In many parts of the biosphere the distribution of ecosystems is determined largely by the temperature regime and the availability of water. Soil fertility is a consequence of ecosystem productivity and nutrient cycling, rather than a key determinant of the distribution of vegetation types. The occurrence of complex rainforests on pure white sands is good evidence of the importance of nutrient cycling in supplying nutrients for growth, rather than direct uptake from soil. Tropical ecosystems have a greater proportion of nutrients in biomass than temperate ecosystems, and consequently tropical deforestation (felling plus burning) can result in a relatively infertile soil subject to leaching and degradation. As trees grow and their structures are renewed, plant residues, such as branches, leaves, bark, and fruits, accumulate on the forest floor, and roots die and release organic matter into soil. These organic materials serve as an energy source for the decomposer community. In forest ecosystems only about 1.5 to 5 per cent of primary production is consumed by herbivores, leaving the bulk of organic materials to be consumed by organisms living in the litter and surface soil. This interacting web of biota ranges from the relatively large earthworms and arthropods, that mix detritus between soil layers and break it in to smaller pieces, through to fungal and bacterial microbes that mineralize organic matter, releasing CO2 to the atmosphere and inorganic nutrients into the soil. Nutrients that were once bound in plant and animal structures become available again for plant uptake. This is the process of nutrient cycling that, over decades, centuries, and millennia, acts to concentrate carbon and nutrients in the forest floor and surface soil. The net result of forest growth is the accumulation of carbon and nutrients in litter and surface soil. Much of the annual demand for nutrients is met by tree internal cycling of nutrients such as withdrawal of nutrients prior to leaf fall and from nutrients released again from litter and soil organic matter. Consequently, a large proportion of the annual demand for nutrients is drawn from nutrient cycling rather than uptake from the soil. Forests live on their past accumulation of nutrients.
The process of nutrient cycling involves the crucial step whereby organic matter is decomposed by the microbial component of the soil biota that is responsible for over 90 per cent of decomposition and mineralization. As decomposition proceeds, litter is broken in to smaller pieces by soil fauna, and organic matter derived from microbes is mixed intimately with the soil mineral component in the process of humus formation. Typically the humus that is formed dominates the soil organic matter pool, and it contributes to many of the environmental values of soil. Carbon locked in the organic-mineral complexes that we call humus is resident in soil for decades, centuries, and millennia so that most organic matter in soil is old and important in carbon storage. Because it is resistant to decay, humus also contributes significantly to the formation of stable soil aggregates that increase the diffusion of water and air through soil, and it forms an ion-exchange surface that holds nutrients.
Please view the following video lecture and video for this topic.
1.3 Lecture: Forest Productivity – Nutrient Cycle
Please answer the following self-reflection questions. After formulating your answers, you may post them online at the Knowledge Café for this course as a way to share your ideas and glean knowledge from other students’ responses.
- What are the three main pathways of nutrient cycling in a forest?
- In terms of the main pathway for nutrient cycling, how do N, P, and Ca differ?
- At what stage of forest growth is a forest most susceptible to nutrient loss? Why?
Attiwill, P., & Weston, C. (2006). Soils. In P. Attiwill, & B. Wilson (Eds.), Ecology: An Australian Perspective (pp 141-160). Cambridge University Press.
Attiwill, P.M., & Leeper, G.W. (1987). “Forest Soils and Nutrient Cycles”. Carlton: Melbourne University Press.
Articles in Journals
Attiwill, P.M., & Adams, M.A. (1993). Tansley Review No. 50 Nutrient cycling in forests. New Phytologist 124, 561-582.
Cleveland, C.C. et al. (2011). Relationships among net primary productivity, nutrients and climate in tropical rain forest: a pan-tropical analysis. Ecology Letters 14, 939-947. doi: 10.1111/j.1461-0248.2011.01658.x