ORGANIC-MATTER AND NUTRIENT DYNAMICS IN 3 ECOSYSTEMS OF THE SIERRA-DE-BEJAR MOUNTAINS (SALAMANCA PROVINCE, SPAIN)

We studied the production and decomposition of litter and the return o f nutrient elements to the soil in three forest ecosystems in the ''Si erra de Bejar'' mountains. Three plots representative of these ecosyst ems with similar soil characteristics were studied: a climax Quercus p yrenaica forest, a paraclimax Castanea sativa grove, and a replanted P inus sylvestris forest. To collect litters, ten boxes were placed at r andom in each of the plots. The plant material was later sorted and ch emically analysed. To study the litter decomposition, 2 cm mesh litter bags, containing 50 g of air-dried litter, were buried in the respect ive forest plots; random litter bags were collected at approximately t wo-month intervals over a period of three years. Mean aboveground tota l litter and leaf litter production of the trees are highest in the pi ne forest and lowest in the oak forest, but leaf/needle production is similar (about 3.5 Mg.ha(-1).yr(-1)) in the three ecosystems. Jenny's decomposition index is highest in the chestnut grove (0.22) and lowest in the pine forest (0.13); so, a correspondingly higher loss of organ ic material (36%) is observed in the decomposing chestnut leaves compa red to that of pine needles (30%). However, the release of nutrient el ements from the litter does not relate to decomposition rate. Only in chestnut litter was a net release of nutrients found, so the annual N, P, K, Ca, etc. requirements of trees must be mainly obtained from oth er soil sources, e.g. decomposing roots and mineralisation from humus layers.