Stable Isotope Effects of Biogas Slurry Applied as an Organic Fertilizer to Rice, Straw, and Soil

Biogas slurry (BS) is now increasingly used for organic rice production in China. However, the isotopic response and fractionation of different BS application rates to characterize organic rice cultivation have not yet been investigated. In this study, different fertilizer treatments were applied to rice paddy soil including urea, BS with five different application rates and a control with no fertilizer added. Multiproxy analyses (% C, % N, δ13C, δ15N, δ2H, and δ18O) of rice, rice straw, and soil were undertaken using elemental analyzer–isotope ratio mass spectrometry. Rice, straw, and soil showed only minor isotopic and elemental variations across all fertilizer treatments except for δ15N. δ15N values of rice and straw became more positive (+6.1 to +11.2‰ and +6.1 to +12.2‰, respectively) with increasing BS application rates and became more negative with urea fertilization (+2.8 and +3.0‰, respectively). The soil had more positive δ15N values after BS application but showed no significant change with different application rates. No obvious δ15N isotopic differences were found between the control soil and soils fertilized with urea. 15N fractionation was observed between rice, straw, and soil (Δrice-soil −2.0 to +4.3‰, Δstraw-soil −1.9 to +5.3‰) and their isotopic values were strongly correlated to each other (r > 0.94, p < 0.01). Results showed that % C, % N, δ13C, δ2H, and δ18O in rice displayed only minor variations for different fertilizers. However, δ15N values increased in response to BS application, confirming that BS leaves an enriched 15N isotopic marker in soil, straw, and rice, indicating its organically cultivated status. Results from this study will enhance the stable isotope δ15N databank for assessing organic practices using different fertilizer sources.