Lime and Nutrient Addition Affects the Dynamics and Fractions of Soil Carbon in a Short-term Incubation Study With 13 C-Labeled Wheat Straw Elizabeth C. Coonan, 1,2 Clive A. Kirkby, 1 John A. Kirkegaard, 1 Martin R. Amidy, 2 Craig L. Strong, 2 and Alan E. Richardson 1 ABSTRACT: Lime and nutrients are common agricultural amendments and when applied with fresh organic matter can alter the cycling of carbon (C) in soils. Previous research has focused on assessing either the impact of nutrients on C associated with specific soil fractions or the application of lime on C dynamics in the whole soil, without fully considering the potential interactions of lime and nutrients on changes in soil C. A short-term (77 days) incubation experiment was conducted to investigate the effects of lime and nutrients (nitrogen, phosphorus, and sulfur) when added with 13 C-labeled wheat straw on the incorporation of the wheat straw C into soil C. Respiration was measured throughout the incubation, and soil (a Yellow Chromosol or acidic Alfisol) was analyzed for C in the fine (<0.4 mm) and coarse (>0.4 mm) soil fractions after 77 days. There was greater C loss from the coarse fraction soil across all treatments when straw was incorporated, and lime application increased C loss from both fractions. Lime plus nutrients led to positive soil priming, a decrease in the preexisting soil C tracked through changes in 13 C, and reduced conversion of straw C into the fine fraction soil C as compared with nutrients alone. The lime and lime plus nutrients added with straw reduced the net humification efficiencies (9.3 and 8.1%, respectively), compared with the unlimed or nutrient-only treatments (18.4 and 15.5%, respectively). Lime with sup- plementary nutrients was less effective in promoting the conversion of straw C into the fine fraction soil C compared with nutrients alone in this incubation experiment. Where lime and supplementary nutrients as fertilizer are routinely used to improve agricultural production, the lime may offset any benefit of the nutrients for conversion of straw C into the fine fraction soil C. Key Words: Carbon fractions, fine fraction soil, nutrient stoichiometry, soil organic matter, soil priming effect (Soil Sci 2019;184: 43–51) A gricultural productivity is intrinsically linked to soil organic matter (SOM) dynamics. Mineralization of SOM provides energy for microorganisms and nutrients for microorganisms and plants (Lal 2004; Petersen and Hoyle 2016). Stabilization of SOM improves soil structural stability and soil water holding capacity and reduces soil bulk density (Stockmann et al., 2013; Murphy 2015; Petersen and Hoyle 2016). Soil organic matter is a complex contin- uum with a range of chemical and physical properties and turnover times (Lehmann and Kleber 2015). Therefore, the partitioning of SOM into active and stable pools is useful because it provides a basis for analysis of the SOM dynamics and allows for improved under- standing of the mechanisms of mineralization and stabilization (Cambardella and Elliott 1992; Simonsson et al., 2014). Soil organic matter content of soils has declined substantially under modern agricultural practices leading to growing interest in management practices that can increase conversion of fresh organic matter (FOM) into SOM and reduce SOM mineralization (Dalal and Chan 2001; Guo and Gifford 2002; Sanderman et al., 2017). Management practices that influence soil properties and mi- crobial function, such as the application of nutrients and liming, may influence SOM dynamics (Li et al., 2015). Addition of supplementary nutrients alongside FOM can increase incorporation of carbon (C) into stable SOM (Kirkby et al., 2014). One study by Moran et al. (2005) reported that addition of mineral nitrogen (N) or residue N increased the accumulation of residue C into stable SOM. Similarly, Chen et al. (2018) reported that N fertilizer added to straw residues increased soil C particularly in the more stable O-alkyl and anomeric C compo- nents. However, these findings were limited to addition of supplemen- tary N without additions of supplementary phosphorus (P) and sulfur (S). Previously, research has suggested that consistent ratios of C:N:P: S are required for conversion of FOM into SOM (Kirkby et al., 2011). The 56-day incubation study by Kirkby et al. (2014) found that application of supplementary nutrients to the fine fraction soil C (<0.4 mm) increased the conversion of FOM into SOM along with higher net humification efficiency. However, the findings from Kirkby et al. (2014) have not been tested in an incubation of whole soil and have not been tested in soils in response to lime addition. Liming alters the soil pH and calcium or magnesium content and subsequently increases microbial activity leading to increased SOM mineralization (Halstead et al., 1963; Haynes and Naidu 1998; Paradelo et al., 2015). Increased respiration following lime applica- tion has been shown to be associated with increased SOM mineral- ization by bacteria (Halstead et al., 1963; Neale et al., 1997). Lime application also has significant effects on microbial community structure and may increase physical protection of SOM through in- creased aggregation via increased root growth, higher microbial ac- tivity, and formation of calcium ion bridges between organic matter and clay particles (Haynes and Naidu 1998; Chan and Heenan 1999; Hati et al., 2008; Fornara et al., 2011). Using a 100-day incubation, Xiao et al. (2018) found that increased soil pH altered the soil fungal community and increased SOM mineralization. Further, Chan and Heenan (1999) reported a loss of organic C due to liming, which oc- curred predominantly in the more active fraction of soil C. These studies, however, did not assess changes in the physical fractions of C or the potential interaction with nutrients. In practice, lime 1 CSIRO Agriculture & Food, Canberra, Australia. 2 Fenner School of Environment and Society, Australian National University, Acton, Australia. Address for correspondence: Dr. Alan E. Richardson, CSIRO Agriculture & Food, PO Box 1700 Canberra, ACT 2601, Australia. E-mail: alan.richardson@csiro.au Financial Disclosures/Conflicts of Interest: E.C.C. is supported by an Australian Gov- ernment Research Training Program Scholarship, an ANU Dean's Merit HDR Sup- plementary Scholarship in Science and a CSIRO postgraduate scholarship. For the remaining authors, none were declared. Received August 30, 2019. Accepted for publication October 7, 2019. Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's website (www.soilsci.com). Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved. ISSN: 0038-075X DOI: 10.1097/SS.0000000000000249 TECHNICAL ARTICLE Soil Science • April 2019 • Volume 184 • Number 2 www.soilsci.com 43 Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.