Measuring decomposition, nutrient turnover, and stores in plant litter

Decomposition processes represent a major flux of both fixed carbon (C) and nutrients in most terrestrial ecosystems, and quantifying ratesof litter mass loss and the concomitant changes in nutrients bound in the litter are important aspects of evaluating ecosystem function. Plant litter decomposition plays an important role in determining carbon and nutrient accumulation, as well as the rate and timing of nutrient release in forms available for uptake by plants and soil biota. Litter decomposition and nutrient dynamics are controlled to varying degrees by substrate quality (litter morphology and chemistry), abiotic conditions (temperature, moisture, soil texture), and biotic activity (microbial and faunal; Kurcheva 1960; Heath et al. 1964; Bunnell et al. 1977; Bunnell and Tate 1977; Parton et al.1987). Thus, decomposition processes can serve as "integrating variables" for evaluating ecosystem function, for comparing different ecosystems, and for evaluating management practices or other anthropogenic influences (Coleman and Crossley1996).
Decomposition involves not only mass loss but also changes in the nutrient content of plant litter and the eventual release of nutrients therefrom. Decomposition involves leaching of soluble organic and inorganic components, catabolic breakdown of organic matter, and comminution or physical fragmentation of litter (Swift et al. 1979). These processes ultimately transform senescent plant material into both labile and stable organic matter both above- and belowground. Methods used for quantifying rates of mass loss often can be used to determine changes in nutrient content as well. The dynamics of nutrients in decomposing litter can be complex,and decomposing litter can alternately act as either a nutrient sink or a source. This varies as a function of the nutrient under consideration, litter quality, biotic activity, exogenous nutrient inputs, and stage of decomposition.
In addition to quantifying rates of mass loss and nutrient dynamics of decomposing litter, it is often desirable to quantify the stores, or standing stocks, of various plant litter pools. Standing stocks of both coarse (i.e., woody) and fine (i.e.,leaves, fine roots, etc.) plant litter represent important carbon and nutrient reservoirs in terrestrial ecosystems. The sizes of these reservoirs are influenced by both rates of litter production and decomposition, and are sensitive to changes in either process. Unfortunately, there are relatively few large scale direct measurements of plant litter stores, and regional, national, and global estimates of these pools are often modeled based on input and decomposition rate data (e.g., Birdsey 1992; Harmon and Chen 1992; Kurz et al. 1992; Turner et al. 1995).
Our understanding of, and ability to model, litter decomposition, soil organic matter formation, and the storage of carbon and nutrients in ecosystems will be much improved if researchers design decomposition experiments and conduct inventories that lend themselves to broader synthesis. Our goals in this chapter are to present standard protocols for quantifying decomposition dynamics and standing stocks of most pools of plant litter. Two important exceptions are soil organic matter and very fine roots (