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Evaluating how decomposition responds to temperature is important for understanding the soil's response under a changing climate. Our objective in this study was to evaluate the effect of temperature and addition of residue at different soil depths on decomposition of the residue and native soil organic matter (SOM) (i.e. priming). Soil samples from depths of 5–10, 20–25 and 40–45 cm were incubated at 5, 15 and 25 °C with and without 13C‐labelled barley (Hordeum vulgare L.) residue. Respiration, microbial biomass, phospholipid fatty acids (PLFAs) and mineral N were measured during the incubation. Respiration from residue and SOM increased with temperature in all treatments. The activation energy (Ea) was smaller in the control subsurface soil, indicating a more labile substrate, which resulted in greater respiration than surface soil when respiration was standardized to the amount of C. With the addition of residue, the subsurface soil respired less than the surface soil because of preferential use of residue‐C over soil organic carbon (SOC) and mineral N limitation. This resulted in negative priming of native SOC in the subsurface soil when residue was added. All PLFA biomarkers increased in the subsurface soil when incubated, even without the addition of residue, indicating a positive response of microbial activity to the optimized incubation conditions. Subsurface soil with small OC content and with small Ea had an active and efficient microbial community and relatively large rates of respiration. Highlights: The response of residue and SOM decomposition to temperature in soil with a C gradient was evaluated. We assessed residue turnover with 13C in surface and subsurface soils under varying temperatures. Activation energy was smaller in subsurface than surface soil. Temperature response of subsoil with small SOC content was similar to that of topsoil. [ABSTRACT FROM AUTHOR]

Plant roots are important sources of new organic matter in soil, but measuring their mass and composition is sometimes made difficult by adhering soil particles. We propose a technique, based on carbon in ‘pure’ root tissue, to correct mathematically for soil contamination in measurements of root mass, nutrient concentrations, and isotopic abundance. At least in some applications, the method may allow researchers to measure root yields and composition without the tedium and losses sometimes incurred during rigorous cleaning. [Copyright &y& Elsevier]

Differences in the location of residues in soil under conventional and no-tillage systems may affect microbial control of C flow through soil. Understanding the response of the microbial community to residue management in these systems may be useful in developing adaptation strategies to climate change and C sequestrations in soils. Our objective in this study was to compare the assimilation of C by the microbial community in soils where the residue was either incorporated to a depth of 10 cm or applied to the surface of soil. Labeled 13C barley residues were incorporated or surface-applied in soil under humid (Ottawa, ON, Canada) or semi-arid (Lethbridge, AB, Canada) climates. Soils were sampled in the surface layer over a two year period and crop residue C uptake by microorganisms was measured using phospholipid fatty acid 13C analysis. The results showed that soil with residues incorporated retained 13C in the biomass similarly in humid and semi-arid climates. However, when residues were left on the soil surface, C retention differed (≤50% compared to residues that were incorporated into the soil) and was more variable over time. As expected the overall residue C turnover through the microbial biomass was slower under semi-arid conditions. The effect of residue placement on community composition was site-specific with a gradual shift toward a defined community structure of the labeled biomass that reflected whether residues had been incorporated into the soil or applied to the surface two years earlier. Our findings show that climate has a greater influence on microbial utilization of residues when the residues are placed on the surface than when they are mixed into the soil. Consequently, residue C dynamics may show much higher-site-to-site variability in no-tillage systems than in conventionally tilled systems. This observation may help to explain why no-tillage favors C accumulation in some environments but not in others. [ABSTRACT FROM AUTHOR]

Earth hummocks are the most common patterned ground feature in the Subarctic region and subduction of organic matter by cryoturbation (i.e., mixing of soil layers by repeated freezing-thawing) physically protects soil organic carbon (SOC) from decomposition. Climate models predict that Subarctic regions will experience disproportionate rates of warming, which may accelerate rates of decomposition of stored SOC and thus cause increased release of CO2 . Our objectives in this study were to characterize and develop relationships between the chemistry and bioavailability of SOC in the horizons of Subarctic earth hummocks. Biodegradability was assessed in a controlled mineralization study in the laboratory. The chemical composition of soil organic matter (SOM) was characterized by X-ray absorption near-edge structure (XANES) spectroscopy at the carbon (C) K-edge and its thermal stability was determined by Rock-Eval pyrolysis. The mineralization bioassay showed that buried organic horizons were less susceptible than surface SOM to biodegradation, and not significantly more susceptible than the adjacent mineral soil. Analysis by XANES showed the accumulation of ketones in buried organic horizons, and the loss of carbohydrate, phenolic and carboxylic compounds. This suggests that ketones can be used as biomarkers for microbially transformed SOM. In contrast, SOM in perennially frozen mineral soils (i.e., below the permafrost table), was more susceptible to biodegradation than that in buried mineral and organic soils in the annually frozen active layer. The SOM in these horizons did not show ketone signals but instead showed strong phenolic content. Analysis by pyrolysis indicated that the thermolabile fraction was related to the bioavailability of C, and that in perennially frozen soils, this fraction contained proportionately higher oxygen-containing functional groups. These results point to a pool of labile SOC, relatively rich in phenolic compounds, in perennially frozen soils which may be susceptible to decomposition in a warming climate. Future warming-induced C losses, therefore, may mostly occur not from annually-frozen SOM buried by cryoturbation, but from perennially-frozen C made accessible by falling permafrost table. [ABSTRACT FROM AUTHOR]