Effects of Pulp Wastewater Irrigation on Soil Enzyme Activities and Respiration from a Managed Wetland

A long-term field irrigation (4 years) experiment and short-term laboratory experiment (25 days) were carried out to evaluate the effect of pulp wastewater on soil enzyme activities and respiration of a managed wetland. Long-term irrigation increased the pH, electrical conductivity (EC), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), available phosphorus and potassium in the soil. A significant increase of β-glucosidase (115.3%) was found in wastewater-treated soil. Acid and alkaline phosphatase activities were increased by 26.2% and 45.9%, respectively. The protease was 12.6% higher than the control, whereas no significant change was observed in urease (an increase of only 4.2%), and this inhibition effect was believed to be due to the increase in concentration of heavy metals resulting from the wastewater. However, there was also a significant increase in the concentration of Na, an important factor for aggravating salinity problems, which cannot be overlooked. But for short-term irrigation, all the enzyme activities and respiration were increased compared to the control, and were well correlated to CODCr values of the irrigated wastewater. The activity of all the enzymes was significantly inter-correlated, which indicated that wastewater irrigation has similar effects on the activity of these enzymes involved in C, N and P cycling in soils. It is suggested that pulp wastewater can be used to irrigate wetland with a continuous monitoring of the effluent quality to avoid contamination.