CROP FUNCTIONAL STRUCTURE INFLUENCES THE C:N RATIOS OF PRUNED BIOMASS AND LITTER IN SUCCESSIONAL AGROFORESTRY SYSTEMS

Abstract

Functional identity (dominant values of plant traits) and functional diversity (variation in trait values) are part of the functional structure of a plant community and may direct species choice in intercropping to promote multiple agroecosystem services. The objective of this study was to evaluate the effect of the functional structure of polycultures on the mass and C/N ratio of pruned crop biomass and litter layer in low–input Successional Agroforestry Systems on low–fertility sandy soil in Santa Catarina, humid subtropical Brazil. We established a long–term field experiment in 2016, in randomized complete blocks with six replications and three treatments that varied the functional structure of polycultures. We collected four traits of all intercropped plants: leaf N concentration (LNC), leaf pH, specific leaf area (SLA) and leaf dry matter content (LDMC). To characterize the functional identity, we calculated the trait mean, weighted by the relative biomass in each intercrop community (CWM). Functional diversity was calculated by Rao’s quadratic entropy, also weighted by each crop’s relative biomass. We performed principal components analysis and fitted the data using linear mixed models. Two principal components jointly explained 79% of data variation that measured the functional structure of communities. The first component (PC1) was strongly negatively related to the functional diversity indices, while the second (PC2) was strongly positively related to the functional identity indices (CWM of pH, LNC and SLA). As predicted by the mass–ratio hypothesis, intercrops dominated by species with large and thin leaves (high specific leaf areas), high leaf N concentrations and high leaf pH resulted in plant communities with low C:N ratio in leaves and stems of biomass available for pruning and produced litter layers with low C:N ratio. This study suggests that functional identity may be more important than functional diversity to manage biomass and litter quality in agroforestry systems on nutrient–poor soils.