Using ¹⁵N tracing technique to study the yield and environmental effect of decreasing N fertilization on cotton in different fertility fields

ObjectivesIn the medium fertility field of cotton area in the Yellow River catchments, the North China Plain, the economic optimum nitrogen (N) rate for cotton is 300 kg/hm², and the result was derived from cotton yield response to N fertilizer rate in field trials for many years, but the N application rate did not fully consider residual N in the soil of cotton field. This paper was to explore effects of different N application rates and ratios on seed cotton yield and N use efficiency under low fertility soil, and effects of the same N application rate under low, medium and high fertility soils in field conditions aimed to provide a theoretical basis for N fertilization management of cotton.

MethodsField trials of nitrogen fertilizer management were carried out using ¹⁵N tracer technique and cotton was used as tested material. Fields with soil total N content of 0.83, 0.74 and 0.60 g/kg were taken as high (S1), media (S2) and low (S3) fertility fields for the experiment. Three N application rates were set up for the low fertility field: 113 kg/hm²(N1), 225 kg/hm²(N2) and 338 kg/hm²(N3). The N fertilizer was divided into equal two parts and topdressed at the seedling and flowering stages. A treatment of twice N topdressing in ratio of 1∶2 was also setup in the low fertility field with low N rate. Cotton and soil (0–60 cm) were sampled at 70% boll opening stage, the contribution of N absorbed from fertilizer and soil were determined using ¹⁵N tracer technique. The seed cotton yield, and ¹⁵N recovery rate of cotton plants were investigated.

ResultsFor the low N rate, the soil fertility had no significant effect on seed cotton yields, the plant N uptake proportion deprived from fertilizer was declined with the increase of soil fertility, while the N uptake proportion deprived from soil nitrogen was increased. The ¹⁵N recovery rate of cotton plants was decreased significantly with the increase of N application rate, and declined with increase of the soil fertility. There were no significant differences between the low soil fertility and medium soil fertility for ¹⁵N recovery rate of cotton plant, but the recovery rate under the low soil fertility was significantly higher than that of the high fertility soil. The ¹⁵N residual rate of high fertility soil was higher than those of the low fertility soil and the medium fertility soil. The ¹⁵N loss rate was increased significantly with the increase of N application rate and soil fertility. Seed cotton yield for the low N rate at the ratio of 1∶2 was higher than that at the ratio of 1∶1. Under the condition of low fertility soil, seed cotton yield for N 225 kg/hm² was relatively higher than those for N 113 kg/hm² and N 338 kg/hm² because of more ¹⁵N uptake by cotton plants.

ConclusionsUnder the condition of lower soil fertility, the seed cotton yield and ¹⁵N recovery rate for the moderate N rate (225 kg/hm²) were higher than those for high N rate (338 kg/hm²), and the N fertilizer loss rate was less, which demonstrated that it was feasible for improvement of both N use efficiency and seed cotton yield by decreasing input of N fertilizer. Low N input under the condition of high fertility soil could decrease N fertilizer loss.