[Input and Distribution of Photosynthesized Carbon in Soil-Rice System Affected by Water Management and Nitrogen Fertilization]

Fertilizer and water management are two key factors for rice growth. A better understanding of the carbon (C) cycling in paddy soil requires investigation into the input characteristics and distribution dynamics of photosynthesized carbon in rice-soil system. We grew rice (Zhongzao 39) in PVC pots and used the ¹³ C-CO₂ continuous labeling method to quantify the allocation of photosynthesized carbon in rice-soil system under two regimes(Drying-rewetting vs. continuous watering) and N fertilization (250 mg·kg^-1vs. no addition). The results showed that nitrogen fertilizer application increased rice shoot biomass and the amount of C and N, but had no significant influence on rice root biomass. Thus, nitrogen fertilizer application decreased rice biomass root/shoot ratio significantly. Drying-rewetting with N fertilizer treatment resulted in higher total C and N amount by 22% and 33%, respectively, in the shoot, and by 36% and 44%, respectively in the root than continuous watering with nitrogen fertilizer treatment. These results indicated that nitrogen fertilizer application promoted the growth of rice shoot. Nitrogen fertilizer application significantly increased the ¹³ C content in rice shoot by 32%-83% over the control without N addition. Nitrogen fertilizer application also increased the ¹³ C recovery in rice shoot by 6%-32%, but decreased that in the root by 18%-59%. Pertaining to water effect, drying-rewetting with N application increased the amount of ¹³ C in rice shoot and root. However, without N addition, the amount and the recovery of ¹³ C in shoot dropped by 10.3 mg·pot^-1 and 12%, respectively, compared with the continuous watering treatment. The root, on the other hand, recorded increases in both the amount and the recovery of ¹³ C by 1.9 mg·pot^-1 and 57%, respectively. Furthermore, the deposition of assimilated C into rhizosphere-soil increased by both the individual and the interactive effects of N fertilizer application and drying-rewetting treatment. Thus, combining N fertilizer and drying-rewetting water management led to more increased allocation and deposition of photosynthesized carbon in soil-rice system compared with combined continuous flooding and N application. This study was able to quantify the partitioning and allocation of rice photosynthesized carbon into different plant and soil pools under different water and N fertilizer treatments, and can serve as a useful guide for better water and nutrient management practices in paddy-rice production that can achieve both sustainable high yield and sequestration of more C within the paddy soil system.