Soil biochemical properties as indices of performance and sustainability of effluent irrigation systems in new zealand—a review

Abstract

In New Zealand, there have been a number of investigations of the effects on soil biochemical properties of land application of industrial and sewage effluents. In recent years, the rationale for determining these properties has been to ascertain if they have a potential role as early warning indicators of adverse effects of effluent irrigation on treatment sustainability and/or soil health. In this review, I summarise the findings from these studies and attempt to establish whether the data do support this role. Assessment of biochemical effects of the application of effluents to land under crops, forest, or scrub is complicated by previous land management and by site characteristics. Consequently, only investigations of effluent application onto pastoral soils have allowed an assessment of the potential value of soil biochemical properties as early‐warning indicators of adverse effects. Generally, these studies have shown that effluent application has had a beneficial effect on soil properties and plant growth and this is reflected by enhanced soil biochemical activities. Where an adverse effect did occur in response to a drastic change of effluent amount and composition, soil biochemical properties were markedly reduced. However, soil chemical properties and aggregate stability were unaffected. This suggests, therefore, that there could be a role for biochemical properties as indices of performance and sustainability of land‐based effluent irrigation systems. However, with most studies showing that most effluent application is beneficial, such a role may be limited to situations where the effluent is to be applied at an amount, or has a composition that has not been previously tested. The main conclusion from this review is that when irrigation schemes have been running for a number of years and are functioning well, soil biochemical properties reflect the soil health enhancements provided by the water and nutrients added. Such enhancements are generally manifested slowly and, therefore, monitoring is required over a longer duration than has occurred in several of the studies examined. Adverse effects attributable to effluent irrigation are more difficult to recognise and interpret unless a drastic change has occurred, due mainly to methodological limitations and our lack of understanding of the true meaning of what we are measuring or its relevance to soil functioning. Until our understanding improves markedly, a predictive role for these properties as an early warning of adverse effects of effluent irrigation will remain elusive. © 2002 Taylor & Francis Group, LLC.