| Planting cover crops in orchard does not only enhance the ecological service,but also produce forage and improve land use efficiency.The southern area of Qingyang is one of the major apple production areas in China,apple orchards in this area are mainly under rain-fed conditions,and the effects of cover crop on the system production and soil water dynamics are still uncertain.This study was conducted in a 11-year-old apple orchard（Malus pumila M.cv Qingguan）which is located in Xifeng,Qingyang（local annual precipitation is 527 mm）,Gansu Province of China.The experiment included two treatments of intercropping cocksfoot in orchard and one treatment of clean tillage.The two intercropped patterns were:cocksfoot harvested with a normal frequency（NF:2-4 times during the apple tree growing season）,and cocksfoot harvested with a high frequency（HF:4-6 times during the apple tree growing season）.The system yields,canopy structure,light utilization,soil water dynamics,fine root length density（FRLD）,and evapotranspiration（ET）of apple tree/cocksfoot agroforestry system were monitored in the apple tree growth period of 2016-2018（the rainfall were 478,746 and574 mm,respectively）,with the purpose of clarifying the mechanisms of light and water utilization in apple tree and cocksfoot agroforestry,and providing a theoretical basis for the promotion and application of cover crops in orchard in the Loess Plateau.The main results are as follows:1.Planting cocksfoot in orchard showed no significant effect on the fruit yield,single fruit weight,fruit diameter,soluble sugar content and titratable acid content of apples in 2016-2018（P>0.05）.The dry matter yield of cocksfoot in apple tree/cocksfoot agroforestry systems ranged from 2.07 to 6.32 t ha-1 in three years.The dry matter yield of cocksfoot under HF was 11.5%and 45.9%higher than that under NF in 2016 and2017,respectively.There was no significant effect of mowing frequency on the yield of cocksfoot in 2018（P>0.05）.Planting cocksfoot in apple orchard changed the distribution and interception pattern of photosynthetically active radiation（PAR）and improved the radiation utilization efficiency in apple orchard.During 2016-2018,the cumulative PAR intercepted by apple trees accounted for 37.8%-44.3%of the total PAR,respectively,and the cumulative PAR intercepted by cocksfoot accounted for 18.2-24.3%.The leaf area index（LAI）of cocksfoot under HF was decreased by 14.1%-34.0%,compared to that under NF.PAR interception by cocksfoot under HF was also reduced by 8.7%-16.5%compared to that under NF,and the light utilization efficiency under HF was 6.3%-74.0%and higher than that under NF,respectively.2.The effect of sowing cocksfoot on soil water dynamics and soil water balance in apple orchard was affected by rainfall distribution during the apple tree growing period.The effect of sowing cocksfoot on soil water content in 0-80 cm soil layer was greater than that in 80-160 cm soil layer.The existing of cocksfoot reduced the soil water content in 0-80 cm soil layer in interrows in the drought period,while increased the soil water content in 0-80 cm soil layer in interrows after heavy rainfall.In 2016,cocksfoot cover crop not only significantly increased the soil water depletion in 0-100cm soil layer in interrows in 2016（P<0.05）,but also significantly increased soil water depletion in 0-200 cm soil layer in tree row（P<0.05）,no significant differences were found in deep soil layers.In 2017,cocksfoot significantly reduced the water replenishment in 100-300 cm soil layers in tree rows（P<0.05）.In 2018,soil water consumption increased only in 0-100 cm soil layer in inter-rows,while soil water content in deep soil layers in inter-rows and the entire soil layers in the tree rows were not affected.Increasing the mowing frequency of cocksfoot reduced the water consumption in 0-100 cm soil layer in inter-rows and tree row in 2016,while increased the soil water replenishment in 100-200 cm soil layer of the tree row in 2017.3.Planting cocksfoot in apple orchard changed the partitioning of ET between evaporation and transpiration,but it did not significantly affect the total system ET of the orchard under HF treatment.ET under the tree canopy under NF increased by 19.1%,6.4%and 11.4%compared to that under CT in 2016,2017 and 2018 respectively,and that under HF was 11.8%,2.3%and 9.0%higher.The total system ET under NF was significantly higher than that under HF and CT only in the dry year of 2016,and there was no significant difference among the three treatments in other years（P>0.05）.The transpiration of apple trees under NF and HF was significantly lower than that under CT only in 2018（P<0.05）,and there was no significant difference under three treatments in the other two years.4.Continuously measured apple tree transpiration in 2018 showed that the monthly average transpiration under NF was significantly lower than that of CT only during the drought period（P<0.05）.The transpiration rate of apple trees was significantly correlated with LAI of apple tree,solar net radiation,atmospheric temperature,water vapor pressure difference and soil water content in 0-160 cm soil layer.The dual crop coefficient method,calibrated according to the LAI of cocksfoot,was capable to estimate the ET and its components in apple/cocksfoot systems.5.Compared with CT,the increase of FRLD of apple trees in deep soil layers（100-300 cm）was significantly promoted in 2016 after sowing cocksfoot in the apple orchard,and the effects were significant in tree row（P<0.05）.In 2018,the FRLD of apple trees in deep soil layers in interrows was significantly inhibited（P<0.05）,but there was no significant difference in deep soil layers in tree rows（P>0.05）.The response of FRLD of apple tree in the agroforestry system to rainfall variation was helpful to improve the water use strategy of apple trees in different rainfall years.In conclusion,this study showed that planting cocksfoot in orchard on the Loess Plateau can improve the light interception and utilization efficiency,reduce soil evaporation,and improve rainwater utilization efficiency.We clarified the mechanisms of light and water utilization in the apple tree based agroforestry by analyzing both aboveground and belowground plant structure and resources utilization.Therefore,we suggest applying cover crops in orchard to simultaneously improve the economic and ecological benefits of the fruit tree planting systems in the Loess Plateau of China. |
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