Water,Salt and Heat Influences on Carbon and Nitrogen Dynamics in Seasonally Frozen Soils in Hetao Irrigation District,Inner Mongolia,China

To investigate carbon(C)and nitrogen(N)dynamics in seasonally frozen soils under saline and shallow groundwater supply conditions,in-situ lysimeter experiments with different groundwater table depths(WTD=1.8 and 2.2 m)were conducted in Inner Mongolia,China during the wintertime of 2012-2013.Changes in soil organic C and total N in multiple layers during various periods,as well as their relationships with soil water,salt,and heat dynamics were analyzed.Accumulation of soil organic C and total N during freezing periods was strongly related to water and salt accumulation under temperature and water potential gradients.Water and salt showed direct influences on soil C and N dynamics by transporting them to upper layer and changing soil microbial activity.Salt accumulation in the upper layer during freezing and thawing of soil affected microbial activity by lowering osmotic potential,resulting in lower C/N ratio.Nitrogen in soil tended to be more mobile with water during freezing and thawing than organic C,and the groundwater table also served as a water source for consecutive upward transport of dissolved N and C.The changes in C and N in the upper 10 cm soil layer served as a good sign for identification of water and salt influences on soil microbial activity during freezing/thawing.