Effect Of Simulated Nitrogen Deposition During Seasonal Freeze-thaw Action On Soil Fauna And Soil Properties In Songnen Leymus Chinensis Grassland

Soil fauna community structure and function play key roles in the nutrient cycle andenergy flow in the ecosystem. On a global scale, the studies of soil fauna communities indifferent ecosystem have been widespread concerned. Atmospheric nitrogen deposition byhuman activities increased in recent years had influenced the global ecosystem processes.Songnen grassland played an key role in northeast ecological construction, climate regulationand biodiversity conservation. However, there were few studies focus on grassland ecosystemprocesses during freeze-thaw period. Soil fauna community dynamics and diversities areimportant parts in soil processes. Effect of nitrogen deposition during freeze-thaw action onsoil fauna communities and diversities in grassland ecosystem remained poorly understood.Thus, this thesis investigated the effect of simulated nitrogen deposition during freeze-thawprocess on soil fauna communities and soil properties, and revealed the relationship betweensoil fauna and soil properties. The main results are listed below:Through the samplings in December2013(freeze-thaw period), January2014(frozenperiod), March2014(thaw-freeze period) and June2014(growth season),2604soil faunaindividuals (21categories) were collected, belonging to a4classes,14orders (suborder),16families. The dominant groups of soil fauna included: Isotomidae, Oribatida, Prostigmata andAstigmata, accounting for78.61%of total soil fauna. Common groups of soil fauna included:Onychiuridae, Culicidae, Formicidae and Sminthurinae, accounting for16.24%of total soilfauna. Dominant groups and common groups accounted for94.85%of the total soil fauna,which constituted the main soil fauna community in the chinensis grassland. There were12categories soil fauna classified into rare groups, such as Curculionidae, Carabidae,Staphylinidae and so on. Different sampling times influenced soil fauna communities, thenumber of groups and individuals of soil fauna showed a trend of decline first and then rise.The lowest temperature in January2014leads to lowest groups and individuals of soil fauna,while those were the highest in June2014. Soil fauna communities also showed a surfacegathering characteristics, which had a vertical distribution in three different soil layers. Thenumber of groups and individuals of soil fauna gradually decreased with the soil depth. Low dose of nitrogen addition increased soil fauna groups and individuals, while high doses ofnitrogen showed an inhibition effect. Clipping treatment had a certain impact on soil faunacommunities, there were differences in different sampling time, soil depth and nitrogenaddition treatments between clipping treatment and non-clipping treatment.Shannon-Wiener diversity index, Simpson dominance index, Pielou evenness index andMargalef richness index of soil fauna were also affected by sampling time, soil depth andnitrogen addition treatment. Sampling time had an impact on soil fauna communities, allindices decreased first and then increased, with lowest occurring in January2014. Diversityand richness of soil fauna reduced with soil depth, while dominance index and evenness indexincreased with soil depth. Diversity and richness index of soil fauna communities increasedwith the nitrogen addition first and then reduced. Nitrogen addition resulted in a decrease ofdominance index and evenness index，and diversity index and richness diversity increased firstand then decreased along with the nitrogen addition. The effect of clipping treatment andnon-clipping treatment on soil fauna indices was not significant.Soil enzyme activities and soil properties changed with different treatments of samplingtime, soil depth and nitrogen addition. Soil urease, protease and invertase activities increasedfirst and then decreased with the nitrogen addition, and were positively correlated withtemperature changes, negatively correlated with soil depth. Soil catalase had little change indifferent treatments. Soil pH, increased with the freeze-thaw action in autumn-winter alternateperiod and winter-spring alternate period, decreased in growth season. The surface soil pH isgenerally higher than the underlying soil, and soil pH decreased with the nitrogen addition.Soil inorganic nitrogen content increased with the freeze-thaw action, but factors like springsnow melting lead to a decline in soil nitrate content. Soil inorganic nitrogen content reducedwith soil depth and increased with nitrogen addition. Soil organic matter content decreasedwith soil depth and freeze-thaw action, increased with the nitrogen addition.There were interactions in soil fauna communities, soil enzyme activities and soilproperties. Soil enzyme had different effect on soil fauna due to the soil enzymatic product.Invertase had the greatest effect on soil fauna, proteasewasthe second, and urease and catalase had little effect on soil fauna. Effect of soil properties on soil fauna was similar to that of soilenzymes. Soil inorganic nitrogen had the greatest influence on soil fauna, soil pH and soilorganic matter content also had effect on soil fauna. Differences of soil properties in samplingtime, soil depth and nitrogen addition treatments had impacts on soil enzyme activities, amongwhich soil organic matter content had a greatest influence, followed by soil pH and soilinorganic content. The results could provide reference and basis for the ecological processchanges in response to nitrogen deposition during freeze-thaw action in high latitude grasslandecosystem.