Nitrogen use in mixed tree crop plantations with a legume cover crop.

Abstract

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. (cupuacu) 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 cupuacu, peachpalm or pueraria together with fertilizer N usually applied (95.4 and 42.4 g N tree−1 for cupuacu 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 cupuacu (27–40%) and peachpalm (34–47% of the total N uptake by pueraria). Cupuacu 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) 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.