Response of vegetable yields and soil nitrogen balance to nitrogen regulators in a greenhouse cucumber-purple cabbage rotation system

Objectives In this study, we analyzed the effects of urease and nitrification inhibitors on vegetable yield, soil N distribution, and soil-vegetable N balance in a greenhouse facility. The aim was to provide a scientific basis for optimizing N nutrient management techniques and reducing N loss in the facility farmland.

Methods In Zhuozhou, Hebei Province, a field controlled experiment of cucumber-purple cabbage rotation was conducted in a greenhouse for 8 years using no N input as the control (N₀). The treatments were no inhibitor (N), plus application of urease inhibitor (N+UI), plus application of nitrification inhibitor (N+NI), N and combined application of urease + nitrification inhibitor (N+UI+NI). All treatments were applied with the same quantity of P and K fertilizer. In cucumber season, organic fertilizer (41500 kg/hm²) and chemical fertilizer N (56.3 kg/hm²) were applied as a basal fertilizer, and N 304 kg/hm² in toal was applied since 30 days after planting in frequency of every 10–15 days. In purple cabbage season, only N 135 kg/hm² was used as a basal fertilizer before transplanting. We investigated the differences in vegetable yield, economic benefits, soil N distribution, nitrate leaching characteristics, and soil-vegetable N balance among the treatments.

Results Compared with N treatment, the cucumber yield in N₀ did not decrease significantly; however, the yield of purple cabbage decreased by 12.7%. The cucumber yield of N+UI, N+NI and N+UI+NI treatments did not increase significantly, but the N uptake significantly increased by 7.6% to 11.9% compared with N treatment. The apparent N utilization efficiency, agronomic efficiency and harvest index of nitrogen fertilizer significantly increased by 34.5%–53.3%, 13.5–22.5 times, and 1.5–3.6 percentage points, respectively. N+UI+NI treatment had the hightest agronomic efficiency and harvest index. The purple cabbage yield of N+UI, N+NI and UI+NI were significantly higher than that of N treatment, and the yield of N+UI+NI was markedly higher than those of N+UI and N+NI. N uptake and agronomic efficiency of purple cabbage were significantly increased by 18.0%–21.4% and 34.8%–68.3%, respectively. The accumulated N use efficiency and agronomy efficiency of cucumber-purple cabbage were significantly increased by 76.9%–94.2% and 52.8%–98.3%. In addition, the application of urease/nitrification inhibitors could substantially increase NO₃^–-N content in 0–20 cm depth during the growing period of cucumber-purple cabbage. At harvest, the 0–100 cm soil layer in purple cabbage recorded a 215.1%–275.2% increase in NO₃-N content, 15.1%–17.8% decrease in N surplus amount, and the N surplus rate remained at 35.5%–37.1%.

Conclusions In greenhouse, vegetables are planted for many years with continuous N input, leading to high soil N supply. The use of N fertilizer and combined application of N inhibitors do not improve vegetable yield, N utilization efficiency, and economic benefits. Under optimal N supply, the combined application of N fertilizer with urease/nitrification inhibitors could increase vegetable yield, N utilization efficiency, and economic benefits and effectively retain N in the 0–100 cm soil layer. Consequently, the N surplus rate is considerably reduced. However, the large quantity of N accumulated in vegetable field soil may still be leached and lost through flooding. Therefore, further studies are needed on N fertilizer optimization and regulation techniques to produce greenhouse vegetables under high soil fertility levels in a continuous cropping system.