عنوان مقاله [English]
The soils of arid and semi-arid regions of Iran contain small amount of organic matter due to the lack of sufficient vegetation. Improving the soil organic matter due to the rapid decomposition of the added materials requires the continuous return of organic materials to the soil. Addition of organic amendments such as straw and stubble, manure, sewage sludge and other waste to the soil increases the amount of organic matter in the soil. Sewage sludge is one of the most important organic wastes and byproducts of wastewater treatment processes, and due to the increase in their production in recent decades, its management has become one of the key tasks in the environmental policies of many countries. The use of sewage sludge in agriculture provides a high amount of the nutrients to plants. But potential environmental hazards such as the presence of microbes and heavy metals in sewage sludge is considered as a limiting factor, thus sewage sludge transformation to biochar is a desirable way to manage them. Biochar is a product of thermal decomposition of organic materials in the absence of air (pyrolysis). It has been shown that biochar production from sludge reduce the volume of sludge and removes some heavy metals from the sludge. Biochemical properties of soil are more sensitive to changes in soil management compared to chemical and physical properties. Among them, soil enzymes play an important role in nutrient cycling in nature, additionally they are sensitive indicators to agricultural operations, and respond faster than other soil biological characteristics to changes in soil management and agricultural operations. Therefore, the aim of this study was to investigate the effect of sewage sludge and its biochar on the activities of soil urease, alkaline phosphatase and sucrose enzymes.
Materials and Methods
A factorial experiment was conducted in a completely randomized design with three replications. Effects of sewage sludge biochar at three level of 0 (B0), 2 (B2) and 4 (B4) percent, the sewage sludge at three level of 0 (S0), 4 (S4) and 8 (S8) precent and four different incubation time were evaluated on soil urease, alkaline phosphatase and sucrase activities. Nine treatments including 1) control; zero level of biochar and sludge (B0S0), 2) 2% w/w of biochar (B2), 3) 4% w/w of biochar (B4), 4) 4% w/w of sludge-sewage (S4) 5) 8% w/w of sewage sludge (S8), 6) 2% w/w of biochar + 4% w/w of sewage sludge (B2S4), 7) 2% w/w of biochar + 8% w/w of sewage sludge (B2S8), 8) 4% w/w Biochar + 4% w/w of sewage sludge (B4S4), 9) 4% w/w of biochar + 8% w/w of sewage sludge (B4S8) was incorporated in the soil. Each of the treatments was mixed with one kilogram of soil samples and then transferred to plastic containers with a capacity of 1.5 kg and their moisture was kept 60-70% of the field capacity. Then the lid of the containers was closed and five holes with a diameter of approximately 2 mm were installed on each for air exchange. During the incubation the moisture of the samples was kept constant by regularly weighing the containers. Sub-samples were taken at intervals of 2, 15, 30 and 60 days. In these samples, pH, organic carbon, total nitrogen, available phosphorus and activities of sucrase (invertase), urease and alkaline phosphatase enzymes were measured.
Results and Discussion
The activity of urease in various treatments showed a high fluctuation during the incubation time. At the beginning of incubation, the addition of amendments caused a significant increase (p˂0.01) in the activity of this enzyme compared to the control. On day 15, the highest urease activity of urease in was observed in S4 (950 µg NH4+ g-1 h-1) and B2S8 (954 µg NH4+ g-1 h-1). After one month of incubation, the activity of this enzyme in B4, S4 and B4S8 decreased significantly compared to the control, and in other treatments, its amount was significantly higher than that of the control. However, at the end of the incubation time, the maximum activity of this enzyme was achieved in B2S4 (1196 µg NH4+ g-1 h-1). Alkaline phosphatase activity fluctuated highly in different treatments during the incubation period. After one month from the beginning of the experiment, the activity of this enzyme increased significantly in all treatments compared to the control, and the highest amount alkaline phosphates was observed in B2S4 (3348 µg pNP g-1 h-1) and B4S8 (3342 µg pNP g-1 h-1). Nevertheless, by Day 60 the activity of alkaline phosphatase decreased significantly in all treatments compared to the control. Sucrase enzyme activity increased on Day 30 days in all treatments compared to the control. The highest amount of the activity of this enzyme was obtained in S8 at the rate of 2574 µg glucose g-1 h-1. But at the end of the incubation time, the maximum activity of this enzyme was observed in B2S4.
The highest pH was observed in control and B2 treatments. Increasing the dosage of sludge and biochar did not have a significant effect on soil pH, perhaps because the pH of sludge and biochar was lower than soil pH. The highest amount of organic carbon was observed in the B2S8. Increasing the dosage of biochar did not have a significant effect on organic carbon, but increasing the dosage of sludge caused an increase in organic carbon. The highest amount of total N was achieved in B2S8 treatment. Maximum amount of available was observed in B4S8. No significant difference was recorded in higher doses of sludge and biochar, which may be due to stabilization or surface absorption of phosphorus. The highest amount of urease activity was observed in B2S4 and B4S8. In higher doses of biochar, urease activity relatively remained constant, however, higher doses of biochar reduced it. The maximum activity of alkaline phosphatase was achieved in S8, B2S8 and B4S8. The highest amount of sucrase activity was recorded for S8. Overall, our study showed that co-application of sewage sludge and biochar (B2S4) seems suitable to improve the soil urease, alkaline phosphatase and sucrase activity.