Variation in fine root traits reveals nutrient-specific acquisition strategies in agroforestry systems

Abstract

Changes in root functional traits reveal important nutrient acquisition strategies, with well documented patterns in root trait expression within complex communities or along gradients of singular nutrients. In this field study, we investigate intra-root functional trait expression with six soil macro- and micro-nutrients in Theobroma cacao agroforestry systems. Using image, chemical, and spatial analysis, the fine root distribution, architecture, and morphology of T. cacao were compared to localized soil nutrients on two-dimensional soil profiles with conspecific and heterospecific neighbours. Fine-scale variation in soil nutrients was observed within the range of T. cacao root systems. Higher NH4+ and Ca2+ was associated with greater root length and biomass densities, coupled with greater investment to individual roots, expressed as increased fine root tissue density and diameter and lower specific root length. Conversely, NO3− had the opposite effect. Overall, roots tended towards higher acquisitive trait values when next to a shade tree. Plants generally employ several concomitant and at times opposing strategies for nutrient acquisition in heterogeneous soils. We show that fine-scale root plasticity is highly linked to localized nutrient-specific and neighbour-specific effects, driving patterns of nutrient acquisition in agroforestry systems.