Plant nutrient resorption efficiency in a Central Amazon rainforest

Abstract

Nutrient resorption efficiency (RE) occurs before leaf abscission, with nutrients being actively transported via phloem through abscission zones from senescent leaves to be used in other parts of the plant. It is an essential nutrient conservation strategy for tropical plants growing on soils depleted of P (phosphorus) and other rock-derived elements (K; potassium, Ca; calcium, and Mg; magnesium), influencing nutrient cycling in these ecosystems. The environment heavily influences resorption; therefore, a better understanding of nutrient resorption processes in tropical trees, which act as a carbon sink, is important when facing the rapid climatic changes. Thus, our objective was to investigate the resorption of five macro elements (C; carbon, N; nitrogen, P, K, Ca, and Mg) and three micronutrients (Fe; iron, Zn; zinc, and Mn; manganese), and the effects of leaf longevity, foliar nutrient concentration, and canopy position in RE in a lowland forest tree community in Central Amazon. The study was conducted in two experimental plots at the AmazonFACE Program (Free-Air CO2 Enrichment) in Manaus, Amazonas, Brazil. Two 40 m scaffolding towers in the center of the plot granting access to the canopy of the twelve tree species studied. Young, mature, and senesced leaves were collected, totaling 188 leaves, from 2018 to 2019 for nutrient laboratory analysis. We found that K, P, and N were the most resorbed nutrients (42%, 33%, and 7%, respectively), while Zn, Fe, and Ca were the most accumulated (-65%, -27%, and -21%, respectively). Additionally, we found that C, N, P, Fe, and Zn resorption positively correlated with their concentration in leaves. Likewise, P, N, Mg, K, and C resorption positively correlated with leaf longevity. On the other hand, canopy position influenced the resorption of three elements: C, K, and Zn resorption. Our results suggest that P was the scarcest nutrient stored in leaves; the higher resorption efficiencies for K and P than for N suggest higher plant internal nutrient recycling of K and P, likely due to their scarcity in the soil during leaf senescence, species with longer leaf life span are often assumed to have higher nutrient resorption efficiency than species with short leaf life span to reduce nutrient loss, species in the community studied can optimize leaf anatomy and physiology to make the best use of the variable light encountered regarding of its position on the forest vertical profile. These trends suggest that nutrient resorption from senescent leaves may be a general adaptive strategy for conserving nutrients by plants in tropical forests growing on nutrient-poor soils, which should be considered when predicting future scenarios.