Changes in CO 2 emissions after crop conversion from continuous maize to alfalfa Giorgio Alberti a, *, Gemini Delle Vedove a , Michel Zuliani a , Alessandro Peressotti a , Simona Castaldi b , Giuseppe Zerbi a a Department of Agriculture and Environmental Sciences, University of Udine, via delle Scienze 208, 33100 Udine, Italy b Department of Environmental Sciences, Second University of Naples, via Vivaldi 43, 81100 Caserta, Italy 1. Introduction Management strategies to increase the carbon sink of agricultural soils have gained great importance since the Kyoto Protocol was signed in 1997. Croplands represent about 12% of the earth’s surface (Wood et al., 2000) and can have equal or greater net ecosystem production (NEP) than several natural ecosystems (Law et al., 2002; Hollinger et al., 2004). An important consequence often associated with the conversion of native ecosystems to croplands is the reduction of soil organic carbon (SOC) (Houghton et al., 1983). Therefore, different agronomic strategies have been suggested and tested in order to reduce CO 2 emissions and increase SOC. These measures include the conversion of arable land to perennial forage crops and the implementation of crop management practices, such as the elimination of tillage and the introduction of rotations based on N 2 -fixing plants (Lal and Bruce, 1999; Lal et al., 1999; Lal, 2002). For example, the conversion from mouldboard ploughing to no-till was found to increase the C sink up to 57  14 g C m 2 y 1 (West and Post, 2002). Similarly, when continuous maize crops are converted to grasslands, an increase of soil C storage between 0.5 and 1.0 t C ha 1 y 1 was reported by IPCC (2000), calculated over 50 years. Although several studies investigated changes in soil carbon stocks associated with changes in crop management or in land use (Deen and Kataki, 2003; Su, 2007), limited information exists regarding the potential of C sequestration of high yield maize under different management options (i.e. Robertson et al., 2000). Also lacking are short- term studies examining C losses/gains associated with the conversion of maize crops to alfalfa. Two parameters are particularly important when a compre- hensive C balance for agro-ecosystem is performed. The first is net biome production (NBP), which is a measure of C storage in the ecosystem and also accounts for carbon losses, due to harvest material (Anthoni et al., 2004). The second is net ecosystem exchange of carbon (NEE) which expresses the potential of each agro-ecosystem to fix C or to release C by both autotrophic and heterotrophic respiration. The traditional way of addressing net carbon exchange of an ecosystem over multiple years involves quantifying temporal changes in biomass and soil carbon. However, changes in soil organic matter become apparent after decades rather than years and thus soil sampling techniques does not allow going insight changes in the short term. The eddy covariance technique has been used widely to determine NEE (Baldocchi, 2003) because it is scale appropriate and provides a method to assess net CO 2 exchange at ecosystem level, it produces a direct measure of net ecosystem exchange (NEE), it is able to measure ecosystem CO 2 exchange across a spectrum of timescales and the area sampled by this technique (footprint) range between hundred meters to several kilometres. This technique has been Agriculture, Ecosystems and Environment 136 (2010) 139–147 ARTICLE INFO Article history: Received 25 August 2009 Received in revised form 9 December 2009 Accepted 11 December 2009 Available online 6 January 2010 Keywords: Maize Alfalfa NEP NBP C cycle ABSTRACT Mitigation strategies for the reduction of carbon dioxide emissions are the central focus of the Kyoto Protocol and international scientific efforts. Agriculture plays a substantial role in the balance of the most significant greenhouse gases (CO 2 ,N 2 O, CH 4 ), mostly attributed to management practices. In this study, we present data on the effects of a conversion from a cropland (Zea mays L.) to N 2 -fixing grassland (Medicago sativa L.) on C cycle in an agricultural area of Northern Italy. Net ecosystem production (NEP) and net biome production (NBP) have been followed for 2 years by mesuring CO 2 fluxes by paired eddy covariance stations (EC) and continuous soil respiration measurements (SR). Root exclusion subplot replicates were also used to estimate heterotrophic respiration (Rh). The comparison between the net primary production (NPP) inventory estimation and NPP based on measured CO 2 fluxes (EC and Rh) showed excellent agreement for both land uses. An increase in NEP was observed 2 years after conversion from corn to alfalfa (+281 g C m 2 ), however, in terms of NBP, maize was a lower source of C (96 g C m 2 ) than alfalfa (354 g C m 2 ). From the present study, it appears that this type of land conversion is not an effective measure of C sequestration in the short term (1–3 years). ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +39 0432 558608; fax: +39 0432 558603. E-mail address: alberti@uniud.it (G. Alberti). Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee 0167-8809/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2009.12.012