SOIL NUTRIENT STATUS OF MOIST·DECIDUOUS FORESTS AT SIGIRIYA SANCTUARY, SRI LANKA ACROSS A CHRONOSEQUENCE OF FOREST AGE

Soil nutrient status of moist-deciduous forests across a chronosequence of forest age wasdetermined at Sigiriya sanctuary, Sri Lanka to determine the patterns of nutrient build up ~with forest succession. Forty 20 x 20 m experimental plots were established at differentages of sucessional forests grown after swidden farming and in mature forests older than75 years. Two soil samples were collected from in each established experimental plots.Total carbon, nitrogen, phosphorus and mineralizable nitrogen, carbon, sodium,potassium, calcium, magnesium and clay contents were estimated.Results revealed that the total nitrogen content increases with the process of successiondespite the fact that fragmented soil litter results in exaggerating the total nitrogencontents. Fragmented soil litter accumulates in all forest due to high termite activity.Mineralizable nitrogen was found to be immobilized in the soil litter. Total carboncontent of the soils of young successional forests increased with the increase of forestbiomass and subsequent leaf litter accumulation with the process of succession. The hightotal carbon content in 10-15 year old forest soils is due to the incorporation of largeamounts of litter after the self-thinning of plant individuals at about 6-7 years of age.However, it takes a longer time to build up the labile carbon contents within the soil. Itseems that the clay content in the soil decreases with the increase of forest age, but it isactually due to the dilution of clay with organic matter. Both phosphorus and calciumcontents in the soils of young successional forests were lower but higher in 20-25 yearssuccessional and mature forests despite the fact that these were somewhat high in I yearold fallow forest. These fallow forests are the grazing grounds for domestic cattle andbuffaloes and this may cause to add some nutrients in the soil. Potassium content in thesoil is high in 10-15 years old forests and this may also be due to the incorporation ofJitter due to self-thinning of individuals.It seems that swidden farming practices result into reducing the soil nutrients but after10-15 years following swidden farming, the soil nutrient contents have increased at leaststarted building up. Therefore, forest degradation inevitably takes place if the rotationperiod of swidden farming is less than 15 years. The soil nutrient contents in 20-25 yearsold successional forests are some what similar to mature forests implicating that theseforests can be easily converted or rehabilitated into climax forests.ReferenceAnderson, J.M. and Ingram, J.S. 1993. Tropical Soil Biology and Fertility: a handbookof methods. Second edition, CABl Publishing, UK