Warming but not elevated CO2 depletes soil organic carbon in a temperate rice paddy

Global climate change has the potential to alter soil organic carbon (SOC) stocks in rice paddies, because increases in temperature and atmospheric carbon dioxide concentration ([CO₂]) both influence the primary input (i.e., net primary production, NPP) and output (i.e. heterotrophic respiration) of carbon (C). We used two types of open-top chambers representing present conditions (+0℃, +0 ppm CO₂) and projected climate change conditions (+2℃, +200 ppm CO₂) to investigate the net effect of climate change on SOC stock in rice paddy. Additional chambers with elevated temperature only (+2℃, +0 ppm CO₂) allowed us to quantify the individual effects of temperature and [CO₂]. We calculated changes in SOC stock using net ecosystem C balance (NECB) analysis (i.e., the balance between C inputs and outputs). Compared to present conditions, projected climate change did not change grain yield due to a trade-off between the effects of warming and [CO₂] on grain yield components. NPP during the fallow season significantly decreased under combined warming and CO₂, as the impact of warming outweighed that of elevated [CO₂]. However, rice NPP remained unchanged during the cropping season. Warming plus elevated CO₂ increased SOC mineralization by 157–429 %, particularly through warming-induced soil CO₂ emission during the fallow season. Consequently, climate change conditions decreased (119–271 %) NECB values compared to present conditions, primarily through the response to warming. Our findings demonstrate that rice paddies represent positive feedback on climate change, because accelerated C release from warmed soils will override C gains from NPP under elevated CO₂. Reducing SOC depletion in rice paddy agriculture under a changing climate therefore requires conservative soil management practices during the fallow season.