Assessment of short- to medium-term impacts of bench terrace construction on soil water repellency in north-central Portugal

Abstract

Agrarian terraces are surely recognized as a traditional soil and water conservation technique, however the use of these methods in forestry are poorly studied, especially the hydrological impacts of bench terracing for forest plantations. The employment of heavy machinery in forests have been described to increase soil bulk density and compaction which might reduce soil water infiltration capacity, and thus, increase runoff generation. However, in the case of terraces, the flattening of the terrain may increase the infiltration and re-infiltration of runoff due to consecutive slope discontinuity. The present study assesses the short to medium-term impacts of bench terrace construction on soil water repellency and soil water infiltration in eucalypt forest plantations. A chrono-sequence approach was used to analyse a time series of sites terraced at different periods with neighbouring mature long-undisturbed sites. Therefore, Methanol droplet test was employed in-situ in four different periods of time-since-terracing (0, 5, 10 and 17 years after terracing). Three pairs of nearby terraced and non-terraced eucalypt plantations were studied for each period, counting a total of 24 study sites. Our results revealed clear differences in SWR between terraced and non-terraced eucalypt plantations, with all non-terraced sites showing extreme SWR values while at the terraced sites highly variable values were found. In addition, the median SWR increase with time elapsed since terrace construction, i.e. from very wettable immediately and 5 years after terracing, and increase to strong and moderate water repellent at 10 and 17 years following terracing. The infiltration variables were observed inversely correlated with SWR following terrace construction and were watched to decrease with time-since-terracing. The litter and the Oh humified layer were found positively correlated to the SWR as well as the stone and bulk density variables, while the percentage of organic matter and the soil moisture content in the soil were poorly correlated.