3.1 Effect mechanism of optimal water and fertilizer management on soil microbial biomass C and N of Panax notoginseng
This study found significant differences (p<0.05) in soil microbial biomass C and N and enzyme activities at different growth periods under the same water and fertilizer optimal management, and the changes tended to increase first and then decrease with the growth time of Panax notoginseng. The soil microbial biomass C and N content ranged from high to low in flowering, fruiting period, seedling period, and rooting period, with the highest in flowering (rainy season) and the lowest in rooting period (dry season). Bargali, K et al. found that soil microbial biomass carbon of different forest types showed significant seasonal changes, with the minimum value in winter and the maximum value in the rainy season(Bargali et al.,2018).Lepcha, N.T. et al.studied the effects of land use types and seasonal changes in the eastern Himalayas on soil microbial biomass carbon and nitrogen and found that soil microbial biomass carbon and nitrogen of different land use types showed strong seasonal differences (p<0.001), with the peak occurring in the rainy season and the lowest in winter(Lepcha et al.,2020). In this study, the highest soil microbial biomass C and N content occurred in the flowering season (rainy season) and the lowest in the root growth period (dry season) because the dry and wet seasons in the study area were distinct, with rainy summer, dry winter and spring, abundant rainfall in summer, humid air, and small evaporation. The soil temperature and humidity were suitable for the growth, reproduction, and decomposition activities of soil microorganisms, and microbial increase promoted the conversion and accumulation of soil microbial biomass C and N content. Huang et al. found that the interaction between increased irrigation amount and season had significant effects on MR, MBC, and MBN but significantly increased in summer and autumn, with no effect in spring and winter. Rehydration in summer and autumn increased bacterial biomass as well as bacterial and fungal diversity and abundance(Huang et al.,2018), which was in contrast to the finding by Li et al. that soil microbial biomass carbon and nitrogen in the Mediterranean agro-ecological system had the lowest value in summer and the highest value in autumn(Li et al.,2014) because of the different climatic types in the two regions. The Mediterranean region was hot and dry in summer and warm and rainy in winter, in contrast to the rainy summer and dry winter and spring in the studied region.
This study found significant differences (p<0.05) between soil microbial biomass C and N and enzyme activities in the same growth period under different water and fertilizer optimal management, and the soil microbial biomass C and N content increased with the increase of irrigation and fertilizer application amount, of which, the W2F4 treatment at the rooting period and the W3F4 treatment at the seedling, flowering and fruiting periods greater than the other treatments in all reproductive stages. The study showed that long-term fertilization improved soil microbial biomass carbon (SMBC) and dehydrogenase activities, and organic fertilizers had a greater effect on SMBC and dehydrogenase activities than mineral fertilizers(Luo et al.,2015).Increases in lower soil elemental and microbial biomass stoichiometry (SMBC:SMBP and SMBN:SMBP) and LAP were mainly due to the combined use of manure and mineral fertilizers accelerating SOC and N mineralization(Yang et al.,2015).In this study, the worst treatments for soil microbiomass carbon, nitrogen and enzyme activity were W1F2 and W1F1 for both the rooting and seedling periods, respectively, and W2F1 and W1F1 for both the flowering and fruiting periods, respectively.