Effects of the combined application of organic and inorganic fertilizers on soil microbial biomass and soil respiration in saline soil

Microorganisms can adapt to the salinity by ingesting energy, further to synthesize organic osmolytes in soil. However, microbial demand for energy may change, as the soil salinity changes. In this study, a field experiment was carried out to explore the effects of the combined application of organic and inorganic fertilizers on soil respiration and microbial biomass in saline soil. The samples were taken as the mild saline soil, S1 (electrical conductivity (EC) 0.46 dS/m) and moderate saline soil, S2 (electrical conductivity (EC) 1.07 dS/m) in Hetao Irrigation District of Inner Mongolia, China, in 2018. Soil respiration rate was measured by a li-8100 soil carbon flux automatic measurement system. Surface soil samples of 0-20 cm depths were collected under various fertilization regimes in mild and moderate saline soil. Calcium chloride and chloroform fumigation extraction were used to investigate the effect of various fertilization regimes on the mineral nitrogen contents and microbial biomass in soil. Six treatments were set, including no fertilization (CK), urea (U1), and 25%, 50%, 75%, as well as 100% of urea N substituted by organic fertilizers U3O1, U1O1, U1O3, and O1, respectively, during the second growing season (April-September). The parameters were measured under equal N application rates of 240 kg/hm2, the dynamics of soil microbial biomass (i.e., microbial biomass carbon MBC and microbial biomass nitrogen MBN), and soil respiration (i.e., soil respiration rate, soil autotrophic respiration rate, and soil heterotrophic respiration rate). The results showed that the contents of mineral nitrogen in S1 soil were higher in the U1 treatment during the early growing stage (seedling and stem elongation stage), but higher in the U1O1 treatment in the later stage. S2 soil showed that the soil mineral nitrogen contents were higher in the whole growing stage, as the application rate of organic fertilizer increased. Furthermore, the increase of soil salinization resulted in the decrease of microbial biomass and microbial activity in the soil. The MBC in S1 soil increased by 12.01%-68.81%, while the MBN increased by 14.31%-58.58%, and the soil respiration flux increased by 11.75%-54.71%, compared with S1 soil. Furthermore, the organic fertilizer significantly increased the microbial biomass and microbial activity under different degrees of saline soils. The S1 saline soils treated with U1O1 presented the higher MBC, MBN, and soil respiration flux, indicating a significant increase compared with U1 treatment (P<0.05). The performance of S2 saline soil after the O1 treatment was better than that after the U1 treatment, where the MBC, MBN, and soil respiration flux increased (P<0.05). The highest maize yield was also achieved in the S1 and S2 soil treated with U1O1 and O1, which were 11 902.91, and 7 609.67 kg/hm2, respectively. A correlation analysis was found that the soil respiration presented a significant positive correlation with the MBC and MBN (P<0.05). The soil temperature and mineral nitrogen had a significant positive correlation with the soil respiration, MBC, and MBN (P<0.05). Regression analysis showed that there was a significant exponential relationship between the soil respiration and temperature (P<0.05), but the relationship between soil respiration rate and soil moisture content was not significant. Additionally, there was a significant nonlinear relationship of soil microbial biomass and respiration rate with the organic fertilizer rate and soil salt concentration. The regression coefficient demonstrated that the appropriate organic fertilizer rate contributed to maximizing the soil microbial biomass and respiration rate in different saline soils. Optimal organic and chemical fertilizer management models were achieved for the mild saline soil in the Hetao irrigation area. Specifically, the mild saline soil (120 kg/hm2 urea+120 kg/hm2 organic fertilizer), and moderate saline soil (240 kg/hm2 organic fertilizer) can be expected for the higher corn yield under the improved soil microbial environment.