Net nitrogen immobilization in soil induced by small additions of energy sources

Abstract

This study investigated whether small additions to soil of primary paper-mill sludge, a wood fibre residue from paper production (fibre sludge), caused temporary N immobilization and thereby reduced the amount of inorganic nitrogen leached from agricultural land. This was achieved by measuring respiration and immobilization of N in incubation studies at 8°C, with fibre sludge added at rates varying from 63 to 1000?mg?C?kg^?1 soil. Glucose added at rates of 63–250?mg?C?kg^?1 soil was used as a reference. Respiration in soil after glucose addition followed an exponential course with the highest rates on days 2–4. During this period maximum peaks of net N immobilization were measured. Even addition of only 63?mg glucose-C?kg^?1 soil caused significant immobilization of N in soil. Fibre sludge additions to soil caused lower respiration activities, characterized by two initial peaks followed by somewhat higher respiration rates during the remaining incubation than for glucose. It was likely that hemicellulose, which amounted to 14% of the total C, was the initial available energy source in the sludge as concentrations of water-soluble C were very low. Addition of at least 250?mg?C?kg^?1 soil as fibre sludge was required to cause significant N immobilization in soil corresponding to 5?kg?N?ha^?1. Both nitrate and ammonium were immobilized. Relating maximum N immobilization data during days 2 to 10 to corresponding respiration data for glucose and fibre sludge revealed that microbes utilised similar amounts of C per unit N immobilized. On average, 175.6±74.8?mg CO₂-C were respired to immobilize 1?mg?N and the relationship between C respiration and N immobilization was linear (R ²=0.984). To make soil application of fibre sludge a realistic counter-measure against N leaching from agricultural soils, pre-treatment is necessary to increase the content of energy readily available to microbes.