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Title: Grazing land intensification effect on soil C dynamics in aggregate size fractions of a Spodosol

item SILVEIRA, MARIA - University Of Florida
item XU, SUTIE - University Of Florida
item ADEWOPO, JULIUS - University Of Florida
item BUONADIO, GUILHERME - Universidad De Sao Paulo
item Franzluebbers, Alan

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2013
Publication Date: 6/2/2014
Citation: Silveira, M.L., Xu, S., Adewopo, J., Buonadio, G., Franzluebbers, A.J. 2014. Grazing land intensification effect on soil C dynamics in aggregate size fractions of a Spodosol. Geoderma. 230-231:185-193.

Interpretive Summary: Rising carbon dioxide in the atmosphere is of huge concern to society due to its role in altering climate and causing economic disasters. Storage of carbon from the atmosphere into soil is a strategy that can help mitigate greenhouse gas emissions. Land use can have a major impact on storage of carbon in soil. A group of scientists from the University of Florida, University of São Paulo, and the USDA-Agricultural Research Service reported on changes in soil organic carbon storage and its relative permanence in soil under three different land management systems. Storage of carbon in soil was greater under improved pasture (fertilized bahiagrass) and silvopasture (mixture of trees and bahiagrass) than under native rangeland in Florida. Carbon was found to be preferentially stored in small aggregates under improved pasture. Although more carbon was stored in improved pastures, it was also susceptible to rapid mineralization under ideal decomposition conditions, suggesting that continuation of beneficial agro-ecological systems are necessary to keep carbon from being released to the atmosphere. These results will be beneficial to producers in Florida and other subtropical regions to adopt better management approaches to optimize grazingland productivity and mitigate negative consequences to the environment.

Technical Abstract: Conversion of native rangeland ecosystems into intensively managed pastures can have important implications on the amount and characteristics of soil organic C storage. This study investigated the impacts of grazing land intensification on soil organic C stocks and the distribution and decomposition rates of C associated with aggregate sizes. Experimental sites consisted of three grazing land biomes (native rangelands, pine-bahiagrass silvopasture, and improved pastures) under similar soil type and climate conditions. Carbon stocks (0 to 20 cm depth) were generally greater in the improved and silvopasture (44 Mg C/ha) than the native rangeland ecosystem (24 Mg C/ha). This response was due the relatively high biomass production of the warm-season C4 grass augmented by N fertilization. Delta 13C signature of the improved pasture (-14.7 to -18.8‰) reflected the influence of introduced C4 pasture grass, while the native rangeland and silvopasture showed more negative delta 13C values (-20.3 to -22.7‰). Relatively greater C responses to management were observed in the smallest aggregate size fraction (<0.053 mm). This was coincident with the large proportion of C associated with this fraction. Greater C mineralization rates were generally observed in improved pasture, particularly in the 0.25-2 mm and <0.053 mm fractions. Relatively greater susceptibility of C to mineralization in the improved pasture than less intensively managed ecosystems indicates that C was more labile.