The Productivity Of Agroforestry Systems In Northern Wei River And Responses Of The Soil And Roots

Agroforestry is a way of land use pattern that takes trees, fruit treeser or shrubs into agriculture or animal husbandry production systems on the basis of synthesizes social, economic and ecological factors. Agroforestry has the advantage of output diversity providing food, feed, other forest by-products, etc. Therefore, agroforestry was widely used in northern Wei River of loess area. However, agroforestry system, as a kind of artificial system, its stability and sustainability in the region were urgent to be studied. Soil ecosystem is the key to the system sustainability of ground vegetation, especially in arid and semi-arid loess area. The research of soil ecosystem of agroforestry system can help to understand the mechanism and the key to the sustainability, and to provide theoretical guidance for management of agroforestry systems that have better sustainability and high stability of output. Therefore, we took a walnut(Juglans regia) – wheat(Triticum aestivum) intercropping system as the research object, with the two species monoculture systems for the control, discussed the influence of agroforestry system on soil water infiltration, soil water content, shallow soil temperature, soil physical and chemical properties, and root activities; analyzed the relationship between soil ecological factors and the sustainability, and output of the agroforestry system. The purposes of this research were to clarify the effects of soil and roots on productivity of agroforestry system, and to provide theoretical basis for managing the agroforestry system scientifically.The main results were as follows:The annual water consumption of the walnut-wheat intercropping system was higher than that of walnut and wheat monoculture systems, respectively. But the walnut-wheat system could reduce runoff, increase soil water infiltration, and catch more water in rainy months and years. These could alleviate the occurrence of dried soil desiccation and thus achieve sustainable development.Compared with the two monoculture systems, the walnut-wheat system could effectively reduce the 5, 10 and 25 cm soil temperature of high temperature period on summer days. The monthly average soil temperatures of 5, 10 and 25 cm of the walnut-wheat system were lower than those of the two monoculture systems in growing season(March~October), and there were significant differences, the maximum difference appeared in the 5 cm. The walnut-wheat system reduced the surface soil temperature, which had great significance for reducing ineffective evaporation.The initial infiltration rate and constant infiltration rate of the three land use patterns characterized by the walnut-wheat system > wheat monoculture system > walnut monoculture system. Compared with the two monoculture systems, the walnut-wheat system could increase soil water infiltration rate significantly since the 9th year after planting, but the effective depth of the influence was the 40 cm. During the growing season, the infiltration rates in the walnut-wheat system had a peak in between July and September, and the rainfall distribution in the region also had a peak during this period, the two peaks had a good coupling relationship with each other. The coupling relationship could let soil retain more water in rainy season.The improvement of walnut-wheat system on soil physical properties mainly appeared in the 0~40 cm soil layer. The agroforestry system could prevent the bulk density of soil from rapid rising caused by trampling of human and animal as well as lack of plowing in the surface soil(0~20 cm), but the situation in the walnut monoculture system was the opposite. Compared with the wheat monoculture system, the plow pan in the 20~40 cm soil layer could be significantly improved by the walnut-wheat system. The walnut-wheat system had continuous improvement on the soil field capacity in each soil layer as the planting age increased. The soil field capacity of walnut-wheat was higher than that of the two monoculture systems in each soil layer since the 5th year after planting, except in the 20~40 cm soil layer it was slightly lower than the walnut monoculture system. The agroforestry system also had continuous improvement on soil porosity in each soil layer, but mainly in the 0~20 and 20~40 cm soil layer. The improvement of the soil porosity is the foundation of water infiltration performance improved.As the planting age increased, the soil organic carbon content showed a trend of increasing gradually in all soil layers in the walnut-wheat system. There were significant differences between the agroforestry system and the two monoculture systems since the 7th year after planting. The accumulation rates of soil organic carbon content presented an inverted U-shaped curve, and reached maximum at around the 7th year in all soil layers. The walnut-wheat system had higher carbon sequestration potential, and the most prominent advantage was that the carbon could be sequestrated in the deeper soil layer. This has great significance for soil carbon sink. The results can provide help to establish carbon cycle model of agroforestry systems, terrestrial ecosystems or global.The fine roots of walnut in the wanlnut-wheat system had a big growth peak in May, and a small peak in September; those of wheat in the system also had two growth peaks, respectively in April and November. In the big roots growth peaks period overlapped in most of time between the walnut and wheat, mainly competition between the two occurred in the period of big roots growth peaks. The growth peaks of fine roots of walnut staggered with those of the wheat. The length density of fine roots of walnut in the walnut-wheat system and the walnut monoculture system all showed the tendency of increase gradually, as the planting age increased. The length desity of fine roots of walnut in the walnut-wheat system was lower than that in the walnut monoculture system in each soil layer, and there were significant differences since the 7th year after planting. There were significant impacts on the length density of wheat roots in the walnut-wheat system in each soil layer. In shallow soil layer(0~20 cm), the walnut-wheat system could significantly increase the length density of wheat roots since the 7th year after planting, but the contrary appeared below 20 cm soil layer.The depth of vertical distribution center of fine roots of walnut in the walnut-wheat system and the walnut monoculture system showed a trend of more and more deep with the age of planting, respectively increased from 20 cm to 38.85 cm, and 35.26 cm during the 11 years of observation. The depth of vertical distribution center of wheat root in the increased system showed a trend of more and more shallowed during the 11 years of observation, which was from 27.15 cm to 16.75 cm.In walnut- wheat intercropping system, the wheat gain competitive advantage by its root rapid growth occupied soil spatial in the short term, however, the walnut trees gain competitive advantage via root accumulated year by year. In 0~20 cm soil layer, the walnut root cannot accumulated year by year because of tilling disturbance, this provided opportunities for wheat root competition strategy, so the two species to coexist. Interspecific competition of agroforestry system via human disturbance can achieved new balance, namely, "competition- disturbance- rebalance" management strategy. It is an important factor for sustainable of productivity.The walnut-wheat system improved the productivity of walnut, and reduces wheat productivity; the main reason of wheat productivity decreased should be the light competition between walnut and wheat, the cause of the walnut productivity increased from soil ecosystem. Walnut production of the increased system was significantly higher than that of the walnut monoculture system, since the 5th year after planting. Compared with the walnut monoculture system, walnut’s output of the increased system was increased by 15.47%. Wheat yield of the increased system was significantly lower than that of the wheat monoculture system since the 5th year after planting. But the yield reduced basically stable at around 30% since the 7th year after planting. The economic benefits of the increased system were significantly higher than those of the two monoculture systems respectively since the 3the year after planting, and had no the process of economic benefits reduce compared with the walnut monoculture system.The economic benefits water use efficiency(BWUE) of the walnut-wheat system was 111.50 yuan· ha–1·mm–1 in the 11 th year after planting, which was 3.78 and 1.24 times as much as that of the wheat and walnut monoculture systems respectively. But the yield water use efficiency(YWUE) of the three land use patterns showed wheat monoculture > walnut-wheat system > walnut monoculture system. Therefore, the BWUE is more suitable for evaluating an agroforestry system with multi-output agricultural products considering the unify yield and value.In general, the first, the root activity and litter return in the agroforestry system could improve soil physical and chemical properties, and further improve soil water infiltration capability, and then improve the water-holding capacity of soil, reduce water loss and soil erosion, finally improve soil fertility and increase the quantity and quality of system productivity. Second, the root competition among the different species in the agroforestry system could let root ecological niche separation, which could help to realize the multilevel utilization of resources in time and space, therefore, improve the efficiency of resource utilization and the productivity of agroforestry system. Third, adopted "competition-disturbance- rebalance" management strategy in agroforestry system, it is also an important factor for sustainable of productivity.