Small scale stratification of microbial activity parameters in Mediterranean soils under freshwater and treated wastewater irrigation

Abstract

Using treated wastewater for agricultural irrigation is a common practice in many water-scarce countries. Apart from the beneficial effects associated with the nutrient load and organic substrates, some negative effects on soil properties, such as increased water repellency or priming effects have been observed. The objective of this study was to determine if negative effects are associated with differences in microbial activity parameters. Soil samples were collected from three sites in Israel (IL) and Palestine (PA): (i) a lysimeter experiment near Lachish (IL) with three soils irrigated with four different water qualities; (ii) a long-term field experiment in a citrus orchard (Batsra, IL) irrigated with fresh water (FW) and treated wastewater (TWW), and (iii) an arable field experiment with also FW and TWW irrigation (Gaza, PA). Samples were collected from 0 to 5 (top-crust) and 5–20 mm (sub-crust) and analyzed for biological and chemical properties.The results show little influence of water quality on the tested parameters. Only total phosphorus content in soils was enhanced due to TWW irrigation at all sites. Soil organic carbon showed a trend to lower contents under TWW irrigation compared to FW irrigation. Microbial biomass showed no significant influences of irrigation water quality while enzyme activities were higher in FW irrigated soils compared to TWW at Gaza. Instead, depth stratification within the top 20 mm was very pronounced at all sites, independent of irrigation water quality. Associated with a 13–34% higher SOC content in the top-crust, microbial biomass, basal respiration and enzyme activities were greatly elevated compared to the underlying sub-crust. At the arable sites (Lachish, Gaza), amino-peptidases dominated in the top-crusts while β-glucosidase activity dominated in the orchard soil (Batsra) independent of irrigation water qualities. These differences are attributed to the occurrence of phototrophic N-fixing organisms (i.e. cyanobacteria, lichens) on the surface of the lysimeter and Gaza soils.Overall, no significant effect of TWW irrigation on the microbial activity within the C-, N-, P-, and S-cycle was observed in this study. Instead, soil crusts formed on light-exposed soil surfaces are characterized by high microbial activity forming a hotspot of fertility which should be protected to maintain soil functionality.