Search

A general effect of climate on litter decomposition has been well established for the early stage of several litters. Thus, higher annual actual evapotranspiration (AET) and mean annual temperature (MAT) have been related positively to litter mass-loss rate. Still, the effect of climate on mass-loss rate appears limited to the early stage of several litter species, among them Scots pine. However, this effect starts decreasing as the litter enters the late phase and may even be overruled by that of substrate quality. After some decomposition has taken place there appears to be a shift from sensitivity to climatic variables to a greater or even dominant sensitivity to substrate quality. For some litter species (e.g. Norway spruce and some oak species) the initial chemistry may decrease the influence of climate so much that it is not measurable. Thus, the initial chemical composition is important and for spruce litter, manganese (Mn) concentration has been positively related to first-year mass loss. We also note that the decomposition of main part of all studied litters’ mass is ruled by substrate quality regardless of the litter’s initial climate sensitivity.

Design of experiments is important, and we have summarized our experience with long-term measurements. We discuss how to plan experiments, how to prepare the litter for incubation and share thoughts about sampling intensity and number of replicate samples as well as consequences of too few samples. The concept period mass loss or annual mass loss is presented and discussed. This chapter also discusses some common analytical methods for lignin/Acid Unhydrolyzable Residue (AUR) with emphasis on the approach of using C-NMR.

We have identified two decomposition patterns; one based on the three-stage model (Type I) with pine litter as model substrate; the other being a two-stage pattern (Type II) with spruce and oak litter as model substrates. The Type I decomposition pattern means a decomposition of non-lignified tissue in an early stage with concentrations of e.g. N and P being rate regulating together with climate. In contrast—decomposition pattern Type II is regulated by just Mn concentration. For both decomposition patterns the rate slows and a limit value may be calculated. These limit values range between c 50 and 100% accumulated mass loss. So far we have seen a positive relationship between limit values and litter Mn concentration based on 21 litter species indicating that higher levels of available Mn will reduce the amount of potential humus or long-term residue, whereas N concentration appears to suppress limit values, but only for decomposition pattern Type I. We also note that the building units of lignin, including guaiacyl, syringyl, and p-hydoxyphenol, vary in their relative abundance in the lignin molecules of different plant species or even genotypes of a single species. The variability of lignin composition may influence decay rates.

Wood is a major component of forest biomass but woody litter has been difficult to study because of its enormous range in size and its temporally erratic input due to both natural and human-caused events. Cones also are erratic in their input to the forest floor as litter but can sometimes represent a large percentage of organic matter input. This chapter reports a basic long-term study on cone decomposition including chemical analyses. Wood decomposition studies in natural settings include both studies of wood blocks and wood chips as well as studies of intact twigs and logs. Log studies often rely on the use of decay classes and several decay classes can exist simultaneously in a single decaying log. Nutrient concentrations in wood are very low compared to foliar litters and the chapter describes some of the nutrient dynamics observed in decaying wood. Mass loss rates in coarse woody debris change dramatically over the course of decay, often beginning very slow, then becoming faster as the wood is invaded by fungi, and then slowing again as the more easily decomposed components are depleted.

Models have been used to describe decomposition processes. There are types of models that serve different purposes. Empirical models are statistical in nature and attempt to fit empirical data to mathematical equations. Mechanistic models are theoretical in nature, using a system of equations to describe complex processes. Simulation models simulate the behavior of a system, allowing researchers to manipulate aspects of the model to investigate potential outcomes. This chapter focuses on empirical models, presenting commonly used equations for decomposition patterns: the single exponential and the asymptotic function. The decomposition pattern, indicated by e.g. initial rate and limit value for decomposition is related to litter chemical composition and environmental factors. Nitrogen (N) and manganese (Mn) may be important nutrients for the shape of the pattern. Thus, asymptotic functions have different limit values depending on the litters’ initial Mn concentration with a positive relationship between Mn concentration and limit values ranging from 55 to 100%. Thus, higher initial concentrations of Mn in litter can result in smaller amounts of long-term residue. In contrast, high N availability, both in litter and in soil, e.g. in fertilization experiments has been related to lower limit values and a larger residue.

Carbon (C) sequestration in organic layers has been studied in single forest stands and in continental scale gradients and this chapter focuses on gravimetric determinations, which mainly have been carried out in managed stands and chronosequences. Carbon sequestration rates may be based on two main factors, namely litter fall and size of the stable litter fraction formed during decomposition. We discuss and describe linear relationships between stand age and amount of humus accumulated in a humus layer. We also question the concept ‘steady state’. It appears that pine species gave a dominant C sequestration rate for humus on top of the mineral soil (primary sequestration). Recent data indicate that other species (e.g. Norway spruce) may store less in primary sequestration but a higher fraction in the mineral soil (secondary sequestration). Coniferous species appear to accumulate C in the humus layer faster than deciduous ones. Further, there appears to be a positive relationship between C accumulation rate in a humus layer and mean annual temperature with rates ranging from 0 to 1106 kg C ha−1 yr−1. The chapter also considers factors that influence the accumulation and distribution of soil organic matter in the soil profile.

Decomposition of fine-root litter is much less studied than that of foliar litter although fine root litter is a major contributor to litter input to the forest soil. This chapter is devoted to decomposition of root tips and fine roots. Root litter decomposition may be studied using litter bags, through following remaining root mass in soil cores, or by direct observation. Recent studies have allowed to set up a model for root tips’ decomposition. Also, substrate quality appears important for the decomposition of the finest roots and climate considerably less. Furthermore, the finest root tips appear to decompose and approach a limit value. Limit values for fine roots can be influenced by AUR concentration and N availability. For somewhat coarser roots, say 2 mm diameter or greater, climate may have an effect on decomposition. Root decomposition in climatic gradients has been related to mean annual temperature (MAT) and at a global level using several species there is also a positive response to calcium (Ca) concentration. Recent studies have allowed estimation of limit values for root litter decomposition.

Two regional case studies on carbon (C) sequestration rate for the forested land of Sweden are compared; one is based on direct measurements of the humus layer and the other a theoretical study based on calculable stable residue (limit-value approach). For both studies, the country was subdivided into grid cells and Kriging interpolation was used. In the first case study, Kriging was used to interpolate humus-layer thickness over 41 years, giving an increase all over the region which was positively related to temperature sum. Thickness of the humus layer was converted to amount of C using bulk density and in a next step to increase in humus-layer C. Over 41 years, the average rate for the country was 251 kg C ha−1 year−1. With just two dominant tree species, a comparison was made. In humus layers, Scots pine stands sequestered C at a higher rate than Norway spruce, whereas Norway spruce appears to sequester more in the mineral soil, which suggests different mechanisms for the two systems. The second case study was a theoretical one based on measured data. Actual evapotranspiration (AET) was used to estimate foliar litter fall for pine, spruce, and deciduous trees in 1 × 1 km grid cells. Estimated stable fractions were used to calculate the annual contribution of stable fraction (based on limit values), which corresponds to stable humus and a multiplication by 0.5 gave the sequestered amount of C.