Water dynamics of cover crops: No evidence for relevant water input through occult precipitation

Abstract

Rising temperatures and disruption of rainfall patterns due to climate change make water a limiting growth factor even in regions of temperate climates. Thus, producing ‘more crop per drop’ is of utmost importance. While cover crops provide many benefits to cropping systems, their influence on soil water is discussed controversially. While an increase in transpiration can lead to soil water depletion, the reduction of evaporation in combination with possible additional water inputs could provide a water benefit for a succeeding crop. Occult precipitation could be such an additional water input. The objective of this study was to quantify whether cover crops provide a net water benefit over a bare fallow due to the occurrence of occult precipitation. In a 2-year experiment, seven different cover crops were cultivated in pure stands and as a mixture under semi-controlled conditions in a container experiment. Water fluxes and meteorological conditions were closely monitored. Although favourable conditions occurred during both vegetation periods, we found no evidence of occult precipitation. In autumn, soil water was depleted by fast-growing cover crops. In winter, soil water was recharged due to the early preparation of a mulch layer combined with high winter precipitation while in early spring rising temperatures increased transpiration losses of a winter-hardy cover crop, leading to a reduction of soil water. For middle European conditions, this shows that (1) living cover crops do not provide any water benefits and that (2) soil water recharge in winter is highly dependent on meteorological conditions and cover crop management. From a water budget viewpoint, negative effects on a succeeding cash crop can only be prevented if cover crops are terminated early enough for replenishment of soil water.