Effects of irrigation water and N fertilizer types on soil microbial biomass and enzymatic activities

Effects of irrigation water and N fertilizer types on soil microbes and enzymes are poorly understood.This study aimed to evaluate the effect of different irrigation water and N fertilizer types on soil microbial biomass and enzymatic activities.A 2-year experiment(2015-2017)was conducted at the Tongzhou Experimental Station of Tongzhou District, Beijing,China(39°42'6.93″N, 116°41'2.31″E).The experiments utilized a factorial randomized complete block design with 8 treatments(2×4)in 3 replicates.The first factor had 2 irrigation water types(fresh water(FW)and reclaimed water(RW)), and the 2nd factor had 4 N fertilizer types(no nitrogen, urea, ammonium sulfate, and slow-release urea).Physicochemical, microbe and enzyme analyses from soil and water samples were determined.The results showed the contents of SOC, SON, DOC, DON, TN,NO_3~--N, and NH_4~+-N in different treatment soils were significantly different across crop seasons(P<0.05).The average contents of DON in soils irrigated with RW are significantly higher than that in soils irrigated with FW(P<0.05).Fertilization increased the SOC, SON, DON, TN, and NO_3~--N contents in the soils significantly(P<0.05).No significant interaction between irrigation and fertilization on the soil SOC, SON, DOC, DON, TN, NO_3~--N, and NH_4~+-N contents.the DON content in the soils irrigated with RW was average 8.33% significantly higher than in the soils irrigated with FW(P<0.05).The SOC, SON, DON, TN and NO_3~--N contents in the soils fertilized with inorganic N was on average 8.35%, 10.13%, 11.75%, 11.60% and 92.99% significantly higher,respectively, than that in the soils fertilized with no nitrogen(P<0.05).Soil bacterial biomass under different treatments ranged from 9.59 to 12.18 nmol/g.The soil actinobacterial biomass values under different treatments ranged from 1.01 to 1.24 nmol/g.The soil G+ bacterial biomass values under different treatments were 6.10- 6.73 nmol/g.Compared with FW and RW irrigation significantly increased the soil bacterial, actinobacterial, and gram-positive(G+)bacterial biomasses and total phospholipid fatty acids(PLFAs)by 7.60%, 10.48%, 4.97% and 4.88%, respectively.N fertilization significantly increased soil bacterial biomass by 13.42%-17.34% and increased the total PLFAs by 8.12%-11.19%.Compared with FW, RW irrigation did not significantly increase soil urease, catalase and invertase activities.N fertilization did not significantly increase soil urease, catalase and invertase activities.Soil microbes and enzymes were more active in the soils fertilized with slow-release urea.These results indicate that actinobacteria and G+ bacteria grew more quickly and could more efficiently utilize the DON introduced by the RW and that the soil bacteria could more efficiently utilize NO_3~--N introduced by the fertilizers than the other microbes.RW irrigation and slow-release urea fertilization were more effective to increase the soil microbes and enzymes.For better soil quality, higher yields and save water resources, drip irrigation with RW under slow-release urea fertilization was recommended for summer maize-winter wheat crop rotation.