Soil phosphorus availability and fine root proliferation in Amazonian agroforests 6 years following forest conversion

Abstract

In the Amazon Basin, where deforestation rates are among the highest in the world, raising land productivity with perennial crop-based agroforestry systems may allow small farmers to meet economic demands with less forest clearing. Although tree-based agroecosystems may cycle nutrients more efficiently than other agricultural systems, difficulties maintaining phosphorus (P) availability to plants growing in tropical Ultisols and Oxisols may threaten the long-term sustainability of Amazonian agroforests. To determine how soil nutrient dynamics are altered when primary forest is converted to perennial crops, soils from eight 6-year-old peach palm ( Bactris gasipaes Kunth)–cupuassu ( Theobroma grandiflora) plantation agroforests were compared to those of adjacent native forests in Acré, Brazil. The response of fine roots to P microsite enrichment by agroforest and native forest plants on eight farms was measured using a root ingrowth bioassay to determine if a P-limitation already threatened agroforest sustainability. Localized root proliferation by individual agroforest species was also examined as a component of interspecific competition. The impact of agroforest adoption by small farmers on forest clearing was qualitatively examined through interviews and focus groups held with farmers participating in the field studies. Six years after establishment on sites initially occupied by native terra firme forest, exchangeable base cations, ECEC and pH were greater in agroforest soils than in those of adjacent forests. Extractable inorganic P was 30–50% lower in agroforest soil, suggesting that P uptake by the aggrading agroecosystem outpaced its restoration in the soil solution by other P pools. After 100 days, cupuassu root length and mass were greater in P-treated cores buried in agroforest alleys, but not in cores buried in rows. While a P deficiency in any of the agroforest components could not be inferred using the root ingrowth technique, the study of root proliferation demonstrated both the potential for interspecific competition among peach palm and other agroforest components, as well as opportunities for alleviating the impact of competition through management practices. Discussions with farmers revealed that this type of commercial plantation agroforest, unless managed to maintain productivity, may actually contribute to more forest clearing on small farms because of a lack of farmer confidence in the future of the system. Thus, despite more efficient nutrient cycling in tree-based agroecosystems, the decline in agroforest soil P following establishment must be addressed through management practices if plantation agroforests are to remain a sustainable land use in Amazônia.