Changes in CO₂ emissions after crop conversion from continuous maize to alfalfa

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 (CO₂, N₂O, CH₄), mostly attributed to management practices. In this study, we present data on the effects of a conversion from a cropland (Zea mays L.) to N₂-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 CO₂ 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 CO₂ 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 (+281gCm⁻²), however, in terms of NBP, maize was a lower source of C (−96gCm⁻²) than alfalfa (−354gCm⁻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).