Scales that matter: guiding effective monitoring of soil properties in restored riparian zones

Abstract

Considerable effort has been directed at restoring riparian zones to ensure they continue to provide ecosystem services and one of the most common aims of these activities is to reduce nutrients (in either water or soil) entering waterways. Vegetation plays a major role in nutrient interception, but nutrients in terrestrial ecosystems are strongly influenced by edaphic factors. Therefore understanding the effectiveness of riparian restoration efforts is dependent on knowledge of the complex and highly dynamic nature of nutrient cycling processes in riparian soils and their adjacent landscapes. Our primary aim was to assess the potential utility of a range of common soil indicators for monitoring responses to riparian restoration, and to use this information to provide guidance for more effective monitoring. A range of soil physiochemical properties in riparian zones and adjacent paddocks as a comparison were measured, incorporating both structural (e.g., bulk density) and functional (e.g., nitrogen) variables likely to differ in terms of both their responsiveness to restoration, and degree of natural spatial and temporal variation. Soil properties across the three spatial scales considered here (among creeks, among sites and within sites) varied considerably, particularly levels of phosphorus, ammonium and nitrate. Total organic carbon and total nitrogen were less variable and more uniform across all scales. Potential explanations for these patterns were explored by examining relationships between soil properties and vegetation measures, and between a subset of the most promising indicators (carbon, total nitrogen and bulk density, based on inherently low spatial variability) and adjacent land-use. Fertiliser inputs appear to be a strong determinant of soil phosphorus but otherwise soil properties were not strongly related to vegetation or adjacent land-use. For mineral N this is likely a reflection of the highly spatiotemporally dynamic nature of nutrient cycling in riparian zone soils. A better understanding of natural variability in soil properties will greatly aid in developing more effective monitoring programmes to assess potential changes in riparian soil properties. Management of riparian systems to recover soil ecosystem services will depend upon identifying effective ecological indicators that can be used as measures of progress towards restoration goals. This study represents a necessary first step towards guiding meaningful monitoring of soil properties at riparian zones subject to restoration efforts.