04017naa a2200385 a 450000100080000000500110000800800410001910000160006024501660007626000090024230000140025150000270026552028070029265000170309965000220311665000130313865000250315165000090317665000190318565000260320465000210323065000240325165000290327565000160330465000090332065300170332965300130334665300110335965300110337070000160338170000270339770000150342470000240343977301680346316681772022-04-04       1999    bl uuuu     u00u1 u    #d1 aSCHROTH, G.  aSpatial and temporal patterns of N availability and N mineralization under tree crops and a cover crop in a multi-strata agroforestry system of central Amazonia.  c1999  ap. 58-89.  aRelatório Anual 1998.  aUnder rainforest vegetation, the Oxisols and Ultisols which dominate Amazonian uplands are characterized by relatively high availability of N in relation to other nutrients. When the rainforest vegetation is converted into agricultural systems, the disruption of the nutrient recycling mechanisms of the forest under conditions of intensive rainfalls and permeable soils increases the potencial for N leaching. In a previous study, substantial amounts of mineral N were found in the subsoil of central Amazonia Oxisol under various tree crops and a legumenous cover crops, indicating that even under perennial cropping systems, much of the mineralized soil and fertilizer N may be lost unproductively. We hypothesized that the spatial and temporal patterns of N availability and N mineralization in the soil of a polycultural system would influence the efficiency with which N is taken up by the trees instead of being leached. The analysis of such pattens coul lead to strategies for improving this efficiency and reducing the dependence of the system on external nutrient inputs. The concentration of mineral N and the net N mineralization in the topsoil were measured at five occasions during 10 months in a multi-strata agroforestry system on an Oxisol in central Amazonia, Brazil. The system was composed of three tree crop species and a leguminous cover crop, and was studied at two fertilization levels. The net N mineralization was highest under the cover crop, intermediate under a tree crop where the soil was also covered by the cover crop. This resulted in higher mineral N concentrations in the soil under the cover crop than under the tree crops during part of the rainy season. The increased N mineralization under the cover crop was due to higher total N  in the soil, higher soil moisture and presumably a larger pool of readily mineralizable N components in the soil compared to 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 fertilization level had no significant influence on N mineralization. As most of the N was mineralized under the cover crop at some distance from the trees, the uptake of the mineralized N by the tree crops could presumably be increased, and N leaching reduced, by a higher tree density and an altered management of the cover crop to increase N mineralization in the soil close to the trees. In view of the high total N mineralization rates in the system (349 kg ha-1 yr-1) and unclear yield responses of the tree crops to N fertilization at this site, it is concluded that after an establishment phase, the fertilization with N of tree crops with well-developed root systems may not always be necessary on this soil type.  aagroforestry  amultiple cropping  anitrogen  anitrogen fertilizers  asoil  asoil fertility  atropical rain forests  aCultivo Multiplo  aFertilidade do Solo  aFloresta Tropical Úmida  aNitrogênio  aSolo  aAgrofloresta  aAmazonas  aBrasil  aManaus1 aSALAZAR, E.1 aSILVA JUNIOR, J. P. da1 aSEIXAS, R.1 aMACEDO, J. L. V. de  tIn: SHIFT PROJECT ENV 23 (Manaus-AM). Recuperacao de areas degradadas e abandonadas, atraves de sistemas de policultivo. Manaus: Embrapa Amazônia Ocidental, 1999.