Why does carbon increase in highly weathered soil under no-till upon lime and gypsum use? Thiago Massao Inagaki a , João Carlos de Moraes Sá b, ⁎, Eduardo Fávero Caires b , Daniel Ruiz Potma Gonçalves b a Technical University of Munich, Chair of Soil Science, Institute for Advanced Study, Email-Ramann Str. 2, 85354 Freising, Bayern, Germany b Department of Soil Science and Agricultural Engineering, State University of Ponta Grossa, Av. Carlos Cavalcanti 4748, Campus de Uvaranas, 84030-900, Ponta Grossa, Paraná, Brazil HIGHLIGHTS • Lime and gypsum use in field and incu- bation experiments increases soil C. • Increase of calcium content and soil bio- logical activity resulted in SOC gains. • Calcium and labile SOC formed complex with mineral soil fractions. • Associations between calcium and labile SOC can be the pathway to increase C sequestration. GRAPHICAL ABSTRACT abstract article info Article history: Received 26 January 2017 Received in revised form 24 April 2017 Accepted 29 April 2017 Available online xxxx Editor: Ajit Sarmah Field experiments have been used to explain how soil organic carbon (SOC) dynamics is affected by lime and gyp- sum applications, however, how SOC storage occurs is still debatable. We hypothesized that although many studies conclude that Ca-based soil amendments such as lime and gypsum may lead to SOC depletion due to the enhance- ment of microbial activity, the same does not occur under conservation agriculture conditions. Thus, the objective of this study was to elucidate the effects of lime and gypsum applications on soil microbial activity and SOC stocks in a no-till field and in a laboratory incubation study simulating no-till conditions. The field experiment was established in 1998 in a clayey Oxisol in southern Brazil following a completely randomized blocks design with a split-plot ar- rangement and three replications. Lime and gypsum were surface applied in 1998 and reapplied in 2013. Undis- turbed soil samples were collected before the treatments reapplications, and one year after. The incubation experiment was carried out during 16 months using these samples adding crop residues on the soil surface to sim- ulate no-till field conditions. Lime and gypsum applications significantly increased the labile SOC stocks, microbial activity and soil fertility attributes in both field and laboratory experiments. Although the microbial activity was in- creased, no depletion of SOC stocks was observed in both experiments. Positive correlations were observed between microbial activity increase and SOC gains. Labile SOC and Ca 2+ content increase leads to forming complex with min- eral soil fractions. Gypsum applications performed a higher influence on labile SOC pools in the field than in the lab- oratory experiment, which may be related to the presence of active root system in the soil profile. We conclude that incubation experiments using lime and gypsum in undisturbed samples confirm that soil microbial activity increase does not deplete SOC stocks under conservation agriculture. © 2017 Elsevier B.V. All rights reserved. Keywords: Carbon sequestration Soil biology Acid soils Soil organic matter Soil correction Incubation Science of the Total Environment 599–600 (2017) 523–532 Abbreviations: SOC, Soil organic carbon; SL, Surface applied lime; HWEOC, Hot-water extractable organic C; POXC, Permanganate oxidizable organic carbon; TOC, Total organic carbon. ⁎ Corresponding author. E-mail address: jcmsa@uepg.br (J.C. de Moraes Sá). http://dx.doi.org/10.1016/j.scitotenv.2017.04.234 0048-9697/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv