Simulation Of Alfalfa-Grain Crop Rotation Effects On Semi-Humid Areas Of The Loess Plateau

Alfalfa(Medicago sativa L.)is a perennial legume grass which was cultivated widely on semi-humid areas of the loess plateau.Due to its strong consumption of soil water,soil desiccation and grassland degradation happened quite frequently at this zone.In order to overcome and slow down the effects of high-intensity water consumption,then promote deep soil moisture to recover gradually,We found that these kinds of grasslands should be treated through fallow during rainy season or rotated with cereal grains that consumed little soil water, which helped to update and efficiently reuse the dried alfalfa grassland.We used the field soil water observation combined with EPIC model simulation to study soil desiccation characters of alfalfa grassland and effects of its water recovery on the semi-arid areas of the loess planteau.Firstly,we investigated soil moisture on different growth age alfalfa grasslands and analysed soil desiccation characters,at the same time,the deep soil moisture of 10-year-old alfalfa field after rotated with different years crops were measured, and effects of soil water restoration by alfalfa-grain rotation were explored.Secondly, we used EPIC0509 model developed by USA to simulate the productivity and soil desiccated effects of 1¹6-year-old alfalfa at Zhenyuan station,on the basis of creating different kinds of databases,such as the daily weather elements series,the typical profile of physicochemical properties of soil,the crop growth parameters,the field tillage mearsures,the fertilizer characteristic parameter and cropping system database.Seven disposals were designed, concluding 21 years continuous cultivation of spring soybean,winter wheat and spring corn, and the same years of rotation,which were"winter wheat-winter wheat-spring soybean (WWS)","winter wheat-winter wheat-spring corn (WWC)","spring soybean-spring soybean-winter wheat (SSW)","spring soybean-spring soybean-spring corn(SSC)"and so on,after plowing 10-year-old alfalfa.Which could help quantitatively analysis the dynamic change of annual yield and the restoration rule of soil moisture in deep soil layer.According to the quantitative simulation of long period results,we could select the optimal alfalfa-grain rotation pattern that adapted to Zhenyuan station.The main progresses were as follows:(1)Field experimental results in Zhenyuan showed as following: A: The mean soil moisture in the 0¹ 000 cm soil layers of alfalfa fields 15²8 years old at semi-humid zones was 10.20%,the soil desiccation rate 34.2 mm per year and the maximum distribution depth of desiccated soil layer over 1 400 cm,and only 10 years old or younger alfalfa fields were fit for tillage and crop rotation.B: Soil moisture in the desiccated soil layer of the alfalfa fields could be restored gradually after 3²5 years of tillage and crop rotation,with the thickness of the desiccated soil layers reduced to 583 cm or by 77.3 mm·a^-1 and the average soil water restoration index (SWRI)reaching up to 83.3%,an extremely significant level.It took at least 8 years for a 15 year-old alfalfa field to restore its soil moisture to a stable level(15.5%)common in the region by following alfalfa-grain crop rotation.Therefore,alfalfa-grain crop rotation for over 8 years was a suitable pattern to restore soil moisture in 10-year-old alfalfa fields.(2)EPIC model simulation results showed as following:A: The simulation results of the productivity and soil moisture change of alfalfa fields on semi-humid of the loess plateau showed that,after alfalfa 10 years'growth,the available water content in 0¹ 000 cm soil layers fluctuated at a low value,soil desiccation was serious,which resulted in the instability of alfalfa yield and soil moisture situation.Therefore,proper measures should be taken to ease soil water deficit when alfalfa was planted for 10 to 12 years,such as alfalfa-grain crop rotation.B: The simulation results of soil moisture restoration under different alfalfa-grain rotation measures showed that,the restoration effects of alfalfa-grain crop rotation were excellent,the longer the year,the better the effects.Among the seven disposals,the year for continuous cultivation of spring soybean,WWS and SSW to reach full restoration degree was 15 years,and it needed 21 years for continuous cultivation of winter wheat,WWC and SSC.We could find that WWS has the remarkable restoration of soil moisture,and the effect of rotation was better than other continuous cultivation of crops.C: The simulation results of cereal crop yield under different alfalfa-grain rotation measures showed that,The yield played "inverted U"-type movement.15-year was suitable for crop rotation after alfalfa plowed in terms of yield components.Because the soil water restoration reached to a good level at this time.So we could plant alfalfa again after 15-year crops rotation.D: The simulation result of soil nutrient evolution showed that,planting alfalfa was conductive to the accumulation of soil organic carbon and organic nitrogen.The year of alfalfa were longer,it would accumulate more,however,that was different for organic phosphorus, labile phosphorus and NO₃-N.Seven disposals all consumed soil organic carbon,organic nitrogen and organic phosphorus.And the continuous cultivation of winter wheat had the maximun loss of organic carbon,while the continuous cultivation of spring soybean had the least loss.Then if spring soybean rotated with winter wheat,which could relieve the loss of organic matter.All of those indicated that"WWS"was beneficial to balance soil nutrient.E: In sum,we must think over soil moisture dynamics and soil nutrient dynamics combined with yeild dynamics by various rotation measures,then determined the suitable alfalfa-grain rotation pattern for Zhenyuan,which was"10 years alfalfa→15 years WWS".