| Loess tableland area situated in the northwest of China,is the main apple production area of the loess plateau.However,in the region,the water resource is scarce,and more than 80% of the orchard is rainfed agricultural area.Researches indicated that there has been serious desiccation in deep soil under apple orchards,and the desiccation depth below can be as deep as 10 m,which seriously restricted the apple tree growth and production.Therefore,the objective of this study is to investigate the mechanism of rainfall infiltration and the effects of roots on rainfall infiltration in apple orchards on Loess Tableland.Previous studies on mechanisms of rainfall infiltration and effects of roots on water infiltration have mostly focused on natural forestry and artificial forest.Little has been reported on the highly-managed orchards on the Chinese loess plateau.Dye tracing experiments were carried out with three water application methods(sprinkler(replicated 6 times),flood(replicated 3 times),stemflow simulation(replicated 6 times)to simulate a regular rainfall event(50 mm),intense rainfall event(50 mm)and stem flow(18 L),respectively,in apple orchards of 20 years old in Changwu tableland using Brilliant Blue FCF solution of 4 g·L-1..The main results were listed as follow:(1)For sprinkler water application,the average dye-stained depth in the excavated soil profiles was 0.28 m,indicating that water infiltration was shallow.The average and median values of uniform infiltration fractions were over 0.65 and 0.7,respectively,indicating that water infiltration was quite uniform.The above suggested rainfall infiltration was dominated by piston flow in apple orchards on Loess Tableland.This might mainly result of soil physical property,clay illuviation,mechanical tillage and soil compaction.(2)For flood water application,the average and maximum dye-stained depths were also shallow(0.53 m and 0.59 m,respectively),suggesting that intense rainfall event that activated macropore flow,did not lead to much deeper infiltration in a short time.(3)It was more uniform for sprinkler water application than flood water application(P<0.05),indicating that the extreme heavy rain will increase heterogeneity of water infiltration and the proportion of preferential flow.(4)When the apple roots were fine,water infiltrate vertically in a one-dimensional way and it was less influenced for water lateral infiltration.The maximum dye-stained depth induced from stemflow simulation was 0.46 m,which was also relatively shallow,suggesting that it was difficult for rainfall to recharge deep soil along the root by stemflow.(5)Live roots(?2 mm)had minor effects on water infiltration,while decayed roots(?2 mm)could create water infiltration paths and facilitate preferential flow.In this study it didn't find abundant loess macropores and soil pore due to fine roots'(< 2 mm)seasonal death.(6)Based on the statistical analysis on the depths at which branching roots growing from the anchoring roots and angles of roots with the horizontal plane,we found most of the apple roots were horizontal in the soil surface.Because piston flow was the main mechanism in apple orchards on loess tableland area,it can make the rootsabsorb rainfall in the soilto the greatest extent.Furthermore,the degraded roots in surface soil were more likely to facilitate lateral preferential flow,thus presenting rainfall from infiltrating into the deep soil.In summary,piston flow is the main recharge mechanism for deep soil water from rainfall in apple orchards on Loess Tableland,and soil macropores and degraded vertical roots may also be an approach to deep soil water recharge.It was a necessary way to maintain sustainable production of the orchards through farmland-orchard rotation and intermediate cuttings.The results of the study is is of great significance to reveal soil water recharge mechanism,soil desiccation formation,and the effects of more frequently extreme rainfall for water recharge mechanism under climate change. |
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