The effect of soil moisture content on nitrogen transformation using OECD test guideline 216

Abstract

One of the most important microbial processes in soil is the mineralisation of nitrogen in organic matter and this is vital for the maintenance of soil fertility. When performing a risk assessment for a substance that may enter the soil environment it is important to consider the potential effects the substance may have on the soil's microbial activity. The standardised test produced by the OECD (Organisation for Economic Co-operation and Development) ( OECD, 2000) to address this requires that the soil moisture content should be maintained at between 40% and 60% of the maximum water holding capacity (MHC) with a range of ±5%. However, our investigations have suggested that the soil moisture content may adversely affect the performance of the test. The current research aimed to identify the moisture content range, within which the measured concentration of nitrate-N was sufficiently above background levels in soil. This moisture content range, which will be different for soils of different texture types, focused on a sandy loam soil fulfilling the criteria of the OECD 216 guideline ( OECD, 2000). The nitrate-N yield and the EC 50 (the concentration causing 50% inhibition of nitrogen transformation) of the reference substance, nitrapyrin (2-chloro-6-(trichloromethyl)-pyridine), were determined at varying soil moisture contents. The results indicate that the current OECD recommended upper limit for the soil moisture content (60% MHC) may be too high. The yield of nitrate-N and the EC 50 for nitrapyrin, were relatively constant between 20 and 40% MHC. Other reported data ( Roberts et al., 2003) suggests that 50% MHC produced similar results. Therefore, we suggest that the OECD test be conducted between 20 and 50% MHC.