Nutrient Movement During Decomposition

Soils contain many of the same elements as found in their underlying substrate of rock, but the proportions differ greatly. Elements such as calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) are lost as soluble cations during weathering, depending on climatic conditions (especially precipitation). Some other elements, such as iron and aluminum, are resistant to leaching losses and their proportions may increase compared to rocks. Movements of cations are governed by the exchange properties of the soil, properties dependent upon the nature of the clays and amount and type of organic matter. Exchangeable cations in soils include Ca2+, Mg2+, K+, NH4+ and Na+, affinities for exchange sites (i.e., energy of adsorption) decreasing approximately in that order. Certain anions are not as tightly held in soils, again depending on the nature of clay colloids and on soil pH. Phosphate ions, multiply charged, are more tightly fixed by anion exchange properties than are singly charged ions such as nitrate (Bowen, 1979; Foth, 1990).

During the decomposition process, elements are converted from organic to inorganic forms (mineralized) and may enter the exchangeable pools, from which they are available for plant uptake or microbial use. Cellulose and hemicellulose account for more than 50% of carbon in plant debris and help to fuel microbial processes such as transformations of nitrogen (Fig. 5.12) and sulfur (Fig. 5.13), which gradually reduce the carbon-nitrogen and carbon-sulfur ratios in decomposing materials.

As plant litter decomposes, the elemental mix changes because of differential mobility and biological fixation. Carbon is lost through micro-bial respiration, as cellulose and other labile organic compounds are hydrolyzed and utilized in growth and maintenance. Potassium is highly mobile until it encounters exchange sites, where it can become fixed. Sodium ions, which are more mobile in soils, are not accumulated in plants but are essential for animals. The "herbivore exclusion hypothesis" (McNaughton, 1976; McNaughton et al., 1998) proposes that plants discriminate against sodium and thereby limit herbivory. Sodium does

FIGURE 5.12. Soil nitrogen (N) cycle showing active (1-1.5 years), slow (10-100 years), and passive (100-1000 years) fractions. Flow diagram for the nitrogen submodel of the Century model (from Parton et al., 1987, with permission).

accumulate in food chains, often increasing by a factor of 2-3 between trophic transfers.

The nitrogen content of decomposing litter increases during the initial stages of decomposition and then declines (Fig. 5.14) (Berg and

FIGURE 5.13. Conceptual model for microbial and faunal components of the sulfur cycle in soil (from Gupta, 1989).

Staaf, 1981). Nitrogen is mineralized during decomposition and is simultaneously immobilized by microbes, resulting in an increase in the concentration of nitrogen in the litter, and in the absolute amount of nitrogen if it is transported into the litter from soil or by atmospheric nitrogen-fixation. As decomposition proceeds, the carbon-nitrogen ratio declines until the substrate becomes more suitable for microbial action. In some forests, the period of nitrogen increase may extend for 2 years or more (Fig. 5.15) (Blair and Crossley, 1988). Phosphorus and sulfur also show increases in absolute amounts during decomposition of some species of tree leaf litter (Fig. 5.16) (Blair, 1988a), even though mass is

Time (weeks)

FIGURE 5.14. Rates of nutrient immobilization and mineralization from decomposing Scots pine (Pinus sylvestris) litter over a 5-year period (from Staaff and Berg, 1982). Note initial influx of nitrogen (N) into litter.

Time (weeks)

FIGURE 5.14. Rates of nutrient immobilization and mineralization from decomposing Scots pine (Pinus sylvestris) litter over a 5-year period (from Staaff and Berg, 1982). Note initial influx of nitrogen (N) into litter.

being lost. Calcium and magnesium concentrations in decomposing litter change only slightly through time. There may be an initial decrease in concentration followed by a slight increase (Blair, 1988b). Thus the absolute amounts of these elements during decomposition approximately track the loss of mass (Cromack, 1973). Potassium is not a structural element, and is lost via solubilization more rapidly than mass is lost from decomposing leaf litter. Decomposing woody litter, in contrast, accumulates calcium and phosphorus, evidently as a result of fungal invasion and translocation from soil.

The nitrogen pool in decomposing litter is a dynamic one. Although nitrogen is accumulating, there is evidently a large amount of turnover taking place. When tracer amounts of 15N [as (NH4)2SO4] were added to leaf litter, significant losses of tracer took place even as total nitrogen accumulated (Fig. 5.17) (Blair et al, 1992). Nitrogen evidently became incorporated from exogenous sources, in amounts greater than those lost through biotic factors. Inputs of nitrogen via rainfall or canopy throughfall are a potential source of added nitrogen. However, these would appear to be inadequate to account for the amount of nitrogen immobilized in litter. Fungal translocation from lower layers (F, H, or mineral soil) is another possibility. Finally, lateral transport to and from "hot spots" in the forest floor may contribute to the dilution of tracers.

As noted in Chapter 2, one of the main sources of particulate organic matter to soils is that from decomposing roots. Researchers have often

FIGURE 5.15. Mean percentage of initial mass and nitrogen remaining over time in (a) Cornus florida, (b) Acer rubrum, and (c) Quercus prinus litter on uncut WS 2 (solid line) and clearcut WS 7 (dashed line) at Coweeta Hydrologic Laboratory from January 1975 to January 1977 (from Blair and Crossley, 1988).

Days in the field

FIGURE 5.15. Mean percentage of initial mass and nitrogen remaining over time in (a) Cornus florida, (b) Acer rubrum, and (c) Quercus prinus litter on uncut WS 2 (solid line) and clearcut WS 7 (dashed line) at Coweeta Hydrologic Laboratory from January 1975 to January 1977 (from Blair and Crossley, 1988).

0 0

Post a comment