Decomposition and nutrient release from Norway spruce coarse roots and stumps – A 40-year chronosequence study

• Decomposing Norway spruce coarse roots and stumps are long-term carbon and nutrient pools.
• Stumps decompose and release carbon faster than coarse roots.
• Nitrogen releases slowly from coarse roots and stumps.
• Stumps act as phosphorus sinks for several decades during their decomposition.

Stumps and coarse roots form the largest part of the coarse woody debris in managed boreal forests but their contribution to nutrient cycling and carbon balance of forest ecosystems is poorly understood. Decomposition and nutrient (C, N, P, K, Ca) release from Norway spruce (Picea abies) coarse roots (diameters 5–10 cm and >10 cm) and stumps were studied in southern Finland in a chronosequence of stands clear-cut 0, 5, 10, 20, 30 and 40 years ago. Density, mass and the amount of C decreased significantly faster from stumps than from coarse roots. The average annual decomposition rate constants (k-values) for the whole 40-year study period were 0.040 for stumps, 0.024 for >10 cm diameter roots and 0.034 for 5–10 cm diameter roots. The release of N was extremely slow since stumps, >10 cm roots and 5–10 cm roots still contained 97%, 107% and 96% of the initial amounts of N, respectively, after 40 years of decomposition. The amount of P was significantly higher in 40-year decomposed stumps (115%) than in >10 cm (71%) and 5–10 cm (61%) roots. Stumps, >10 cm roots and 5–10 cm roots lost 79%, 79% and 81% of their initial amount of K, and 51%, 47% and 45% of their initial amount of Ca, respectively, during the 40-year period. The results indicate that coarse roots and stumps are long-term C pools and sources of nutrients for vegetation in boreal forests.