Forest Soils

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Introduction

Forests represent the 31% of the total land area and support more than 1.6 million people around the world. At the same time, forests are important because they are home of 80% of the terrestrial biodiversity which include plants, animals and microorganisms [1]. The interactions of the terrestrial biodiversity are integrated in the food web process. Those processes include the transfer of energy, community interactions and nutrient cycling. These process are also linked with abiotic factors as soil, water and climate and determine the productive characteristic of each niche.

The most common soil definition is "Soils as a media for plant growth" [2]. However, soils have several functions that determine the ecological roles in the environment:

• Recycling system for nutrients and organic waste
• Modifier if the atmosphere
• Habitat for soil organisms
• Engineering medium
• System for water supply and purification

Soil formation is a pedogenic and biogeochemical process that is determined by space and time, and depends of the translocation, biocycling and transformation of materials deposited and degraded by organisms on the uppers soil layers. These processes are developed along time and space, and have as final product the formation of soils with different physical and chemical characteristics.

• Vegetation
• Terrestrial organism interaction
• Climate.

The early soil classification made by the USDA on 1938, classified forest soils in the Zonal soil Order, as Pedalfers, that are soils with the following suborder:

• Soils of forest-grassland trnasition
• Light-colored podzolized soils of timberland regions
• Lateritic soils of forested warm-temperature and tropical regions.

The Soil Science Society of America does not have a specific definition about forest soils. However, Comerford (2006) defined forest soils as a soil that has been developed under forest cover, or a forest that has been disturbed but once was covered by forest vegetation [3].

Physical environment

Soil Formation

The physical characteristics observed in forest soils, are determined by the soil formation factors:Climate,Organisms,Parent Material, Time and Topography.

The process mentioned above, determine the type of soil that will be formed. For forest soils in particular, the principal characteristic is the development of an organic layer on the soil surface, usually referred as forest floor [3]. This particular horizon contains fresh organic materials from dead plants and animals, that are easily degraded by microorganisms, especially by bacteria and fungi. This layer is represented with the letter "O" that means organic, due to the accumulation of organic mater.As the soil depth increase, the availability of organic matter decreases, having as result a more mineral soil layer. However, the thickness of each layer varies depending of the topography, vegetation and parental material in which the forest has been established.

The transformation and translocation of organic and inorganic material by physical and chemical process along the soil profile, are the basic processes for the soil formation.

Vegetation

The most important role of the vegetation on soil, is the accumulation of organic matter. The tree leaves represent the most important source of carbon in the soil. The type of vegetation is also an important factor because it affects the soil acidity. For example, litter from conifer trees require more time to be decomposed, having as result a slow recycling of nutrients. On the other hand, litter from deciduous trees are easily decomposed due that they contains more less strong carbon bonds, that microorganisms can brake faster. As result, microorganisms release organic acids, that promote the soil formation by the desegregation of rocks. This process requires time, and at the long term, it will determine the thickness of the soil horizons and the fertility of the forest soil.

Climate

Temperature and precipitation regimes have a direct impact on the forest vegetation, decomposition rate and soil formation process.

Tropic Forest Soils

Soils formed under high temperature and precipitation rates from volcanic parent material are usually characterized for having a fragile nutrient cycle. In fact, tropical forest soils have high clay content and lower soil organic matter accumulation. A good example of these, are the Tropical Rainforest which usually have low pH and low plant nutrient due to the translocation of clay particles that carried non acid cations into the soil. Some examples of these soils are the Ultisols [|[8] and Oxisols [|[9]].

Temperate Forest Soils

Forest soils developed under temperate climate, usually have higher organic matter because they have lower temperature which slow down the decomposition rate. However, these soils are highly weathered due to the seasonal temperature and precipitation rate along the year. Alfisols and Spodosols are the most common soils observed under these conditions. These soils are characterized by the accumulation of Calcium carbonate and clay particles in deeper horizons, due to leaching nutrient from the parent material rocks and decomposition of litter which released strong organic acids that reduce the soil pH and with a CEC up to 35%.

Chemical environment

C/N ratio determine the bacteria community distribution in the soil. Carbon quality tend to be an important component of the microbial communities distribution. Johnson (2001)[|[7]] studied different forest management and their effect on the C and N soil storage. The results shown that saw-log forest removal tend to increase the C/N ratio in soil at the short term. This process is due to the rapid incorporation of small size carbon material into the soil, that allow microorganisms to decompose the carbon molecules and release the excess of nutrients to the soil. In this way, soil fertility increased until the carbon source is depleted.

Microbial communities

Fungi

The microbial community in forest soil is dominated by fungi. Due that most of the organic compounds in forest soils stayed on the soil surface and there is plenty oxygen, fungi communities are able to extend their filaments called hyphae. Fungi are more efficient than bacteria, in terms of decomposing organic compounds. In fact, 50% of the decomposed material is transformed in fungi tissue. The formation of hyphae is what improve soil fertility by the formation of humus.

Bacterias

Bacterias have an important role in the nutrient cycling. The distribution of bacterias in forest soils is mostly determined by vegetation and soil chemical characteristics. For example, the study conducted by Hackl, E.(2004), compared the bacterial communities on six forests under different pine and oak vegetation. The results shown that Gram-positive bacteria communities, especially actinomycetes, were more abundant under conifer forests than under oak coverage. This results suggest that bacterial communities are adaptive to the soil chemistry [4].

Microbial processes

What microbial processes define this environment? Describe microbial processes that are important in this habitat, adding sections/subsections as needed. Look at other topics in MicrobeWiki. Are some of these processes already described? Create links where relevant.

Decomposition

Microbial communities degrade most of the organic material that is settled on the forest soils. For these reason, most of forest soil have a fragile nutrient cycle. An example of these process can be seen when mature forest soil is disturbed, often the disturbance include, deforestation, fire and soil removal for agricultural activities. The first observation is that soil nitrate under natural conditions, does not represent a big part of the N pool, however, when is disturbed, nitrifier bacteria population increases, which increase soil fertility by the making nitrate more accessible for plants. That is one of the reason why poor farmers on the Tropics had adopt the slash and burn [5] practice to enhance soil fertility. However, at the long range the soil fertility is depleted due to the disruption of the nutrient cycle.

Current Research

Enter summaries of recent research here--at least three required

References

[Sample reference]

[1][1]

[2][Soil Genesis and classification 2003. 5th Edition. Blackwell Publishing Company]

[3]

[4][Comerford, B., 2005. Forest Soil. Encyclopedia of Soil Science, Second Edition]

[5][2]

Hackl, E., Zechmeister-Boltenstern, S., Bodrossy, L., and Sessitsch, A. "Comparison of Diversities and Composition of Bacterial Populations Inhabiting Natural Forest Soils". Appl Enviro Microbiol. 2004 September; 70(9):(5057-5065)|[4]

Johnson,D., and Curtis, D.(2001)Effects of forest management on soil C and N storage:meta analysis. Forest Ecology and Management 140: 227-238 |[7]

Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

Edited by Tito Lavaire, a student of Angela Kent at the University of Illinois at Urbana-Champaign.