Nitrogen use in mixed tree crop plantations with a legume cover crop
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In a multi-strata agroforestry system in central Amazonia, we studied the nitrogen (N) use of two indigenous fruit tree species, Theobroma grandiflorum Willd. (ex Spreng.) K. Schum. (cupuaçu) and Bactris gasipaes Kunth. (peachpalm) for heart of palm production, and a legume cover crop, Pueraria phaseoloides Roxb. (Benth.) (pueraria). 15N was applied at a rate of 1 kg ha−1 twice at the beginning and at the peak of the rainy season, in a split plot design under either cupuaçu, peachpalm or pueraria together with fertilizer N usually applied (95.4 and 42.4 g N tree−1 for cupuaçu and peachpalm, respectively). Plant and soil 15N content and total 15N uptake were measured for 1 year. The highest N uptake by the trees occurred from areas underneath their canopy being more than 70% of their total N uptake. During the dry season, pueraria also took up most of its N (more than 70%) from the area underneath its own canopy. During the rainy season, however, pueraria utilized N from the area under cupuaçu (27–40%) and peachpalm (34–47% of the total N uptake by pueraria). Cupuaçu took up between 12 and 26% of its N from the area covered by pueraria, peachpalm slightly less with 10 to 18% (significant only at the end of the rainy season; P<0.05). Competition for N uptake between the trees was negligible. The above-ground recovery was highest in cupuaçu (15% of the applied 15N), followed by pueraria (11%) and peachpalm (3%). Pueraria proved to be very important for the N cycling in the mixed tree cropping system recovering most (31%) of the applied 15N in plant and soil in comparison to cupuaçu (20%) and peachpalm (21%). However, the natural 15N abundance of the tree leaves did not show a significant transfer of biologically fixed N2 from pueraria to the trees (P>0.05) and the cover crop did not improve tree N nutrition. The investigated fruit trees did not benefit from biologically fixed N2 of the legume cover crop due to their low lateral root activity and the high available soil N contents largely being an effect of the amount and placement of mineral fertilizer.
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