Humus buildup in boreal forests: effects of litter fall and its N concentration

This synthesis paper presents a model for estimating the buildup of soil organic matter in boreal deciduous and coniferous forests. A basic model was developed using data from a well-studied Scots pine (Pinus sylvestris L.) forest (SWECON site) and based on limit values for litter decomposition and amounts of litter fall. A local validation gave a calculated humus accumulation that differed by 8% from the amount measured in the stand. This model was further validated using data for humus accumulated for 2984, 2081, and 1106 years, predicting an accumulation close to the measured amount, and for needle litter the missing fractions were 16, 17, and -6%, respectively, for the three groups. The limit value for litter decomposition is negatively related to the litter's initial N concentration; thus, N-rich litter should have a larger resistant fraction left than N poor. This relationship was validated using nine paired stands of monocultures: eight pairs of Scots pine and Norway spruce (Picea abies (L.) Karst.) and one pair of red alder (Alnus rubra Bong.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). The measured amount of SOM was related to foliar litter fall and its N concentration. In all cases the more N-rich litter gave in all cases the more N-rich Norway spruce litter gave a significantly higher accumulation of humus for Norway spruce in spite of a higher litter fall for Scots pine. Also, red alder gave more SOM than Douglas-fir and in an expected relation to the litter N concentration. A consequence of this would be that C sinks of different efficiencies or capacities would tend to accumulate SOM at different rates.