Planting Trees on Sandy Saline Soil Increases Soil Carbon and Nitrogen Content by Altering the Composition of the Microbial Community

The remediation and exploitation of sandy saline soils, an underutilized resource, can be enhanced by a greater comprehension of the impact of plants and microorganisms on nutrient cycling. However, there is scant research information on the capacity of different trees and shrubs to improve carbon and nitrogen cycling in saline soils at different depth layers. This study investigated the effect of the trees <i>Zelkova serrata</i> (ZS) and <i>Ligustrum lucidum</i> (LL) and shrub <i>Hibiscus syriacus</i> (HS) on the carbon and nitrogen fractions, soil enzyme activities and microbial communities in sandy saline soils. Planting ZS, LL or HS improved soil quality, increased soil carbon and nitrogen content, changed rhizosphere soil metabolites and enhanced soil enzyme activities and microbial abundance and diversity. Compared to values in the bare soil, the highest reduction in soil salinity was noticed under <i>Zelkova serrata</i> (49%) followed by <i>Ligustrum lucidum</i> (48%). The highest increase in total soil organic carbon (SOC) was noted under <i>Ligustrum lucidum</i> and <i>Hibiscus syriacus</i> (62% each), followed by <i>Zelkova serrata</i> (43%), as compared to levels in the bare soil. In the 0–10 cm soil layer, the total N in bare soil was 298 ± 1.48 mg/kg, but after planting LL, ZS or HS, the soil total N increased by 101%, 56% and 40%, respectively. Compared with that of the bare soil, <i>cbbL</i> sequencing showed that the relative abundance of <i>Bradyrhizobium</i> increased and that of <i>Bacillus</i> decreased due to planting. Similarly, the <i>nifH</i> sequencing results indicated that the relative abundance of <i>Bradyrhizobium</i> and <i>Motiliproteu</i> increased and that of <i>Desulfuromonas</i> and <i>Geoalkalibacter</i> decreased. These findings suggested that soil microorganisms could play a pivotal role in the carbon and nitrogen cycle of saline soils by influencing the content of soil carbon and nitrogen.