Combined effects of freezing-thawing cycles and livestock excreta deposition complicate soil nitrogen and phosphorus nutrient dynamics in an alpine steppe of northern Tibetan Plateau, China

Abstract

Abstract Background and Aims: Livestock excreta (dung and urine) is one of the significant sources of soil nitrogen (N) and phosphorus (P) nutrients in grazed grasslands, and freezing-thawing cycle (FTC) during non-growing season are considered to complicatedly affect soil N and P dynamics by changing excreta nutrients release, soil physicochemical, and microbial properties. However, the combined effects and functional mechanisms of FTCs and excreta deposition on the availability, transformation, and migration processes of soil N and P nutrients in alpine grasslands need further clarifications. Methods: A 277-day experiment was conducted to investigate the response of dung N and P nutrients release, soil available N and P dynamics, and N transformation process to FTCs at different freezing-thawing periods, including treatments of control (CK), soils respectively treated with yak dung (YD), Tibetan sheep dung (TSD), yak urine (YU), and Tibetan sheep urine (TSU) in an alpine steppe of northern Tibetan Plateau. Results: Enhanced mineralization during the stable freezing period significantly (P < 0.05) increased alpine steppe soil ammonium-N (NH4+-N) and available P (AP) concentrations at 0–5 cm depth. The significant (P < 0.05) increase in soil nitrate-N (NO3--N) concentration at 0−5 and 5−10 cm depths during the thawing period was a response to the improvement in nitrification. The YU and TSU deposition respectively resulted in the 0−5 cm soil NH4+-N concentration reached 127 and 89.4 mg/kg and the corresponding NO3--N concentration reached 90.5 and 29.6 mg/kg, with these peaked values occurred during freezing period significantly (P < 0.05) greater than other freezing–thawing periods. Interactions between excreta treatment and freezing–thawing periods significantly (P < 0.001) affected soil NH4+-N and NO3--N concentrations, and soil AP variation was strongly (P < 0.001) correlated with freezing–thawing periods. Trade-off between NH4+-N and NO3--N was closely related to the influence of nitrification and denitrification, which were affected by frequent FTCs during the winter freezing and spring thawing. Conclusion: Increases in mineral N and AP concentrations during the spring thawing period likely altered the phenological properties of vegetation and the productivity of grassland ecosystems to varying degrees. Our study would provide scientific guidance for optimizing the management mode of livestock excreta during freezing–thawing periods and promoting the sustainable development of alpine grassland ecosystems.