Soil C turnover

In terms of improving our ability to predict soil C turnover, we identify five priorities for research (1) The interactive effects of temperature and moisture on microbial decomposition rates, because soils will experience novel and transient conditions (2) the mechanisms governing protection of OM through interactions with mineral surfaces and due to spatial structure (3) the mechanisms leading to slower OM turnover times with depth (4) the potential for nonlinear responses of decomposition to C availability—for example, the role of labile C inputs in stimulating decomposition of less labile OM (i.e., priming) and density-dependent microbial behavior and (5) how the chemical characteristics of organic compounds, as inputs from different plant species, charred (black) carbon, or microbial cell walls and by-products, influence mechanisms of stabilization and turnover. 

Little is known on the catalysis of the Maillard reaction and especially the integrated polyphenol-Maillard reaction by natural soils and sediments. Further work is warranted on this subject matter to advance our understanding of the role of abiotic catalysis in the formation of humic substances and related C turnover and N transformations in the environment. 

The turnover time (x) of a reservoir is its mixing or refresh rate, and is the time it would take for the reservoir to completely empty if there were no further inputs. For soils, it is a measure of the first-order kinetics for decay (x = Ilk). At steady state, it is calculated as the inventory divided by the total inputs (or total outputs) to the reservoir. To calculate the turnover time for a soil C reservoir at steady state, we would divide the mass of SOM (C) by the total carbon fluxes (.S ) from the reservoir or x = C/S. Fluxes would include decomposition to C02 and leaching of dissolved organic. 

No single satisfactory method yet exists by which to separate soil C from the complex soil matrix into discrete components with different turnover times. Instead,... 

Plant productivity is determined by factors such as plant species composition, moisture, soil fertility, growing season length, and solar radiation—many of which are affected by human activities. All else equal, increases in primary productivity and production of plant tissues will lead to increases in soil C stock, while decreases will lead to decreases in soil C stock. The rate of change in soil C stock is determined by the difference between C inputs and outputs, as well as the turnover times of the soil C, which are often not known. Here we review briefly how some environmental factors are expected to alter productivity and explore how the effects on stock depend on the number of soil carbon pools and their turnover times. 

Six, J., Elliott, E. T., and Paustian, K. (2000a). Soil macroaggregate turnover and microaggregate formation A mechanism for C sequestration under no-tillage agriculture. Soil Biol. Biochem. 32(14), 2099-2103. 

These changes in soil C stocks are relatively rapid, due to rapid turnover of soil organic matter (SOM) in these tropical soils. Using radiocarbon derived from atmospheric testing of nuclear weapons in the 1960s as a tracer, Trumbore et al. (1995) estimated that the mean residence time of C in the top 10 cm of soil is about 3 years for 30% of the SOM and 10-30 years for 60% of the SOM. Only about 10% of the C in the top 10 cm of soil cycles on a millennial time scale. This very old C fraction increases to 40-80% in the... 

The surfaces of Mn oxides promote the oxidative polymerization of many polyphenolics, the polycondensation of pyrogallol and glycine, and the formation of humic substances. Many organics are oxidatively decomposed by Mn oxides during the reduction of Mn(III) or Mn(IV). The role of Mn oxides in C turnover and N transformations should, thus, be studied in depth. Besides natural organics, the kinetics of the degradation of certain xenobiotics by Mn oxides and oxyhydroxides in soils and related environments warrant investigation. 

Modeling experiments allowed us to control for factors that might cause variation in field-based estimates of woody plant age-SOC relationships. Model estimates of SOC accumulation were comparable to field estimates for upland patch types and substantially lower than field estimates for lowland patch types (Table 4). Model estimates of soil N accumulation were substantially lower than field estimates, especially in lowlands. Given that woody patch age explained only 26-68% of the variance in soil C and N content, our field estimates of accumulation rates cannot be taken as definitive. Model results underestimated field observations, especially for N. Reliability of model estimates of soil carbon could likely be improved with a better understanding of how turnover of the substantial root mass (Table 2) might differ among patch types. Model estimates of soil N are likely constrained by lack of information on inputs associated with N, fixation, atmospheric N deposition, translocation between uplands and lowlands, and root turnover. 

Any positive effects of Ca on tree gixwth, as suggested by the correlation between Ca supply and forest growth, cannot thus be direct, but can potentially occur in a longer perspective, provided increases in soil pH increase soil N turnover. However, in typical boreal forest soils with a C/N ratio > 30 in the inor-layer, the reverse, i.e., increased N immobilization, commonly occurs after liming (Persson and Wiren, 1996) and is most likely the reason why forest growth often declines over a period of several decades after liming (Fig. 5 Derome et ciL, 1986). 

Bmun, E.W., Ambus, P., Egsgaard, H., Hauggaard-Nielsen, H., 2012. Effects of slow and fast pyrolysis biochar on soil C and N turnover dynamics. Soil Biology and Biochemistry 46, 73—79. http //dx.doi.0rg/lO.lOl6/j.soilbio.2Oll.ll.Oi9. 

J. Swinnen, J. A. Van Veen, and R. Merckx, Root decay and turnover of rhizodepos-its in field-grown winter wheat and spring barley estimated by C pulse labelling. Soil Biology and Biochemistry 27 211 (1995). 

Several authors have applied in situ pulse labeling of plants (grasses and crops) with C-CO2 under field conditions with the objective of quantifying the gross annual fluxes of carbon (net assimilation, shoot and root turnover, and decomposition) in production grasslands and so assess the net input of carbon (total input minus root respiration minus microbial respiration on the basis of rhizodeposition and soil organic matter) and carbon fixation in soil under ambient climatic conditions in the field. 

J. N. Ladd, M. Amato, P. R. Grace, and J. A. Van Veen, Simulation of turnover through the microbial biomas.s in soils incubated with C-labelled residues. Soil Biol. Biochem. 27111 (1995). 

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