Leaching of nitrogen in Swedish agriculture - a historical perspective

There is a need to examine long-term changes in nitrogen leaching from arab le soils. The purpose of this study was to analyse variations in specific l eaching rates (kg ha(-1) per year) and gross load (Mg per year) of N from a rable land to watercourses in Sweden from a historical perspective. The sta rt of the study was set to 1865 because information on crop distribution, y ield and livestock has been compiled yearly since then. The SOIL/SOILN mode l was used to calculate nitrogen leaching. Calculations were done for cerea ls, grass and bare fallow for three different soil types in nine agricultur al regions covering a range of climatic conditions. Results indicate that b oth specific leaching rates and gross load of nitrogen in the middle of 19t h century were approximately the same as they are today for the whole of so uth and central Sweden. Three main explanations for this were (1) large are as of bare fallow typical for the farming practice at the time; (2) enhance d mineralisation from newly cultivated land; and (3) low yield. From 1865, i.e. the start of the calculations, N leaching rates decreased and were at their lowest around 1930. During the same period, gross load was also at it s lowest despite the fact that the acreage of arable land was at its most e xtensive. After 1930, average leaching increased by 60% and gross load by 3 0%, both reaching a peak in the mid-1970s to be followed by a declining tre nd. The greatest increase in leaching was in regions where the increase in animal density was largest and these regions were also those where the natu ral conditions for leaching such as mild winters and coarse-textured soils were found. Extensive draining projects occurred during the period of inves tigation, in particular an intensive exploitation of lakes and wetlands. Th is caused a substantial drop in nitrogen retention and the probable increas e in net load to the sea might thus have been more affected by this decreas e in retention than the actual increase in gross load. (C) 2000 Elsevier Sc ience B.V. All rights reserved.