Nitrogen and phosphorus pools and fluxes in upland and seasonally flooded forests and woodlands of the Cerrado-Pantanal transition of Brazil

Abstract

Brazilian tropical savanna (referred to as Cerrado) has enormous structural variability ranging from tree- and shrub-free grasslands to tall, dense forests that are found in both well-drained upland and seasonally flooded (hyperseasonal) landforms. These structural and hydrological variations cause variations in aboveground carbon (C) storage, and presumably nitrogen (N) and phosphorus (P) storage and fluxes; however, the effects of these structural and hydrological variations on ecosystem N and P pools and fluxes are less well-known. Thus, our objective was to analyze N and P pools and fluxes for a spectrum of Cerrado physiognomies in both upland and hyperseasonal landforms. N and P pools and fluxes were compiled over an 8-10 year period in structurally different forms of Cerrado, including both upland and hyperseasonal grass dominated (campo sujo), typical wooded (sensu stricto) and dense wooded cerrado, and forested Cerrado stands located in the Cuiaba Basin and Northern Pantanal of southern Mato Grosso, Brazil. We found that aboveground N and P storage was generally higher in forested Cerrado than in non-forest landcover types, regardless of hydrology; however, these patterns were not consistent as wood N and P storage was just as high in typical- and dense-wooded savanna as in forested savannas. Wood typically made up the largest aboveground vegetation N and P reservoir, even for woody-grass savanna (campo sujo) which had a substantially lower tree density than wooded or forested savanna, followed by litter and roots, and then foliage. However, the vast majority of N and P was found in the upper 50 cm soil layer, accounting for 82-98% and 72-98% of the total ecosystem N and P, respectively, and even if only the 0-10 cm soil layer is considered, more than half of the ecosystem N and P was found in soil. Nutrient return via litter production was the largest flux followed by mineralization and root uptake, and nutrient cycling in litter production tended to be higher for forests than for wooded and woody-grass savannas. Savanna woodlands and forests in the Pantanal had higher N and P storage than comparable woodlands and forests in the Cuiaba Basin, presumably because of the shallower and stonier soils of the Cuiaba Basin. However, while the vegetation types studied here had large differences in structure (vegetation height, leaf area index, and density), the forested and wooded savannas had comparable belowground and wood N and P pools.