Effect of plant cover type on soil water budget and tree photosynthesis in jujube orchards

The traditional soil management practice used in rainfed orchards in the Chinese Loess Plateau is clear cultivation (bare soil). This system leads to severe water erosion and considerable loss of water as runoff during heavy rainfall, especially on steep slopes. Plant cover systems represent a controversial alternative to the clear cultivation system. Although cover plants strongly promote water infiltration during rainfall, they may also compete with the trees for limited soil water. In this study, water regimes and inhibition of jujube (Ziziphus jujube Mill.) tree photosynthesis by water stress were compared across two types of soil management strategy: 1) a traditional clear cultivation system (CC); and 2) three plant cover systems (PCS) using different species, namely birdsfoot trefoil (Lotus corniculatus L.), cocksfoot (Dactylis glomerata L.) and white clover (Trifolium repens L.), in constructed soil macrocosms (0.8 × 0.8 × 2.0 m) under simulated rainfall regimes. The PCS resulted in the infiltration of significantly more water during a rainfall event of about 60 mm (infiltration coefficients 86%-91% for PCS and 68% for CC). The superior infiltration achieved in the PCS treatments was then gradually offset by their extra water consumption compared with that of CC. During a light rainfall event (rainfall amount: 30 mm), runoff became negligible (less than 1.5% of the total rainfall amount) and the plant cover systems had mainly negative effects, including faster development of drought and water-mediated photosynthesis inhibition in jujube leaves. The white clover cover treatment showed the lowest runoff (runoff coefficient: 15% for the heavy rainfall and 3% for the light rainfall), the most conservative water consumption and the weakest inhibitory effects on jujube photosynthesis (net assimilation rate: >8 μmol CO2 m−2 s−1) among the three plant cover treatments. The white clover cover treatment is thus recommended as the most suitable choice for jujube orchards in the Loess Plateau area.