Understanding the interactions between salinity and fertilizers is of significant importance for enhancing crop yield and fertilizeruse efficiency. In this study a complete block design experiment was performed in the Hetao Irrigation District of Inner Mongolia,China, to evaluate the effects of interactions between soil salinity and nitrogen(N) application rate on sunflower photosynthesis and growth and to determine the optimum N application rate for sunflower growth in the district. Four levels of soil salinity expressed as electrical conductivity(0.33–0.60, 0.60–1.22, 1.2–2.44, and 2.44–3.95 dS m-1) and three application rates of N fertilization(90, 135,and 180 kg ha-1) were applied to 36 micro-plots. Soil salinity inhibited the photosynthetic rate, stomatal conductance, transpiration rate, plant height, leaf area, and aboveground dry matter of sunflowers. The intercellular CO2 concentration first decreased and then increased with increasing soil salinity in the seedling stage, and the instantaneous leaf water-use efficiency fluctuated with soil salinity. The stomatal and non-stomatal limitations of sunflowers alternated in the seedling stage; however, in the bud, blooming,and mature stages, the stomatal limitation was prevalent when the salinity level was lower than 2.44 dS m-1, whereas the nonstomatal limitation was predominant above the salinity level. The application of N fertilizer alleviated the adverse effects of salinity on sunflower photosynthesis and growth to some extent. During some key growth periods, such as the seedling and bud stages, a moderate N application rate(135 kg ha-1) resulted in the maximum photosynthetic rate and yielded the maximum dry matter. We suggest a moderate N application rate(135 kg ha-1) for the Hetao Irrigation District and other sunflower-growing areas with similar ecological conditions.
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.
