Influence of Fallowing Practices on Soil Water and Precipitation Storage

Abstract

In semiarid drylands of Central Aragon, the cereal-fallow rotation, with mouldboard ploughing as main cultivation, is the most traditional farming system, in which a weed-free long fallow period (16-18 months) is practised to increase the amount of water available to the next crop. However, the ability of long fallowing for soil water conservation has been questioned in some dryland regions including Central Aragon. This research was aimed to quantify soil water losses (E), soil water storage (SWS) and precipitation storage efficiency (PSE) of long fallow under three management systems (conventional tillage, CT; reduced tillage, RT; no-tillage, NT). The PSE of long fallow relative to short fallow (5-6 months) was also evaluated. Over four experimental years (1999-2002), soil water balance for both short and long fallow periods was calculated from fallow seasonal precipitation and volumetric soil water content (0-70 cm depth). During long fallowing, primary tillage implemented in CT and RT plots induced significant E losses from the plough layer for the first 24 h after tillage. However, secondary tillage under CT and RT appeared to have a positive effect on soil water conservation at the end of fallow. Despite a different soil water dynamics among treatments, total E at the end of fallow was similar for the three fallow management systems. The partitioning of long fallow into three sub-periods showed that the early phase (July-November) was the most efficient in terms of SWS, having the lowest average daily evaporation rate (Er) per day (0.65 mm day-1). The overwinter period (December-May) and the late period (June- November) had greater Er values (0.93 and 1.09 mm day-1, respectively) and variable SWS as a function of rainfall pattern in those periods. In general, PSE, either for the fallow phases or the entire fallow period, increased when most of seasonal effective rainfalls ( 10 mm day-1) were received in the last two months of each period. Overall, long fallow PSE was small (11% on average). Neither SWS nor PSE were significantly affected by the tillage system. The average additional soil water at sowing after long fallow compared with short fallow was 20 mm. Correspondingly, the average PSE of long fallow relative to short fallow was only 5.3%. These findings indicate that the use of long fallowing, as an agronomic practice to enhance soil water storage for the subsequent crop, should not longer be justified in semiarid Aragon.