Carbon dioxide exchange at four intensively managed grassland sites across different climate zones of Japan and the influence of manure application on ecosystem carbon and greenhouse gas budgets

Abstract

To investigate the influence of different fertilization regimes on carbon dioxide (CO2) exchange and the effect of manure application on the carbon balance, we established two experimental plots, one with chemical fertilizers only and one with manure and supplementary chemical fertilizers, at each of four intensively managed grassland sites across the range of climate zones in Japan. By using eddy covariance CO2 flux and biometric measurements, we evaluated gross primary production (GPP), ecosystem respiration (RE), net ecosystem production (NEP), and the net ecosystem carbon balance (NECB), which accounts for carbon input through manure application and carbon loss through harvest. The sites in warmer zones showed larger annual GPP and RE, but annual NEP did not display any clear temperature dependence. The annual NEP was positive at all study sites and plots. The annual GPP and autotrophic respiration differed only slightly between the two plots despite the different fertilization regimes, but the decomposition of applied manure increased the annual heterotrophic respiration, thus causing a reduction in the annual NEP. At all the study sites, NECB of the plots with application of chemical fertilizers only was negative, indicating the loss of carbon. Manure application increased NECB, and its effect was larger at the cool temperate sites than at the temperate and warm temperate sites because of the greater amount of applied manure and larger sequestration rate at the cool temperate sites. Thus, manure application increased the accumulation of carbon in grassland, which is favorable from the viewpoint of ecosystem carbon management. Even when the enhanced nitrous oxide emission from the plots with manure application was taken into account, our findings suggest that manure application is a favorable option for mitigating greenhouse gas emission from Japanese grasslands and effectively managing livestock waste.