Nitrogen sinks following two intensities of harvesting in a Sitka spruce forest (N. Wales) and the effect on the establishment of the next crop

Abstract

The importance of the ground vegetation and soil microbial biomass as sinks for nitrogen following conventional (CH) and whole-tree (WH) harvesting in a Sitka spruce plantation in N. Wales (UK), were investigated. Lysimeters containing the organic horizons were established with the treatments brash, Agrostis capillaris (L.) and/or Sitka spruce ( Picea sitchensis (Bong.) Carr.) seedlings in a limited factorial design. Following a period of 15 months, during which chemical analysis of the lysimeter leachates was carried out, the lysimeters were destructively sampled and the vegetation and soil microbial biomass nitrogen pools measured. The effect of the different treatments on the decomposition rate of the organic matter and the growth and nutrient status of the young Sitka spruce trees was also determined. A comparison of the nitrogen pools and the cumulative leachate results identified the grass as the major sink for nitrogen with uptake over a 15 month period equivalent to 2.6 g N m −2 at 50–75% cover. Uptake by the Sitka spruce was also significant with net uptake ranging from 0.6 g N m −2 when growing with the grass to 1.74 g N m −2 when planted within a pile of brash. The microbial biomass nitrogen pool was unchanged by the presence of the grass species but was significantly increased in the lysimeters containing brash and/or Sitka spruce (possibly owing to the presence of mycorrhizal fungi). The microbial N pool was greatest in the treatment containing the combination of the trees planted within the brash, with 3.49 (±0.52) g N m −2 as microbial biomass N compared with 1.99 (±0.24) g N m −2 in the control lysimeters containing neither brash nor trees. The growth of the Sitka spruce was highly variable within treatments, however, the mean biomass m −2 doubled if planted within the brash, and also resulted in elevated foliar concentrations of nitrogen, potassium and phosphorus. When in competition with the grass, tree biomass was halved, tree needles appeared chlorotic and phosphorus concentrations were halved. The management implications for harvesting intensity on upland sites are considered.