The effect of Norway spruce stump harvesting on net nitrogen mineralization and nutrient leaching

Abstract

Stumps of conifer trees are a prospective source of bioenergy and stump harvesting is a novel practice in forestry management in the Baltic and Nordic countries. However, as stump harvesting may cause possible environmental risks there has emerged a clear need for research focusing on sustainable forest management.Three Norway spruce (Picea abies) clear-cut areas on different soils in Estonia were selected for the present study. We analysed the effect of stump harvesting on net nitrogen mineralization (NNM) and on nutrient leaching. On dry and sandy Endogleyic Arenosol (Oxalis site type), stump harvesting reduced the annul NNM flux significantly; 134 and 202kgNhayr−1 at the harvested and at the control site, respectively. In clear-cut area where Endogleyic Cambisol was dominating (Hepatica site type), stump harvesting had no effect on NNM (92 vs 88kgNhayr−1). However, in a clear-cut area where the soil type was Endogleyic Albic Podzol (Myrtillus site type), stump harvesting increased the total annual NNM flux: 102 vs 70kgNhayr−1 at the harvested and at the control site, respectively. Stump harvesting affected also the proportion of nitrification and ammonification processes in NNM. At the Myrtillys site type stump harvesting increased the annual nitrogen (N) leaching flux. One year after stump harvesting, leaching at the harvested site was 11.7 vs 4.5kgNha−1yr−1 at the control site. In the second year N leaching decreased and the difference levelled off. Increased N leaching was induced by a larger amount of water; average N concentration of the harvested and control sites did not differ. Although at the Oxalis site N leaching was larger at the harvested than at the control site, the total annual leached N flux was small (∼2kgNha−1). At the fertile Hepatica site type treatment had no impact on N leaching, which was only ca 1kgNha−1yr−1. Phosphorus (P) leaching was very small in all study areas, being below 0.1kgPha−1yr−1.The effect of stump harvesting on annual NNM as well as on N leaching was soil specific and highly variable. Stump harvesting affected also the proportion of the nitrification and ammonification processes in total NNM. Considering the first short-term results obtained from different site types, we can conclude that harvesting of spruce stumps does not induce serious environmental hazards in relation of N cycling.