Effect of wheat residue enriched with Pleurotus on microbial characteristics of a saline soil

Soil microorganisms play an important role in maintaining soil quality through the decomposition of organic matter and nutrients cycling. The quantity of plant residue has a positive effect on the accumulation of organic carbon in the soil. One of the most important problems hampering the release of nutrients from plant residues is the high content of lignocellulose in their structure. Therefore, biological treatment has been considered as a candidate to improve lignocellulosic conversion and more release of nutrients from them. Salinity reduces microbial biomass and decreases their activity in decomposition of soil organic matter and organic matter input into soil. Due to the importance of the role of microorganisms in the storage and release of energy and nutrients in the soil, in recent years, increasing attention has been paid to the estimation of microbial activity and biomass in soil. Therefore, the aim of this study was to study the effect of salinity, inoculation of Pleurotus astreatus and wheat residue on respiration, microbial biomass carbon, organic carbon, carbon availability index and metabolic quotient.

The experiment was conducted as a completely randomized design with factorial arrangement in three replications under controlled laboratory conditions at Gorgan University of Agricultural Sciences and Natural Resources. Factors included three salinity levels (0, 8 and 15 dS m-1), two fungal levels (0 and 5%) and two wheat residue levels (0 and 1%, w/w). Salinity treatments including (control), 8 and 15 dS m-1 was applied using a mixture of salts (NaCl, KCl and MgCl2 with a molar ratio of 3:2:1). Wheat straw was treated pleurotus fungus and the treated straw was then thoroughly mixed into the soil. To activate the microbial population, soil moisture was adjusted to about 70% of the field capacity and the containers were pre-incubated at room temperature for 2 weeks. The samples were incubated at 25±2°C for 90 days. Microbial biomass carbon, organic carbon was measured at monthly intervals, microbial respiration was measured weekly and substrate-induced respiration (SIR) was measured once at the end of the incubation period.

The results show that salinity has a negative effect on microbial activity and population, but wheat residues reduce the effect of salinity stress on soil microbial community. Inoculation of Pleurotus into the soil also increased the respiration and microbial biomass. The interaction of wheat residues and Pleurotus on microbial activity in saline soil was greater than their effect alone. According to the results, the simultaneous addition of Pleurotus and wheat residue increases organic carbon (%98), microbial respiration rate (90%), substrate respiration (69%) and microbial biomass carbon (79%) and decreases the metabolic coefficient (6%). Salinity reduced respiration (78%), microbial biomass carbon (81%) and carbon availability index (23%), which indicates a decrease in carbon for microbial activity in saline soils. The lowest and highest microbial activity and biomass were in saline soil (15 dS m-1) not treated with wheat residues and Pleurotus (S2F0R0) and in non-saline soils treated with wheat residues enriched with Pleurotus (S0F1R1), respectively. The results showed that higher salinity level (15 dS m-1) further decreased the measured characteristics including carbon availability index, respiration and microbial biomass carbon compared with 8 dS m-1 salinity level in all treatments. In non-treated soil with wheat residue and Pleurotus, salinity level of 8 dS m-1 reduced MBC by 43, 46 and 44 % compared to control (non-saline) soil. The results showed that there was a significant negative correlation between microbial respiration rate and salinity (P <0.01, r = - 0.87). Salinity reduced microbial respiration rate and the effect of salinity on reducing microbial respiration rate of soil with EC 15 dSm-1 was higher than lower salinity level (8 dSm-1). Also, inoculation of Pleurotus in soil led to increase microbial respiration rate compared with non-treated one. According to the results, salinity levels of 8 and 15 dSm-1 reduced carbon availability index in soil treated with Pleurotus and wheat residue by 18% and 23%, respectively, compared to non-saline soil.

The addition of wheat straw enriched with Pleurotus astreatus increased microbial respiration, organic carbon, microbial biomass carbon, substrate-induced respiration and carbon availability index due to the increase of available substrate. Therefore, in saline soils with carbon restriction, increasing the level of organic matter, increased microbial activity and biological potentials in the soil. However, further information on responses of microbial indicators to the joint effect of salinity and Plant residues enriched with other microorganisms is required.ReceivedReceived in revised formAcceptedKey words:Carbon availability index, Microbial biomass carbon, Microbial respiration rate, Soil organic carbon, Substrate-induced respiration