Short-term effects of grassland renovation on CO2 exchange of grasslands in a temperate humid region

Abstract

To investigate the short-term effects of grassland renovation on carbon dioxide (CO2) exchange of intensively managed grasslands in Japan, we conducted CO2 flux measurements during the renovation process by using the eddy covariance and closed chamber methods in 2007, 2012, and 2013. The flux measurements were conducted at three grassland fields: two fields used for cutting, one receiving only chemical fertilizer (CF) and the other receiving composted cattle manure (CM) annually, and one field used for grazing (GM). Chamber measurements revealed flushes of CO2 after plowing (14.45-23.94 μmol m-2 s-1) and subsequent disk harrowing (7.36 μmol m-2 s-1, in CF) followed by rapid drops of CO2 flux, which were local and temporary phenomena. The mean CO2 losses during renovation periods were calculated as 4.52-4.74, 6.00-6.81, and 4.57 g C m-2 d-1 in CF, CM, and GM, respectively; CF and CM calculation is based on eddy covariance measurements, including temporal flux variations and representing footprint areas. The amount of carbon input during renovation including non-harvested grass biomass (stubble and roots) and applied manure was estimated as 2.24-3.50 and 6.17-8.77 Mg C ha-1 in CF and CM, respectively. Among them, carbon derived from plowed roots and manure is presumably resistant in soil, contributing to long-term soil organic carbon (SOC) accumulation. Our results also indicate that grassland renovation work does not affect short-term net CO2 loss significantly, although it affects CO2 emissions to a certain extent. We can therefore say that net CO2 loss during renovation is mainly brought by the absence of vegetation in this site. Grasses contribute SOC accumulation through biomass allocation belowground; and it is thus recommended to shorten grassland renovation period, the duration without photosynthesis, to reduce CO2 loss associated with grassland renovation.