Influence of fallowing practices on soil water and precipitation storage efficiency in semiarid Aragon (NE Spain)

The use of long fallowing (16–18 months) for soil water conservation has been questioned in semiarid drylands of Central Aragon. We quantify the soil water loss, soil water storage (SWS) and precipitation storage efficiency (PSE) of long fallow under three management systems (conventional tillage; reduced tillage; no-tillage). The precipitation storage efficiency of long fallow relative to short fallow (5–6 months) was also evaluated. Over 4 experimental years (1999–2002), the soil water balance was calculated from fallow seasonal precipitation and volumetric soil water content (0–70 cm depth). During long fallowing, primary tillage implemented in conventional tillage and reduced tillage plots induced significant soil water losses from the plough layer (0–40 cm depth) for the first 24 h after tillage. However, secondary tillage appeared to have a positive effect on soil water conservation at the end of fallow. The division of long fallow into three sub-periods showed that the early phase (July to November) was the most efficient in terms of soil water storage. Both for the individual fallow phases and the entire fallow period, the precipitation storage efficiency increased when most of the seasonal effective rainfalls (≥10 mm day−1) were received in the last 2 months of each period. Long fallow precipitation storage efficiency was low (11% on average). Neither soil water storage nor precipitation storage efficiency was significantly affected by the tillage system. The average additional soil water at sowing after long fallow compared with short fallow was 20 mm. Accordingly, the average gain in precipitation storage efficiency of long fallow relative to short fallow was only 5.3%. We conclude that long fallowing might not be suitable for enhancing soil water storage in Aragon.