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 (Δ_rice-soil -2.0 to +4.3‰, Δ_straw-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.