Soil organic carbon is the primary below-ground plant derived carbon (dead plant residues or carbon released by living roots). However, the effect of rice plant age and chase period (time from labeling to sampling) on the relationship between carbon photosynthesized by rice plant and allocated into paddy soil has not been studied yet.

In order to accurately trace allocation of photosynthate carbon depending on plant age and chase period in nitrogen-fertilized and non-fertilized rice-soil systems, as well as during crop growth, and estimate the net plant-derived carbon input to belowground, researchers from the Institute of Subtropical Agriculture (ISA) of the Chinese Academy of Sciences (CAS) and University of Goettingen conducted a pot experiment in the lab using fresh topsoil (0-20 cm) of a rice field after removing roots and plant residues. All the pots were fertilized by base phosphorus (P) kalium (K) fertilizers, with rhizosphere and bulk soils separated by rhizosphere bag.

20-day-old rice seedlings were transplanted to the rhizosphere bag in each pot and exposed to ¹³CO₂ pulse labeling (6 h) at the tillering, elongation, heading, and filling stages. The seedlings were allowed to grow with or without nitrogen (N) fertilization until mature. Soil and plants were sampled immediately after each labeling and at the end of the experiment.

Their results indicated that the rice photosynthesized carbon allocated to belowground (root and soil) strongly decreased with plant age. They also found an increase of carbon allocation to root in the first 10 days after labeling, which remained stable thereafter.

Immediately after each labeling, the carbon allocated to root, shoot, rhizosphere soil as well as bulk soil was independent on nitrogen fertilizer. Whereas N input increased the ¹³C remaining in the rice-soil system during the whole rice growth period.

They suggested that the great variation among chase periods and accurate trace of the carbon allocation in rice-soil systems required multiple ¹³CO₂ pulse labeling and subsequent sampling along rice growth, with nutrient availability taken in consideration.

The study, published in Plant and Soil, was supported by the National Natural Science Foundation of China and the Youth Innovation Team Project of ISA, CAS.