Limitations of xylanase assays in plant material and organic-matter-rich soils

The potential activity of enzymes involved in carbon cycling in soil is often determined by an enzyme assay described by Schinner & Von Mersi (1990) and Schinner et al. (1996) . This method measures the amount of reducing sugars produced in a sample with ample substrate added, after subtraction of a control without any substrate addition. Theoretically, there might be a problem with the control treatment when measuring organic material, as plant material itself contains considerable amounts of polymers that release reducing sugars. This paper addresses the effect of naturally occurring substrates on xylanase activity when working with plant materials and soils with a large organic matter content. We tested the Schinner method for measurement of xylanase activity on five plant roots and 18 grassland soils and compared the results with measurements of β-xylosidase activity on substrate containing fluorescent compounds (4-methylumbelliferone, MUF). The results strongly indicated problems associated with the control in the Schinner method for plant root materials with a large substrate-to-enzymes ratio and large bioavailability, causing the difference between substrate saturated sample and control to become marginal, at times even negative. In the grassland soils, we did not observe negative activities, but no correlation was found between the xylanase and β-xylosidase activity. A better correlation was found when no controls were subtracted from the xylanase activity. We therefore recommend using MUF substrates when measuring enzyme activities on plant materials and soils with large organic matter contents.