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Legume cover crops have various important effects on nutrient cycling in tropical fruit tree production: they (i) fix atmospheric N 2 and may amend the soil and trees with N, (ii) recycle nutrients, and (iii) affect soil nutrient availability. Aboveground N accumulation usually is very large in legume cover crops as seen from Pueraria phaseoloides being 8 to 14 times higher than in intercropped fruit trees. Biomass production and N turnover will often exceed these values. Nutrient cycling may be significantly enhanced by the cover, leading to less nutrient leaching and higher nutrient contents in the topsoil, such as K. The amount of biological N fixation is an important factor for the N cycling. As a result, total soil N contents may often increase and N availability is enhanced. For nutrients other than N which cannot be supplied by the legume from external sources like P, the cover crop may induce nutrient competition resulting in lower tree nutrition.

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Fruit tree production is gaining an increasing importance in the central Amazon and elsewhere in the humid tropics, but very little is known about the nutrient dynamics in the soil-plant system. The present study quantified the effects of fertilization and cover cropping with a legume (Pueraria phaseoloides (Roxb.) Benth.) on soil nitrogen (N) dynamics and plant nutrition in a young guarana plantation (Paullinia cupana Kunth. (H.B. and K.) var. sorbilis (Mart.) Ducke) on a highly weathered Xanthic Ferralsol. Large subsoil nitrate (NO3-) accumulation at 0.3-3 m below the guarana plantation indicated N leaching from the topsoil. The NO3- contents to a depth of 2 m were 2.4 times greater between the trees than underneath unfertilized trees (P<0.05). The legume cover crop between the trees increased soil N availability as shown by elevated aerobic N mineralization and lower N immobilization in microbial biomass. The guarana N nutrition and yield did not benefit from the N input by biological fixation of atmospheric N2 by the legume cover (P>0.05). Even without a legume intercrop, large amounts of NO3- were found in the subsoil between unfertilized trees. Subsoil NO3- between the trees could be utilized, however, by fertilized guarana. This can be explained by a more vigorous growth of fertilized trees which had a larger nutrient demand and exploited a larger soil volume. With a legume cover crop, however, more mineral N was available at the topsoil which was leached into the subsoil and consequently accumulated at 0.3-3 m depth. Fertilizer additions of P and K were needed to increase subsoil NO3- use between trees.

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