Impact of Eucalyptus PF1 and Pinus caribaea on soil properties, and potential for silvopastoral systems in southern Congo

Deterioration of soils chemical and physical properties are concerns of great importance in intensive cropping systems of tropical environments. Management and conservation of soils productive capacity are prerequisite for both sustainable production of crops and the environment. Nevertheless, the complexity of soil reactions in tropical ecosystems to various plant production methods as a sink and buffer to pollutant compounds are often ignored by land managers and planners.; The overwhelming literature indicates that plantations of fast growing tree species are causing under certain silvicultural practices severe degradation of soil properties particularly in tropical regions. A regular and periodic assessment of soil conditions under intensive forest plantation systems are advisable for the following purposes: (1) detect and predict changes in soil conditions; (2) develop models capable of assessing ecosystem functioning; (3) and as the basis to design and implement Best Management Practices (BMPs) to ameliorate productive capacity of the soils. The presented research addresses the above concerns.; Soil fertility parameters including pH, exchangeable Al, Ca, Mg, and K, available P, soil organic matter (SOM), organic carbon (OC), total nitrogen (TKN), cation exchange capacity (CEC), carbon and nitrogen ratio (C/N) and bulk density (BD) were comparatively assessed in monospecific stands of Eucalyptus PF1 and Pinus caribaea at the age of four, eight and twelve years, and under native grass vegetation in southern Congo. The experimental design of the study was a randomized complete block with split plot of species containing nine replications and two soil horizons, A and B.; The presented study is comprised of three parts. The first quantifies changes of soil acidity over time; the second correlates soil organic matter with plantation type and forest practices; and the third analyzes short and long term problems of the plantations and recommends a silvopastoral system as appropriate land-use system.; The results show significant differences between eucalyptus and pine stands, and less variability under grass vegetation. Soil pH, exchangeable Ca and Mg increased with stand age under pine but decreased under eucalyptus stands. The above properties did not change under grass vegetation. Soil exchangeable K, SOM, OC and TKN were greater under eucalyptus and grass vegetation than under pine stands. There were no treatments effects on CEC of soils. The bulk density increased significantly in the A horizon following tree harvesting at the age of six years. Soil pH and exchangeable Al were more affected under tree plantations. Changes of soil pH remained within the Al/Fe buffer range. The observed changes may be attributed to temporal disruption of the ions cycle caused by forest activities, plant nutrient uptake, decomposition and mineralization processes. A partial restoration of soil pH, build up of organic matter and base cations can be achieved by incorporating livestock, grass-legume pasture mixed with trees. The recommended silvopastoral system is less environmentally manipulative. Conceptually, this system is functionally more sustainable for production of the crops and the environment compared to the current monoculture of eucalyptus and pine plantations. This agroforestry system may also yield greater socioeconomic and other environmental benefits.