Climatic factors and mulching affect soil salinity dynamics in coastal areas

To clarify the impact of climatic factors on seasonal variations in soil salinity and to explore the effects of vegetation cover and straw mulching on soil desalination and salinity control in coastal areas, a field experiment was conducted in the medium-heavy saline coastal area of Rudong, Jiangsu Province. Four treatments were set in the study-control (bare land, CK), straw mulch (at 15 t·hm^-2, SM), vegetation cover (PC) and PC combined with SM (at 7.5 t·hm^-2, PC+1/2SM). Climatic factors (rainfall and air temperature) and topsoil salinity dynamics were determined for the period from May 2014 to May 2015. The results showed that:1) seasonal variations of soil salinity to some extent occurred in bare lands in coastal areas, with obvious salt accumulation during the period from October to December with the highest EC_1:5 (3.90 dS·m^-1) in October. 2) Correlation analysis showed that change in soil salinity under CK treatment was significantly negatively correlated with 7-day cumulative rainfall before sampling (P < 0.01) and also with 15-day cumulative rainfall before sampling (P < 0.05), but no so obvious significant correlation were found under SM, PC and PC+1/2SM treatments (P> 0.05). Under CK treatment, multi-factor interphase analysis of climatic factors indicated that increase in rainfall promoted soil desalinization (P < 0.01) and increase in air temperature exacerbated soil salt accumulation in surface soil (P < 0.01). Simultaneously, interaction between rainfall and air temperature had a positive effect on soil salt accumulation (P < 0.01). 3) Surface mulching (including PC and SM) significantly altered the effects of climatic factors on seasonal variations in soil salinity. Under PC and PC+1/2SM treatments, the relationship of the change in soil desalinization ratio (Y) with treatment time (X) followed the quadratic functions of Y_PC=0.001X²-0.345X + 54.41 (R²=0.456, P < 0.01) and Y_PC+1/2SM=0.001X²-0.293X + 57.121 (R²=0.526, P < 0.01), respectively. Under SM treatment, the relationship was a Logistic curve with an equation of Y_SM=94.912/(1+e^1.482-0.052X) (R²=0.980, P < 0.001). In addition, soil desalination rate increased rapidly after 29 days of SM treatment (that is June to August) and the trend stabilized as the rate reached 94.91% in 70-80 days after straw mulching. Moreover, large amount of straw mulch (15 t·hm^-2) had a more obvious effect on soil desalination, but also caused more obvious alkalinity problems. To therefore control desalination and salinity, a suitable amount of straw mulching (such as straw cover of 15 t·hm^-2) or suitable amount of straw mulching combined with vegetation cover (such as Sesbania cannabina Pers. and Brassica juncea L. Czern. et Coss.) were recommended in coastal area with seasonal concentrated high rainfall. This promoted soil desalinization and future soil reclamation under warming-wetting climate in coastal areas.