Monitoring the variation of soil quality with sewage sludge application rates in absence of rhizosphere effect

Agricultural soils in semi-arid regions have frequently been degraded due to adverse climatic conditions, organic matter depletion, and poor farming practices. To enhance soil quality, this study examines the reuse of sewage sludge (SS) as an available source of organic matter in a typical Mediterranean sandy-loam soil. Accordingly, we studied the cumulative effect of two annual applications of 40, 80 and 120 tons of sludge per ha on soil quality in absence of vegetation. The dose-dependent improvement of organic matter content was the most significant event that reflected sludge application rates, and consequently influenced other soil properties. Accordingly, soil structural stability increased by 13.3%, 28.8% and 59.4% for treatments SS-40, SS-80 and SS-120 respectively as compared to unamended control. Structural stability improvement was also confirmed by the dose-dependent variation of other edaphic factors including calcium content, the microbial quotient as well as Welt and C:N ratios. These parameters are involved in cementing soil aggregates by cation bridging, the formation of microbial mucilage, and clay-humic complexes. Soil magnetic susceptibility (SMS) was measured in situ as a possible rapid tool to evaluate soil condition. SMS showed significant correlation with sludge dose and stability amelioration testifying to the aggregation role that can play Al2O3 and particularly Fe2O3 minerals added by the hematite-rich sludge. Besides, analytical results and field observations revealed no trends of soil salinization or acidification by excessive sludge amounts. By avoiding the rhizosphere effect, outcomes could reflect the resilience and intrinsic capacity of the soil to cope with excessive sludge loads.This study was financially supported by a research grant from the Tunisian Ministry of Higher Education and Scientific Research. The authors would like to thank the National Sanitation Utility (ONAS) for providing urban sewage sludge. The technical support of Rym Ghrib is hereby acknowledged

Impact of urban sewage sludge on soil physico-chemical properties and phytotoxicity as influenced by soil texture and reuse conditions

Urban sewage sludge (USS) is increasingly applied to agricultural soils, but mixed results have been reported because of variations in reuse conditions. Most field trials have been conducted in cropping systems, which conceal intrinsic soil responses to sludge amendments due to the rhizosphere effect and farming practices. Therefore, the current field study highlights long-term changes in bare soil properties in strict relationship with soil texture and USS dose. Two agricultural soils (loamy sand [LS] and sandy [S]) were amended annually with increasing sludge rates up to 120 t ha−1 yr−1 for 5 yr under unvegetated conditions. Outcomes showed a USS dose-dependent variation of all studied parameters in topsoil samples. Soil salinization was the most significant risk related to excessive USS doses. Total dissolved salts (TDS) in saturated paste extracts reached the highest concentrations of 37.2 and 43.1 g L−1 in S soil and LS soil, respectively, treated with 120 t USS ha−1 yr−1. This was also reflected by electrical conductivity of the saturated paste extract (ECe) exceeding 4,000 µS cm−1 in both treatments. As observed for TDS, fertility indicators and bioavailable metals varied with soil texture due to the greater retention capacity of LS soil owing to higher fine fraction content. Soil phytotoxicity was estimated by the seed germination index (GI) calculated for lettuce, alfalfa, oat, and durum wheat. The GI was species dependent, indicating different degrees of sensitivity or tolerance to increasing USS rates. Lettuce germination was significantly affected by changes in soil conditions showing negative correlations with ECe and soluble metals. In contrast, treatment with USS enhanced the GI of wheat, reflecting higher salinity tolerance and a positive effect of sludge on abiotic conditions that control germination in soil. Therefore, the choice of adapted plant species is the key factor for successful cropping trials in sludge-amended soils.This project was supported by a national grant from the Ministry of Higher Education and Scientific Research of Tunisia. The authors thank the Center of Material Science at Borj Cedria Technopark, Tunisia, for inductively coupled plasma analysis