Immediate changes in soil chemical and biological fertility and short-term changes in soil biological activity following bench terrace construction for eucalypt reforestation

Abstract

Over the last decades, eucalypt plantations have increasingly involved intensive soil mobilization by heavy machinery. In north-central Portugal, bench terracing with bulldozers has become a common practice for eucalypt re-/afforestation. Its impacts on soil properties and functions, however, have received little research attention so far.The present study aimed to address the changes in topsoil fertility from immediately before to immediately after the construction of terraces. This was done for two contrasting micro-sites (tree-patch and inter-patch). Topsoil fertility was characterized in terms of both total and labile carbon and nitrogen as well as available phosphorous. In addition, this study aimed to assess the short-term impacts of terracing on soil respiration, as an indicator of soil biological activity. Soil respiration was monitored at monthly intervals during the first year following terracing at the terraced as well as at a nearby, non-terraced eucalypt plantation.The construction of bench terraces led to a significant immediate reduction in topsoil contents of total and labile carbon and nitrogen as well as available phosphorus. The reductions were inferred to result from the mixing of the A, B, C and R horizons, in combination with a priming effect due to soil aeration and resulting exposure of carbon and nitrogen compounds to soil micro-organisms. Bench terrace construction furthermore increased spatial heterogeneity in topsoil fertility, turning differences in carbon, nitrogen and phosphorus contents between tree and inter-patches significant immediately afterwards. Likewise, soil respiration tended to be significantly different between tree-patches and inter-patches at the terraced site but not between tree-patches and inter-patches at the neighbouring non-terraced site. These short-term impacts of bench terracing on the spatial patterns in soil respiration are likely to reflect its immediate impacts on the spatial patterns in soil fertility. The present findings plainly justified further research on the medium and long-terms« impacts of bench terracing on eucalypt soil properties and functions.