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.
Wednesday, September 29, 2010
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.
Trinsoutrot et al. 2000
Trinsoutrot I, Recous S, Mary B, Nicolardot B. 2000. C and N fluxes of decomposing 13C and 15N Brassica napus L.: effects of residue composition and N content. Soil Biology and Biochemistry 32: 1717-1730.
These authors studied the decomposition process by soil microorganisms when isotope-labelled crop residues were added to soil. The crop used, oilseed rape Brassica napus (also known as canola) varies its nitrogen content of tissues, and the C:N ratio, depending on levels of N inputs by fertilization. This allows variation in input organic matter quality by manipulation of growing conditions; in this experiment, both carbon and nitrogen inputs to the plant included stable-isotope labels, in the form of 13C-CO2 and 15N-KNO3. Plant residues were added to soils and incubated for 168 days.
Initial C:N ratio and especially the labile-C fraction of organic-matter inputs are major controls of both the rate of decomposition and fate of matter through the system. Additionally, temperature, particle size of residues, and water content in the soil also strongly influence decomposition processes.
Here, N mineralization (the formation of NO3- and NH4+ pools in the soil from organic-N precursors) occurred in two phases. In the early phase, up to about 3 weeks, the N cycle resulted in net mineralization. Later, mineral N pools were depleted and N was immobilized, that is, incorporated into the tissues of microbial cells.
Carbon dioxide release during the experiment occurred through two pathways. The more direct route was rapid mineralization of organic matter, which I interpret as non-incorporation of organic matter by microbes, consuming such material but metabolizing it rapidly through respiration. The second, presumably slower route was through metabolization of material after incorporation into cells through respiration. Either way, the ultimate fate of much of the organic-C in the residues was release as CO2.
Differences in the N-content of residues affected decomposition rates early in the experiment, but by about 4 months the differences between high-N and low-N residues had evened out. Only a small fraction of labelled N from residues ended up in soil mineral-N pools; the majority was either immobilized into microbial cells or remained in recalcitrant organic matter fractions. Immobilization of unlabeled, SOM-derived N was enhanced by the addition of C through a substitution effect.
These authors conclude that 15N labelling was fraught with difficulties, and both under- and overestimated some pools and processes. However, the use of their model, named NCSOIL, improved their ability to trace the fate of added material through the system. This paper represents a study similar in some ways to our planned course activity in the special topics in soil science course, fall 2010.
These authors studied the decomposition process by soil microorganisms when isotope-labelled crop residues were added to soil. The crop used, oilseed rape Brassica napus (also known as canola) varies its nitrogen content of tissues, and the C:N ratio, depending on levels of N inputs by fertilization. This allows variation in input organic matter quality by manipulation of growing conditions; in this experiment, both carbon and nitrogen inputs to the plant included stable-isotope labels, in the form of 13C-CO2 and 15N-KNO3. Plant residues were added to soils and incubated for 168 days.
Initial C:N ratio and especially the labile-C fraction of organic-matter inputs are major controls of both the rate of decomposition and fate of matter through the system. Additionally, temperature, particle size of residues, and water content in the soil also strongly influence decomposition processes.
Here, N mineralization (the formation of NO3- and NH4+ pools in the soil from organic-N precursors) occurred in two phases. In the early phase, up to about 3 weeks, the N cycle resulted in net mineralization. Later, mineral N pools were depleted and N was immobilized, that is, incorporated into the tissues of microbial cells.
Carbon dioxide release during the experiment occurred through two pathways. The more direct route was rapid mineralization of organic matter, which I interpret as non-incorporation of organic matter by microbes, consuming such material but metabolizing it rapidly through respiration. The second, presumably slower route was through metabolization of material after incorporation into cells through respiration. Either way, the ultimate fate of much of the organic-C in the residues was release as CO2.
Differences in the N-content of residues affected decomposition rates early in the experiment, but by about 4 months the differences between high-N and low-N residues had evened out. Only a small fraction of labelled N from residues ended up in soil mineral-N pools; the majority was either immobilized into microbial cells or remained in recalcitrant organic matter fractions. Immobilization of unlabeled, SOM-derived N was enhanced by the addition of C through a substitution effect.
These authors conclude that 15N labelling was fraught with difficulties, and both under- and overestimated some pools and processes. However, the use of their model, named NCSOIL, improved their ability to trace the fate of added material through the system. This paper represents a study similar in some ways to our planned course activity in the special topics in soil science course, fall 2010.
Monday, September 27, 2010
Smith et al. 2003
Smith KA, Ball T, Conen F, Dobbie KE, Massheder J, Rey A. 2003. Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes. European Journal of Soil Science 54:779-791.
These authors present a broad review paper of the role of soil physical factors, mainly temperature and water-filled-pore-space, in controlling soil emissions of the greenhouse gases CO2, CH4, and N2O. The paper’s goal is stated to be to expose a variety of researchers to the links between soil physics and soil biology, as well as the importance of these fields to current research in many disciplines on global warming.
All three gases are produced and consumed in soil primarily by microorganisms, which respond to variation in soil physical parameters in different ways. In general, both temperature and WFPS impact GHG production. Higher temperatures almost always result in increased production of gases, though the Q10 values (measuring the magnitude of response to a change of 10ยบ of temperature) vary widely in the literature for all three gases. The effect of WFPS is different, involving upper and lower bounds, though in the middle range increasing WFPS generally promotes increased gas production. Microbes are limited in their tolerance of dry conditions, such that gas production falls rapidly below some critical WFPS value; for CO2 this threshold is near 20%. WFPS is also indirectly important, through its effects on soil diffusivity. Higher WFPS as well as higher bulk density are associated with lessened CH4 oxidation, due to reduced diffusivity of both CH4 and atmospheric O2. Very high WFPS values are associated with reduction of N2O to N2, partly by limiting O2 supplies and creating larger anaerobic microsites, and partly by preventing the escape of N2O gas into rapid-diffusion pathways; it is trapped in the vicinity of microbes capable of using it as an electron acceptor.
There are other factors controlling net GHG emissions, such as the relationship between plant productivity and water table position, which will change the relationship between rates of soil organic matter oxidation to CO2 and the removal of CO2 from the atmosphere by plants; trees in particular can lower local water tables, increasing SOM oxidation while simultaneously consuming more CO2 than the previous wetland vegetation community.
I read this paper on the suggestion of my coworkers in the special topics class of fall 2010, but it applies well to the general area of my research. The reference list includes multiple interesting papers addressing particular specialties within this large topic.
These authors present a broad review paper of the role of soil physical factors, mainly temperature and water-filled-pore-space, in controlling soil emissions of the greenhouse gases CO2, CH4, and N2O. The paper’s goal is stated to be to expose a variety of researchers to the links between soil physics and soil biology, as well as the importance of these fields to current research in many disciplines on global warming.
All three gases are produced and consumed in soil primarily by microorganisms, which respond to variation in soil physical parameters in different ways. In general, both temperature and WFPS impact GHG production. Higher temperatures almost always result in increased production of gases, though the Q10 values (measuring the magnitude of response to a change of 10ยบ of temperature) vary widely in the literature for all three gases. The effect of WFPS is different, involving upper and lower bounds, though in the middle range increasing WFPS generally promotes increased gas production. Microbes are limited in their tolerance of dry conditions, such that gas production falls rapidly below some critical WFPS value; for CO2 this threshold is near 20%. WFPS is also indirectly important, through its effects on soil diffusivity. Higher WFPS as well as higher bulk density are associated with lessened CH4 oxidation, due to reduced diffusivity of both CH4 and atmospheric O2. Very high WFPS values are associated with reduction of N2O to N2, partly by limiting O2 supplies and creating larger anaerobic microsites, and partly by preventing the escape of N2O gas into rapid-diffusion pathways; it is trapped in the vicinity of microbes capable of using it as an electron acceptor.
There are other factors controlling net GHG emissions, such as the relationship between plant productivity and water table position, which will change the relationship between rates of soil organic matter oxidation to CO2 and the removal of CO2 from the atmosphere by plants; trees in particular can lower local water tables, increasing SOM oxidation while simultaneously consuming more CO2 than the previous wetland vegetation community.
I read this paper on the suggestion of my coworkers in the special topics class of fall 2010, but it applies well to the general area of my research. The reference list includes multiple interesting papers addressing particular specialties within this large topic.
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