P.H.B. de Visser

Risk of damage to crops in the direct neighbourhood of ammonia sources

In the Netherlands ammonia is emitted from several thousand sources (animal buildings, manured pastures, slurry storage facilities, etc.). Obviously, the ammonia concentrations are higher close to the source, but at this short distance vegetation has no special value in terms of nature conservation, as it mostly consists of ammonia-resistant agricultural crops. However, three crop categories include relatively sensitive species (fruity culture, glasshouse crops and arboriculture). This paper presents an estimation of the risk of ammonia damage to sensitive crops as related to the distance to the source, based on the mean and variation in emission, dispersion, regional background concentration, landscape characteristics and plant sensitivity. Some attention is paid to damage of natural vegetation and to risk-reducing measures (elevation of emission point, wind fence around the source). The goal is to provide information for regional and local officials and farmers who are dealing with risk evaluation, claims for damages, and environmental policy matters like stable reallocations etc.

Nutrient and proton budgets in four soil-vegetation systems underlain by Pleistocene alluvial deposits.

Proton budgets of four soil-vegetation systems in a 3 ha woodland with acidic and calcareous soils were derived from three years of hydrochemical monitoring of rain- and canopy throughfall water, soil solutions at various depths and measurements on litterfall and tree growth. Soil water fluxes were obtained from dynamic simulation of water transport, calibrated with measured water potentials and water contents in the soil. Atmospheric input is predominantly ammonium sulphate. Only 5–20% of the annual uptake of nutrient elements from the soil is stored in wood, bark and roots of trees. Free H+ inputs from the atmosphere are almost completely neutralized in the tree canopy by exchange with cations from the vegetation. The forest floor and the 0– 10 cm mineral surface layer at the acidic soil sites, however, produce free H+, mainly by nitrification, partly by organic acid dissociation. Most of this H+ is neutralized at greater depth by nitrate uptake or denitrification, and by dissolution of cations from soil minerals, mainly aluminium in the acid soils. The dominant role of N transformations in the H+budget of these soil vegetation systems is clearly due the high anthropogenic inputs of ammonium sulphate. As a result, the rate of weathering of acidic soils has probably increased severalfold.

Effects of water and nutrient applications in a Scots pine stand to tree growth and nutrient cycling

In a Scots pine forest stand, demineralized water and a complete set of nutrients with water were applied to the soil by means of frequent irrigation for four years in order to eliminate water and nutrient shortage of the trees. Apart from this optimization, dissolved (NH4)2SO4 was irrigated at a rate of 120 kg N ha-1 y-1 to create a situation of N excess. Effect of treatments on tree growth and chemical composition of soil water and vegetation were monitored. From the first treatment year onwards basal area growth increased by ca. 35% as a result of the increased water supply. Nutrient applications increased K and P concentrations in pine needles immediately, but growth was enhanced only in the fourth treatment year and coincided with an improved K supply. Most of the applied P and K was retained in the soil, and only 6% was recovered in the vegetation. Tree nutrient status did not respond on Ca and Mg applications, whereas Ca and Mg seepage losses were increased with ca. 5 kg ha-1 y-1. The applied NH4 was mostly retained in the 0–20 cm surface soil and caused a drastic increase of Al in soil solution. Tree growth was stimulated initially by extra NH4, but was hampered after three years obviously because of a decreased P nutrition. The applied base cations were absorped to the soil and the accompanying anions were leached, thus temporarily increasing the acidification of the soil solution.

The 'Exman' project - Experimental manipulations of forest ecosystems.

The“EXMAN” project is a collaboration between the universities in Lyngby DK, Wageningen NL, Munich DE, Gottingen DE, and Dublin IRL. The project started formally in 1987, but joint research efforts already existed between some of the partners before that time. The idea of the project is to perform comparable manipulations of forest ecosystems at different sites in Europe under a wide range of soil and atmospheric conditions. This is done in order to quantify the biogeochemical cycling of elements and the turnover of biomass, and to assess the effects of atmospheric deposition of S and N compounds. The research sites are Klosterhede DK, Harderwijk and Kootwijk NL, Hoglwald DE, Soiling DE, and Ballyhooly IRL. At least two of the following types of manipulation are performed at each site: fertilization, liming, irrigation, experimental acidification, removal of water and element inputs by roof constructions. For the time being, in the middle of the project period, we present our tentative findings on precipitation, throughfall, soil water and tree growth, and discuss the effects of the different experimental treatments.

Effects on forest ecosystems

Publisher Summary This chapter discusses the results of research on the effects on forests, which was central in the Dutch Priority Programme on Acidification (DPPA). The chapter deals with the results of laboratory experiments and of the monitoring and manipulating of forest stands—including the modelling of these field results, with regional inventories of forest vitality and soil chemistry and with research on non-tree biotic components of forest. Seedlings of Douglas fir were used in a pot experiment to study the effect of fertilizing with 100 kg N ha -l y -1 on the biomass production and in particular on the specific root length (length/weight). The results of these experiments indicate that applying nitrogen can lead to an increase in the shoot/root ratio and can also cause damage to the root system. It also appears that the response depends not only on the amount of nitrogen supplied to the plant but also on the form in which it is offered. The chapter focuses particularly on tree physiology and research on the chemistry of soil and water, together with the tree vitality. Various aspects of forest undergrowth and forest fauna are dealt and the results are evaluated in the light of critical levels and loads.

Thematic report on effects (with special emphasis on combination stress)

Publisher Summary This chapter describes the Dutch Priority Programme on Acidification (DPPA) projects and focuses on ecophysiological effects. The results of DPPA-III are presented in three thematic reports: deposition, combination stress, and modelling. The chapter focuses on the second theme, combination stress. In the scope of the assessment of damage caused by air pollution it is relevant to compare this with the damage caused by natural stresses like insect pests, fungal diseases, frost, drought and nutrient deficiencies, and to understand interactions of air pollution and natural stresses are also relevant. Damage by natural stresses in Dutch forests has not been quantified as yet. Both NH 3 and O 3 are major air pollutants in the Netherlands. Their effects and interactions were studied in several experiments with young trees. Based on experimental evidence current concentration levels of O 3 is assumed to reduce stomatal aperture, CO 2 assimilation and biomass production, especially that of the roots. Reduced stomatal aperture may reduce transpiration and thus drought sensitivity. But on the other hand, a reduced root/shoot ratio may result in increased drought sensitivity.

Optimal nutrition in two forest stands exposed to acid atmospheric deposition

A Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) and a Scots pine (Pinus sylvestris L.) stand on acidified soils are manipulated by means of irrigation and optimal nutrition. The aim is to assess effects of different soil conditions on tree growth in order to quantify indirect effects of acid deposition on trees. High atmospheric inputs of nitrogen in the soil are observed, resulting in soil acidification, high N availability, high aluminium concentrations and unbalanced nutrient supply. The soil solution composition is changed gradually in the manipulated soils. After one year treatment no significant growth differences have been observed yet.