Nitrogen migration along the soil-forage-livestock interface for Sorghum bicolor×S. sudanense

In this paper, we discussed the nitrogen migration rule of the soil-forage-livestock interfaces for Sorghum bicolor×S. sudanense to provide a theoretical basis for rational fertilization of the grass. A field experiment in which the plot contained micro area was conducted and different ¹⁵N application rates, including 0 kg∙hm⁻² (CK), 90 kg∙hm⁻² (N90), 180 kg∙hm⁻² (N180), 270 kg∙hm⁻² (N270), and 360 kg∙hm⁻² (N360), were set up. The indexes of nitrogen migration rule were analyzed using the ¹⁵N isotope labeling method under different ¹⁵N treatments. The indexes included plant utilization rate, soil residual rate, loss rate, and distribution rate to stem or leaf of ¹⁵N at the interface of forage-livestock; and the digestibility, utilization rate, and residual rate of ¹⁵N of sheep and cows at the interface of forage-livestock for S. bicolor×S. sudanense. The results showed that the total hay yield in the N180 treatment was significantly higher than that in the CK and N90 treatments (P<0.05), but there was no significant difference from those in the N270 and N360 treatments. At the interface of soil-forage, the plant utilization rate and soil residual rate of ¹⁵N were the highest, while the loss rate was lowest under the N180 treatment, and there were significant differences between N180 and N360, but there were no significant differences among the N180, CK, and N90 treatments. At the interface of forage-livestock, the ¹⁵N digestibility of sheep under the N180 treatment was significantly higher than that under the N360 treatment, but there was no significant difference with those under other treatments; and the ¹⁵N utilization rate of dairy cows under the N180 treatment was the highest, but the difference was not significant. On average, the ¹⁵N migration rule of the soil-forage interface was as follows: the soil residual rate was 25.22%, the utilization rate was 23.95%, and the loss rate was 50.83%; the distribution rate of leaf (54.35%) was higher than that of stem (45.65%) for S. bicolor×S. sudanense. After the S. bicolor×S. sudanense was ¹⁵N-labeled and further digested by ruminants, the ¹⁵N migration rule of soil-forage-livestock (sheep) interface was as follows: the soil residual rate was 25.22%, the residual rate of forage was 4.49%, the utilization rate of sheep was 19.46%, and the loss rate of ¹⁵N was 50.83%, while at the interface of soil-forage-livestock (dairy cows) was 25.22%, 5.78%, 18.17%, and 50.83%, respectively. These results provide a theoretical basis for rational fertilization and transformation of soil-forage-livestock for S. bicolor×S. sudanense.