Interactive effects of land-use change and topography on asymbiotic nitrogen fixation in the Brazilian Atlantic Forest

Abstract

The conversion of tropical forests to other land use forms is known to affect biogeochemical processes, but its influence on asymbiotic nitrogen fixation (ANF) remains poorly understood. In this study, we investigate patterns of soil ANF and its biogeochemical controls across multiple land uses and topographic positions in the Brazilian Atlantic Forest. We report ANF rates derived from observational and manipulative laboratory experiments using 15N-labeled dinitrogen (15N2) incubations of soils from primary forest, secondary forest, eucalypt plantations and pastures sampled at three hillslope positions (summit, backslope, and footslope). Our field observations revealed significant interactions between land use and topography on ANF rates, which among 14 measured soil parameters were primarily correlated with total nitrogen and available iron (explaining 69% of ANF variance). Complex non-linear relationships were observed between ANF and (mineral and organic) nutrient pools in forest soils, whereas simple positive linear relationships were found between ANF and total soil nitrogen in managed pastures and plantations. Consistent with this observation, experiments with nitrogen and phosphorus addition did not affect ANF rates in pasture or plantation soils, possibly as a result of previous fertilization and homogenization of plant and microbial communities, but variable positive and negative ANF responses were measured in primary and secondary forest soils depending on hillslope position. These findings show that ANF can be simultaneously affected by multiple variables that are sensitive to environmental conditions and demonstrate the need for considering land-use change and topography to improve predictions of terrestrial biogeochemical cycles.