Litter fall

Input rates of organic C into the soil system are hard to quantify, particularly for natural ecosystems and to a lesser extent for agricultural ecosystems. Whereas quantity and quality of carbon inputs via litter fall and plant residues after harvest might be directly measurable, inputs via roots and rhizodeposition are more difficult to assess. 

The DOC/enzyme/microbe interaction (DEMI) model divides bacterioplankton into two functional guilds, opportunists and decomposers, and DOC into two pools, labile and recalcitrant. In the context of the model, labile DOC is defined as directly assimilable monomers (saccharides, amino acids, and organic acids) and readily hydrolyzed polymers (polysaccharides, proteins, and nucleic acids). Because these substrates turn over rapidly, thus are unlikely to be transported far, most of the carbon in this pool will be autochthonous lysates and exudates, or allochthonous leachates from storms or seasonal litter fall. Recalcitrant DOC is defined as humic substances created by oxidative reactions among proteins, polysaccharides, hydrocarbons, and phenolic molecules. For inland waters, recalcitrant DOC is largely of allochthonous origin. 

Furthermore, a minimum catalytic turnover can be estimated from the above ratio C/Mg 1,000, that is, how long chlorophyll and rubisco molecules must last before there is loss of Mg to the environment besides photodegradation (bleaching, occurs with chlorophyll) or reconstitution of the molecules, both by rainwater leaching of leaves still alive (Franzle and Schimming 2008) and by litter-fall. Supplies and sinks are thus linked to the stoichiometric ratio once again which leads us to estimate it. 

Cuevas, E., and E. Medina. 1986. Nutrient dynamics within Amazonian forest ecosystems. I. Nutrient flux in fine litter fall and efficiency of nutrient utilization. Oecologia 68 466-472. 

In regions of the world where litter accumulates on the forest floor, annual decay rates can be estimated simply by measuring the litter fall (L) and the standing stock of recognizable litter (Olsen, 1963) ... 

Regina I. S. (2001) Litter fall, decomposition and nutrient release in three semi-arid forests of the Duero basin, Spain. Forestry 74(4), 347-358. 

Grace, J. R. 1986. The influence of gypsy moth on the composition and nutrient content of litter fall in a Peimsylvania oak forest. Forest Sci. 32 855-870. 

Fig. 1.3 Annual litter fall and litter transport in the Bangrong mangrove forest, Phuket, Thailand. The litter is separated into leaves, apexes, twigs, fruits and miscellaneous (misc., unidentified) material. Modified from Poovachiranon etal. (2003).

Field studies of leaf removal by sesarmide crabs in mangrove forests in Thailand have shown that the crabs can remove about 75% of the total daily litter fall and green, yellow as well as brown leaves were consumed (Fig. 1.12). It was estimated that the total population of sesarmide crabs could consume 58% of the total leaf litter per year, in this case 1130tons (Thongtham etal., 2003). A more detailed study of the economically important sesarmide crab... 

Bunt, J.S. (1995) Continental scale patterns in mangrove litter fall. Hydrobiologia, 295, 135—140. 

Wafar, S., Untwale, A.G. and Wafar, M. (1997) Litter fall and energy flux in a mangrove ecosystem. 

Table 11-5. Rales of Plant Litter Fall, Mass of Plant Litter, and Organic Soil Carbon in Different Ecosystem Types 1... 

Slightly condensed from Ajtay et al. (1979). Conversion factors for carbon in dry organic matter litter fall 0.45, litter mass 0.5. h Note 17.5 x 106 km2 is perpetual ice, lake and streams, etc., which are not included. c Dry mass. 

Dead material shed by plants is called litter. Reiners (1973) made an attempt to estimate total litter fall using four different procedures and derived values ranging from 37 to 64 Pg C/yr. The results are based largely on the estimates provided by Whittaker and Likens (1973) for the areal extent, primary productivity, and mean biomass of different ecosystems. Ajtay et al. (1979) repeated Reiner s calculations using their own data and obtained for the annual litter fall values between 37 and 49 PgC/yr. One of their detailed estimates is presented in Table 11-5. These results demonstrate that the global primary production of the biosphere of about 55 PgC/yr turns mainly into plant litter. Ajtay et al. further estimate a consumption by herbivores of about 6 Pg C/yr, including consumption by domestic livestock. This leaves only about 10% of net primary productivity to be incorporated annually into living biomass. 

F56 is the flux of carbon in litter fall, such as dead leaves. 

Under forest fallow the rate of turnover of the nutrient elements is very rapid, according to Nye and Greenland (1960), and after a few years exceeds the rate of storage in the fallow. In addition to the nutrients in the litter fall, considerable phosphorus and potassium are leached from the green leaves, and the dead wood contains considerable calcium. About five tons per acre of dry litter falls on the soil annually, and most of it decomposes rapidly. The amount of nutrients added in the rain and as dust is very small in comparison with the amounts involved in natural cycling. 

The amount of carbon present in the biosphere is estimated to be 700 Pg in the living phytomass and about 150 Pg in the dead phytomass (Bolin et al, 1979) (Table 5.2). Net primary production of the land biota is estimated to be 63 Pg C year It is also estimated that 80% of the net primary productivity may contribute to annual litter fall. The total amount of carbon in the soil is estimated... 

As plants senesce, a portion of the aboveground biomass, and in some cases all of the aboveground biomass is returned to detrital pool, where it undergoes decomposition. In emergent marshes (such as freshwater and saltwater), most of the organic matter produced is converted to detrital pool. In forested wetlands, litter fall is the primary source of detrital pool. For example, litter fall accounts... 

At the ecosystem scale, POM is generated through litter fall (in forested wetlands) and detrital production. This is the major reservoir of organic carbon above the soil surface and constitutes approximately 95% of the total carbon. The POM is broken down into simple readily utilizable organic forms, followed by simultaneous mineralization to inorganic carbon. 

There are several loss processes for mercury from this ecosystem. Terrestrial vegetation was found to play a significant role in mercury movement as evidenced by the large amount of mercury uptake found in this study (132 g). The importance of terrestrial vegetation is not fully explored since three important mercury fluxes were not measured (i) uptake from roots, (ii) volatilization from leaves, and (iii) litter fall. 

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