EFFECT OF DROUGHT EXPERIMENTS USING ROOF INSTALLATIONS ON ACIDIFICATION NITRIFICATION OF SOILS/

Within the framework of the EU-funded EXMAN (EXperimental MANipulation of Forest Ecosystems) project drought experiments were conducted at f our different European Norway Spruce (Picea abies (L.) Karst.) plantat ions in the year 1992-1995. The aim of the project was to investigate if then is an additional risk of soil acidification due to nitrificati on/acidification pulses after extended periods of summer droughts. The sites included (Ballyhooly/BH, southwest of Ireland, Klosterhede/KH, west coast of Denmark, Hoglwald/HW, northwest of Munich,and Solling/SL , central Germany) cover considerable gradients of climatic and air po llution regimes. Artificial droughts were produced by using different types of roof constructions installed below the forest canopy (non-per manent constructions in BH and HW, permanent installations in KH and S L). Each drought was started in spring time and dry conditions were ma intained until the soil water tension was below a potential of -700 hP a at 70 cm depth in the mineral soil. Results given are focused on cha nges in the soil solution concentrations during the rewetting of sever ely dried out soils. No marked nitrification pulses. were observed aft er any of the droughts carried out at any site. Only single lysimeters /sample locations showed the hypothesized reaction with increasing alu minum concentrations and decreasing pH values, but spatial heterogenei ty was high and during rewetting. In the fil st soil solution samples after the drought, occasionally distinct peaks of NH4+, DOC and K+, an d paltry also N-org,,., and HPO42- appeared. For the BH site the respe ctive potassium concentrations increased 10 to 20 times compared to th e level of the control plot. Presented data indicate a certain potenti al for nitrification pulses at the BH and KH site, but probably due to a fast root uptake. below the looting zone no nitrate was determined. At the HW site, the drought induced reactions in the soil solution co mposition were only visible in humus water samples. At SL: nitrate con centrations were generally reduced at the drought plot. During rewetti ng in 1992 samples fi om a single lysimeter indicated a clear nitrific ation pulse. Applied flux calculations for SL showed a significant inc rease of the nitrate budget but a distinct decrease of the potassium b udget. It is concluded that drought phases can influence the element c ycling, but there seems to be no risk that forest soils w ill be subje cted to pronounced acidification after summer droughts. (C) 1998 Elsev ier Science B.V.