Spatial variability of soil characteristics across a restoration planting in Glorit, New Zealand

Abstract

Native forest restoration is an increasingly common practice in New Zealand, due to its potential to sequester carbon, benefit wildlife conservation and provide recreational opportunities. The majority of forest restoration in New Zealand occurs on pasture and has the potential to affect soil characteristics and soil organic carbon stocks. Although there has been extensive research into soil characteristics under native forests, little is known about the soil characteristics in young restoration plantings in New Zealand. This study quantified the biomass of the organic layer and estimated soil carbon stocks (to 30cm) at a 24 ha restoration planting in Glorit, Kaipara Harbour, New Zealand. Further, the relationship between soil carbon and soil texture, bulk density and pH was investigated. The effect of planting age on soil carbon stocks and soil characteristics was tested, and the spatial distribution of soil characteristics was mapped using GIS. Soil and organic layer samples were also taken from remnant and regenerating bush at the site, and from adjacent pasture, for comparison. Organic carbons stocks (0-10 cm) varied across the 24 ha restoration planting ranging from 3.1-9.5 kg m-2. Organic carbon concentration and bulk density were significantly correlated with slope and elevation. Spatial analysis such as kriging and cokriging helped to establish the major influence slope and elevation had on their distribution. The past land-use of pasture appeared to have some effect on organic carbon concentration. This was likely due to a decrease in belowground organic matter input from fine grass roots. pH (0-10 cm) ranged from 4.6-6.9 and showed legacy effects of pastoral farming. In this study it was established that planting age had a have minimal effect on soil characteristics. Combining correlation and spatial interpolation, using ordinary kriging and cokriging, provided an in-depth assessment of the soil characteristics across a complex landscape. This study highlights that past land use and topography had a stronger effect on soil characteristics than the present vegetation across a young restoration planting. The findings of this study can help to inform future restoration projects across complex landscapes.