What Controls Decomposition Rate in Conifer Forests?

Source: Litter decomposition, climate, and litter quality. Link.

Nutrients:  Mn and N

Manganese and Nitrogen control forest decomposition, but in an unexpected way. Mn is an essential component of ligninolytic enzymes important for degrading litter in the later stages of decomposition.  But high available N can limit decomposition. The most efficient degraders of lignin and humic acids are wood-rotting or litter-decomposing white-rot fungi (Hintikka 1970; Hatakka 2001).  For several of the lignin-degrading white-rot fungi, high concentrations of low-molecular weight N compounds suppress the synthesis of the lignin-degrading enzymes (Keyser et al. 1978; Eriksson et al. 1990; Carreiro et al. 2000). 

Further, N has repeatedly been reported to react with remains of degrading lignin to form recalcitrant condensation products. Such products form chemically (Vahtras 1982; Stevenson 1982) rather than biologically.  Spaccini (1999) suggested that such bonds create a hydrophobic surface thereby resisting decomposition. The higher the concentrations of lignin and N in a litter material the more likely it seems that such covalent bonds will be formed. Using 106 sets of foliar litter comprising 21 tree species (both coniferous and deciduous) representing a wide range in chemical composition, Berg (2000) found a highly significant negative relationship between limit values and initial N concentrations in litter.

Many examples exist in which addition of N to a N-deficient system slows down decomposition, especially where organic matter with high lignin content is present (Verhoef and Brussaard, 1990; Carreiro et al., 2000). Nitrogen and other fertilizers may negatively influence specific groups of organisms, particularly microbes.  Any shift in microbial composition can have a negative effect on other soil fauna.  As a result, decomposition and mineralization may decrease. Additionally, plants can compete successfully with decomposers for nutrients.  (Neher 2003.)

What about pH?

In a review of 58 studies, Wardle (1998) found that temporal variability in soil carbon (C) was related most closely to soil N content in forests and soil pH in arable and grassland ecosystems. (Neher 2003.)

But a study by Parn et al adding ash to a pine forest to increase soil pH did not observe increased decomposition.  A large meta-review noted that 22-85% of treatment trials have failed to affect decomposition: “This analysis shows that Ca additions are not universally beneficial and provides insight into when Ca additions to forest soils are likely to be most effective." (Evaluating the effects of liming and wood-ash treatment on forest ecosystems through systematic meta-analysis. Carolyn Reid, Shaun A. Watmough)

Decomposition Rate and Ecosystem Integrity

Neher 2003 "Effects of disturbance and ecosystem on decomposition." is a great review of this topic.  (PDF)  They conclude that "decomposition of organic matter is a useful indicator of soil condition because it is measured easily and serves as integrator of the collective activities of organisms within the soil food web."  For example, slowing rates of decomposition serve as an early warning sign of pathology in forest ecosystems (Bormann and Likens 1979),  A difference in decomposition rate between similar sites implies either a change in the decomposer community or quality of the biotic and abiotic resources at a site. 

This WSU website has a great review of pine forest decomposition.