Litter cycle

Forest systems also act as sources of CO2 when controlled or uncontrolled burning and decay of litter occur. In addition, release of ethylene occurs during the flowering of various species. One additional form of emission to the atmosphere is the release of pollen grains. Pollen is essential to the reproductive cycle of most forest systems but becomes a human health hazard for individuals susceptible to hay fever. The contribution of sulfur from forests in the form of dimethyl sulfide is considered to be about 10-25% of the total amount released by soils and vegetation (12). 

Hg concentrations in forest soils, mosses and fungal fruiting bodies are variable, and are influenced by many factors, such as the extent of forest-based capture of atmospheric Hg deposition, transmission of Hg from the forest canopy to the litter layer whether covered with mosses or not, and type of moss and soil layer conditions and configurations. Within the fungal fruiting bodies, further alternation of the Hg cycle occurs on account of mycelia substrate preferences and Hg allocation to stalk and caps, according to developmental stage. 

In the Mixed Forest ecosystems a soil fraction less than 1 pm contains most of the elements previously confined in the forest litter and gradually involved in the biogeochemical cycle. In this fraction Cu and Mo forms account for 60-70% of the total soil content. The metals, poorly absorbable by plants, for example, Cr and V, occur in finely dispersed soil fraction in smaller amounts, about 20-30%. 

The reservoir of energy and mineral nutrients represented by litter is a very important resource in natural ecosystems with closed nutrient cycles. The growth of new green plant tissue depends on the slow release of nutrients by decomposer organisms. In agroecosystems geared for high production, litter is often removed or burned, and fertilizer is added to the soil the nutrient cycle is open and subsidized. 

In summary, it is anticipated that decreasing litter production by green plants experiencing pollutant stress would result in a similar reduction in the inventory of nutrient elements held within the system, owing to the interruption of cycling pathways and mechanisms of nutrient conservation. ... 

As shown in Table I, the plastic component of MSW has increased dramatically. Past development of synthetic plastic formulations has focused on reducing the photo, chemical, and biological degradation of the plastic polymers. However, the persistence of plastics in the environment as litter, potential marine hazard, and with concern for global carbon cycling 1,6) has focused recent attention on the recycling of plastics or development of new biodegradable plastic formulations. 

However, combining different phases of the reproductive cycle into a single study design increases the risk of an adverse event compromising the evaluation of subsequent stages, e.g., any impact on fertility of the male or female could result in too few litters for the embryo-fetal development evaluation. Under such circumstances, a separate embryo-fetal development study would have to be scheduled with the associated added cost and use of animals. Further, a separate embryo-fetal development study is recommended if the test item is poorly tolerated with repeated dosing in order to limit the duration of the administration period. 

Flaxseed consumption did not affect the pregnancy of the rats (Collins et al., 2003 Tou et al., 1998). Flaxseed (20% and 40%) and flaxseed meal (13% and 26%) diets did not affect fertility, litter size, or survival of offspring (Collins et al., 2003). However, length of gestation, anogenital distance and index, onset of puberty, and estrous cycles were affected by flaxseed consumption. 

Fauna also influence soil carbon cycling. Bioturbation mixes and aerates soil, physically breaks down litter, creates flow paths for water in soil, and can reduce surface litter stocks and enhance erosion (Bohlen et al., 2004). For example, along a gradient of European earthworm (Lumbricus terrestris) colonization in a deciduous forest of northern Michigan, earthworms are associated with a decrease in litter-layer thickness, apparently mixing some forest floor organic matter into the mineral soil. Thus, fauna can create spatial patterns in SOM stocks. 

Figure 6.7. Simplifed soil carbon cycling scheme. Major inputs (plant litter) to and outputs (respiration and erosion) from the soil carbon reservoir. The observed flux of C out of the soil can be modeled by assuming three pools of carbon an active pool with a turnover time on the order of years, an intermediate pool with a turnover time on the order of decades to centuries, and a passive pool with a turnover time on the order of millennia. The decomposition constant is k = 1/t. Subscripts a, i, and p refer to the active, intermediate, and passive C pools, respectively. Adapted with permission from Amundson, R. (2001). The carbon budget in soils. Annu. Rev. Earth Planet. Sci. 29, 535-562.

Natural Abundance Stable Carbon Isotopes (I3C). There are trends in 13C of plant, litter, and organic constituents in soil that can be used to investigate carbon cycling, but to date the trends have proven too subtle, the variation too high, or the mechanisms too poorly understood to exploit these patterns definitively. The 813C of plant litter and particulate SOM is typically close to that of the plant source,... 

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