The Effect Of Alternate Of Wetting And Drying On Denitrification In Farmland Drainage Ditch Sediments In Hilly Areas

With economic development and population growth,the amount of "active" nitrogen added by human activities has caused a serious imbalance in the global nitrogen cycle,and has caused a series of environmental problems such as eutrophication of water bodies and increased greenhouse gas emissions.Farmland drainage ditch is an important channel for agricultural irrigation and drainage.Because it has a biogeochemical environment that is conducive to nitrogen reduction,many researchers believe it is an important "nitrogen" sink.Denitrification is one of the main ways to remove nitrogen from ditch sediment,and there are many factors that affect denitrification.Among them,wet and dry alternation is one of the important factors affecting the denitrification of sediments.At present,there are a lot of researches on denitrification at home and abroad,but they mainly focus on rivers,lakes and ditches in the plain area.The dry and wet alternation phenomenon caused by the undulations of the ditch in the hilly area has not been reported,especially the mechanism is insufficiently understood.In this paper,field in situ observations and indoor simulation experiments are used to study the effect of dry-wet alternation on denitrification in farmland drainage ditch,in order to find out the characteristics of sediment denitrification rate change in dry-wet alternation process,and to explore the denitrification effect in dry-wet alternation process.The impact mechanism provides a scientific basis for improving the removal efficiency of active nitrogen in ditches in hilly areas and reducing the input of active nitrogen in the surface water of the upper Yangtze River.The main conclusions of the paper are as follows:?1?The wet and dry alternation significantly affects the physical and chemical properties of ditch sediments.The redox potential?Eh?is shown as alternating wet and dry areas?333.7m V?> intermittent water stagnation area?268.1m V?> long-term flooding area?169.1m V?.The content of NH₄⁺-N,DOC and organic carbon in surfacelayer and sub-surface layer were significantly different.The content of NO₃^--N in surface layer of different regions was significantly different,and the difference of TN in sub-surface layer of different regions was significant?P <0.05?.The dry-wet alternation can significantly reduce the NH₄⁺-N and DOC content in the sediment,while the NO₃^--N content in the sediment increases first and then decreases.?2?The wet and dry alternation can significantly affect the denitrification rate of ditch sediments.In farmland drainage ditch,the denitrification rate of sediments in alternating wet and dry areas: 94.55 ?g·m-2·h-1 on the surface layer and 66.17?g·m-2·h-1 on the subsurface layer;the denitrification rate of sediments in intermittent water stagnation areas: 287.9 ?g·m-2·h-1 on the surface layer and 191.41 ?g·m-2·h-1 on the subsurface layer;sediment denitrification rate in long-term flooded area: 225.27?g·m-2·h-1 on the surface layer and 125.6 ?g·m-2·h-1 on the subsurface layer;the overall spatial performance is: intermittent water stagnation area > long-term flooding area > wet and dry alternating area;the intensity of dry and wet alternation can also significantly affect the rate of sediment denitrification.The average value of denitrification rate under different dry and wet alternating intensity treatments is medium intensity?266.42 ?g·m-2·h-1?> high intensity?199.1 ?g·m-2·h-1?> low intensity?152.93 ?g·m-2·h-1?> long-term flooding?9.57 ?g·m-2·h-1?.In addition,the study found that the denitrification rate of surface sediments in farmland drainage ditches accounted for 62.8% of the overall denitrification rate.In order to improve the denitrification efficiency of the sediments in the dry and wet alternating area of the ditch,it was suggested that the management mode of the farmland drainage dry and wet alternating area can be treated with medium intensity.?3?The functional genes of denitrification in farmland drainage ditch sediments are spatially expressed as follows: the nir S gene is significantly higher in the wet-dry alternating zone and intermittent water retention area than the long-term flooded area?P <0.05?,and the nos Z gene is significantly higher in the intermittent water retention area than the long-term flooded area?P <0.05?.The copy number of denitrification function genes in sediments in different zones showed in time: different denitrification function genes in the dry and wet alternating regions showed an increasing trend,inwhich nir K and nos Z increased significantly?P <0.05?;although the denitrification function genes in intermittent stagnation areas and long-term flooded areas showed a decreasing trend,there was no significant difference?P >0.05?.The intensity of dry-wet altered the copy number of the denitrification function genes in the sediment.Compared with the control group,the dry-wet altered treatment increased the copy number of the denitrification function genes.Multiple comparison analysis showed that the treatment of nar G gene before and after the experiment was significantly high and medium intensity,and the treatment of nir K gene before and after the experiment was low and medium intensity.?4?Pearson correlation analysis showed that the denitrification rate of trench sediments had a positive correlation with water content,NO₃^--N,and DOC?P <0.05?;the denitrification rate of trench sediments was related to the TN,TDN,NO₃^--N,DOC showed a significant positive correlation,a negative correlation with DO,and no correlation with NH₄⁺-N;the denitrification rate of trench sediments had a significant positive correlation with the denitrification function gene nir K.Structural equation model results show that the nir K genes in the sediment and NO₃^--N in the water body are the main factors affecting the denitrification rate of farmland drainage ditch sediments.