Paddy soil, the largest anthropogenic soil on earth, plays a strategic role in the global carbon sequestration. However, an understanding of how paddy soil plays its role in mitigating the global increase in atmospheric CO2 concentrations requires investigation into the management of SOC sources, pools, spatial distribution, and stabilisation processes.

As a major crop grown on paddy soil, rice depends on fertilizer N inputs and good water management for its success. An estimated 90% of the total irrigated water allocated to crops goes into rice production. However, regular irrigation requires enormous energy input, and supplying fresh water for continuously flooded paddies is increasingly unsustainable due to competitive demands from urban and industrial fronts.

The perceived benefits of efficient water-use and improved yield of alternating flooding and drying water management has made it popular in rice cultivation. Drying-rewetting cycles, however, have major implications on below-ground plant-soil-microbe interactions, such as instantaneous C and N mineralization, as well as shifts in microbial use and stabilisation of rhizodeposited nutrients.

Further, increased photosynthate partitioning and allocation belowground have been reported under drying-rewetting in rice. Despite these consequences, little is known about the combined effects of water management and N fertilization on the partitioning and allocation of rice photosynthates in above-and belowground paddy soil systems.

Also, the distribution of rice-derived C across different aggregate fractions as it relates to water management and N fertilisation is poorly understood.

The research group from the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA), therefore, investigated how water management (continuous vs. alternating flooding-drying) and N fertilisation interact to affect the partitioning and stabilisation of newly plant-derived C in the rice-soil system.

They continuously labelled rice (‘Zhongzao 39’) with 13CO2 under a combination of different water regimes (alternating flooding-drying vs. continuous flooding) and N addition (250 mg N kg-1 urea vs. no addition), and then followed 13C incorporation into plant parts as well as soil fractions.

They hypothesized that an alternating water regime and N fertilisation would increase rhizodeposition via enhanced root activity compared with continuous flooding. It was expected that the surge in microbial activities, and hence their increased use of rhizodeposits under flooding-drying episodes would reduce C stabilisation. They then hypothesized that N addition would increase rhizodeposition through enhancing photosynthesis, and the associated larger input of available OC would increase macroaggregation in rhizosphere soils under both water regimes.

The researchers found out that the interactive effects of water regimes and N fertilisation increased rice shoot biomass, as well as the allocation and stabilisation of newly the plant-derived C in the rice-soil system.

Moreover, N application was more effective in the alternating flooding-drying treatment than in continuous flooding, causing a larger increase to recent assimilate deposition in rhizosphere macroaggregates, microaggregates, and silt and clay-size fractions.

Thus, combining N application with a drying-rewetting water management stabilized rhizodeposited C in soil more effectively than other tested conditions. Hence, in addition to benefits such as cost reduction, water use efficiency, and yield increase, the positive impact on sequestration makes this combined management system desirable for rice cropping.

The study entitled "Rice rhizodeposition and carbon stabilization in paddy soils is regulated via drying-rewetting cycles and nitrogen fertilization" can be accessed online in Biology and Fertility of Soils.

It was supported financially by the National Natural Science Foundation of China and Royal Society Newton Advanced Fellowship.