International Day of Forests – Forest Soils

A bird's eye view of a a forest pathway- Credit UAVisonary

It is difficult to overestimate the value of forests and the numerous benefits they provide to people all over the world. They provide a wide range of ecosystem services, including timber production; vast net primary productivity, which is a central ecological function; carbon, organic matter, water, and pollutant regulation; and even pure intrinsic and cultural values (Brandt et al, 2014).

Forest ecosystems also support enormous amounts of biodiversity, with nearly every phylum of living things represented in its soils alone (Weil and Brady, 2017). Microbes are particularly abundant in soils, where one gram is estimated to hold up to 1010 – 1011 bacteria (Horner-Devine et al, 2003).

Just as we cannot survive without forests, forests could not function without the vital soil foundations on which they stand. These soil foundations account for half of the total land area of the planet (Wilde, 1947), and not only do they provide anchorage, water and nutrients for the trees; but they also regulate critical ecosystem processes, such as nutrient uptake, which is critical to the overall productivity of the woodland ecosystem (FAO, 2015). Given the numerous benefits that forest soils provide to humanity, they can be considered imperative to the functioning of our planet. Combine this with soils irreplaceable nature of the soil, and you have the reason why it is critical to study and conserve soil, as well as understand the processes that regulate activity within forest soils.

A bird's eye view of a a forest pathway- Credit UAVisonary
Photo credit: UAVisionary

Plant – Soil Interactions

Plants and vegetative inputs, which rely on soil conditions for growth, have a strong influence on soil conditions. This close interaction between soil and its vegetation is known as a plant-soil feedback system (Miki, 2012). Vegetation cover can influence important physical and chemical soil properties such as nutrient availability and pH, which can serve as useful soil quality indicators. Soil organic matter (SOM) has the potential to be the most important soil quality indicator because it is directly related to a variety of other soil properties. SOM is primarily formed by litterfall from the forest vegetation, which is then processed by microbes and their enzymes, converting the nitrogen in the organic matter to its inorganic forms, allowing the nitrogen to be available for plant uptake (Gilmour, 2011). This results in an equilibrium that is influenced by both land use and vegetation type.

Soil – Microorganism Interactions

Because of their ability to support an abundance of microorganisms, forest soils are extremely effective at stabilising and driving the fertility of the entire ecosystem. Not only do the microbes influence the cycling of nutrients, but they also influence soil structure (Lucas-Borja et al, 2016). In addition to leaf litter and vegetative inputs influencing the soil microbial community; soil characteristics such as pH, soil depth and soil moisture all have an influence on how effective the microbes can do their jobs.

Most of the soil nitrogen (95-99%) exists in an organic form which is inaccessible to  plants. It is the role of microbes to not only break down these large organic molecules into smaller compounds, but also to secrete enzymes which release ammonium ions which can then be oxidised to form nitrate ions, which plants can then use. Often the enzymes that carry out this process are excreted by the microbes, and soil acts as the ideal solution for the enzymes to carry out this work (Weil and Brady, 2017). Nitrogen mineralisation (NM) refers to the process of converting organic nitrogen in SOM to inorganic nitrogen (Gilmour, 2011).

Soil – Microorganism Interactions

Difference Between Coniferous and Deciduous Forest Soils

Research indicates conifers and broadleaves have distinctly different biogeochemical signatures on soil carbon stock, carbon:nitrogen ratio and pH, particularly in the forest floor and top mineral soil layer (Dawud et al, 2016). Tree species differ in terms of productivity, canopy structure, litter quality and quantity, and thus affect soil properties. Previous research has shown that tree species can have a significant impact on soil temperature and moisture, soil fertility, and microbial communities (Li, et al, 2017).

Ecologists have also proposed that deciduous trees increase soil fertility, while evergreens decrease soil fertility, which explains why evergreens can thrive on infertile soils (Mueller et al, 2012). The hypothesis, however, assumes deciduous tree soils have higher nitrogen mineralisation rates than evergreen soils, despite the fact that solid evidence on plant functional type effects on nitrogen soil cycling is limited (Mueller et al, 2012). This assumption is based on higher nitrogen concentrations in deciduous leaf litter and faster rates of deciduous litter decomposition (which is driven by microbial activity). Even if these differences between deciduous and evergreen soils are present, soil nitrogen availability is influenced by a wide range of plant traits, biogeochemical processes, and environmental factors; not just by dominant plant group (Mueller et al, 2012).

More research into the mechanisms underlying the difference between coniferous and deciduous soils would be beneficial in understanding why disparities exist between different plant-soil feedback systems. This research is critical because it has the potential to improve current ecosystem models and assist people in making better informed land management decisions in the future, thereby reducing human impact on forest soil quality; an irreplaceable natural resource.

References

Brandt, P., Abson, D.J., DellaSala, D.A., Feller, R., von Wehrden, H. (2014). Multifunctionality and biodiversity: Ecosystem services in temperate rainforests of the Pacific Northwest, USA. Biological Conservation. 169, 362-371.

Dawud, S.M., Raulund-Rasmussen, K., Domisch, T., Finér, L., Jaroszewicz, B. and Vesterdal, L. (2016). Is Tree Species Diversity or Species Identity the More Important Driver of Soil Carbon Stocks, C/N Ratio, and pH?. Ecosystems. 19 (4), 645–660.

Food and Agricultural Organisation of the United Nations. (2015). Forests and forest soils: an essential contribution to agricultural production and 38 global food security. Available: http://www.fao.org/soils-2015/news/news-detail/en/c/285569/. Last accessed 10th Oct 2017.

Gilmour, J. (2011). Soil testing and nitrogen mineralization from soil organic matter. Available: https://www.soils.org/files/certifications/certified/education/self-study/exam-pdfs/295.pdf. Last accessed 25th Oct 2017.

Horner-Devine, M.C., Leibold, M.A., Smith, V.H., Bohannan, B.J.M. (2003). Bacterial diversity patterns along a gradient of primary productivity. Ecological Letters. 6 (7), 613–622.

Miki, T. (2012). Microbe-mediated plant–soil feedback and its roles in a changing world. Ecological Research. 27 (3), 509–520.

Li, W., Bai, Z., Jin, C., Zhang, X., Guan, D., Wang, A., Yuan, F. and Wu, J. (2017). The influence of tree species on small scale spatial heterogeneity of soil respiration in a temperate mixed forest. Science of The Total Environment. 590-591, 242-248.

Lucas-Borja, M.E., Hedo, J., Cerdá, A., Candel-Pérez, D., Viñegla, B. (2016). Unravelling the importance of forest age stand and forest structure driving microbiological soil properties, enzymatic activities and soil nutrients content in Mediterranean Spanish black pine (Pinus nigra Ar. ssp. salzmannii) Forest. Science of The Total Environment. 562, 145-154.

Mueller, K.E., Hobbie, S.E., Oleksyn, J., Reich, P.B. and Eissenstat, D.M. (2012). Do evergreen and deciduous trees have different effects on net N mineralization in soil?. Ecology. 93 (6), 1463-1472.

Weil, R.R. and Brady, N.C (2017). The Nature and Properties of Soils. 15th ed. Harlow, Essex: Pearson Education. 220.

Wilde, S.A. (1947). Forest Soils and Forest Growth. The Scientific Monthly. 64 (3), 271-272.

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