Microbial respiration of soil has received considerable attention because it can be used as a soil quality indicator Brendecke, J. et al 1993 (1) and it is one important variable to quantify soil microbial activity Alef, K 1995 (2). Soil respiration is considered a good estimator of overall biological activity and has been proposed as a descriptor of soil quality Doran, J.W. and T.B. Parkin. 1994 PDF (3). Soil respiration measurements are increasingly used in studies of soil C cycling to detect early changes in decomposition rate of soil organic matter in response to various soil or crop management practices Jensen et al., 1996 (4); Rochette & Angers, 1999 (5).
What is soil respiration?
Soil respiration is a measure of carbon dioxide (CO2) released from the soil from decomposition of soil organic matter (SOM) by soil microbes and respiration from plant roots and soil fauna USDA NCRS 2016 PDF (6). It depends on substrate quality and quantity, maximum activity of enzymes, demand for temperature and moisture Myrold et al. 1989 PDF (7).
Under field conditions, soil respiration is a combination of autotrophic respiration from roots and heterotrophic respiration from microbes, fungi, bacteria, etc. Root respiration is strongly linked to aboveground production and allocation over rapid timescales Lee, M.S. et al., 2003 (8), while heterotrophic respiration is strongly influenced by the amount of decomposable plant material Parkin, T.B. and Kaspar, T.C. 2003 (9).
The two sources of respiration may be largely independent and it is important to distinguish between autotrophic and heterotrophic contributions to soil respiration because only SOM consumed by heterotrophs can be stored as soil carbon and autotrophic respiration is rapidly released from the soil Edwards N.T. and Norby R.J. 1998 PDF (10).
Autotrophic respiration evolves from bacteria capable synthesizing their food from simple inorganic nutrients. It is associated with the metabolic energy expended in the synthesis of new plant tissue and in the maintenance of living tissue. We also consider respiration by mycorrhizae to be ‘autotrophic’, if they receive carbohydrate directly from the roots Hogberg, P et al. 2001 (11). Plant root respiration comes from the rhizosphere and consists principally from root respiration and from rhizomicrobial respiration using plant-derived substrates. These bacteria utilize CO2 (from atmosphere) as carbon source and derive energy either from sunlight (photoautotrophs, eg. Chromatrum. Chlorobium. Rhadopseudomonas) or from the oxidation of simple inorganic substances present in soil (chemoautotrophs eg. Nitrobacter, Nitrosomonas, Thiaobacillus) AgriInfo.in 2015 (12).
Heterotrophic respiration evolves from soil microorganisms. There are several distinct but the most important include bacteria, fungi, actinomycetes, algae, protozoa and viruses. Their relative proportions / percentages in the soil are: Bacteria-aerobic (70%), anaerobic (13 %), Actinomycetes (13%), Fungi /molds (03 %) and others (Algae Protozoa viruses) 0.2-0.8 % AgriInfo.in 2015 (12). They depend on organic compounds for their energy and the carbon required to build their biomass. Majority of soil bacteria are heterotrophic in nature and derive their carbon and energy from complex organic substances/organic matter, decaying roots and plant residues. They obtain their nitrogen from nitrates and ammonia compounds (proteins) present in soil and other nutrients from soil or from the decomposing organic matter USDE Office of Science 2009 PDK (13).
Mechanics of heterotrophic respiration:
Heterotrophs (i.e. living organisms that use carbon compounds directly from plants and other organisms) are comprised largely of bacteria. Bacteria are single cell organisms, prokaryotes, with no internal structure or nucleus, only a cell membrane (outside capsule) and (DNA) and ribosome, that produces the protein. Bacterial population is one-half of the total microbial biomass in the soil. A typical acre of soil contains 10 to 40 pounds of earthworms and 400 to 4,000 pounds of bacteria National Wildlife Federation 2011 PDF (14).
The decay of organic residues by soil organisms leads to incorporation of part of the carbon (C) into microbial tissue with the remainder being liberated as carbon dioxide (CO2). Heterotrophic respiration, therefore, involves the breakdown of simple organic carbon molecules, and the end product is CO2 and water. This process yields energy and some of this energy is used to keep the bacteria alive Nichols, K. 2012 Webinar (15).
The bacteria primarily feed on organic matter, labile carbon, that is easy to break down. When Bacteria ingest a peptide, an organic nitrogen molecule, this compound consists of 5 carbon (C) atoms and 2 nitrogen (N) atoms. The microbe sees this compound and what it really wants is the C. C is essential to energy generation. The C atom is used for two purposes: generate energy (when they do this, they breath out CO2) The other purpose for C is to build biomass (C is used for repairing cells and growth – causing the microbe to divide and reproduce). When bacteria build biomass, they need a protein, N. The average bacteria needs 4 C atoms to 1 N atom (C:N ratio of 4:1) Yarwood, S. 2015 Webinar (16).
Energy that is utilized in respiration requires a supply of organic carbon molecules, (including nitrogen and phosphorus containing molecules) There is a proportional link between the carbon and energy, and if either is limiting, microbial growth is restricted accordingly Davies, P.S. and Murdoch, F. 2014 PDF (17).
Respiration rate is a predictor of the rate of residue decomposition (biodegradation). In heterotrophic bacteria, the carbon taken up is divided between that which is broken down in respiration and that which is built up into macromolecules for growth (biomass)Davies, P.S. and Murdoch, F. 2014 PDF (17). The relationship between respiration, growth and total carbon uptake (which equals biodegradation) is shown in Figure 1 below:
By measuring the respiration rate (A), it is possible to calculate the amount of organic carbon (B) that is being used to provide energy. "A" can therefore be used to predict "B". Respiration rate (A) can also be used to predict the energy used in growth. It can be assumed that there is a proportional relationship between this energy and the amount of carbon being used in growth (C). "A" can therefore be used to predict "C" also. B + C equals the total carbon uptake which equals the rate of biodegradation. Since Respiration rate (A) predicts B and C, it can be used to predict rate of biodegradation Davies, P.S. and Murdoch, F. 2014 PDF (17).
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