Effects of Dairy Cattle Slurry Application on Alfalfa Biomass: Photosynthetic Characteristics and Nitrogen and Phosphorus Use Efficiency

With the rapid development of the animal farming industry in China, the large amount of manure has caused a systematic environmental problem, while the demand for high-quality feed continues to increase. The application of dairy cattle slurry to alfalfa fields is a simple and inexpensive solution to the problems above. A repacked soil column study was conducted to investigate the effect of slurry nitrogen (N) on alfalfa biomass, as well as its photosynthetic characteristics. Dairy cattle slurry N or mineral fertilizer N was applied in two dressings at the first cut, with a target amount of 90 kg ha−1. A non-fertilization control (CK), a single mineral fertilizer N (MIN), and a slurry substitution for mineral N fertilizers (with equivalent N rate: FPS, 50% N from dairy cattle slurry; SLU, 100% N from dairy cattle slurry) were used. The results show that the slurry N increased the alfalfa biomass by 16.40–36.36% and the SPAD value by 30.27–61.34% with FPS and SLU treatments, respectively. Compared to the CK treatment, the FPS and SLU treatments meaningfully increased the net photosynthetic rate by 19.97–60.04% and 3.03–89.48%, the stomatal conductance by 10.53–57.14% and 15.38–88.89%, the intercellular CO2 concentration by 5.78–24.92% and 7.21–32.53%, and the transpiration rate by 13.16–103.50% and 16.44–111.19%. More specifically, compared with the CK treatment, the N absorption of the SLU treatment increased by 6.78–12.30%, and the use efficiency increased by 30.98–46.60% in the SLU treatment. Similarly, phosphorus (P) absorption of the SLU treatment increased by 36.73–52.57%, and the use efficiency increased by 30.98–46.60%. Overall, the dairy cattle slurry N was utilized efficiently as mineral N for alfalfa biomass, improved the photosynthetic characteristics of alfalfa leaves, and increased the N and P use efficiency. Our results clarify the optimal amount of dairy cattle slurry to be applied and provide a scientific basis for the use of dairy cattle slurry in agricultural systems.