Optimization of removal and recycling ratio of cover crop biomass using carbon balance to sustain soil organic carbon stocks in a mono-rice paddy system Md. Mozammel Haque a , Sang Yoon Kim b , Gil Won Kim a , Pil Joo Kim a, c, * a Division of Applied Life Science (BK 21 Program), Gyeongsang National University, Jinju 660-701, South Korea b Netherlands Institute of Ecology (NIOO-KNAW), Department of Microbial Ecology, Wageningen, The Netherlands c Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 660-701, South Korea A R T I C L E I N F O Article history: Received 26 March 2014 Received in revised form 16 March 2015 Accepted 21 March 2015 Available online xxx Keywords: Green manure Methane emission Carbon dioxide emission Carbon mineralization loss A B S T R A C T The cultivation of a winter cover crop as green manure is strongly recommended to improve soil quality in mono-rice paddy systems; however, the biomass is largely removed to feed cattle in many Asian regions. To determine the minimum recycling ratio of the biomass that can sustain soil organic carbon (SOC) levels and produce more fodder for cattle, the SOC balance, which is the difference between OC input and output during rice cultivation, was evaluated with the various levels of biomass addition. The sources of OC input included cover crop biomass and fertilizer, and the OC outputs were estimated by the losses from the mineralization of C (emissions of CH 4 and CO 2 ). A mixture of barley (75% of the recommended dose, RD) and hairy vetch (hereafter, vetch, 25% of the RD) seeds were broadcast after rice harvests in 2011 and 2012, and the aboveground biomass (11.5–12 Mg ha 1 , based on dry weight) harvested in the following years was incorporated at different ratios (0–100%) into soils one week before transplantation of rice with the same chemical fertilization. The incorporated OC was lost primarily through emissions of CO 2 (73–85% of the OC output). However, the proportion of CH 4 loss increased significantly with an increase in the rate of aboveground biomass application, which was caused by the development of anaerobic soils. A negative SOC balance, which implied soil fertility was at risk from a decreasing stock of SOC, was observed with total aboveground biomass removal. However, the balance of SOC increased significantly with an increase in level of biomass recycling and reached a sustainable level at approximately 28–30% recycling of aboveground biomass; thus, the current levels of SOC could be sustained. In conclusion, more than 30% of the aboveground biomass of the cover crop (3.4–3.6 Mg ha 1 dry weight) should be incorporated as a green manure to sustain levels of SOC in mono-rice cultivation systems with chemical fertilization. ã 2015 Elsevier B.V. All rights reserved. 1. Introduction The intensified cultivation of rice may pose risks to long-term sustainability and productivity (Lal, 1997), primarily because of low levels of soil organic carbon (SOC), low soil fertility and an imbalance in the management of nutrients (Reddy and Krishnaia, 1999). The use of green manure is an important soil management practice with the potential to maintain SOC contents and to reduce dependence on mineral fertilizers (Elfstrand et al., 2007). In the mono-rice cultivation systems of temperate zone countries such as Korea, the cultivation of winter cover crops as a green manure is strongly recommended. These crops include N-fixing plant species such as hairy vetch and Chinese milk vetch and non-leguminous species such as rye, barley and wheat, which generally produce high-biomass yields (Yasue, 1991; Singh et al., 1999; Cho et al., 2003). The winter cover crops are seeded after rice harvest in the late fall and are then incorporated in situ as green manure before rice transplanting in the following year. The addition of leguminous cover crop biomass sustains the vegetative growth of rice, but the rice grain quality deteriorates, and the addition of non-leguminous cover crops with high C/N ratios decreases rice productivity because of slow mineralization; therefore, the combined cultivation of the two different cover crops is broadly used in paddy soils in Korea (Jeon et al., 2008; Ryu * Corresponding author at: Gyeongsang National University, Division of Applied Life Science, 900 Gajwa-dong, Jinju 660-701, South Korea. Tel.: +82 55 772 1966; fax: +82 55 772-1969. E-mail address: pjkim@gnu.ac.kr (P.J. Kim). http://dx.doi.org/10.1016/j.agee.2015.03.022 0167-8809/ ã 2015 Elsevier B.V. All rights reserved. Agriculture, Ecosystems and Environment 207 (2015) 119–125 Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journal homepage: www.elsev ier.com/locate /agee