Growth and nutrition of Douglas fir, Scots pine and pedunculate oak in relation to soil acidification

In a Douglas fir ( Pseudotsuga menziesii (Mirb.) Franco) and in a Scots pine ( Pinus sylvestris L.) stand on sandy soil in the Netherlands, inputs of water, nutrients and acid loads were changed for four years. Effects of soil changes on growth and nutrition were compared with similar foreign experiments, and with some soil-tree relationships in an oak ( Quercus robur L.) stand, which had different soil types within the site. Water limitations in the stands were demonstrated by a 40% to 50% increase in tree basal area growth from water additions of 3 mm d ^-1or more. Needle N and K contents were depressed and needle shedding was retarded in autumn by irrigation in the Douglas fir stand. In both coniferous stands mineralization and nitrate leaching were enhanced by the moister soil conditions. The modest growth response to nutrient applications in addition to irrigation, proved that no nutrient deficiencies were present at the sites, although foliar P and K increased instantly. The application of dissolved K, Mg and Ca to both acid forest soils increased Al dissolution and decreased Ca/Al ratios in soil solution. This soil reaction was even stronger when applying dissolved (NH ₄ ) ₂ SO ₄ and was explained by the combined effect of soil exchange processes and root uptake of the applied cations, both increasing soil acidification. The applied (NH ₄ ) ₂ SO ₄ initially enhanced tree growth, later followed by growth reduction in the forest stand and tree death in a pot trial at similar NH ₄ supply rates.

Douglas fir growth was higher at optimal water and nutrient applications when atmospheric loads were excluded. The reduced NH ₄ inputs resulted in a decrease of NO ₃ leaching, in a lower needle N concentration and tended to decrease Al concentrations and Al/Ca ratios in soil solution relative to the treatment with atmospheric loads.

Transpiration of Douglas fir, as measured from sapflow, was drastically reduced during summer drought. Irrigated trees showed optimal transpiration rates and were found to absorb water more readily after rain events relative to previously water-stressed trees. NH ₄ applications in a pot trial initially increased shoot growth and transpiration of juvenile Douglas fir, but strong soil acidification later impaired root functioning and water uptake.