Characteristics Of Nitrogen Cycling Microbial Populations And Nutrient Stoichiometry In Vegetable Field

In recent years,China has become one of the most important vegetable producing countries in the world with increasing planting area and yield.Greenhouse vegetable field has the typical characteristics of high water,high fertilizer,high temperature,high replanting index and so on,which leads to many ecological and environmental problems,such as soil acidification,water eutrophication,increased emissions of nitrous oxide and excessive nitrate in groundwater.As soil ameliorant,organic fertilizer is widely used in farmland in China,because of increasing yield and improving soil quality.Nitrogen(N)is the most important nutrient in agricultural production,and its different forms and conversion rates will determine whether N can be used efficiently by crops.Microorganisms are the main drivers of soil N cycle.The nutrient relationships between soil resources and microbes will affect the activity and growth of microorganisms,and then change the soil N cycle rate and crop yield.Nutrient stoichiometry is an important method to characterize the nutrient relationships between microorganisms and soil resources.These nutrient relationships will be inevitably influenced by organic fertilization and frequent irrigation.Therefore,it is of great significance for the precise agriculture and model prediction to explore the nutrient stoichiometry,and to study the regulation of organic fertilization and frequent irrigation on soil N-cycling functional microbial groups and crop yield in greenhouse vegetable field.Based on a vegetable field ecosystem following organic fertilization,four fertilization treatments with the same amount of N,phosphorus(P)and potassium(K)(240 kg ha-1crop-1)and control treatment were set up in this study,including N-free treatment,single chemical fertilizer treatment,20% substituting chemical N with organic fertilizer,50%substituting chemical N with organic fertilizer treatment,and 50% substituting chemical N with bio-organic fertilizer treatment.Based on ecological stoichiometry theory,the ecoenzymatic stoichiometry was used to determine the status of microbial nutrient limitation,and then revealed the regulation of microbial nutrient limitation on the nitrifiers and denitrifiers.At the same time,the resistance and resilience of N-cycling functional microbial groups,nutrient stoichiometry and their relationships to dry-wetting stress under different environments were investigated by combining multi-factor experiments(i.e.temperature,fertilization and dry-wetting cycle).The main results are as follows:1.Microbial communities presented a high N:P imbalance with strong N:P homeostasis,but a low carbon(C):N(or P)imbalance between microbial communities and their available resources with weak C:N(or P)homeostasis.Soil microorganisms were in a N limitation,and 20% and 50% organic substitutions significantly aggravated microbial N limitation compared with single chemical fertilization.Microbial N limitation was significantly correlated with available soil nutrient resource ratio and microbial biomass ratio.It suggested that enzymatic stoichiometry is useful in evaluating nutrient relationships between microbial community and soil resources.2.The abundance of functional genes for nitrifying and denitrifying microorganisms was on average 5.5% lower for the 20% organic substitutions and 14.2% lower for the 50%organic substitutions compared to the chemical fertilizer alone.20% and 50% organic substitutions were not significantly different from the chemical fertilizer alone in terms of vegetable yield(three-year dry matter accumulation),while the vegetable yield was significantly higher for the 50% organic substitutions than for the 20% organic substitutions by 6.1%.Both N-cycling functional gene abundances and vegetable yields were influenced by the soil-microbial nutrient imbalance relationship.3.Under the stress of dry-rewetting cycles,the resistance of N-cycling functional microbial groups following 50% organic substitution was higher than that of chemical fertilization at all temperatures(15℃,25℃,and 35℃),while 50% bio-organic substitution showed inconsistent resistance at different temperatures compared with single chemical fertilization.N-cycling functional microbial groups showed no resilience at all temperatures following organic fertilization,while other fertilization treatments showed relative strong resilience only at certain temperatures.At low temperature(15℃),the N-cycling functional microbial groups showed strong resistance and no resilience.There were some complementary relationships between resistance and resilience of N-cycling microorganisms under dry-rewetting cycles.There was the significant correlation between the resistance of N-cycling functional microbial groups and the C:N:P nutrient stoichiometry,while the resilience only related significantly with the C-related indexes.Soil N-cycling microorganisms in vegetable fields showed a good stability overall under frequent dry-rewetting cycles.To sum up,50% organic substitution may be recommended in intensive greenhouse vegetable production because of stable yield and emission reduction.The main reasons are as follows.50% organic substitution changed the nutrient supply and demand relationship between microbes and available soil resources,which made microbes be in an appropriate nutrient limitation,and then reduced the functional gene abundances of nitrification and denitrification.These results lead to the reduction of N losses.At the same time,50%organic substitution could improve the resistance of N-cycling functional microbial groups to dry-rewetting cycle compared with single chemical fertilizer application,thus to ensure the stability of N-cycling related ecological functions in greenhouse vegetable field.Based on the above regulatory mechanisms,50% organic substitution stabilized crop yield and reduced environmental pollution compared to single chemical fertilization.