Soil nitrogen mineralization under tree crops and a legume cover crop in multi-strata agroforestry in central Amazonia: Spatial and temporal patterns

Under rainforest vegetation, the central Amazonian Ferralsols are character ized by relatively high availability of N in relation to other nutrients. A fter forest clearing, several tree crops also have not shown yield response s to N fertilizer. To elucidate the mechanisms of this apparent N sufficien cy, the mineralization of soil N was measured under three tree crops and a leguminous cover crop (Pueraria phaseoloides) in a multi-strata agroforestr y system at two fertilizer input levels on a Xanthic Ferralsol in central A mazonia. In situ incubations of topsoil (0-10 cm) were carried out using th e buried-bag method on five occasions over ten months. The highest minerali zation rates were found under the cover crop, intermediate rates under rubb er trees (Hevea brasiliensis) where the soil was also covered by the cover crop, and lowest rates under peach palm (Bactris gasipaes) and cupuacu (The obroma grandiflorum) with no cover crop. The increased N mineralization und er the cover crop was due to more total N in the soil, higher soil moisture and, presumably, a larger pool of readily mineralizable soil N compared wi th the soil under the tree crops. Other fertility parameters also differed significantly between sampling positions within the plots, but this had no major influence on net N mineralization. Also, the input of NPK fertilizer and dolomite had no significant influence on N mineralization, indicating t hat N mineralization was not nutrient-limited. High total N mineralization rates in the soil (approximately 350 kg ha(-1) a(-1) at 0-10 cm depth! expl ained earlier observations of nitrate leaching into the subsoil under multi -strata agroforestry at this site. Considering the spatial patterns of N mi neralization with maximum values under the cover crop, the exploration of t he soil volume by crop roots should be maximized to increase the uptake of mineralized soil N by the crops and reduce nitrate leaching. Appropriate me asures are narrow tree spacing, use of annual and semi-perennial intercrops and encouragement of the lateral root development of the trees. In additio n, the mineralization of soil N close to the tree crops can be influenced t hrough the management of the cover crop. in view of the high total N minera lization rates in the system and unclear yield responses of tree crops to N fertilizer, the application of N fertilizer to tree crops with well-develo ped root systems and a well-managed cover crop may often be unnecessary on this soil type. This may facilitate the further development of tree crop ag riculture in the region.