Interactive effects of land-use change and topography on asymbiotic nitrogen fixation in the Brazilian Atlantic Forest
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.
KeywordsBiogeochemistry Deforestation Disturbance Free-living diazotrophs Generalized additive model Hillslope Tropical soils
This research was funded by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, CAPES 99999.001261/2013-04, Henry A. Jastro Award 2015-2016 and UC Davis Academic Federation Award (Silva LCR). The first author thanks the Horwath Nutrient Cycling Laboratory at University of California, Davis and the Soil-Plant-Atmosphere Laboratory at University of Oregon, Eugene for academic advisory and financial support throughout the project. The authors also thank colleagues and professors of the Soils and Biogeochemistry Graduate Group at UC Davis for support throughout the project, and Flavio Guercio, Toby Maxwell, Natalie Treyz, Anna Britzmann, Jamie Wright, Julianna Hello, John Hadish and Zheng Chen for field and laboratory assistance. Special thanks to João Batista Fernandes for granting the authors full access to the farm and providing logistic support during the field data collection. The MODIS data used was retrieved from https://www.dsr.inpe.br/laf/series/index.php, courtesy of the NASA Land processes Distributed Active Archive Center (LP DAAC), located at the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center (lpdaac.usgs.gov). The authors thank the anonymous reviewers for the valuable contribution to improve the manuscript from its original version.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest.
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