Nutrient cycling over five years of mixed-species plantations of Eucalyptus and Acacia on a sandy tropical soil

Mixed-species plantations have the potential to enhance nutrient cycling and increase the overall biomass of the stand. In this study, we evaluate the dynamics of nutrient accumulation and exports in mixed-species plantations of Eucalyptus urograndis (Eucalyptus urophylla S. T. Blake × Eucalyptus grandis W. Hill ex Maiden) and Acacia mangium Willd. after five years of rotation. Monocultures of Eucalyptus urograndis, with or without nitrogen fertilization (120 kg N ha−1) (E100 and E100 + N, respectively), and of Acacia mangium (A100) were established in a randomized block experimental design. Two arrangements with these species in mixed stands were also established: one with 50% of the stand density composed by each species (E50A50; 1,111 trees ha−1), and one high-density system, containing double the population of each species (E100A100; 2,222 trees ha−1). Aboveground biomass and litter nutrient contents, nutrient retranslocation from leaves, and nutrient export through wood harvesting were measured over a full rotation. Eucalyptus in E50A50, despite having half of the population of trees (555 trees ha−1), accumulated the same amount of nutrients in total aboveground biomass in relation to E100, suggesting a lower nutrient limitation to the growth of Eucalyptus in this mixed stand. Conversely, Eucalyptus in E100A100 did not accumulate proportionally larger amounts of nutrients in relation to E50A50, possibly due to intra- and interspecific competition in this high-density arrangement. The deposition of N and K via litter was higher in mixed-species stands than in Eucalyptus monocultures. Also, P, Ca and Mg depositions were higher in mixed-species stands than Acacia monocultures. These results suggest higher nutrient cycling and availability in the mixtures, especially after 30 months. The retranslocation of N in Eucalyptus and Acacia leaves decreased with age. At 60 months, Eucalyptus trees in E100A100 retranslocated less N than the trees in E100, probably reflecting the higher availability of soil N in relation to E100. At this same age, the Eucalyptus trees planted in E50A50 exported less N, P, K and Mg in relation to E100. However, at stand level, mixtures exported more N due to the enrichment of this nutrient in the soil promoted by Acacia. This study shows the importance of introducing Acacia in Eucalyptus plantations to promote a positive balance of nutrients for subsequent rotations and additional ecological benefits to the ecosystem due to N2-fixation from Acacia trees.