The effect of chitosan application on some microbial population and eco-physiological indices in a Pb contaminated soil

Contamination of agricultural soils with heavy metals is one of the major challenges worldwide. Heavy metal stress can affect soil living community and, consequently, different soil processes. The aim of this study was to evaluate the effect of chitosan application levels and soil contamination with Pb on soil biological activity.

In order to investigate the effects of different levels of Pb (non-contaminated (Pb0), 250 (Pb250) and 500 (Pb500) mg kg-1 of soil) and chitosan application (without chitosan (Ch0), 0.6% (Ch0.6) and 1.2 % (Ch1.2)) on the available Pb (DTPA-extractable Pb) and some biological properties in soil, a factorial experiment was conducted in a completely randomized design. Different levels of lead and chitosan were added in pots containing 400 g of soil in three replications and and soil moisture was maintained at 70% field capacity for 45 days. At the end of the incubation period, DTPA-extractable Pb and the population of microbial groups including heterotrophic bacteria, fungi, spore-forming bacteria and actinomycetes were determined. Moreover, microbial biomass of C (MBC) and N (MBN), basal respiration (BR) and substrate-induced respiration (SIR) were measured and some ecophysiological characteristics including microbial metabolic quotient (qCO2), microbial respiration quotient (QR) and MBC: MBN ratio were calculated.

The results showed that Pb250 treatment did not show a significant effect on the population of the studied microbial groups compared to the control. Application of Pb500 treatment significantly reduced the population of spore-forming bacteria and actinomycetes but did not lead to a significant change in the faungi/bacteria ratio. BR, SIR, MBC and MBN were significantly reduced by Pb500 treatment. Under Pb500 treatment, qCO2, QR and MBC:MBN ratio increased. Application of chitosan at both levels decreased the available Pb concentration of lead in the soil so that application of 1.2% chitosan reduced DTPA-extractable Pb by 42% in Pb250 treatment and by 12% in Pb500 treatment. Application of chitosan increased BR, especially under Pb500 treatment, and caused a decrease in qCO2 (p ≤ 0.05).

It can be stated that the use of chitosan at low levels of soil pollution was more effective in reducing the Pb bioavailability. It also seems that the positive effects of the application of this biopolymer on the biological properties of the soil have been partly due to its biodegradation in the studied soil. In general, the results indicate the importance of chitosan application as a biodegradable adsorbent to reduce available Pb in soil and its potential to enhance biological activity in Pb-contaminated soil.