Mechanism of subsurface organic amendment combined with plastic mulching increasing soil fertility and microflora in an arid saline soil

[Objectives]: The soils in arid Northwest China are unfertile and have high salinity in surface tillage layers. Applying organic fertilizers into 10-30 cm of subsurface layer and plastic mulching were attempted in order to constructing a fertile subsurface layer in saline-alkali land. [Methods]: A field micro-plot experiment was conducted in Wuyuan County, Inner Mongolia, using sunflower as tested crop. Four treatments were designed, as conventional ploughing (CK), ploughing plus plastic mulching (PM), applying 106.8 t/hm² of manure in 10-30 cm subsurface (OM), and subsurface manure application plus plastic mulching (OMP). Before and after spring irrigation and after harvest, the pH, total salt, organic matter and available nutrient contents in 0-100 cm soil profile were determined, and the microflora in moderate saline soil were investigated during sunflower growth period.[Results]: Before spring irrigation, the OM and OMP treatments significantly increased the salt contents in the 10-30 cm soil layer (P < 0.05), the increase to CK was by 37.1%-52.9%, and to PM by 32.1%-47.2%. After spring irrigation, there was no significant differences among all treatments. Plastic mulching strongly inhibited salt returning, and the effect of OMP was better than of PM. After harvest, the salt return rate in 0-10 cm soil layer under OMP was reduced by 131%, 77.6% and 106%, compared with CK, PM and OM; and the salt return rate under PM was reduced by 53.5% and 28.7%, compared with CK and OM (P < 0.05), respectively. Compared with CK, the pH values under OM and OMP were decreased by 0.36 and 0.60 units in 10-30 cm soil layer, and by 0.45 and 0.44 units in 30-40 cm soil layer, respectively (P < 0.05). Compared with CK, the organic matter contents in 10-30 cm soil layer under OM and OMP were increased by 100% and 127%, and the contents of available N, P and K under OM and OMP were increased by 88.7% and 105%, 564% and 514%, 453% and 400%, respectively (P < 0.05). Compared with CK, the organic matter content in the 30-40 cm soil layer under OMP was increased by 33.6%, and the available P and K contents under OM and OMP were increased by 517% and 604%, and 191% and 157%, respectively (P < 0.05). The high-throughput results showed that all the treatments had an effect on the content of the horizontal bacterial population to a certain extent. The OM and OMP treatments significantly increased the abundance of dominant bacteria such as Bacillus, Altererythrobacter, Steroidobacter, Streptomyces and Nocardioides (P < 0.05), but the differences in diversity of soil microbial community (Shannon index and Simpson index) and abundance (ACE index and Chao1 index) among all the treatments were not significant. Correlation analysis showed that the relative contents of the above five bacteria were significantly and positively correlated with organic matter and available nutrient contents in 0-40 cm soil layer, significantly and negatively correlated with pH value (P < 0.01), not significantly correlated with the total salt content.[Conclusions]: Subsurface manure application combined with plastic mulching is proved of effective in reducing the salt accumulation on soil surface, and increasing the soil organic matter and available nutrient contents rapidly, and improving the soil microbial structure and abundance of dominant bacteria at the same time, which stimulate the mineralization of organic matter consequently.