Charles McClaugherty

Decomposition of litter and soil organic matter—can we distinguish a mechanism for soil organic matter buildup?

This synthesis paper presents a model for estimating the buildup of soil organic matter in various types of coniferous forests. The knowledge used was obtained from a well‐studied forest with good Iitterfall data, decomposition information and validation measurements of the soil organic matter layer. By constructing a simple model for litterfall, and the information on maximum decomposition levels for litter, we could estimate the annual increase in soil organic matter and extend this to encompass stand age. The validation measurement and the estimated amount of soil organic matter differed by about 8 or 26% over a 120‐yr period, depending on the litterfall model. The estimated increased storage of soil organic matter as a consequence of climate change was found to be drastic. We thus found that the soil organic matter layer would grow about four times as fast as a result of the needle component only. This estimate was based on a comparison between latitudes with a difference of 17°.

Decomposition as a process

Decomposition of plant litter involves a complex set of processes including chemical, physical and biological agents acting upon a wide variety of organic substrates that are themselves constantly changing. Due to the immense diversity of possible factors and interactions, decomposition in a natural setting can be described in general terms only. In spite of this complexity, several major processes are involved and general trends can be outlined.

Decomposition of Fine Root and Woody Litter

Decomposition of wood and fine root litter is much less studied than that of foliar litter. This chapter gives a brief overview to a few representative studies, giving some method approaches. For logs, there are decomposition classes given, which allow a description of the process. Accumulated mass loss may be determined by water displacement as decomposition produces more space. Further, wood chips as a model substrate may be placed in litter bags. In contrast to foliar litter, wood-decomposition rate may increase with time and be highest after 2–3 years, after which the rate may decrease. Root litter decomposition may be studied using litter bags, through following remaining root mass in soil cores or by direct observation. Root decomposition in climatic gradients has been related to mean annual temperature (MAT) and at a global level using several species also to calcium (Ca) concentration. Recent studies have allowed estimation of limit values for root litter decomposition.

Decomposition as a Process: Some Main Features

The principles for litter decomposition are introduced. With focus on pine spp. needle litter, we present a three-stage model giving a sequence of distinct steps in the decomposition process with some main rate-regulating factors. The early stage in which solubles and non-lignified holocellulose are degraded appears to be limited by access to the main nutrients nitrogen (N), phosphorus (P), and sulfur (S) as well as by climate. The late stage encompasses the lignified part of the litter and includes a shift in rate-regulating factors. In this stage, N has a rate-suppressing effect and manganese (Mn) a rate-enhancing effect. As decomposition proceeds, the rate often decreases and may approach zero, resulting in a limit value for decomposition. This value may be estimated, and we may distinguish a ‘stable’ litter fraction, the size of which is determined by the ‘limit value,’ at which the rate is zero. For pine needle litter, a model substrate, the limit value is higher and the ‘stable’ fraction smaller when the litter Mn concentration is higher. The stability of this ‘stable’ fraction is not known, but may be increased by a higher N concentration.

Changes in Substrate Composition during Decomposition

The mass loss of the organic compounds appears to be selective with water solubles degraded (or leached) first, followed by cellulose and hemicelluloses. For these compounds, the concentrations decrease or remain about constant whereas that of Acid Unhydrolyzable Residue (AUR)/lignin increases as decomposition proceeds. In general, it appears that concentrations of the main nutrients nitrogen (N), phosphorus (P), and sulfur (S) increase. For N, the concentration increases irrespective of changes in absolute amount. Also, concentrations of the main heavy metals increase (e.g., copper (Cu), lead (Pb), and iron (Fe). In contrast, as potassium (K) is leached its concentration decreases heavily. The concentration changes for nutrients such as calcium (Ca), magnesium (Mg), and manganese (Mn) appear to have variable patterns with Mn concentrations decreasing over the early stage whereas it increases in the late stages. A special study on Mn concentrations and Mn release in pine and spruce litter indicated a clear difference between the two genera.

Models that Describe Litter Decomposition

The graphs for accumulated litter mass loss vs time form different patterns, which may be described using a few simple mathematical functions. Generally we may state that litter may decompose completely or leave a stabilized residue, which may decompose very slowly. Complete decomposition may be described using the single exponential, with a constant rate, the same function as for radioactive decay. In another approach the complete decomposition may be described in two steps using a two-factorial ‘double exponential’ with part of the litter decomposing at a high and part at a lower rate. A flexible, decreasing rate and a calculable stabilized residue are described using an asymptotic function. The decreasing decomposition rate allows the calculation of a limit value, often between 50 and 100% accumulated mass loss, using the decomposition rate of zero. Although this estimated rate gives an extremely slowly decomposing ‘stable’ fraction there are few studies on the stability of this residue. Possibly a raised nitrogen concentration may be stabilizing.

Initial litter chemical composition

As regards chemical composition, foliar litter appears to be the most studied group of litter. The main organic litter compounds are cellulose, a group of hemicelluloses and lignin, the latter often measured as acid unhydrolyzable residue (AUR). In addition, there are several polymeric compounds including suberins, tannins, and cutins. Concentrations of the main nutrients vary heavily with litter species. For some species/genera (e.g., pine spp.), we may see that in newly shed litter concentrations of nitrogen (N), phosphorus (P), sulfur (S), and potassium (K) increase with increasing mean annual temperature (MAT) and actual evapotranspiration (AET), whereas manganese (Mn) in pine litter has been shown to decrease. For spruce (Picea) needle litter, no such effect of climate has been observed. Still, an increase in N concentration with MAT/AET appears to be a general phenomenon covering most investigated species. AUR concentration has been positively related to litter N concentration. It appears that AUR concentrations are higher in coniferous litter than in broadleaf, whereas those for N are higher in broadleaf litter.

Decomposition and ecosystem function

The microbial decomposition of plant litter is a basic process in the functioning of ecosystems, not only for the general release of nutrients to plants, but also for the buildup of a stable humus and the accompanying storage of nutrients.

Influence of chemical variation in litter on decomposition

Chapter 5 reviewed the effects of initial litter chemical composition on the pattern of changes that occur during decay. This chapter focuses attention on the influence of changing litter quality on the decay processes, and will show that the influences of selected litter chemical components change dramatically during the process, sometimes even reversing the direction of their effect. For this purpose, the three-phase model introduced in Chapter 2 is applied to explain the effects of chemical variation, changes in mass-loss rates and decomposition patterns. Litter types that have been found to deviate from the general pattern will also be discussed.

Influence of site factors other than climate

Although litter chemical composition and climate appear to dominate as regulating factors in decomposition over a regional scale (Chaps. 4–7), there are numerous factors that are important in regulating decomposition at the local or even micro scale. These are related to soil characteristics, nutrient availability and cycling, plant community composition and structure, soil fauna, and topography. Some of these factors exert their influence by modifying the microclimate. Other factors operate primarily through biochemical or nutritional influences on microbial metabolism. Not only can these site-specific factors influence microbial metabolism, they can also alter the composition of the microbial community, as was discussed in Chapter 3.