Trading water for carbon in agricultural systems

Climate change and agricultural intensification are placing enormous pressure on soil to provide essential services, from water storage and nutrient provision to carbon sequestration. Indeed, storing carbon in agricultural systems is proposed as an effective climate mitigation approach. Yet, storing carbon comes often at a cost in terms of water consumed—water used either to increase productivity and carbon inputs to soil, or to create conditions in the soil that promote carbon storage. These linkages are perhaps most evident in rice paddy systems.

Rice – a staple food for 3 billion people – consumes more water than any other crop, leading to unsustainable water withdrawals. However, this large water consumption allows paddy soils to store more carbon than under other land uses, because flooding of the fields keeps soils saturated and inhibits organic matter decomposition. Therefore, changing water saving approaches such as alternate wetting and drying has the potential to reduce carbon storage and alter the provision of other ecosystem services. But how much can soil organic carbon change across land uses and when water management is altered?

In this contribution, we discuss the mechanisms of carbon storage in paddy fields, using data from a meta-analysis of soil carbon budgets in tropical rice paddies and from a detailed investigation of carbon storage along a gradient of a land use and soil age in a temperate rice system. The meta-analysis shows that, as expected, reducing the time of flooding decreases soil organic carbon, but also results in a net decrease of greenhouse gas emissions. The more detailed study shows that in temperate conditions with relative short flooding time, rice paddies can store as much organic carbon as forest sites, despite the higher carbon inputs of forests compared to rice systems. This higher carbon storage is achieved thanks to decreased soil respiration in anaerobic conditions and increased mineral associations of organic carbon. These results show that water management strongly affects soil carbon storage, and that trade-offs emerge between sustainable water use and long-term provision of soil-related ecosystem services.