1.2 Determination method
The chloroform fumigation extraction method was used for the determination of soil microbial biomass carbon and nitrogen. The chloroform-fumigated and non-fumigated soils were extracted with 0.5mol/L K2SO4 solution at the soil/liquid ratio of 1: 4. The organic carbon content in the extraction solution was determined by FeSO4 solution titration with a conversion coefficient of 0.38, and the organic nitrogen content in the extraction solution was determined by ninhydrin colorimetric method with a conversion coefficient of 5.00(Li et al.,2008).The measurement of fumigation and non-fumigation was repeated 3 times, and the mean value was taken.
Soil enzyme activity was determined by referring to Soil Enzyme and Its Research Method(Guan et al.,1986).Catalase (Cat) was measured by KMnO4 titration as a ml number of 1g.h-1 soil consuming 0.1 mol.L-1KMnO4.Urease (Ure) using phenol-sodium hypochlorite colorimetric method, in 1g of soil after 1d culture generated NH3-N mg number said urease activity; Sucrase (Sue) was colorimetric with 3,5-dinitrosalicylic acid and expressed as sucrase activity in milligrams of glucose formed in 1g of soil after 1d of culture. Acid Phosphatase (Acp) activity was determined using a disodium phosphate colorimetric method using milligrams of phenol released from 1g of soil 1d later. Each soil sample was measured three times, and the average value was taken. The soil moisture water content was determined by the drying method.
Soil microbial biomass carbon and nitrogen and enzyme activity evaluation calculation
In this paper, the entropy method is used to determine the weight of each index. Due to the differences between each index, in order to eliminate the dimension of index data and the positive and negative effects of the index, the original data is standardized to form a new data column. The following formula is the entropy method to calculate the index weight (Huang et al.,2021;Lu et al.,2019):
(1) Data standardization
(1)
Where: is the value of indicator of its sample (=1,2…,n; =1,2…,n);,are the maximum and minimum values of indicator , respectively, and is the normalized ,which were the standardized indicators of soil microbial carbon and nitrogen content and enzyme activity under different water and fertilizer optimal management.
(2)Calculate the weight of thesample data under theindicator for that indicator :
(2)
(3)TOPSIS calculator
①Construction of original evaluation matrix:
There are m evaluation objects, nevaluation indicators,the indicators and the corresponding entropy weightsmultiplied,the data are represented as the original matrix,is the original data of the jth indicator of the ith treatment,is the original data of the jth indicator of the ith treatment:
(3)
Where:i=(1,2,…,m),j=(1,2,…,n), m=13,are 13 water and fertilizer treatment trials.n=7,are indicators of soil enzyme activity, microbial load carbon and nitrogen, and water content。
②Normalization matrix.The maximum and minimum values of each column constitute the optimal and inferior vectors respectively:
(4)
③The distance between the ith treatment and the optimal and inferior solutions are:
(5)
④The ith treatment fits to the optimal solution Ci are:
(6)
Microsoft Excel 2016 and SPSS26.0 were used for statistical analysis of the data, and one-way ANOVA was used to test the differences of soil microbial biomass carbon and nitrogen content and enzyme activity under different water and fertilizer optimal management (=0.05). The entropy method was used to calculate the weight and rank the soil microbial biomass carbon and nitrogen content and enzyme activity. Mantel analysis was performed using R language, and Origin2020b software was used for mapping.

2 Result analysis