Waldkonversion und Bodendegradation im tropischen Tiefland von Ostbolivien

Abstract

The objective of the present study was to assess the main types of soil degradation such as nutrient decline (e.g. soil organic matter) and soil compaction caused by different land use systems, as well as the susceptibility of the soils to wind erosion in the tropical lowlands of Eastern-Bolivia.To obtain information about current and former land uses (period: 1984-2001) and the applied cultivation system (e.g. small holder, industrial farming) several Landsat images were analyzed. The main soil properties (soil texture, soil organic matter, bulk density) were spatially predicted using terrain attributes and climate variables. Relative changes in soil properties (soil organic carbon, total nitrogen and bulk density) following cultivation of previously untilled soils, were derived using data from a literature review and local soil studies. To assess the current state of specific soil properties, the rates of change were related to the respective initial level. The potential risk of wind erosion was assessed by the relative soil erodibility and the wind erosivity.Within the study area, approximately 13.000 km2 forest were converted to agricultural use between 1984 and 2001. About four-fifths of the total deforestation account solely for the period off 1992 to 2001. In the northern sector, the estimated deforestation rates during the 1990s were among the highest in the world. The older cropping areas are located closer to the city of Santa Cruz, in the so-called central zone . Due to ongoing soil degradation processes, approximately 700 km2 of land in this sector are currently not profitable for cultivation anymore and hence mostly turned into extensive pasture land.Conventional tillage (plow-based systems) has been identified as the principal factor responsible of triggering soil degradation in the Santa Cruz region. This technique led to an average loss of about 50 % of organic matter in the topsoil within only two decades of continuous cropping. The analysis of soil compaction in the study area has shown, that under mechanized cultivation severe compaction with a negative impact on plant growth and yields can occur in all soil types. The mean relative increase of topsoil bulk density was between 14 and 43 %. No negative changes in physical soil properties were observed for the traditional slash and burn system.About 14 % of the winds in the central zone are highly erosive, with a proportion of 21 % during the dry season. In contrast, the mean proportion of erosive wind speeds in the northern and eastern part of the study area only accounts for 1-4 %, with a maximum of 8 % during the dry season. Regarding the textural properties, soils that contain 60 % or more fine sand and a proportion of aggregates less than 15 % are most susceptible to wind erosion. The results of the study show, that conventional farming practices under tropical and subtropical conditions lead to a rapid loss of soil fertility. In this context the rapid loss of soil organic matter is of particular importance, because of its positive function in the maintenance as well as improvement of many chemical (e.g. nutrient pool) and physical (e.g. aggregate stability) soil properties. A possible alternative to maintain soil organic matter pools in tropical agriculture consists of the conversion from conventional to conservation tillage management systems, which have already been successfully applied in other tropical or sub-tropical countries for a long time. These techniques should be taken into account for future agricultural practices in Eastern-Bolivia. Only soil conservation practices maintain long-term soil fertility and thus productivity in areas with warm climates.