Interspecific Water Relationship And Its Influence Mechanism Of The Apple-Rape Agroforestry System In The Loess Hilly Region

Agroforestry can not only coordinate the contradiction between the land use of agriculture and forest,but also improve the utilization efficiency of above-and below-ground resources,maintain water and soil,ameliorate soil structure and fertility,and increase biodiversity.The contradiction between water supply and demand for apple orchards in the Loess Hilly Region has been serious.Because,on the one hand,the distribution of the limited rainfall is uneven,which easily cause the surface runoff during the concentrated rainfall period.On the other hand,soil evaporation in the orchard is severe,but,unfortunately,irrigation can not be applied because of the complex terrain.Intercropping-a common form of agroforestry-in the orchard may be a deasible solution,which can reduce soil evaporation,promote rainwater retention and infiltration,and has the potential to improve soil water environment.However,the transpiration of intercrops may aggravate soil water consumption,cause interspecific water competition,and reduce the productivity of apple trees.Therefore,verifying the water consumption process and clarifying the interspecific water relationship and its influence mechanism are the theoretical basis for the implementation of this measure.Because the trade-off between the replenishment and consumption of soil water is affected by the density of intercrops,in this study,different-density Brassica napus L.(B.napus)was introduced into a dryland apple orchard in the loess hilly region,and four intercropping systems was established: apple with low-,medium-,high-density B.napus and tree monoculture(CK).To answer the scientific questions mentioned above,the transpiration of trees and crops,soil evaporation,soil water content,root spatial distribution,and the spatio-temporal dynamics of water absorption by trees and intercrops were measured and simulated by in-situ experiments and the 3D Hi-sAFe model,respectively.The following preliminary conclusions has been made:(1)The consumption and variation of soil moisture under different-density B.napus intercropping conditions were clarified.Compared with CK,intercropping effectively reduced soil evaporation,and the degree of reduction was as follows: high-density intercropping?medium-density intercropping>low-density intercropping.The relationship of the B.napus transpiration between different treatments was: high-density intercropping?medium-density intercropping>low-density intercropping.On the whole,the reduction in soil evaporation caused by intercropping was less than the consumption of B.napus transpiration,and this phenomenon subjected to each treatment.The effect of B.napus intercropping on soil water content was related to planting density: low-density intercropping effectively improved the water content of 60-100 cm soil layer in the cropped area and 20-100 cm soil layer in the uncropped area.On the contrary,medium-and high-density B.napus intercropping reduced the water contents of 60-100 cm soil layer in cropped area and 20-100 cm soil layer in uncropped area.Overall,during the two-year intercropping period,compared with CK,the average soil water content of the low-density intercropping system increased by 8.16%,but the average soil water content of the medium and high-density intercropping systems decreased by 4.43% and 4.67%,respectively.This led to the transpiration of apple trees in the low-density intercropping system was greater than that in the treatment of tree monoculture,while the transpiration of apple trees in medium-and high-density B.napus intercropping systems was lower than that in the treatment of tree monoculture.(2)The effects of different-density B.napus intercropping on the spatial distribution of apple tree's fine roots were investigated.More than 80% fine roots of B.napus were distributed in the 0-40 cm soil layer in the cropped area,which subjected to all treatments.However,the fine root length density(FRLD)were significant differences between treatments: the FRLD of B.napus in the low-density intercropping system was significantly less than that in the medium-and high-density intercropping treatments.This induced different plastic responses of apple trees' fine root systems between uncropped and cropped areas.In the uncropped area,related to the CK,the FRLD of apple trees were respectively increased 49.06%,20.15%,52.08% by low-,midium-and high-density B.napus intercropping in the 0-60 cm soil layer,indicating that planting density of B.napus was not the decisive factor for tree roots increase.In the cropped area,the FRLD of apple trees were respectively decreased 15.42%,31.76%,42.27% by low-,midium-and high-density B.napus intercropping in the 0-60 cm soil layer,and the higher of the B.napus intercropping density,the lower of the FRLD of apple trees.On the whole,B.napus intercropping promoted the distribution of trees' root in the uncropped area,but inhibited its proliferation to the cropped area.(3)The contributions of the soil water in different layers both in cropped and uncropped areas to the transpiration of the apple tree and B.napus was quantified by the stable isotope tracing technology.The results showed that during the vigorous growth period(before cutting),the low-density planted B.napus mainly used shallow(0-20 cm)and medium(20-60 cm)soil water(71.2%-92%),while the medium and high-density planted B.napus mainly absorbed medium(20-60 cm)and deep(60-100 cm)soil water(83%-93.5%).In the low rainfall period,in the low-density B.napus intercropping treatment,more than 60% of the water used by apple trees came from the uncropped area,which is spatially dislocated with the B.napus water sources,and in this condition there was no interspecific water competition.In the medium-and high-density B.napus intercropping systems,more than 50% of the water used by the apple tree came from the cropped area,and there was a high degree of overlap with the main water absorption area of the B.napus(20-100 cm soil layer in the cropped area),indicating the interspecific water competition was strong.In the high rainfall period,the apple trees in the medium-and high-density intercropping systems did not mainly rely on the soil water in the cropped area,indicating that there was no significant water competition between species when soil water is efficient.The relationship between water use patterns of apple trees and soil water/roots distribution was also analysed.In the low rainfall period,there was a significant positive correlation between the soil water content of each layer and its contribution to the apple tree,indicating that the soil water content determines the apple trees' water use stratiges.In the high rainfall period,there was no obvious correlation between the water absorption pattern of apple trees and the spatial distribution of soil moisture.No matter in the low or the high rainfall periods,there was no significant correlation between the proportion of water absorption from each soil layer and the proportion of root system in each soil layer,indicating root distribution is not a decisive factor in determining trees' water absorption.(4)On the basis of the Hi-sAFe model platform,the apple tree-B.napus intercropping module was established and verified.The spatio-temporal dynamics of water absorption by B.napus and apple trees in the low-density B.napus intercropping system were simulated.In the high rainfall period,the main water sources of apple trees and B.napus were misaligned in the horizontal direction,and the water absorption of B.napus and apple trees was different in the vertical direction during the low rainfall period.This was because the low-density B.napus intercropping prompted apple trees to absorb more soil water from areas without B.napus roots or from the deep soil layer in the cropped area where there was litter root systems of B.napus.Therefore,the apple tree and B.napus could synergistically use the increased soil water caused by intercropping.As a result,compared with CK,low-density B.napus intercropping reduced the leaf water potential of apple trees and alleviated water stress.All of above researches show that B.napus intercropping has the potential to synergistically improve the ecological and economic value of rain-fed apple orchards in semi-arid regions,but it is affected by the planted density of the intercrop.In the low-density B.napus intercropping system,the soil water effect was positive,the interspecific water relationship during the entire intercropping period was synergistic,the transpiration of apple trees was promoted,and their water use efficiency was improved(19.23%).Finally the yeild of apple trees was increased by 23.55%,hence,low-density B.napus intercropping is the best choice for apple orchads in the loess hilly region.