Effects of experimental soil frost on the fine-root system of mature Norway spruce

Soil-frost events may influence the dynamics of fine roots and therefore affect root-derived C fluxes to the soil. We studied the impact of soil frost on the fine-root dynamics of Norway spruce (Picea abies [L.] Karst.) by a replicated snow-removal experiment in a mature forest in SE Germany. Snow removal in the three treatment plots reduced soil temperature significantly with minima <-5.5 degrees C in the O layer while the snow-covered control plots never reached temperatures below the freezing point. Sequential soil coring in the O layer at the beginning and at the end of the soil-frost period as well as after thawing revealed that the soil frost treatment increased fine-root mortality by 29%. However, enhanced fine-root production in the snow-removal plots nearly compensated for the fine-root losses caused by low temperatures. These findings were confirmed by minirhizotron observations in the O layer and the upper 25 cm of the mineral soil showing that relative fine-root loss was by far higher in the snow-removal plots than in the control plots. Compensatory fine-root production in the snow-removal plots exceeded fine-root production in the control plots during a period of 8 weeks after the soil frost application by 39% in the O layer while it was similar in both plot types in the mineral soil. Sequential coring and minirhizotron observations led to substantially different fine root-longevity estimates for the soil frost period. However, in both cases, the snow-removal treatment was characterized by a significant reduction in root longevity indicating a faster fine-root turnover. As a consequence, experimental soil frost enhanced the C input to the soil via root death at our study site by approx. 42 g m(-2) and stimulated the C investment towards the root system of the spruce trees due to a higher sink activity.