Effect Of Rice Straw Returning On Si And Zn Absorption And Utilization Of Soil And Crop In Rice-wheat Rotation System

Zinc?Zn?is an essential nutrient element for plants,animals and Humans and extensive studies have been focused on this trace element all the time.Silicon?Si?is usually considered as a “beneficial elements” of plants and research topics,especially the absorption and transportation of Si in higher plants has been widely concerned in recent years.As a big agricultural country,China has more than 840 million tons of agricultural straw resources per year and the rice straw is one of the most abundant crop straws in China.The rice straw returning has been an important agricultural measure for soil amendment and fertility in paddy soils in China.As the typical silicon rich plant,rice straw returning has been considered as a simple and effective measure for soil Si amelioration.But to date,most of studies are based on short-term experiments and there are rare reports on the effect of rice straw returning on absorption of Si and Zn in rice and wheat.On the basis of our long-term experiment in rice-wheat rotation system,we investigated the content changes of available Si and Zn in paddy soils under the long-term rice straw returning and evaluated the effect of long-term rice straw returning on Si and Zn absorption in rice and wheat.We also carried out pot experiment to evaluate the absorption and interaction of rice on Si and Zn under different levels of Si and Zn fertilization.With the simulation experiment of flooded conditions,we characterized the decomposition of rice straw and the release characteristics of Si and Zn from rice straw,aiming to provide scientific basis for nutrition management of Si and Zn with the rice straw returning.The main results in this study were as follows:?1?Effect of rice straw returning on Si and Zn content in paddy soil and rice absorption in long-term experiment in rice-wheat rotation systemIn our long-term experiment in rice-wheat rotation system,we created series of fertilization treatments as rice straw returning?S?,NPK fertilization?nitrgen ? phosphorus and potassium fertilizer?,the combination of rice straw returning and NPK fertilization?NPKS?and the control?CK,no fertilization?.No obvious difference on zinc content was observed in grains and rice/wheat straws between the long-term rice straw returning and the controls.The zinc content in grain was averaged 19.8 mg·kg^-1 in rice and 47.7 mg·kg^-1 in wheat.Based on long-term data?24 years?,the zinc content of grain and straw in rice and wheat remained basically stable in interannual variation under four treatments.The Si content in wheat grain remained basically at the level of 0.16%,and there was no difference between the four treatments.While the rice straw returning significantly improve the Si content in wheat stem and leaves,NPKS treatment could increase the Si content in wheat stem and leaves by 1.4 and 0.8 percentage points respectively,and increase the silicon uptake in aboveground wheat by 47.4% compared with that of NPK treatment.The NPKS treatment could also significantly increase the silicon content of rice glume shell,leaf and stem organ by 2.2,2.8,1.7 percentage points,and increase the silicon uptake in aboveground rice by 27.6%,respectively compared to NPK treatment.It suggested that the wheat and rice absorbed amounts of silicon but just stored them in stems and leaves.The rice straw returning made zinc absorption of rice and wheat in aboveground with a reduced tendency,which might be due to the too much Si absorption by rice and wheat.The NPKS treatment decreased the zinc uptake in wheat aboveground by 4.6%,and in rice by 12.3% compared with that of NPK treatment.The rice straw returning could effectively increased the content of available zinc in paddy soil and alleviate the deficiency situation of silicon and zinc in soils.The NPKS treatment increased soil available zinc by 0.33 mg·kg^-1,and soil available silicon by 8.3 mg·kg^-1 than NPK control.Moreover,rice straw returning with the dosage of 7.5t·hm^-2 could supplement 376.4 g·hm^-2 zinc and 642.0 kg·hm^-2 silicon to the soil per year.?2?Effect of Si and Zn fertilization on the absorption and interaction of Si and ZnWe carried out series of Si and Zn fertilization treatments and evaluated their effect on the rice grain yield in pot experiment,while there was no difference on the rice grain yield between these treatments(Zn fertilization at 0,100,200 and 400 mg·kg^-1 and the Zn fertilization treatments at o and 5 mg·kg^-1).The Zn contents of rice organs are in order as stem > root > seeds > leaves > glume shell,and the silicon contents are in order as glume shell > leaf > stem > grain.At the rice mature stage,Si fertilization led to the obvious decrease in Zn uptake in rice aboveground.The Zn uptake in rice aboveground decreased significantly with the increasing of Si fertilization.It was interesting that the Zn transport into grain was increased with the increasing of Si fertilization.Moreover,the Zn fertilization didn't affect the Si uptake in rice and there was no difference on Si content in rice organs between the Zn fertilization and the control.At rice mature period,silicon and zinc fertilization exhibited positive interaction on soil available Si and Zn.Under zinc fertilization plus less Si fertilization,the content of soil available Si was 47.9 mg·kg^-1,while it was 33.0 mg·kg^-1 if without zinc fertilization.It suggested that Zn and Si fertilization could significantly improve the content of soil available Si to some extent.?3?Decomposition of rice straw and release characteristics of Si and Zn under flooded conditionsUnder the simulated flooded condition?up to 120 days?,the decomposition rates of rice straw(with the dosage of 5,7.5 and 10 g·kg^-1)was approximately 66.1%⁷3.7%.The decomposition rules of rice straw remained basically same even with different dosage: quickly degradation at the early stage,and slowly degradation at the late stage.The decomposition rate decreased with the increasing amount of rice straw.The release characteristics of Si from straw was quite different from that of Zn under flooded conditions.The release of Zn from straw remained quickly at early stage and then slowly at late stage,while the release of Si from straw remained slowly throughout the whole decomposition stages.The averaged release rates of Zn from straw were 81.5% and 87.2% respectively at 15 d and 120 d of flooded conditions.The averaged release rates of Si from straw were 20.4% and 64.6% respectively at 15 d and 120 d of flooded conditions.Straw returning could significantly improve the soil available zinc and silicon.The content of soil available zinc was obviously affected with the supplementation amount of rice straw,while it didn't affect the content of soil available silicon.Under flooded conditions,soil Eh and content of available zinc significantly decreased,while the content of soil available silicon increased.The content of soil available zinc decreased from 1.20 to 0.56 mg·kg^-1 during the 120 d flooded conditions if without straw application,while it decreased to 0.60⁰.66 mg·kg^-1 with straw application.The content of soil available silicon increased from 122.2 to 191.1 mg·kg^-1 during the 120 d flooded conditions if without straw application,while it increased to 241.0²43.8 mg·kg^-1 with straw application.It suggested that soil available silicon could come from the straw released,and the other come from soil itself by silicon activation.All taken together,it was demonstrated here that rice straw returning could significantly promote the silicon uptake of rice and wheat from soil and improve the silicon content in their plant tissues.Although rice straw returning could improve the content of soil available zinc,it didn't promote the silicon uptake of rice and wheat.The excess Si absorption might repress the zinc absorption of rice and wheat and further experiments should be continued to elucidate its interaction mechanism.