title: The temperature response of soil respiration from labile and stable carbon

authors: Numa, Kristyn

subjects: temperature response

abstract: The effect of temperature on the decomposition of labile and stable carbon (C) in soil is a critical factor in understanding soil C dynamics, particularly under a global warming scenario. Despite the temperature response of soil respiration being well-studied, the temperature sensitivity of the labile and stable C pools in soil remains unclear. Here, I conducted a laboratory-based incubation experiment to separate the temperature response of these two C pools. Soil was incubated at 18 discrete temperatures (~8-52°C) for five hours with and without added C substrates. After the incubation period, respiration rates were determined by measuring carbon dioxide (CO₂) production from the soils. Previous studies have attributed differences in the temperature sensitivities between C pools to substrate availability. Therefore, the labile C pool was measured as the CO₂ derived from the added C substrates (450 mM C) to the amended soil (i.e. high C availability), and the stable C pool was represented by the decomposition of soil organic matter (SOM) measured directly from soil without added C (i.e. low C availability).
I applied the macromolecular rate theory (MMRT) to respiration rates (RS) derived from labile and stable C decomposition, to describe their relationship with temperature. I found in all cases, the temperature response of the decomposition of labile C substrates (labile pool) was well fitted with MMRT (i.e. Δ𝐶𝑃‡≠ 0), resulting in clear temperature optima (Topt) and inflection (Tinf) points. While previous studies have been able to fit SOM-RS (stable pool) with MMRT, the equation in this study collapsed to the exponential-like Arrhenius equation (i.e. Δ𝐶𝑃‡ = 0), with no calculable Topt or Tinf points.
Representing the labile C pool, five simple C compounds, a substrate comprising a wide variety of C compounds (yeast extract), and a complex, long-chained glucan (dextran) were added separately to soil and incubated. The temperature response of the simple C compounds (glucose, glutamine, arginine, maltose, and lysine) and yeast extract were largely the same, with average Topt and Tinf points of 37°C and 22°C, respectively. Dextran behaved similarly to the recalcitrant SOM as it could not be fitted with MMRT; thus, no Topt and Tinf parameters were derived. This behaviour was attributed to the high complexity and molecular weight of the compound, reducing its metabolic compatibility with the soil microbes and increasing its physiochemical protection by adhering to soil surfaces. Additionally, I also determined whether the temperature response of these two pools varied with different soil properties by incubating three soils (allophanic, gley, and organic) with and without added glucose. Results suggested that between three very different soils, the temperature response of the decomposition of glucose (labile C) and SOM (stable C) was remarkedly similar, only varying by 3°C (Topt=35°C, Tinf=18-21°C).
A final preliminary experiment determining the soil priming effect was undertaken using a newly developed isotopic analysis method, where 13C labelled glucose was added to soil (allophanic). The results showed that within this study, priming enhanced the native SOM decomposition by 30% and had a similar temperature response to the labile C pool. Priming-RS was well fitted to MMRT suggesting that the C made available by priming was a labile C source for decomposers. Topt and Tinf points for priming-RS (29-31°C and 14-15°C, respectively) were slightly lower than the Topt and Tinf observed for labile C compounds. Additionally, in both relative and absolute terms, the temperature sensitivity of C decomposition induced by priming was, generally, lower than for the added C substrates and SOM. These results suggest that the C made available by priming must be more labile and physically accessible to microbes compared to the labile C compounds and SOM.
Overall, this study found that the temperature sensitivity of the stable C pool was higher in relative terms (i.e. Q10) and lower in absolute terms (i.e. first derivative) compared to the labile C pool. The temperature responses of these two pools were not the same. MMRT accurately described the temperature-respiration relationship for labile C decomposition, whereas the Arrhenius function better described this relationship for stable C. MMRT describes typical reaction rate responses from systems where biological processes dominate (in this case the biological degradation of soil C by microbes), and is a typical response of the temperature-respiration relationship when substrate supply is high in soil. The Arrhenius function, on the other hand, accurately describes chemical reaction rates, which in this case would be physiochemical processes such as, sorption/desorption and diffusion that transport the protected C to the microbes for decomposition. An Arrhenius-like behaviour is commonly exhibited when substrate supply is low for the soil microorganisms. Since the temperature response of the labile and stable C pools did not significantly differ between labile substrate types and soil properties, it could be suggested that a two-pool soil C model might be sufficient for the prediction of soil C dynamics. This two-pool model could also be extrapolated to larger ecosystem models, potentially leading to improved accuracy of climate and soil C storage projections. To determine the adequacy and appropriateness of a simple two-pool C model, further measurements of the temperature response of these two pools from a wide distribution of soils from different locations and under various conditions needs to be undertaken.

text: The temperature response of soil respiration from labile and stable carbon The effect of temperature on the decomposition of labile and stable carbon (C) in soil is a critical factor in understanding soil C dynamics, particularly under a global warming scenario. Despite the temperature response of soil respiration being well-studied, the temperature sensitivity of the labile and stable C pools in soil remains unclear. Here, I conducted a laboratory-based incubation experiment to separate the temperature response of these two C pools. Soil was incubated at 18 discrete temperatures (~8-52°C) for five hours with and without added C substrates. After the incubation period, respiration rates were determined by measuring carbon dioxide (CO₂) production from the soils. Previous studies have attributed differences in the temperature sensitivities between C pools to substrate availability. Therefore, the labile C pool was measured as the CO₂ derived from the added C substrates (450 mM C) to the amended soil (i.e. high C availability), and the stable C pool was represented by the decomposition of soil organic matter (SOM) measured directly from soil without added C (i.e. low C availability).
I applied the macromolecular rate theory (MMRT) to respiration rates (RS) derived from labile and stable C decomposition, to describe their relationship with temperature. I found in all cases, the temperature response of the decomposition of labile C substrates (labile pool) was well fitted with MMRT (i.e. Δ𝐶𝑃‡≠ 0), resulting in clear temperature optima (Topt) and inflection (Tinf) points. While previous studies have been able to fit SOM-RS (stable pool) with MMRT, the equation in this study collapsed to the exponential-like Arrhenius equation (i.e. Δ𝐶𝑃‡ = 0), with no calculable Topt or Tinf points.
Representing the labile C pool, five simple C compounds, a substrate comprising a wide variety of C compounds (yeast extract), and a complex, long-chained glucan (dextran) were added separately to soil and incubated. The temperature response of the simple C compounds (glucose, glutamine, arginine, maltose, and lysine) and yeast extract were largely the same, with average Topt and Tinf points of 37°C and 22°C, respectively. Dextran behaved similarly to the recalcitrant SOM as it could not be fitted with MMRT; thus, no Topt and Tinf parameters were derived. This behaviour was attributed to the high complexity and molecular weight of the compound, reducing its metabolic compatibility with the soil microbes and increasing its physiochemical protection by adhering to soil surfaces. Additionally, I also determined whether the temperature response of these two pools varied with different soil properties by incubating three soils (allophanic, gley, and organic) with and without added glucose. Results suggested that between three very different soils, the temperature response of the decomposition of glucose (labile C) and SOM (stable C) was remarkedly similar, only varying by 3°C (Topt=35°C, Tinf=18-21°C).
A final preliminary experiment determining the soil priming effect was undertaken using a newly developed isotopic analysis method, where 13C labelled glucose was added to soil (allophanic). The results showed that within this study, priming enhanced the native SOM decomposition by 30% and had a similar temperature response to the labile C pool. Priming-RS was well fitted to MMRT suggesting that the C made available by priming was a labile C source for decomposers. Topt and Tinf points for priming-RS (29-31°C and 14-15°C, respectively) were slightly lower than the Topt and Tinf observed for labile C compounds. Additionally, in both relative and absolute terms, the temperature sensitivity of C decomposition induced by priming was, generally, lower than for the added C substrates and SOM. These results suggest that the C made available by priming must be more labile and physically accessible to microbes compared to the labile C compounds and SOM.
Overall, this study found that the temperature sensitivity of the stable C pool was higher in relative terms (i.e. Q10) and lower in absolute terms (i.e. first derivative) compared to the labile C pool. The temperature responses of these two pools were not the same. MMRT accurately described the temperature-respiration relationship for labile C decomposition, whereas the Arrhenius function better described this relationship for stable C. MMRT describes typical reaction rate responses from systems where biological processes dominate (in this case the biological degradation of soil C by microbes), and is a typical response of the temperature-respiration relationship when substrate supply is high in soil. The Arrhenius function, on the other hand, accurately describes chemical reaction rates, which in this case would be physiochemical processes such as, sorption/desorption and diffusion that transport the protected C to the microbes for decomposition. An Arrhenius-like behaviour is commonly exhibited when substrate supply is low for the soil microorganisms. Since the temperature response of the labile and stable C pools did not significantly differ between labile substrate types and soil properties, it could be suggested that a two-pool soil C model might be sufficient for the prediction of soil C dynamics. This two-pool model could also be extrapolated to larger ecosystem models, potentially leading to improved accuracy of climate and soil C storage projections. To determine the adequacy and appropriateness of a simple two-pool C model, further measurements of the temperature response of these two pools from a wide distribution of soils from different locations and under various conditions needs to be undertaken.

year: 2020

title: Decomposing complete tripartite graphs into 5-cycles when the partite sets have similar size

authors: Billington, Elizabeth J.

subjects: graph decomposition

abstract: The problem of finding necessary and sufficient conditions to decompose a complete tripartite graph into 5-cycles was first posed at a conference in 1994 (Mahmoodian and Mirzakhani in Combinatorics Advances, 1995). Since then, many cases of the problem have been solved by various authors; however the case when the partite sets have odd and distinct sizes remains open. In this note, we introduce a new approach to the problem by embedding previously known decompositions into larger ones. Via this approach, we show that when the partite sets have asymptotically similar sizes, the conjectured necessary conditions for a decomposition are also sufficient.

text: Decomposing complete tripartite graphs into 5-cycles when the partite sets have similar size The problem of finding necessary and sufficient conditions to decompose a complete tripartite graph into 5-cycles was first posed at a conference in 1994 (Mahmoodian and Mirzakhani in Combinatorics Advances, 1995). Since then, many cases of the problem have been solved by various authors; however the case when the partite sets have odd and distinct sizes remains open. In this note, we introduce a new approach to the problem by embedding previously known decompositions into larger ones. Via this approach, we show that when the partite sets have asymptotically similar sizes, the conjectured necessary conditions for a decomposition are also sufficient.

year: 2011

title: Cultural diversity and identity discourses in the design and implementation of the New Zealand curriculum key competencies: a Bernsteinian analysis

authors: Glogowski, Sonia Theresa

subjects: Culture

abstract: The key competencies as a curriculum construct have appeared in several national curricula from the early 2000s. Many have their origins in the Definition and Selection of Key Competencies Project (DeSeCo), commissioned by the supra-national Organisation for Economic Co-operation and Development (OECD). A number of international critiques have highlighted tensions between neoliberal ideologies that underpin the key competencies, and broader socio-cultural and social justice issues within their respective countries (e.g., Crick, 2008; Sjøberg, 2016; Takayama, 2013).
This research investigated who and what influenced the design and implementation of the key competencies in the 2007 New Zealand Curriculum, and how students’ cultural diversity and identities were considered as part of this. Data for this case study were collected from three sources: interviews with key policy officials and academic consultants six years after the launch of the official curriculum; analysis of policy documents and discussion papers from government archives, and focus group interviews with teachers from two schools seven years into implementation. Bernstein’s (2000) theory of the pedagogical device, was used to focus analysis on the agents, influences, debates and events that impacted on the design and implementation of the curriculum. Fraser’s (1989) needs’ discourse theory was used to examine interpretations of cultural diversity and identity, and the implications of implicit valued attitudes and behaviours of the key competencies.
Findings indicated that in the New Zealand educational policy context, the roles and interactions between agents and processes in the fields of production and reproduction were more ‘fluid’ and consultative than Bernstein’s (2000) structuralist model would suggest. This may be reflective of New Zealand’s small population and more informal collaboration between agents in different fields. The development of this curriculum prioritised the ‘process’ of understanding curriculum through consultation and feedback, compared to previous curricula which were more focussed on curriculum as a ‘product’.
New Zealand’s socio-political climate, and its bicultural Treaty of Waitangi, appeared to impact on how the key competencies were conceived, and how student diversity and identity were interpreted at different points in the curriculum design and implementation process. Interviews with key policy officials and academic consultants revealed that cultural diversity was largely associated with ethnicity, and in particular, differences between indigenous Māori and Pākehā (New Zealand European). Less attention was given to the concept of multiple identities and intersections between ethnicity, socio economic class, gender and so on, and how these might influence interpretations of the key competencies. This was despite evidence of early discussion papers highlighting these complexities.
At the curriculum implementation level, Bernstein (2000) argues that schools and teachers can often ‘reproduce’ middle class values and expected behaviours through ‘invisible’ pedagogical practices, with little awareness of potential disjuncts for disadvantaged groups. Yet he also states that this is not necessarily deterministic, and the finding for the two school focus groups demonstrated this. Despite no evidence that student diversity was a specific aspect of their professional development on key competencies, the teachers discussed some complexities of student diversity in their schools and how the key competencies might be interpreted in different ways. However, given the case study schools were considered to be particularly ‘diverse’, this type of discussion and consideration may not be typical of other New Zealand schools.
The findings support the notion that the New Zealand curriculum key competencies were ‘weakly classified’ and ‘strongly framed’ (Bernstein, 2000). This means a high level of interpretation was located with school leaders and teachers, within the context of a permissive curriculum and thus presents both opportunities and risks.
This case study highlights the importance of agents in every field understanding the complexities and implications of diversity in education. In addition, if the key competencies genuinely seek to enhance both vocational and social justice outcomes for a more cohesive and equitable society, then education for diversity would seem to be important.
This research contributes new knowledge in relation to the process of curriculum design, highlighting the impact of a unique socio-political context on the interpretation of a curriculum construct, and the implications of ‘valued’ attitudes and behaviours of the key competencies on diverse students. It provides an example of how Bernstein’s (2000) theory of the pedagogical device can be used to identify important influences and processes that impact on the different stages of a curriculum design and implementation process. It also illustrates how the device can be usefully complemented by another theory, in this case Fraser’s (1989) needs discourse theory, to examine more closely the discourse that relates to a particular lens of the research.

text: Cultural diversity and identity discourses in the design and implementation of the New Zealand curriculum key competencies: a Bernsteinian analysis The key competencies as a curriculum construct have appeared in several national curricula from the early 2000s. Many have their origins in the Definition and Selection of Key Competencies Project (DeSeCo), commissioned by the supra-national Organisation for Economic Co-operation and Development (OECD). A number of international critiques have highlighted tensions between neoliberal ideologies that underpin the key competencies, and broader socio-cultural and social justice issues within their respective countries (e.g., Crick, 2008; Sjøberg, 2016; Takayama, 2013).
This research investigated who and what influenced the design and implementation of the key competencies in the 2007 New Zealand Curriculum, and how students’ cultural diversity and identities were considered as part of this. Data for this case study were collected from three sources: interviews with key policy officials and academic consultants six years after the launch of the official curriculum; analysis of policy documents and discussion papers from government archives, and focus group interviews with teachers from two schools seven years into implementation. Bernstein’s (2000) theory of the pedagogical device, was used to focus analysis on the agents, influences, debates and events that impacted on the design and implementation of the curriculum. Fraser’s (1989) needs’ discourse theory was used to examine interpretations of cultural diversity and identity, and the implications of implicit valued attitudes and behaviours of the key competencies.
Findings indicated that in the New Zealand educational policy context, the roles and interactions between agents and processes in the fields of production and reproduction were more ‘fluid’ and consultative than Bernstein’s (2000) structuralist model would suggest. This may be reflective of New Zealand’s small population and more informal collaboration between agents in different fields. The development of this curriculum prioritised the ‘process’ of understanding curriculum through consultation and feedback, compared to previous curricula which were more focussed on curriculum as a ‘product’.
New Zealand’s socio-political climate, and its bicultural Treaty of Waitangi, appeared to impact on how the key competencies were conceived, and how student diversity and identity were interpreted at different points in the curriculum design and implementation process. Interviews with key policy officials and academic consultants revealed that cultural diversity was largely associated with ethnicity, and in particular, differences between indigenous Māori and Pākehā (New Zealand European). Less attention was given to the concept of multiple identities and intersections between ethnicity, socio economic class, gender and so on, and how these might influence interpretations of the key competencies. This was despite evidence of early discussion papers highlighting these complexities.
At the curriculum implementation level, Bernstein (2000) argues that schools and teachers can often ‘reproduce’ middle class values and expected behaviours through ‘invisible’ pedagogical practices, with little awareness of potential disjuncts for disadvantaged groups. Yet he also states that this is not necessarily deterministic, and the finding for the two school focus groups demonstrated this. Despite no evidence that student diversity was a specific aspect of their professional development on key competencies, the teachers discussed some complexities of student diversity in their schools and how the key competencies might be interpreted in different ways. However, given the case study schools were considered to be particularly ‘diverse’, this type of discussion and consideration may not be typical of other New Zealand schools.
The findings support the notion that the New Zealand curriculum key competencies were ‘weakly classified’ and ‘strongly framed’ (Bernstein, 2000). This means a high level of interpretation was located with school leaders and teachers, within the context of a permissive curriculum and thus presents both opportunities and risks.
This case study highlights the importance of agents in every field understanding the complexities and implications of diversity in education. In addition, if the key competencies genuinely seek to enhance both vocational and social justice outcomes for a more cohesive and equitable society, then education for diversity would seem to be important.
This research contributes new knowledge in relation to the process of curriculum design, highlighting the impact of a unique socio-political context on the interpretation of a curriculum construct, and the implications of ‘valued’ attitudes and behaviours of the key competencies on diverse students. It provides an example of how Bernstein’s (2000) theory of the pedagogical device can be used to identify important influences and processes that impact on the different stages of a curriculum design and implementation process. It also illustrates how the device can be usefully complemented by another theory, in this case Fraser’s (1989) needs discourse theory, to examine more closely the discourse that relates to a particular lens of the research.

year: 2020