Litter quality in a north European transect versus carbon storage potential

Newly shed foliar plant litter often has a decomposition rate of ca 0.1-0.2% day(-1), which decreases greatly with time and may reach 0.0001 to 0.00001% day(-1) or lower in litter material in the last stages of decay. The decrease in decomposability (substrate quality) varies among species and is complex, involving both direct chemical changes in the substrate itself and the succession in microorganisms able to compete for substrate with a given chemical composition. In late stages, the decomposition appears very little affected by climate, suggesting that climate change will have little effect on late-stages decomposition rates. Here, we apply a model for the late stages of litter decomposition to address the question of climate-change effects on soil-C storage. Decomposition of litter turning into soil organic matter (SOM) is determined by the degradation rate of lignin. In the last phases of decay, raised N concentrations have a rate-retarding effect on lignin degradation and thus on the decomposition of far-decomposed litter and litter in near-humus stages. The retardation of the decomposition rate in late stages may be so strong that decomposition reaches a limit value at which total mass losses virtually stop. At such a stage the remaining litter would be close to that of stabilized SOM. The estimated limit values for different species range from about 45 to 100% decomposition indicating that between 0 and 55% should either be stabilized or decompose extremely slowly. For no less than 106 long-term studies on litter decomposition, encompassing 21 litter types, limit values were significantly and negatively related to N concentration, meaning that the higher the N concentration in the newly shed litter (the lower the C/N ratio) the more litter was left when it reached its limit value. Trees growing under warmer and wetter climates (higher actual evapotranspiration, AET) tend to shed foliar litter more rich in N than those growing under colder and drier climates. A change in climate resulting in higher AET would thus mean that within species, e.g., Scots pine, a higher N level in the foliar litter may result. Further, within the boreal system deciduous species appear to have foliar litter richer in N than have conifers and within the conifers group, Norway spruce has needle litter more rich in N than, e.g., Scots pine. Thus, a change of species (e.g., by planting) from pine to spruce or from spruce to a deciduous species such as birch may result in a higher N level in the litter fall at a given site. In both cases the result would be a lower limit value for decomposition. The paper presents an hypothesis, largely based on available data that a change in climate of 4degrees higher annual average temperature and 40% higher precipitation in the Baltic basin would result in higher N levels in litter, lower decomposition and thus a considerable increase in humus accumulation.