Growth responses in Swedish boreal coniferous forests after addition of nitrogen as sewage sludge pellets :

Abstract: Every year, large amounts of municipal sewage sludge are produced worldwide. Most of the sludge ends up on landfills, garbage dumps and incineration plants, and only small part is utilized as a fertilizer in agriculture and forestry. Sewage sludge contains a lot of nutrients, especially nitrogen. In boreal forests, a lack of available nitrogen limits the growth, and therefore sewage sludge could be applied into forests to increase the growth. Sewage sludge pellets might offer a good alternative for liquid and dewatered sludge or even for inorganic fertilizers in fertilizing forests. The aims of this study were to investigate the effect of the addition of nitrogen as sewage sludge pellets on tree growth, the effect of dose size and the duration of the growth response. The study consists of two experiments, Pilot and Åheden that had been fertilized with sludge pellets in 1996 and 1998, respectively. In the Pilot experiment two treatments, 0 and 4 tons sludge pellets ha-1 (ca. 100 kg N ha-1), were applied in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) stands. In the Åheden experiment, five treatments, 0, 3.3, 6.6 and 13.2 tons sludge pellets ha-1 (equalling 0, 100, 200 and 400 kg N ha-1, respectively) and 200 kg N ha-1 as ammonium nitrate, were applied in a pine dominated stand. Ammonium nitrate had been applied in order to compare the effects of nitrogen from different sources on tree growth. The breast height diameter (DBH) was measured from the sample trees and cores samples taken with an increment corer. The application of sludge pellets increased the diameter and basal area growth in comparison to control treatment in the Åheden experiment. In the Pilot experiment, addition of sludge pellets did not result in any significant change in basal area in comparison to control treatment, but the spruce stands tended to respond more clearly on sludge pellets addition than pine stands. The results were not statistically significant, however. In the Åheden experiment, the addition of 400 kg N ha-1 as sludge pellets increased the diameter and basal area growth nearly as much as when 200 kg N ha-1 of ammonium nitrate was applied. Additionally, the basal area growth tended to increase with the increasing amount of nitrogen added as sludge pellets. The increased basal area growth tended to last at least the first 5-year period after fertilization on the sludge treated plots in Åheden experiment (and on the spruce stand in Pilot experiment) but these results were not however statistically significant. The study showed that application of sludge pellets in boreal, Scots pine dominated forest increases the forest growth. Since the growth response to the highest dose of sludge pellets was nearly as good as from commercial fertilizer, ammonium nitrate, it seems that pellets do offer a reasonable and attractive alternative for inorganic fertilizers and could even replace them in the future forest fertilizations. However, more research about both environmental and growth effects is needed before sludge pellets can be taken in use in a large scale.