Determination of long-term erosion rates in vineyards of Navarre (Spain) using botanical benchmarks

Grape production is an ancient and important activity, particularly in Spain. Within this country, Navarre is one of the best vine-producing regions. Despite the importance of the vine crop, any quantification of soil erosion rates on a pluri-decennial scale under this world-wide land use is scant. Considering that in Navarre grafting in vines was made until the 1990s directly in the field, and almost at soil surface level, the quantification of erosion/sedimentation rates around a single plant can be performed by using the large, identifiable callus forming around the graft as a palaeo-surface marker. Moreover, by sampling a number of vines evenly spread over the field, an erosion/sedimentation pattern within the study vineyards can also be defined by data interpolation. Our purpose in this paper is thus (i) to assess the erosion rates of vineyards of different ages in a Mediterranean area using the aforementioned botanical signal i.e., grafting callus. and (ii) to determine the topographic changes caused by the erosion processes in these fields. Some vineyards affected by soil erosion were selected to carry out our experiments. In the region of Navarre, 7 rain-fed vineyards of different ages (between 22 and 61 years old) were selected as experimental plots. To find out how far from the ground surface the graft was originally made. we sought vineyards close and (almost) contemporary to the studied ones, whose soil surface level has remained roughly invariant during the time. Also, 2 types of Digital Elevation Models (DEM) were created: (i) A DEM depicting the present land surface by using the actual elevation values of the soil surface, and (ii) another DEM aiming to recreate the former topography of the vineyard (i.e. that present near planting time). By performing in each vineyard a subtraction of the DEM representing the land surface at the moment of planting from the DEM of the present land surface, a new DEM quantifying the local soil loss/gain rates over the entire crop span is obtained. The present work reveals important overall erosion rates in vineyards of Navarre (ca. 3 kg m(-2) year(-1) or 2 mm year(-1)) which, on average, greatly exceed even the most conservative soil loss tolerance thresholds. Moreover, the results suggest that tillage erosion is the leading soil loss process in the study vineyards rather than water erosion. Our innovative way of determining overall rates of soil surface changes in grafted vineyards appears to be a promising method to estimate long-term erosion/sedimentation rates not only in vines but also in other crops, providing a reliable botanical benchmark of the former soil surface. Furthermore, a spatial pattern of soil loss and gain can also be defined in the terrain. Furthermore, this soil loss assessment could be a valuable way, to some extent, to compensate for the lack of information on long-term erosion rates in cultivated areas of Navarre and Spain, as well as to validate long-term erosion models such as USLE or the like. (C) 2009 Elsevier B.V. All rights reserved.