Responses of the ammonia-oxidizing microorganisms' abundance to tunnel evacuation in karst trough volley: a case study in Zhongliang Mountain, Chongqing, China

Abstract:

Ammonia oxidizing microorganisms mediate the oxidation of ammonium nitrogen in the soil, which is the first step of nitrification.[Objective] In the karst areas affected by large tunnel projects, understanding the response of ammonia oxidizing microorganisms to changes in soil moisture and nutrient condition has great significance for researching the changes of the ecological environment and the nitrogen cycle process caused by tunnel construction.[Methods] This study took four land uses (Grass land; bamboo land; mixed forest; vegetable land) of Longfeng trough valley, which is tunnel-affected-area and the Longche trough valley, which is non-tunnel-affected-area in Zhongliang Mountain, Beibei, Chongqing as an example, comparing the abundance of three ammonia oxidizing microorganisms (Ammonia oxidizing bacteria AOB, ammonia oxidizing archaea AOA, nitrite oxidizing bacteria CMX) and combining with the changes of soil water content, pH, and soil nutrient elements, to analyze the changes of ammonia oxidizing microorganisms caused by tunnel construction and its process mechanism.[Results] The results shows:① As the tunnel excavation exposed the underground aquifer, the level of groundwater decreased, the soil moisture content decreased, the pH and the nitrate nitrogen increased. The abundances of AOA, AOB and CMX of the tunnel-affected-area were significantly lower than the non-tunnel-affected zone, the abundances of the latter is 4.8, 4.4 and 3.9 times of the former respectively; ② Affected by the alkaline environment in soil and groundwater of karst area, as well as the vulnerability to leaking of solubles extremely easily, the concentration of substrates such as ammonium nitrogen is not the main influencing factors of ammonia oxidation microorganisms, the abundance of AOA is positively correlated with soil moisture content and soil buffering capacity (P<0.01), the abundances of CMX and AOB are both negatively correlated with soil nitrate nitrogen content (P<0.05), AOB abundance is also negatively correlated with soil pH (P<0.05).[Conclusion] This study reveals that soil physical and chemical properties like the soil moisture content and pH in karst areas are the main factors affecting the abundance of three ammonia oxidizing microorganisms. The leakage of groundwater and the decrease of soil effective moisture caused by tunnel construction are responsible for the decrease of ammonia oxidizing microorganisms abundances, which also change the nitrification process in tunnel-affected-area to some extent. However, the degree of influence and the changes of nitrogen cycle which more microorganisms involve in need to be further studied in detail.