The effects of salinity and Mg2+ alkalinity on the size and activity and structure of the soil microbial community in Mg2+ alkalized soils were investigated.There was a negative exponential relationship between EC and biomass C,biomass N, microbial quotient, indices of microbial activity and the activities of the hydrolases,but the negative relationships with Mg2+/Ca2+ were best described by linear functions. Potentially mineralizable N decreased exponentially with increasing EC and Mg2+/Ca2+. Basal soil respiration rate was very low,and declined exponentially with increasing EC, linearly with increasing Mg2+/Ca2+.The metabolic quotient (qCO2) showed a quadratic relationship with EC and positive linear relationships with Mg2+/Ca2+,indicating that increasing salinity and Mg2+ alkalinity resulted in a progressively smaller,more stressed microbial community which was less metabolically efficient.The biomass C/N tended to be lower in soils with higher salinity and Mg2+ alkalinity,reflecting the bacterial dominance in microbial biomass in Mg2+ alkalized soils.The activities of the enzymes urease,phosphatase,
β-glucosidase,protease-BAA,protease-casein were low and declined exponentially with increasing EC and linearly with increasing Mg2+/Ca2+.The activities of different hydrolases were positively interrelated,and there were also positive
relationships between the measures as good indices for soil microbiological activity and enzymatic activities.In present study there was a positive linear relationship between microbial biomass C and soil organic C and this is generally the case in most situations.Nonetheless,with increasing salinity and Mg2+ alkalinity the conditions were becoming increasingly detrimental to soil microorganisms and this was demonstrated by the decline in microbial quotient. The average microbial quotient is far below the alue suggested by Anderson and Domsch (1989) and Sparling (1992):about 4% of soil organic C belongs to the microbial biomass.Microbial biomass C content in the soil with the highest salt concentration was markedly low, leading to a minimum microbial quotient of 0.53% only. Since basal respiration represents the living component of microbial biomass C, both the active and total microbial communities decreased with increasing salinity and Mg2+ alkalinity. This suggests that CO2-C emission reflects the stress existing in Mg2+ alkalized soils of arid regions.
The positive relationships between qCO2 and EC and Mg2+/Ca2+ found in this study reflected increasing environmental stress as a result of salinity and Mg2+ alkalinity conditions on the soil microbial community.Consequently,increasing salinity and Mg2+ alkalinity resulted in a smaller,more stressed,microbial community which was less efficient in using C resources than its less stressed counterparts.Soil microflora is a composite of several groups of organisms and each microbial group may have a different biomass C/N ratio. The declining trend in biomass C/N with increasing salinity and Mg2+ alkalinity may reflect the bacterial dominance in soil microbial biomass.
The remarkable reduction of FDA hydrolysis rate with increasing salinity and Mg2+ alkalinity might be ascribed to the lower uptake of FDA by the soil microbial cells due to the salinity and Mg2+ alkalinity stress conditions. The similar pattern of decline for arginine ammonification rate as for FDA hydrolysis rate indicates that microbial activity was greatly decreased by increasing salinity and Mg2+ alkalinity.Salinity and Mg2+ alkalinity may decrease organic matter decomposition directly by inhibiting microbial growth and activity. Reduction in soil microbial biomass due to increased salinity and Mg2+ alkalinity caused a substantial exponential decline in potentially mineralizable N. In addition, N mineralization may be affected by species composition of soil microorganisms, as species differ in their ability to degrade various organic compounds.The salinity induced reduction of fungi may reduce the decomposition of complex organic material in saline soils.
The values of hydrolases activity in this study were very low with regards to those of nondisturbed soils.Moreover,the lowest values corresponded to the soils with highest salinity and Mg2+ alkalinity.The decreased enzyme activity may be partially
attributed to the less enzymes released by the smaller, less active microbial biomass due to salinity and Mg2+ alkalinity. In addition, high salt concentrations tend to reduce the solubility and denature enzyme proteins through disruption of the tertiary protein structure which is essential for enzymatic activity.It is worth noting that the inhibitory effect of both EC and Mg2+ alkalinity on the hydrolases activity is enzyme specific. The positive correlation between the hydrolases of the soils studied indicates that although each individual enzyme depends on specific substrates and takes part in specific reactions, the soil degradation induced by salinity and Mg2+ alkalinity affects all the enzymatic activities equally. Enzyme activity was positively related to soil organic C content as was the relationship between the size and activity of the microbial community and organic C.
In general, the negative relationships between the indices analysed and EC were exponential, while the negative relationships with Mg2+/Ca2+ were linear.In addition,measures of the size and activity of the microbial community were more negatively correlated to EC than to Mg2+/Ca2+. Thus, soluble salt concentrations were apparently more important in inhibiting the growth and activity of soil microorganisms than Mg2+ alkalinity.The exponential decline in the size and activity of the microbial community with increasing EC demonstrates the extremely detrimental effect of small increases in salinity.Our data show that high salinity and Mg2+ alkalinity tend to result in a small bacteria dominated soil microbial community with low microbiological activity.