Slash and burn agriculture in the humid forest zone of southern Cameroon

Keywords: crop yields, earthworm cast, Inga edulis , land use, nutrient flows, nutrient stocks, nutrient management, slash and burn A field study was conducted on acid soils in the humid forest zone of Southern Cameroon, to characterize the traditional slash-and-burn land uses, assess the major effects of land use change on soil nutrient stocks, flows, and soil biological quality, and to explore alternatives for sustainable land management. The typical land use chronosequence in the area after forest felling includes essep (cucumber-based), banana, and afup (groundnut/cassava-based), and is interrupted by short (after banana) and long (after afup) fallows. Moreover, farmers have cocoa farms where many shade trees are kept. Yield declines in farmers' fields were attributed to diseases and weed infestation (56%) and soil properties (44%). Burning is practiced prior to essep, and prior to afup. It reduces the weed seed-bank, cleans the field and improves short-term soil fertility, but, together with changing land use, it strongly reduces standing biomass, carbon and nutrient stocks in the vegetation. The forest carbon stock decreased from 199 Mg.ha -1 to 102 Mg.ha -1 in essep , and to 64 Mg.ha -1 in banana farm. Nutrient stocks showed the same trend, but Chromolaena short fallow, that followed banana, recovered most of the P. The cocoa plantation had 53 % of the carbon stocks of the original forest. Soil carbon stock was less affected than vegetation stocks. Burning increased P, K, Ca and Mg available stocks in essep and afup . Lowest 'system' C and N occurred in afup , which is followed by a long fallow to restore soil fertility. The nutrient balance at farm level was strongly negative, i.e., -72.6 kg N, -4.8 kg P and -38.2 kg K ha -1 yr -1 , showing its 'no external input' character, where food and wood are derived from natural stocks. Major losses were due to burning, leaching and the non-recycling of farm residues. Only the cocoa farm had a positive nutrient balance: +9.6 kg N, +1.4 kg P and +7.6 kg K ha -1 . yr -1 , as burning is absent, leaching modest, and deep capture by shade trees providing inputs to the productive system. Simple scenarios showed that recycling farm residues is able to redress the P and K balance, and avoiding burning could even turn the entire nutrient balance positive. Burning also negatively affected earthworm density and casting activities. Up to 95% loss in density was recorded, and casting activity was inhibited during 14 and 19 months in land use systems following afup and essep respectively. Inga fallow proved to favour rapid and intensive casting just as the forest ecosystem. Total cast production was: 5.9 Mg.ha -1 in afup , 3.2 Mg.ha -1 in forest and Inga , and 2.9 Mg.ha -1 in essep after two years, but although the casts were richer in nutrients than the topsoil, the nutrients recycled from casts alone were insufficient for sustained crop production. Inga edulis was found to be a suitable planted fallow, providing several benefits to farmers and follow-up crops. Inga fallows produce more biomass (between 44.5 and 62 Mg ha -1 ), and accumulate more C and N than natural fallow. Maize following Inga fallow yielded 800 to 2200 kg. ha -1 , against a mere 200-400 kg.ha -1 after natural fallow. Burning Inga residues gave ngon (cucumber) production of 300 kg.ha -1 , which is similar to yields obtained in essep following natural forest. Mulching instead of burning, however, only gave 50 kg.ha -1 . An innovative on-farm approach was used in the development and implementation of planted fallows, ensuring high adoption. Inga edulis planted fallow can play a leading role among strategies to fight the gradual process of land degradation in the land use chronosequence, providing wood and fruits, and mimicking the natural forest to a considerable extent.