Abstract. The effect of forest canopy on the chemical composition of throughfall was studied in four forests, two coniferous plantations of Pinus radiata and two deciduous forests (Quercus robur and Q. rubra, respectively) near Bilbao (Bizkaia), Basque Country (Northern Spain) during one year. The study areas are assumed to have different levels of pollution: Manzanal suffers industrial pollution mainly of SO4 2-and N.NO3-, while Durango has industrial and agricultural pollution mainly of N.NH4 + and SO4 2-.The study presents data on bulk precipitation and throughfall, and on the quantities of inorganic N (N.NH4++ N.NO3-), SO4 2-, Ca 2+, Mg 2+, Na +, K + and CI-reaching the forest floor. The nutrient input to the forest floor differed within the four studied sites. The capacity of neutralization of the deposition was better on deciduous forest than on the coniferous. Leaching in association with the canopy washoff were the most significant sources of base cations in deciduous and coniferous throughfall. Despite the pollution levels in Manzanal and Durango no effect on tree leaves were observed during the study period.

Denitrification losses from a poorly drained clayey loamy soil under natural pasture were measured over a two-year period using the acetylene inhibition technique. Plots received two different applications of fertilizer as calcium ammonium nitrate or cow slurry (a total of 145 -290 kg N ha-1 in 1991 and 120-240 kg in 1992). In the first year, N losses in the mineral treatments were about 4 times greater than losses in the slurry treatments. In the second year losses in the slurry treatments increased in such a way that losses in the higher slurry application became similar to those for the two mineral treatments. Soil nitrate was the factor producing differences between treatments. In this way, N mineralization in periods between fertilizations coinciding with high soil water contents was responsible in the second year for the increase in N losses in the slurry treatments. Denitrification rates greater than 0.1 kg N ha -~ day -1 occurred at soil water contents > 33 % (air filled porosity < 26 %) and soil nitrate contents > 1 mg N kg-1 dry soil. Spring and autumn were the seasons of highest risk of denitrification because of N fertilizations coinciding with periods of soil saturation with water. Winter losses were low, but this is a period when there is a risk of denitrification in wetter seasons, particularly for a slurry application management.

Dairy farming is the main agricultural activity of the Basque Country. A dairy farm is characterized as a system with soils and crops, forage, cattle and manure as main components, and in such a system, nutrient cycling is very important to maintain soil fertility and optimize forage production. To quantify nutrient transfers in the cycle, a simple system was developed and has been applied to seventeen farms to examine its ability to achieve a balanced P and K fertilization. These farms have provided data on inputs (fertilizer, feeds, concentrates), pasture and manure management, and outputs (milk production), and soil samples have been taken from farm pastures. Phosphorus and K in excreta and uneaten pasture is used with a relatively high efficiency as suggested by the relatively high efficiency of P and K utilization by the pasture that usually ranges from 70 to 90%. Concentrate feeding (3000 kg cow-t yr-l) represents one of the main P and K inputs in Basque Country dairy farms, averaging 26 and 66 kg ha -l, respectively. Besides, release of K in the soil through slow liberation from non-exchangeable sites was estimated as 30 kg ha -I. Thus, a high efficiency in excreta recycling would diminish substantially P and K mineral fertilizer needs. Farm nutrient budgets appear to be a convenient tool for determining nutrient shortages and surpluses at farm level, and thus they are considered as a first step to support a better management of maintenance fertilization of permanent pastures.

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