all strongly fractionate isotopes of both N and O, with products of reactions significantly depleted in the heavier isotopes. This allows identification of regions of soil producing or consuming N2O by distinguishing between local production and diffusion through the soil layer.
Similar to my own studies, gas concentrations were measured using diffusion wells, in this case probes buried in the soil and sampled by syringe; gas samples were measured by GC-IRMS. Internationally-recognized isotope samples for N2O are not available, so standards for analysis were prepared from commonly available laboratory chemicals and abiotic chemical reactions to completion to avoid fractionation (e.g. reduction of N2O to N2 over copper at 600ºC).
The largest concentrations of N2O were found at the deepest sampling position, 90cm. There was a negative logarithmic relationship between the δ15N value of N2O in the soil and depth, consistent with a relatively large single source of N2O at 90 cm and consumption shallower in the soil combined with vertical-upwards diffusion of N2O; reduction of N2O to N2 enriches the remaining N2O pool for 15N.
Friday, December 3, 2010
Davidson et al. 1991
Davidson EA, Hart SC, Shanks CA, Firestone MK. 1991. Measuring gross nitrogen mineralization, immobilization, and nitrification by 15N isotopic pool dilution in intact soil cores. Journal of Soil Science 42: 335-349.
These authors evaluated the use and limitations of the isotope-pool dilution technique when studying nitrogen dynamics in soil. Because addition of inorganic nitrogen compounds (NH4+, NO3-) can stimulate microbial activity in N-limited systems such as most soils, estimating the rate of these processes by tracking 15N through a system will almost certainly overestimate these rates. The isotope-pool dilution method, on the other hand, measures the dilution of enrichment in the nitrogen pool at the end of a particular process, relying on the assumption that additional product of metabolism will have negligible effects on the magnitude of that metabolism. In this study, immobilization of nitrogen was the main focus of investigation, comparing 15N isotope dilution in pools of either 15NH4+ or 15NO3-.
There are three key assumptions for the isotope-pool dilution method in this context. 1. Microorganisms do not discriminate between 15N and 14N; 2. rates of processes measured remain constant over the incubation period; 3. 15N assimilated during the incubation period is not remineralized. Previously, these assumptions had been evaluated for well-mixed soils, but not for unmixed field-collected soil samples. While fractionation by biological processes certainly does result in discrimination between isotopes of nitrogen, it is of negligible importance when injected solutions are very highly enriched and incubation periods are relatively short; in this case, injections were more than 90% 15N and incubations ran for 24 hours. Rates of measured processes will change if the population and / or activity of microorganisms changes, but again, over a 24-hour incubation period under controlled conditions this is unlikely. Highly enriched injections allow the use of small injection volumes, limiting the impact of nutrient enrichment. These authors were able to measure the remineralization of immobilized 15N, and estimated that between 1.0 and 1.6% of injected 15NH4+ appeared in the 15NO3- pool after 24 hours; they consider this an insignificant amount, but caution that longer incubations would almost certainly result in much more problematic amounts of remineralization.
This paper is clearly a major part of the basis of the project I am currently engaged in with Katherine from our 2009 field season at Alexandra Fjord. I probably should have read this paper long ago. The three main conclusions stated by these authors at the end of their paper I think can be quoted verbatim as justification for both why I (should have earlier) read this paper, and as a reminder to myself to include this paper in the methods & materials section of the eventual manuscript.
These authors evaluated the use and limitations of the isotope-pool dilution technique when studying nitrogen dynamics in soil. Because addition of inorganic nitrogen compounds (NH4+, NO3-) can stimulate microbial activity in N-limited systems such as most soils, estimating the rate of these processes by tracking 15N through a system will almost certainly overestimate these rates. The isotope-pool dilution method, on the other hand, measures the dilution of enrichment in the nitrogen pool at the end of a particular process, relying on the assumption that additional product of metabolism will have negligible effects on the magnitude of that metabolism. In this study, immobilization of nitrogen was the main focus of investigation, comparing 15N isotope dilution in pools of either 15NH4+ or 15NO3-.
There are three key assumptions for the isotope-pool dilution method in this context. 1. Microorganisms do not discriminate between 15N and 14N; 2. rates of processes measured remain constant over the incubation period; 3. 15N assimilated during the incubation period is not remineralized. Previously, these assumptions had been evaluated for well-mixed soils, but not for unmixed field-collected soil samples. While fractionation by biological processes certainly does result in discrimination between isotopes of nitrogen, it is of negligible importance when injected solutions are very highly enriched and incubation periods are relatively short; in this case, injections were more than 90% 15N and incubations ran for 24 hours. Rates of measured processes will change if the population and / or activity of microorganisms changes, but again, over a 24-hour incubation period under controlled conditions this is unlikely. Highly enriched injections allow the use of small injection volumes, limiting the impact of nutrient enrichment. These authors were able to measure the remineralization of immobilized 15N, and estimated that between 1.0 and 1.6% of injected 15NH4+ appeared in the 15NO3- pool after 24 hours; they consider this an insignificant amount, but caution that longer incubations would almost certainly result in much more problematic amounts of remineralization.
This paper is clearly a major part of the basis of the project I am currently engaged in with Katherine from our 2009 field season at Alexandra Fjord. I probably should have read this paper long ago. The three main conclusions stated by these authors at the end of their paper I think can be quoted verbatim as justification for both why I (should have earlier) read this paper, and as a reminder to myself to include this paper in the methods & materials section of the eventual manuscript.
"Three points should be considered when applying the isotope dilution method.
1. Accurate estimation of both 14N and 15N initial pool sizes is important. Abiotic consumption of label, such as by clay fixation, can cause significant errors. A subset of intact cores may need to be destructively sampled directly after adding 15N for estimation of initial pool sizes.
2. Homogeneity of 15N enrichment throughout a soil sample is not possible, and perfectly uniform distribution of added label is not necessary. However, significant errors can arise from a bias in 15N distribution that is concurrent with a non-random
distribution of microbial processes. Distribution of label should, therefore, be as uniform as possible.
3. In situ gross immobilization rates may be overestimated by isotope dilution methods and underestimated by chloroform fumigation methods, depending on which (if any) kN factor is applied to the latter. Gross mineralization and gross nitrification estimates from isotope dilution are more reliable because these rates should not be affected by addition of 15N label in the form of the process products."
Thursday, December 2, 2010
Miller et al. 2008
Miller MN, Zebarth BJ, Dandie CE, Burton DL, Goyer C, Trevors JT. 2008. Crop residue influence on denitrification, N2O emissions and denitrifier community abundance in soil. Soil Biology & Biochemistry 40: 2553-2562.
These authors conducted a factorial experiment using packed soil cores to examine the influence of varying levels of available carbon and nitrogen on the process of denitrification. The treatments consisted of addition of glucose at three levels and KNO3 at four levels in experiment 1, and additions of either red clover or barley straw crop residues with or without additional KNO3. They measured soil chemistry, including extractable organic carbon and NO3- concentration based on K2SO4 extractions, as well as N2O production, the molar ratio of N2O (N2O : (N2O + N2)), and a handful of bacterial genes by qPCR.
The experimental setup was very similar to what we used in SLSC 802 (Special Topics) in the fall of 2010; cylindrical soil cores with gas-exchange holes in the sides were filled with soil at 1 g cm-3 bulk density and a water content of 70% and placed in 1 L canning jars with lids fitted with a perforable septum for gas sampling. One important difference between this experiment and what we have done is that in this experiment, gas measurements were of total cumulative gas production, whereas we flushed each jar with ambient air after each sampling event. Presumably this difference will have important effects on the formation of anaerobic conditions and microbial consumption of N2O previously produced under less anoxic conditions.
Not surprisingly, minimal denitrification activity was found in treatments without added NO3-. Starting NO3- concentrations were 3 mg NO3--N kg-1 soil, and fell in all treatments without added NO3- to less than 1 mg NO3--N kg-1. Once this supply of readily available nitrate was used, it appears the bacteria ceased denitrification activity, or at least it was reduced.
The red clover had a much lower C:N ratio than the barley straw, 13:1 and 45:1, respectively, and more labile carbon. This difference appears to have driven the observed difference in denitrification activity, in a manner that reflects the results of the simple-C-source experiment 1. In general, more labile C and more available N leads to stronger denitrification activity and greater production of N2O; in sealed jars such as these, strong respiration under these conditions leads to anaerobic conditions and a fall in the molar ratio of N2O as nosZ-equipped microbes consume N2O as a terminal electron acceptor.
Extractable organic carbon (EOC) was a relatively poor predictor of denitrification, compared to respiration as measured by CO2 production. EOC is a measure of the instantaneous size of the pool of labile C, while respiration represents carbon that has already passed through a microbe’s metabolism. The distinction here may be between two different pools of carbon, as well as between an instantaneous snapshot measure and a series of measurements readily convertible to an estimate of the rate of a process.
In conclusion, these authors reiterate their finding that available C and available N (especially as NO3-) are strong predictors of denitrifying activity, across a range of C and N sources. I read this paper for the class SLSC 802 in the fall of 2010, but the portions describing denitrification physiology and especially the qPCR information will be generally useful to my other projects.
These authors conducted a factorial experiment using packed soil cores to examine the influence of varying levels of available carbon and nitrogen on the process of denitrification. The treatments consisted of addition of glucose at three levels and KNO3 at four levels in experiment 1, and additions of either red clover or barley straw crop residues with or without additional KNO3. They measured soil chemistry, including extractable organic carbon and NO3- concentration based on K2SO4 extractions, as well as N2O production, the molar ratio of N2O (N2O : (N2O + N2)), and a handful of bacterial genes by qPCR.
The experimental setup was very similar to what we used in SLSC 802 (Special Topics) in the fall of 2010; cylindrical soil cores with gas-exchange holes in the sides were filled with soil at 1 g cm-3 bulk density and a water content of 70% and placed in 1 L canning jars with lids fitted with a perforable septum for gas sampling. One important difference between this experiment and what we have done is that in this experiment, gas measurements were of total cumulative gas production, whereas we flushed each jar with ambient air after each sampling event. Presumably this difference will have important effects on the formation of anaerobic conditions and microbial consumption of N2O previously produced under less anoxic conditions.
Not surprisingly, minimal denitrification activity was found in treatments without added NO3-. Starting NO3- concentrations were 3 mg NO3--N kg-1 soil, and fell in all treatments without added NO3- to less than 1 mg NO3--N kg-1. Once this supply of readily available nitrate was used, it appears the bacteria ceased denitrification activity, or at least it was reduced.
The red clover had a much lower C:N ratio than the barley straw, 13:1 and 45:1, respectively, and more labile carbon. This difference appears to have driven the observed difference in denitrification activity, in a manner that reflects the results of the simple-C-source experiment 1. In general, more labile C and more available N leads to stronger denitrification activity and greater production of N2O; in sealed jars such as these, strong respiration under these conditions leads to anaerobic conditions and a fall in the molar ratio of N2O as nosZ-equipped microbes consume N2O as a terminal electron acceptor.
Extractable organic carbon (EOC) was a relatively poor predictor of denitrification, compared to respiration as measured by CO2 production. EOC is a measure of the instantaneous size of the pool of labile C, while respiration represents carbon that has already passed through a microbe’s metabolism. The distinction here may be between two different pools of carbon, as well as between an instantaneous snapshot measure and a series of measurements readily convertible to an estimate of the rate of a process.
In conclusion, these authors reiterate their finding that available C and available N (especially as NO3-) are strong predictors of denitrifying activity, across a range of C and N sources. I read this paper for the class SLSC 802 in the fall of 2010, but the portions describing denitrification physiology and especially the qPCR information will be generally useful to my other projects.
Wednesday, September 29, 2010
Delgado et al. 2010
Delgado JA, Del Grosso SJ, Ogle SM. 2010. 15N isotopic crop residue cycling studies and modeling suggest that IPCC methodologies to assess residue contributions to N2O-N emissions should be reevaluated. Nutrient Cycling in Agroecosystems 86: 383-390.
These authors reanalyzed two recent reviews of measuring nitrous oxide emissions from agricultural systems and used a model to simulate N2O emissions and NO3 leaching associated with cropping practices in Colorado and Iowa. In general, use of crop residue instead of or in addition to synthetic fertilizers significantly altered patterns of N loss, whereas IPCC recommendations assume no difference between these N sources in regards to N2O emissions. Microbial immobilization of nitrogen, particularly associated with residues with high C/N ratios, is a major factor in these differences, and these authors provide supporting arguments for their suggestion of revisions to IPCC recommendations and modeling.
These authors reanalyzed two recent reviews of measuring nitrous oxide emissions from agricultural systems and used a model to simulate N2O emissions and NO3 leaching associated with cropping practices in Colorado and Iowa. In general, use of crop residue instead of or in addition to synthetic fertilizers significantly altered patterns of N loss, whereas IPCC recommendations assume no difference between these N sources in regards to N2O emissions. Microbial immobilization of nitrogen, particularly associated with residues with high C/N ratios, is a major factor in these differences, and these authors provide supporting arguments for their suggestion of revisions to IPCC recommendations and modeling.
Tuesday, September 28, 2010
Huang et al. 2004
Huang Y, Zou J, Zheng X, Wang Y, Xu X. 2004. Nitrous oxide emissions as influenced by amendment of plant residues with different C:N ratios. Soil Biology & Biochemistry 36: 973-981.
These authors examined the role of residue quality, in the form of C:N ratio and a range of crop residues, on N2O emissions from soils. They also measured CO2 emissions, and found strong correlations between organic-matter decomposition and respiration, and nitrogen cycling.
Gas fluxes of CO2 and N2O were highly correlated across all incubations. To ensure only respiration-derived CO2 was measured, the CO2 released by urea breakdown in urea-treated treatments was calculated and subtracted; respiration in the urea-only treatment was similar to that in the untreated controls. Both gas emissions were negatively correlated with residue C:N ratios. Finally, residue C:N ratios were negatively correlated with dissolved organic carbon concentrations.
Overall, higher C:N ratios in residues seem to result in slow decomposition of mainly recalcitrant organic matter, and low CO2 and N2O emissions. Addition of urea in conjunction with crop residues produces a range of N2O emissions depending on the C:N ratio of the residues.
This short paper may serve as a model for the work I will be doing in the special topics class in soil science, fall 2010.
These authors examined the role of residue quality, in the form of C:N ratio and a range of crop residues, on N2O emissions from soils. They also measured CO2 emissions, and found strong correlations between organic-matter decomposition and respiration, and nitrogen cycling.
Gas fluxes of CO2 and N2O were highly correlated across all incubations. To ensure only respiration-derived CO2 was measured, the CO2 released by urea breakdown in urea-treated treatments was calculated and subtracted; respiration in the urea-only treatment was similar to that in the untreated controls. Both gas emissions were negatively correlated with residue C:N ratios. Finally, residue C:N ratios were negatively correlated with dissolved organic carbon concentrations.
Overall, higher C:N ratios in residues seem to result in slow decomposition of mainly recalcitrant organic matter, and low CO2 and N2O emissions. Addition of urea in conjunction with crop residues produces a range of N2O emissions depending on the C:N ratio of the residues.
This short paper may serve as a model for the work I will be doing in the special topics class in soil science, fall 2010.
Subscribe to:
Posts (Atom)
