Biomass and litter accumulation under managed and natural tropical fallows

Abstract

The accumulation of biomass and litter and changes in vegetation structure by six managed leguminous fallows, planted in a previously cultivated Ultisol (Acrisol) in the Peruvian Amazon, were compared with that of naturally established secondary vegetation. The planted fallows included: Centrosema macrocarpum (CM), Pueraria phaseoloides (PP), Stylosanthes guianensis (SG), Desmodium ovalifolium (DO), Cajanus cajan (CC), and Inga edulis (IE). The aboveground mass of planted legumes, trees, herbaceous vegetation, and litter and root mass were measured at 4, 8, 17, 29, 41, and 53 months after fallow initiation; leaf area index (LAI) and leaf area density (LAD) were measured at 4, 8, and 17 months. After 8 months, total mass in the IE, CC, and DO treatments was significantly greater than in the CM and PP treatments; total mass in the natural fallow (NF) treatment was similar to that in the IE, CC, and DO treatments after 29 months. Aboveground biomass contributed about 75–85% of total mass and was greater when trees were present. Aboveground biomass ranged from 8 (CM) to 63 t ha −1 (IE) at 53 months. Planted legume biomass in most treatments increased until 17 or 29 months, but later declined when many of the legumes were replaced by invading trees. By 53 months, planted legume biomass represented less than 10% of aboveground mass in all treatments except CM and IE. The LAI increased from 4 to 17 months, but there were few differences among treatments at any date; LAD greatly decreased with time in the treatments with trees. The formation and maintenance of dense vegetation canopies with high LAD in the CM, SC, and PP treatments inhibited the invasion and growth of trees. Biomass data at 53 months suggest that the IE and DO treatments accumulated similar amounts of biomass and litter as the NF treatment because of the rapid establishment of planted legumes and the successful invasion of trees and that the CM, SC, and PP treatments performed poorly because of the exclusion of trees. Further research combining trees and other leguminous or non-leguminous vegetation is required to develop management practices that maximize biomass accumulation during managed fallows.