Long‐term consequences of topsoil mining on select biological and physical characteristics of two New Zealand loessial soils under grazed pasture

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Abstract
Soil chemical, biochemical, biological and structural properties were measured in two New Zealand loessial soils that were topsoil‐mined 10 and 25 years ago respectively. Measurements at the 10‐year site were compared to some earlier measurements made at this site and the data combined in a chronological sequence for analysis. Topsoil mining had a large, detrimental impact on the soil microbial biomass, the earthworm populations, easily mineralizable N and soil enzyme activities. However, most of these properties substantially recovered, to 80‐90 per cent of the levels in unmined soils, within 10‐25 years of restoration under pasture. In contrast, while total soil C and N were less affected by topsoil mining, their recovery was much slower. Stabilities of macro‐aggregates of soil had fully recovered within 10‐25 years after topsoil mining. The apparent changes in all the measured properties between 10 and 25 years of restoration were small in comparison with changes between 0‐10 years of restoration after topsoil mining. The total C content of both soils under pasture appeared unlikely to attain the levels present in unmined soils. In soils undergoing restoration, the ratio of microbial C/total soil C may be a useful index of soil ‘biological stability’. Sulphatase activity may reflect the recovery of pasture production.