THE DEGRADATION OF SOIL FERTILITY IN PURE LARCH PLANTATIONS IN THE NORTHEASTERN PART OF CHINA

Soil degradation was found in first generation of Dahurian larch (Lari x gmelinii (Rupr) Rupr.) plantations in northern China and also in pla ntations of other larch species such as Larix olgensis and Larix lepto lepis in the same area. Soil degradation in pure larch plantations may be exacerbated in the second rotation under continuous cropping and w as accompanied by obvious concomitant decrease in tree growth. A study on nutrient cycling revealed that poor quality of larch litter, which decomposed very slowly, was a key factor leading to soil degradation and decline in tree growth in pure larch plantations. The accumulation of forest floor litter in similar to 20-year-old pure larch stands wa s 23-28 ton ha(-1) and this was higher than 9-16 ton ha(-1) in virgin mixed Korean pine/broad-leaf forests. Larch litter took 4.4 years to m ineralise, whereas angiosperm litter lash, birch and poplar) in a pure larch stand took only 0.9-2.4 years. The progressive accumulation of litter on the forest floor acted as a heat-isolating layer which inhib ited thermal conduction and decreased soil temperature and limited soi l microbial activities. Consequently, degradation of soil fertility oc curred and this was exacerbated after two or more rotations under cont inuous cropping. The role of the understorey vegetation was significan t in nutrient recycling and maintaining sustainable soil fertility in larch plantations. A low-cost, effective way to prevent soil degradati on in pure larch plantations is to develop mixed stands formed either by creating a mixture with broad-leaved trees or by introducing unders torey vegetation (herbs and shrubs). Large areas of man-made, pure lar ch stands should be replaced by many small patches of different. uneve n-age stands and crop rotation with different tree species should be a dopted instead of continuous larch cropping. (C) 1998 Elsevier Science B.V. All rights reserved.