The microbial biomass accounts for only 1-3 % of soil organic C but it is the eye of the needle through which all organic material that enters the soil must pass.
— Jenkinson, 1977
Photo by fiulo/iStock / Getty Images
Photo by fiulo/iStock / Getty Images

Soil quality is the capacity of soil to maintain some key ecological functions, such as decomposition and formation of soil organic matter (SOM) Doran, J.W. et al. 1996 PDF (1). Carbon in the soil is in the form of organic matter and provides energy, either directly or indirectly, to all heterotrophs (i.e. living organisms that use carbon compounds directly from plants and other organisms). Soil carbon is produced by autotrophic organisms, such as plants that can fix carbon from the atmosphere by using energy from the sun. The carbon compounds produced by autotrophs eventually become part of a vast warehouse of energy and protein known as soil organic matter. This warehouse functions beneficially in hundreds of different ways, but one essential purpose is to provide energy to soil life Flavel, T.C et al. 2006 (2).

Role of SOM in promoting microbial biomass:

C in the soil is found in the form of organic matter and provides this energy, either directly or indirectly. Soil carbon is produced by autotrophic organisms, such as plants that can fix carbon from the atmosphere by using energy from the sun. The carbon compounds produced by autotrophs eventually become part of a vast warehouse of energy and protein, known as soil organic matter. This warehouse functions beneficially in hundreds of different ways, but one essential purpose is to provide energy to soil life Nichols, K. 2012 Webinar (3).

Substrate supply strongly controls soil respiration (Figure 1), but other environmental factors, such as soil moisture, oxygen supply, and the below ground community are also important. For example, heterotrophic CO2 respiration depends on the availability of soluble, labile C sources, and microbial respiration rates are tightly linked to the chemistry and amount of organic matter entering the soil Gu, L et al. 2004 (4).

  Figure 1.  Conceptual model of the components and responses of CO2 efflux from soil. Both the autotrophic and heterotrophic components of soil respiration are strongly controlled by substrate availability – phloem transport of carbohydrate supply for root and mycorrhizal respiration, and dead organic material for microbial respiration   Gu, L et al. 2004  (4) .

Figure 1. Conceptual model of the components and responses of CO2 efflux from soil. Both the autotrophic and heterotrophic components of soil respiration are strongly controlled by substrate availability – phloem transport of carbohydrate supply for root and mycorrhizal respiration, and dead organic material for microbial respiration Gu, L et al. 2004 (4).

SOM plays a key role in the nutrient cycling. Soil temperature, moisture, pore structure and the proportion of carbon to nitrogen present in organic matter (C:N ratio) are the major controlling factors Nichols, K. 2012 Webinar (3).

C:N ratio:

Microbes require both carbon and nitrogen for food, and while carbon is readily available in plant biomass, nitrogen (in its usable organic forms) tends to be limiting. Therefore, in general, the rate of biomass decomposition can be linked to the quantity of nitrogen available relative to the amount of carbon (the carbon to nitrogen, C:N, ratio). Woody biomass with a high C:N ratio can take a long time to degrade because microbes must find nitrogen from their surrounding environment, rather than directly from the biomass itself, to enable the degradation. Low C:N ratio biomass, such as manure, degrades quickly because there is enough available nitrogen within manure for microbial growth during decomposition; this is why manure makes such a good fertilizer Microbiology Society Charles Darwin House. 2016 (5).

Labile Carbon:

The labile (bio-available) carbon is influenced by ‘new’ organic matter (originating from plants and/or animals) and has a significant role in microbial nitrogen turnover and supply. Since labile carbon turns over rapidly, it is considered a more sensitive indicator of changes in soil quality and function than the percentage of total carbon which includes the more inert fractions Hoyle, F et al. 2008 (6).

Labile carbon influences both the activity and mass of microorganisms (microbial biomass) in soil. The microorganism’s capacity to release plant-available N is influenced by the quality of organic matter inputs, with net release of nitrogen from the labile soil organic matter occurring at a C:N ratio below about 22:1Hoyle, F et al. 2008 (6).

High inputs of more recalcitrant residues can increase the ratio of carbon to nitrogen in this labile fraction and can result in net immobilization of nitrogen, making it unavailable for plant uptake Hoyle, F et al. 2008 (6).

Root exudate and rhizosphere priming effect (RPE):

Carbon input from plant to soil through root exudation is one of the major sources of available C for microorganisms Luo et al. 201 PDF (7). Exudates from living roots stimulate a quick response of soil microbes with acceleration of native soil organic C mineralization, the so-called rhizosphere priming effect (RPE). Soil microbial activities are driven primarily by readily available or labile C provided by root turnover and root exudates influxes Dijkstra et al. 2013 PDF (8).

Effect of moisture and water-filled pore space (WFPS) on soil CO2 respiration:

Soil respiration increases with higher moisture levels until pores are filled with too much water. This limits oxygen availability which interferes with soil organism’s ability to respire (Figure 2). Ideal soil moisture is near field capacity, or when approximately 60 percent of pore space is filled with water. Respiration declines in dry soils due to the lack of moisture for microbes and other biological activity Linn, D. M. and Doran, J.W. 1984 (9).

  Figure 2.  Relative aerobic (respiration, ammonification, nitrification) and anaerobic microbial activity (denitrification) as related to soil WFPS   Linn, D. M. and Doran, J.W. 1984   and   Parkin, T.B   et al., 1996  .

Figure 2. Relative aerobic (respiration, ammonification, nitrification) and anaerobic microbial activity (denitrification) as related to soil WFPS Linn, D. M. and Doran, J.W. 1984 and Parkin, T.B et al., 1996.

As soil WFPS enters the range of 62-78%, soil respiration starts to decline. At the 80% level, water displaces air and restricts oxygen diffusion, reducing aerobic microbial activity. At the same time, anaerobic respiration increases using nitrate (NO3), instead of oxygen. This results in loss of nitrogen, (as nitrogen gases N2 and nitrogen oxides), emission of potent greenhouse gases, yield reduction, and increased N fertilizer expense Linn, D. M. and Doran, J.W. 1984 PDF (9).

Soil texture directly effects WFPS and soil CO2 respiration:

Medium textured soils (silt and loam soils) are often favorable to soil respiration because of their good aeration, and high available water capacity USDA NCRS 2013 PDF (10)

In clay soils, a sizeable amount of SOM is protected from decomposition by clay particles and other aggregates limiting soil respiration and associated mineralization (ammonification) of organic N USDA NCRS 2013 PDF (10).

Sandy soils are low in SOM and have low available water capacity limiting soil respiration and N mineralization USDA NCRS 2013 PDF (10).


References:

  1. Doran, J.W.; Sarrantonio, M.; Liebig, 1996. M.A. Soil health and sustainability. In Advances in Agronomy. Academic Press: San Diego, CA, USA, 56, pp. 25-37.
  2. Flavel, T.C., Murphy, D. V. 2006. Carbon and Nitrogen Mineralization Rates after Application of Organic Amendments to Soil. J. Environ. Qual. 35:183–193 (2006). Technical Reports: Waste Management.
  3. Nichols, K. USDA NRCS. 2012. Webinar: Role of Soil Biology in Improving Soil Quality. Mississippi River Basin Healthy Watersheds Initiative Webinar.
  4. Gu, L., Post, W.M., King, A.W. 2004. Fast labile carbon turnover obscures sensitivity of heterotrophic respiration from soil to temperature: A model analysis. American Geophysical Union. Global Biogeochemical Cycles, Vol. 18, GB1022.
  5. Microbiology Society Charles Darwin House. February, 2016 Issue. Microbial modulation of soil ecosystem processes. Microbiology Today.
  6. Hoyle, F., Murphy, D., Sheppard, J. 2008. Fact Sheets: Labile Carbon. SoilQuality.org.au. Department of Agriculture and Food Western Australia, The University of Western Australia, Avon Catchment Council.
  7. Luo, Y., Xueyong, Z., Olof, A., Yangchun, Z., Wenda, H. 2014. Artificial root exudates and soil organic carbon mineralization in a degraded sandy grassland in northern China. J. Arid Land. 6, 423–431.
  8. Dijkstra, F.A., Carrillo, Y., Pendall, E., Morgan, J.A. 2013. Rhizosphere priming: a nutrient perspective. Frontiers in Microbiology. 4, 216.
  9. Linn, D. M. and Doran, J.W. 1984. Effect of Water-Filled Pore Space on Carbon Dioxide and Nitrous Oxide Production inTilled and Nontilled Soils. Soil Sci. Am. J., Vol. 48, 1984.
  10. USDA NCRS 2013 Soil Respiration. Soil Quality Kit. Guides for Educators.