EFFECTS OF THE AGROFORESTRY LAND-USE SYST EM ON MINERAL NITROGEN-CONTENT AND WATER POTENTIAL IN A HAPLIC-LUVISOL ON THE GROWTH OF ACER-PSEUDOPLATANUS

Results from three years are presented for an agroforestry system with Acer pseudoplatanus without (bare soil, treated with herbicides) and with under cropping, which was either Lolium perenne of Trifolium repe ns. The soil, a Haplic Luvisol derived from loess, had been intensivel y cultivated by agricultural crops before. Without undercropping leach ing of nitrate was observed especially in the first, but also in the f ollowing years of the experimental period. This is due to the low and late N-demand of tress. Remaining contents of mineral nitrogen in the soil were sufficient for an optimal tree growth. With Lolium perenne a s undercrop the mineral nitrogen content in the soil decreased in 0-60 cm depth as a result of N-uptake already in the first year. Leaching was observed from 60-90 cm depth. In the following years mineral nitro gen contents of the soil remained low, and were not sufficient for an optimal tree growth. Tree growth was further affected because of compe tition for water between gras and trees. With trifolium repens mineral nitrogen in the soil was much higher than the demand for trees and un dercrop due to symbiontic N-fixation by clover with subsequent mineral ization. Although low water potentials were recorded similar to gras, competition for water did not diminish tree growth. Hence, optimal tre e growth is guaranteed, but leaching of nitrate cannot be avoided. For all treatments leaching of N occurred also in early spring. Up to now , there was no difference if tree growth between bare soil and Trifoli um repens as undercrop, tree growth was much affected by Lolium perenn e as undercrop.