Organic Matter Decomposition

When suspended particulate, colloidal, and dissolved organic matter (DOM) are scavenged from surface waters and deposited to benthic sediments they encounter the sediment-water interface, an important region in the biogeochemical cycling of many elements and compounds. Most biologically mediated oxidation and mineralization of organic matter, referred to as early diagenesis, occurs within that interface, which typically extends a few centimeters into the surface sediments. This early diagenesis is carried out by microbes using the most energetically favorable oxidant available, which in aerobic environments is molecular oxygen, O2. This is illustrated by the simplified oxidation of organic matter, based on Redfield's ratio of C:N:P of 106:16:1:

(CH2O)106(NH3)i6(H3PO4) + 138 O2 ! 106 CO2 + 16 HNO3 + H3PO4 + 122 H2O

The AG ° of the above reaction is — 3190kJmol—\ which is thermodynamically favorable and yields substantial free energy. The oxidation of organic material via this reaction will be dominated by aerobic bacteria as long as sufficient O2 is available. However, the rate of O2 consumption will exceed the rate of O2 diffusion from overlying waters to sediments at some sediment depth. In highly productive aquatic systems and those impacted by relatively large discharges of organic waste, the depletion of O2 may occur at or above the sediment-water interface.

Microbial depletion of O2 leads to the formation of suboxic conditions in deeper sediments, where further oxidation of organic material continues with microbes using the next most energetically favorable terminal electron acceptor. As one oxidant is consumed, the next most energetically favorable oxidant is utilized. MnO2 and NO— reduction occur following the loss of O2, sequentially followed by iron reduction, sulfate reduction, and methanogenesis. This sequence is illustrated by the following balanced equations for the oxidation of organic material and the Gibbs free energy associated with each terminal electron acceptor:

Manganese oxide reduction: AG ° = — 3090 kJmol—1

(CH2O)106(NH3)16(H3PO4) + 236 MnO2 + 472 H+ ! 23(5 Mn2+ + 106 CO2 + 8 N2 + H3PO4 + 366 H2O

(CH2O)106(NH3)16(H3PO4) + 94.4 HNO3 ! 106 CO2 + 55.2 N2 + H3PO4 + 177.2 H2O

(CH2O)106(NH3)16(H3PO4) + 424 FeOOH + 848 H+ ! 424- Fe2+ + 106 CO2 + 16 NH3 + H3PO4 + 742 H2O

(CH2O)106(NH3)16(H3PO4) + 53 SO4-! 53 S2- + 106 CO2 + 16 NH3 + H3PO4 + 106 H2O

Methanogenesis: AG0 = —350kJmol—1

+ H3PO4

The distribution of redox zones with depth below the sediment-water interface where each of the above respiration pathways occurs varies with physical and bio-geochemical conditions. In naturally eutrophic and contaminated sediments, where inputs of organic matter are relatively high, O2 penetration by diffusion from overlying waters may be limited to a few millimeters. In oligotrophic freshwater and deep-sea sediments, where organic matter inputs are much lower, O2 may diffuse a few centimeters below the sediment-water interface. However, the advection of surface waters into the sediments due to bioturbation commonly accounts for greater and uneven penetration of O2 and other terminal electron acceptors in benthic sediments.

Consequently, the distribution of redox zones in sediments may be highly heterogeneous. Multiple microenvironments often exist within millimeters of each other where aerobic respiration, nitrate reduction, and sulfate reduction are carried out simultaneously at comparable depths. Since the rate of organic matter oxidation is temperature dependent due to its effect on microbial respiration rates, early diagenesis in shallow water sediments also displays seasonal variability.

The oxidation of organic material in sediments during early diagenesis causes a number of chemical changes beyond those in the organic matter itself. The decomposition of the organic matter also releases other nutrients, trace elements, and contaminants which have been com-plexed or associated with it, when it is solubilized.

All of the preceding forms of respiration also produce carbon dioxide (CO2). This increase in CO2 levels is accompanied by a decrease in pH due to the formation of carbonic acid (H2CO3) and bicarbonate (HCO-). That decrease can, in turn, lead to the mobilization of cations adsorbed to the sediments as an increasing number of hydrogen ions compete for negatively charged adsorption sites, through cation exchange.

Phosphorous, principally in the form of phosphate, and nitrogen, principally in the form of nitrate or ammonia, are liberated directly by the mineralization of organic matter, are liberated directly by the mineralization of organic matter, as well as indirectly by the reduction of iron and manganese oxyhydroxides. As a result, those two macronutrients (N and P) are often depleted in surface waters where they are commonly the limiting nutrients for primary productivity and are enriched in subsurface and porewaters, where they are released with the decomposition of organic matter. This nutrient-type profile is also paralleled by those of other trace- and micronutri-ents, and primary productivity is often limited by the subsequent flux of those remobilized nutrients to the euphotic zone in overlying waters.

Flux rates out of anoxic lacustrine and estuarine sediments reported in the literature are generally of the order of 0-20 mgm -2 day-1 for phosphate, 0-10 mgm -2 day-1 for nitrate, and 0-30mgm- day- for ammonium. However, these flux rates vary substantially by environmental setting, as well as spatially and temporally. In addition, these nutrient fluxes may be reversed, from the overlying waters to sediments, under some conditions.

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