Organic Matter Decomposition and Nutrient Release

Iron and manganese reduction in wetland soils and aqnatic sediments is linked to organic matter decomposition. In the absence of oxygen, microbial commnnities nse a wide range of electron acceptors, inclnding nitrate, Mn(IV), Fe(III), sulfate, carbon dioxide, and several simple organic componnds. 

FIGURE 10.30 Distribution of select oxidants and reductants in the water column of Black Sea. (Redrawn from Nealson and Myers, 1992.) 

FIGURE 10.31 Relationship between accumulation of Fe(II) and Mn(II) and ammonium production, showing the relationship between bacterial reduction of electron acceptors and organic nitrogen mineralization. 

A highly significant, positive relationship has been shown between iron oxide reduction and organic nitrogen mineralization in tropical wetland soils (Sahrawat, 2004). The following empirical relationships were reported  

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McLatchey, G. P., and K. R. Reddy. 1998. Regulation of organic matter decomposition and nutrient release in a wetland soil. Journal of Environmental Quality 27 1268-1274. 

Production of roots on top of the mineral soil has been explained as a consequence of the low nutrient availability in Amazon forests (Herrera et al. 1978, Cuevas and Medina 1983, Medina and Cuevas 1989). Vertical root distribution results from differential nutrient availability in the soil profile (Berish 1982, Berish and Ewel 1988). Shallow rooted systems may be a result of litter and soil organic matter production and decomposition rates in systems where nutrient input from litter exceeds that of nutrient release by soil weathering, as is the case of Ca, Mg, and P in terra firme forests (Medina and Cuevas 1989). In the Middle Caqueta region of Colombia, for example, Ca and Mg concentrations in the L and F layers are between 15 and 20 times higher than in the mineral soil (Duivenvoorden and Lips 1995). 

Oxidation-reduction reactions of iron and manganese are also involved in nutrient release in flooded soil and sediments. Fe(III) and Mn(IV) serve as electron acceptors for organic matter decomposition or turnover. Organic nitrogen mineralization results in the release of nutrients such as ammonium nitrogen. Iron reduction is also coupled to phosphorous release in soils dominated by iron redox couples. 

Utilization of By-product. Oil Palm Trunks and Fronds. Under normal plantation practices, the pruned fronds are placed along the palm interrows and act as mulch. Besides conserving soil moisture and reducing soil surface erosion, the fronds on decomposition return organic matter and slowly release plant nutrients to the soil (Table 27). 

During the replanting program, the oil palm trunks and fronds are chipped into small pieces and pulverized using a special pulverizer. The biomass residues are left in the field to allow for decomposition processing, which could then yield organic matter and release of plant nutrients. The placement of tmnk residues on field terraces could also reduce soil erosion. 

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