Long-term fertilization effects on organic carbon fractions in a red soil of China Xiaogang Tong a,b , Minggang Xu a, ⁎, Xiujun Wang a,c , Ranjan Bhattacharyya d , Wenju Zhang a , Rihuan Cong a,c a Institute of Agricultural Resources and Regional Planning, Chinese Academy Agricultural Sciences/Key Laboratory of Crop Nutrition and Fertilization, Ministry of Agriculture, Beijing 100081, China b College of Resources and Environment, Northwest A & F University, Yangling, Shannxi 712100 China c Earth System Science Interdisciplinary Center, University of Maryland Research Park, MD 20740, USA d Scientist (SS) NRL, Indian Agricultural Research Institute, Pusa, New Delhi 110012, India abstract article info Article history: Received 28 February 2012 Received in revised form 22 April 2013 Accepted 8 August 2013 Keywords: Carbon sequestration Long-term fertilization Soil organic carbon Physical fractionation Maize–wheat cropping system Long-term fertilization has a significant impact on total soil organic carbon (SOC) stock. However, fertilization impact on physical fractions of SOC is still poorly understood for red soils in southern China. This study assessed the impact of 17 years (1990–2007) of long-term fertilization on the changes in different SOC fractions under an intensive maize (Zea mays L)–wheat (Triticum Aestivium L) cropping system in a red soil of southern China through various treatments: the unfertilized control (CK), the recommended applied rates of N (N), NP (NP), NPK (NPK), NPK + manure (NPKM), NPK + straw (NPKS) and manure only (M), and a 150% recommended applied rate of NPK + manure (1.5NPKM). Soil samples from 0 to 20 cm soil layer taken in September, 2007, were separated into free particulate organic C (fPOC), intra-microaggregate particulate organic C (iPOC), and mineral associated organic C (MOC) with physical fractionation. In comparison with CK, all the C fractions and maize and wheat yields were significantly increased, except for N and NP treatments. The treatments with ma- nure (M, NPKM, and 1.5NPKM) showed higher C sequestration rates in MOC (323–515 kg ha -1 yr -1 ), fPOC (291–408 kg ha -1 yr -1 ) and iPOC (162–179 kg ha -1 yr -1 ). It was estimated that 8.0 to 35.7% of the gross C input from manure and crop residues over a period of seventeen years contributed to the increase of total SOC stock. Both MOC C sequestration efficiency (CES) and C sequestration distribution (CSD) were the highest among the C fractions for all the treatments. Significantly positive linear correlations were observed between accumulated C sequestrations in all fractions with gross C input and both maize and wheat yields. Our result indicated that MOC was the primary fraction of C sequestration in the red soils. The most efficient fertilization practice for sequestering C in each fraction in the red soils was continuous applications of either manure or manure plus mineral fertilizers. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Red soil occupies an area of 1.13 million km 2 , accounting for 6.5% of the total arable land in China (Xu et al., 2006b). Unfavorable intrinsic properties and inappropriate land use may be responsible for lower productivity in red soils of tropical and subtropical regions of southern China (Zhang et al., 2009a). Soil organic carbon (SOC) plays an impor- tant role in cycling plant nutrients, increasing grain yield and improving the physical, chemical and biological properties of soils (Bhattacharyya et al., 2008, 2010; Manna et al., 2007; Rasool et al., 2008). Therefore, understanding the dynamics and mechanism of carbon sequestration under different management practices in the red soils was a primary way to improve soil fertility and to sustain grain yield production in this region. Soil organic matter (SOM) is a heterogeneous and dynamic sub- stance that varies in C and N content, molecular structure, decomposi- tion rate and turnover time (Oades, 1988). In most current SOM studies, SOM was classified into different pools by their intrinsic decom- position rates and controlling factors, such as microbial biomass C (MBC) (Wu et al., 2005), particulate organic C (POC), potentially miner- alizable C (Cambardella and Elliott, 1992), and KMnO 4 oxidizable C (KMnO 4 -C) (Blair et al., 1995). These C fractions are likely to be more sensitive to management practices than the total SOC and could serve as indicators of future changes in total SOC stock (Campbell et al., 1997). However, these fractions are generally loosely associated with measurable quantities (Six et al., 2002a). Several studies have elucidated the relationship between aggregate and associated SOC dynamics (Jastrow, 1996; Six et al., 1998, 2000). A hypothesis on aggregate hierar- chy was proposed, i.e. microaggregates are bound together into a macro- aggregate and microaggregates are much more stable and less dependent on agricultural managements than macroaggregates. Further, free Catena 113 (2014) 251–259 ⁎ Corresponding author at: Institute of Agricultural Resources and Regional Panning, Chinese Academy Agricultural Sciences, 12 Zhongguancun South Main Street, Beijing, China. Tel.: +86 10 82108661; fax: +86 10 82106225. E-mail address: mgxu@caas.ac.cn (M. Xu). 0341-8162/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catena.2013.08.005 Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena