Growth Response and Nitrogen Accumulation of 5 Native Tree Species 6 Years after Planting of Improved Fallow.

Growth Response and Nitrogen Accumula­­­­tion of 5 Native Tree Species 6 Years After Planting of Improved Fallow

 

Aaron Joslin¹, Daniel Markewitz¹, Lawrence Morris¹, Francisco de Assis Oliveira², Ricardo Figueiredo³, and Oswaldo Kato⁴

Abstract

Small-holding farmers of the Brazilian Amazon often use a rotation of secondary forest, slash-and-burn land-clearing and fallow phase regeneration for agriculture.  In recent decades reduction of the fallow phase from ~20 to ~5 years has limited nutrient accumulation by fallow vegetation to sustain future crop growth.  Slash-and-mulch and improved fallow schemes, including use of native nitrogen-fixing species, have been investigated to address the issue.  In the current study in the eastern Amazon of Brazil, a 7-year old forest site was slash-and-mulched and four treatments applied, no fertilizer, no N-fixer; no fertilizer, N-fixer; P+K fertilizer, no N-fixer; P+K fertilizer, N-fixer.  Manioc was planted in all plots at establishment and harvested after 20 months. The current results only address planted tree growth and soil nutrient status after manioc harvest and four years of fallow.  Use of P+K fertilizer increased tree growth of four of the five planted species.  In the presence of the N-fixer Inga edulis, trends of increased growth and survival among all tree species, except for Parkia multijuga, were observed.   After six years, fertilization with P and K dramatically improved tree growth, estimated volume and biomass.  The N-fixer lowered survival of other species and increased estimated N uptake of planted trees.  Use of P+K fertilizer without N-fixer would allow for commercial harvest of S. amazonicum at the end of one 7-year crop-fallow cycle, and use of P+K fertilizer increased planted-tree N-content 3x in the presence of I. edulis compared to unfertilized.