Dead decomposed organic matter is usually added in the fields before the cultivation of crops.
Apart from providing the mineral nutrients, what additional role does organic matter play in the
cultivation of crops ?
Answers
Answer:
In-stream Decomposition Processes
Decomposition of organic matter in streams is caused by a number of interacting processes (Fig. 4), and their joint effects are usually studied by measuring loss of detrital mass over time. This is not wholly satisfactory, since weight loss [or changes in ash-free dry weight (AFDW)] does not provide direct information on the fate of this material or its uptake and assimilation by consumers. Gessner et al. (1999) propose a more organism-centered perspective on leaf—litter breakdown, which acknowledges that ‚degradation’ may begin even before leaves are shed. In tropical rainforests, many living leaves are colonized by epiphyllic algae and mosses. The surface characteristics and the chemistry of these leaves are also influenced by colonization of fungi and activity of herbivores. While parts of individual leaves may be killed when infected by pathogenic fungi, some fungi appear to maintain leaf activity in order to profit by the products of photosynthesis even after the onset of senescence of infected leaves (Butin, 1995). All these processes interfere with the quality of the leaves before they are shed and eventually reach the water. The term ‚leaching’ describes the extraction of soluble compounds by water. Leaching rates are affected by the integrity of the leaf surface, and leaching may occur during rain when senescing leaves are still attached to the tree. Once fallen leaves enter the stream, osmotic breakage of dead cell walls, penetration by fungal hyphae, and softening of the structural elements by microbial enzymes combined with feeding by invertebrate shredders enhance leaching. Leaching rates generally peak 24–48 h after immersion (Fig. 4) but some leaching continues for weeks (France et al., 1997). The leachates (sugars, amino acids, etc.) are generally energy-rich and easily absorbed by bacteria (Strauss and Lamberti, 2002). Studies in temperate waters have shown that benthic decomposition of leaves enhanced microalgal biomass in the water column, demonstrating the role of allochthonous detritus as a nutrient source for primary production (Fazi and Rossi, 2000). The importance of epilithic algae growing on submerged litter in tropical streams has not been studied, but they could provide a significant enrichment of the food value of leaves consumed by invertebrates. As Fig. 4 shows, a temporary increase of dry weight during decomposition may be caused by the growth of biofilms on the leaf surface (as well as increased endophytic microbial biomass), as has been reported for Syzygium cordatum (Myrtaceae) litter by Mathooko et al. (2000b). Laboratory studies show that light favors biofilm quality on litter and thus the growth of temperate-zone invertebrate shredders (Franken et al., 2005), and there is no reason to assume that this effect does not occur in tropical streams.