Rainforest

Rainforest – Processes

 

The conceptual models were compiled by researchers in collaboration with a wide range of stakeholders from Natural Resource Management groups, universities and government agencies and based on available scientific information[1].

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• Overview
• Inputs
• Processes
• Stocks
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• Effects
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Ammonia volatilisation

Ammonia volatilisation occurs when pH is in the alkaline range (greater than 8 pH) and where concentrations of urea (derived from animals) are relatively high[12].

Biomass accumulation (vegetation)

Rainforest trees can be long-term storages of N, accumulating between 27-384 g of N per hectare of rainforest every day[11].

Mineralisation (Decomposition/Ammonification)

Decomposition may be higher through secondary forest succession and is highly regulated by ammonium[13]. Decomposition is closely associated with rates of denitrification and with forest stage (primary, succession or secondary forest), and season, with the highest rates in the wet season[13][8].

Denitrification

Denitrification in rainforests is mostly controlled by the presence or absence of oxygen, as anoxic conditions are required for this process to take place[13]. After oxygen, nitrate availability and carbon are likely sources of limitation for denitrification in rainforests[13]. Denitrification is likely to be an important process in tropical rainforests, accounting for the loss of 24-53% (39 (21-58) g/ha/day)^* of N, especially in wetter rainforests[6]. Denitrification may vary depending on successional stage, with the lowest values in mid-succession forests[13]. Denitrification rates are relatively small compared to wetlands, because soils are not always anoxic[13].

Food chain transfer

In rainforests most of the carbon and essential nutrients are generally locked up in the living vegetation, dead wood, and decaying leaves. Decaying matter (dead wood and leaf litter) is recycled efficiently by an abundance of decomposing organisms including bacteria, fungi, and termites. These organisms take up nutrients, which are released as wastes when the organisms die, and then taken up by living plants.

Nitrification

Nitrification is seasonal in rainforests in the Great Barrier Reef (GBR) catchments, with higher rates during the wet season[7]. Nitrification is highest when soil moisture is intermediate[7].

Nitrogen deposition from the atmosphere

Nitrogen can be deposited from the atmosphere to the biosphere as gas, dry deposition and precipitation. However, N deposition is only significant in areas with high industrial activity[10]. In the GBR catchments, N deposition is likely to be below 3 g/ha/day[10].

Nitrogen fixation from the atmosphere

Nitrogen (N) fixation in rainforests can occur in the roots of legumes, in shallow soils and litterfall, or in epiphytes which can have associated N fixing bacteria[5][2]. N fixation rates average 91 g/ha/day and may range from 0.4-182 g/ha/day, which is large compared to temperate forests[2][9]. Lowest N fixation values are found in mature forests, and highest in secondary forests[3].

Sedimentation

Rainforests can be erosional or accumulate sediment at a rate of 2-2.5 mm per year (554 (69-1,039) g/ha/day)^*[1]. Unlike wetlands, most of the accumulation of N is derived from locally produced litter and not from external sources[4]. The accumulated sediment can be rapidly decomposed[13][8].

^*_{Nitrogen quantities are displayed as an average followed by a minimum and maximum (range), e.g. “average (min. of range - max. of range) units”.}

References

1. ^ Bowman, DMJS, Cook, GD & Zoppi, U (2004), 'Holocene boundary dynamics of a northern Australian monsoon rainforest patch inferred from isotopic analysis of carbon, (14C and d13C) and nitrogen (d15N) in soil organic matter', Austral Ecology, vol. 29, no. 6, pp. 605-612.
2. ^ a b Farnkranz, M, Wanek, W, Richter, A, Abell, G, Rasche, F & Sessitsch, A (2008), 'Nitrogen fixation by phyllosphere bacteria associated with higher plants and their colonizing epiphytes of a tropical lowland rainforest of Costa Rica', ISME Journal, vol. 2, no. 5, pp. 561-570.
3. ^ Gehring, C, Vlek, PLG, De Souza, LAG & Denich, M (2005), 'Biological nitrogen fixation in secondary regrowth and mature rainforest of central Amazonia', Agriculture, Ecosystems and Environment, vol. 111, no. 1-4, pp. 237-252.
4. ^ Gell, P, Fluin, J, Tibby, J, Hancock, G, Harrison, J, Zawadzki, A, Haynes, D, Khanum, S, Little, F & Walsh, B (July 2009), 'Anthropogenic acceleration of sediment accretion in lowland floodplain wetlands, Murray–Darling Basin, Australia', Geomorphology. [online], vol. 108, no. 1-2, pp. 122-126. Available at: https://linkinghub.elsevier.com/retrieve/pii/S0169555X09000397 [Accessed 2 November 2020].
5. ^ Hedin, LO, Brookshire, ENJ, Menge, DNL & Barron, AR (2009), 'The nitrogen paradox in tropical forest ecosystems', Annual Review of Ecology, Evolution, and Systematics. [online], vol. 40, no. 1, pp. 613-635. Available at: http://www.annualreviews.org/doi/10.1146/annurev.ecolsys.37.091305.110246.
6. ^ Houlton, BZ, Sigman, DM & Hedin, LO (2006), 'Isotopic evidence for large gaseous nitrogen losses from tropical rainforests', PNAS, vol. 103, no. 23, pp. 8745-8750.
7. ^ a b Kiese, R, Hewett, B & Butterbach-bahl, K (2008), 'Seasonal dynamic of gross nitrification and N2O emission at two tropical rainforest sites in Queensland, Australia', Plant and Soil, vol. 309, pp. 105-117.
8. ^ a b Maggs, J (1991), 'Nitrogen mineralization and nitrification in rainforests of contrasting nutrient status and physiognomic structure near Lake Eacham, northeast Queensland', Austral Ecology, vol. 16, pp. 47-51.
9. ^ Parrota, JA, Baker, DD & Fried, M (1994), 'Application of 15N-enrichment methodologies to estimate nitrogen fixation in Casuarina equisetifolia', Canadian Journal of Forest Research, vol. 24, pp. 201-207.
10. ^ a b Phoenix, GK, Hicks, W, Cinderby, S, Kuylenstierna, J, Stock, W, Dentener, F, Giller, K, Austin, A, Lefroy, R, Gimeno, B, Ashmore, M & Ineson, P (2006), 'Atmospheric nitrogen deposition in world biodiversity hotspots: the need for a greater global perspective in assessing N deposition impacts', Global Change Biology, vol. 12, pp. 470-476.
11. ^ Preece, ND, Crowley, GM, Lawes, MJ & van Oosterzee, P (2012), 'Comparing above-ground biomass among forest types in the Wet Tropics: Small stems and plantation types matter in carbon accounting', Forest Ecology and Management, vol. 264, pp. 228-237, Elsevier B.V.
12. ^ Reddy, KR & DeLaune, RD (2008), Biogeochemistry of Wetlands, p. 774, CRC Press, Taylor and Francis Group, Boca Raton, Florida.
13. ^ a b c d e f g Robertson, GP (1989), 'Nitrification and denitrification in humid tropical ecosystems : potential controls on nitrogen retention', in Mineral nutrients in tropical forest and savannah ecosystems, ed. J. Proctor, Blackwell Scientific, Cambridge, Massachusetts, USA, pp. pp-55-69.

Last updated: 2 August 2021