Response of soil nutrients, enzyme activities, yield and saponin content of Panax notoginseng to different water and fertilizer combinations

Objectives We studied the soil nutrient contents, enzyme activities, P. notoginsen yield and saponin content under different water and fertilizer combinations at the main growth stages of P. notoginseng, to optimize the water and fertilizer combination for the P. notoginseng production.

Methods The experiment was conducted in the P. notoginseng Base of Luxi County, Yunnan Province, China, from 2018 to 2021. The treatment was composed of 3 watering (W) and four fertilization (F) quotas, forming 12 combination treatments: W1F1, W1F2, W1F3, W1F4, W2F1, W2F2, W2F3, W2F4, W3F1, W3F2, W3F3, and W3F4, taking the local irrigation and fertilization as control (CK). The three water quota was 120 mm (W1), 240 mm (W2), and 360 mm (W3). The four fertilizer quota were 48 kg/hm² (F1), 72 kg/hm² (F2), 96 kg/hm² (F3), and 120 kg/hm² (F4). The tested fertilizer was a water-soluble fertilizer with a N-P₂O₅-K₂O ratio of 15-15-30. Soil samples were collected at rooting, seedling, flowering and ripening stage for the measurement of nutrient content and enzyme activity. Plant samples were collected at the ripening stage for the analysis of yield and saponin content of P. notoginseng. The entropy weight method was used to calculate the weight of each measurement index, and the TOPSIS method was used to comprehensively evaluate the effect of each treatment combination.

Results Water and fertilizer quota significantly affected the content of NO₃⁻-N, NH₄⁺-N, available P and available K, and the activities of urease, sucrase, acid phosphatase, and catalase (P<0.05). At rooting stage, the highest soil NO₃⁻-N, NH₄⁺-N, and available K content was all in W2F4 treatment (34.6 mg/kg, 21.0 mg/kg and 132.1 mg/kg). At seedling stage, the highest NO₃⁻-N content was in W2F3 (39.4 mg/k), and the highest NH₄⁺-N and available K content in W3F2 (25.4 mg/kg, 136.0 mg/kg). At flowering stage, the highest soil NO₃⁻-N, NH₄⁺-N, and available K content was appeared in W1F4 (49.9 mg/kg), W3F3 (30.2 mg/kg), and W2F4 treatment (178.1 mg/kg), respectively, and at ripening stage, appeared in W2F4 (41.8 mg/kg), W3F3 (25.7 mg/kg), and W2F4 treatment (158.0 mg/kg), respectively. The available P content increased with the increase of fertilizer application rate at the same irrigation level in all the four growth stages, and the highest available P content at the rooting, seedling, flowering and ripening stage was all determined in treatment W3F4 in value of 23.6 mg/kg, 25.9 mg/kg, 30.1 mg/kg, and 26.7 mg/kg, respectively. The four enzyme activities had a tendency to increase with the increasing of fertilizer rate at the three irrigation quota, and the maximum activity of urease, sucrase, acid phosphatase and catalase was measured in treatment W3F4 at the four growth stages as well. The highest yield was in W2F4 treatment (2800 kg/hm²). Water use efficiency was maximum in W1F3 and W1F4 treatments (2.0 kg/m³), and fertilizer partial productivity was highest in W2F1 and W3F1 treatments (1.7 kg/kg). The highest Rg₁ and Rb₁ saponin contents were in W3F2 treatment, while the highest Rd and R₁ saponin content were in W1F4 treatment. Yield were significantly correlated with soil NO₃⁻-N, available P and available K, urease, acid phosphatase, catalase and sucrase, and Re (P<0.05), while Rg₁ and fertilizer partial productivity was positively correlated with each other at extremely significant level (P<0.01). According to the TOPSIS analysis, W3F4 treatment ranked first in terms of soil nutrients, enzyme activities, and saponin content at the rooting, seedling, and fruit stages, while W1F4 treatment ranked first at flowering stage.

Conclusions Based on the comprehensive evaluation of soil nutrient availability, enzyme activity, yield and saponin content of P. notoginseng, and the water and fertilizer use efficiency, water 360 mm and fertilizer 120 kg/hm² (W3F4 treatment) is the optimum combination.