Changes Of N And P Caused By Soil Microbial Activities And The Effect On The Groundwater Quality

In karst area,the groundwater resources are abundant and it is the origins of drinking water in many places all over the world.The karst groundwater is easily polluted for its special hydrological and geological background.The transformation between surface water and groundwater is quick in karst area,which causes the contaminant to be moved into the groundwater system easily.In agricultural region,the lost of nitrogen and phosphorus from soils are universal,that make the water qualities worse.In karst area,nitrogen and phosphorous can be carried into the groundwater system by surface run off through the swallow holes or rock crevasses.Qingmuguan karst valley is selected as the study area in this research.In this region,the main human activities are agriculture.The changes of N and P in soils can affect the groundwater qualities.During the process of N and P changing,the soil microbe plays a vital role.The relationship between microbial activities and changes of nitrogen and phosphorus are close.This research consists of three parts.Firstly,we discuss the changes of N and P contents in five different land use soils,and the microbial activities as well.Secondly, we observe the changes of nitrogenous and phosphorous form during the process of downward migration in the soil.At the same time,we also study the response of the related bacteria.Thirdly,we monitor the variations of hydrochemistry of the Qingmuguan groundwater,especially the nitrogenous and phosphorous substances. According to our results,we discuss the ways that N and P substances come into the groundwater system and the measures that we should take to protect the groundwater.The conclusions of this research can be summed up as follows:1.Organic matter,total amount of N,P and K,effective amount of N,P and K in five different land use types' surface soils(0～40cm) are tested.The five land use types are paddy fields,abandoned fields,vegetable plots,bush lands and bamboo lands.The number of bacteria,including ammonifying bacteria,nitrobacteria,denitrifying bacteria,phosphate-solubilizing bacteria and activity of phosphatase are also tested. The results show that human activities change the distributions of nutrient elements in the soils,thus cause the activities of microbes change to respond it.Furthermore, human activities also make the soil environment change and this change affects the microbial activities.2.Two typical land use types' soils(vegetable plots and bush land) from 0 to 80cm in depth are selected to observe the process of N and P downward movement in the soils,some microbial indicators are analyzed in this research.The result shows that the trends of these indicators between the two types' soils are similar except the surface soils from 0 to 30cm for the human activities' influences.With the soils deeper,the quantities of ammonia nitrogen,organic phosphor and inorganic phosphor become less because they are absorbed by the colloid or metallic ion and fixed in the soils.The amounts of ammonifying bacteria and phosphate-solubilizing bacteria are also decreased in deeper soils.While the quantities of nitric nitrogen and nitrite nitrogen appear an accumulated process in deeper soils.The number of nitrobacteria is decreased in deeper soils for the change of soil environment.The number of denitrifying bacteria has a same trend as the variation of nitrite nitrogen.3.The hydrochemistry of Qingmuguan groundwater is monitored for one year. Comparing with high water period and low water period,the characteristic of the hydrochemistry are different,because of the influence by precipitations.During the high water period,the concentrations of NO₃^-,NO₂^- and PO₄^3- are high for the fertilizer enters the groundwater system through the swallow hole.In the low water period,the concentration of PO₄^3- is lOW for the PO₄^3- is easily fixed in the soils.However,the nitric nitrogen and nitrite nitrogen can mix in the soil water,then enter the groundwater through rock crevasses.So the groundwater still has the risk of nitrogen pollution even in the low water period.We should take different measures to protect water from contaminations of N and P.For phosphorus contamination,on the one hand,it is essential to monitor the hydrochemistry of the groundwater,especially in rainy season; on the other hand,we should study on how to improve the utilization of soil phosphorus and reduce the use of phosphate fertilizer.For nitrogen pollution,besides the measures above,we also need to consider how to inhibit the process of nitrification and denitrification.