Agriculture, Ecosystems and Environment 137 (2010) 294–307 Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee Microbial biomass, and dissolved organic carbon and nitrogen strongly affect soil respiration in different land uses: A case study at Three Gorges Reservoir Area, South China Javed Iqbal, Ronggui Hu ∗ , Minglei Feng, Shan Lin, Saadatullah Malghani, Ibrahim Mohamed Ali College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China article info Article history: Received 23 November 2009 Received in revised form 23 February 2010 Accepted 25 February 2010 Available online 24 March 2010 Keywords: CO2 fluxes Dissolved organic carbon and nitrogen Land use Microbial biomass Subtropical Three Gorges Reservoir Area abstract In order to better understand the limiting factors and substrate affecting soil CO 2 flux, we measured total organic carbon (TOC), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) from seven sites of four land-use types (1 vegetable field, 3 uplands, 2 orchards, 1 pine forest) of subtropical soils in Three Gorges Reservoir Area, China. Objectives were to (1) evaluate the separate and interactive relationship of TOC, MBC, MBN, DOC, and DON with soil CO 2 flux, in addition to the relationship obtained by environmental variables (soil temperature and moisture), and (2) investigate the seasonal and annual CO 2 fluxes from different land uses. Annual CO 2 fluxes ranged from 5.4 to 9.5 Mg CO 2 ha -1 year -1 . Vegetable field had the highest CO 2 emission, while pine forest had significantly lower CO 2 emission than cultivated land uses. Different quantities of MBC and MBN significantly regulated the CO 2 emission among different land uses, relatively weakly correlated with DOC, while not being correlated with DON. However, temporal fluctuations of CO 2 flux were significantly regulated by MBC, MBN, DOC and DON, in one model of variation, in all land uses. But, when all the variables were included in the multiple stepwise regression analysis, different trend of dominancy was observed for soil temperature (two sites), MBC (one site), MBN (one site), DOC (two sites) and DON (one site). Our results indicate that (1) there can be a significant shift of microbial biomass with land-use change, which in turn, caused to shift in CO 2 flux, and (2) apart from the soil temperature, microbial biomass and dissolved organic substances must be considered in a warming future as these can explain a major part of temporal variation of soil CO 2 fluxes. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Different land uses can have a significant impact on changes of CO 2 fluxes from the soil-to-atmosphere (Iqbal et al., 2008). Land- use changes directed to an emission of 1.7 Pg C per year in the tropics (Reay and Pidwirny, 2006). In China, afforestation area was 0.45 million km 2 in the past 30 years (Houghton, 2002) that included the planting of trees for timber, shelterbelts, fuel wood, and orchards. Such changes may dramatically alter soil organic car- bon dynamics (Li et al., 2002) and, in turn, affect exchanges of CO 2 between the soil and the atmosphere (Zhou et al., 2004). Therefore, CO 2 emissions from different land uses are important for our under- standing that how environmental variables affect soil C turnover ∗ Corresponding author at: Room #509, College of Resources and Environment, Huazhong Agriculture University, Wuhan 430070, Hubei, PR China. Tel.: +86 27 87282152; fax: +86 27 87396057. E-mail addresses: solvia uaf@yahoo.com (J. Iqbal), rghu@mail.hzau.edu.cn, hronggui@163.com (R. Hu). processes and associated soil CO 2 emissions. Furthermore, fore- casts with changes in soil respiration with changes in climate are obviously needed, but they remain highly uncertain in subtropical region. Factors affecting soil respiration include soil temperature (Hu et al., 2004; Iqbal et al., 2009a), soil moisture (Tang et al., 2006), root exudation (Kuzyakov, 2002) and aboveground plant litter (Raich and Schlesinger, 1992). However, several other soil C and N pools such as soil microbial biomass (SMB) including microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN), dissolved organic substances (DOS) including dissolved organic carbon (DOC) and dissolve organic nitrogen (DON) should also be considered while estimating soil respiration budgets. SMB is the most active component of soil organic carbon that regulates biogeochemical processes in terrestrial ecosystems (Paul and Clark, 1996). Although total soil microbial biomass carbon worldwide is approximately 1.4% of the world’s total soil organic carbon, its turnover represents a significant contribution to the global carbon cycle (Wardle, 1992). Numerous studies on microbial biomass have been conducted in temperate ecosystems (e.g. Vance and Chapin, 2001). However, 0167-8809/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2010.02.015