| Zinc(Zn)is an essential micronutrient and plays acrucial role in the growth and development of human beings.Currently,Zn deficiency in human has aroused great concern throughout the world.In the soil-plant-animal system,low Zn concentration in plant due to soil Zn deficiency leads to animal and human Zn malnutrition.Wheat(Triticum aestivum L.)is a staple food for people living in northern China and widely planted in the calcareous,potentially Zn-deficient soil(0.5~1.0 mg kg-1 DTPA-Zn),where the wheat grain with the poor Zn nutritional quality can not meet the daily recommended demand of Zn intake in human.Accordingly,improving the Zn availability in calcareous soil is of great significance to enhance Zn concentration in wheat grain and consequently alleviate human Zn deficiency.Zn uptake by plant roots depends on both the concentration of available Zn(DTPA-Zn)and the diffusion of DTPA-Zn in soil.Soil Zn fertilization is an effective practice to increase soil DTPA-Zn concentration,but causes the slight increase of Zn uptake by winter wheat and Zn concentration in wheat grain,which might be attributed to the low diffusion of DTPA-Zn in calcareous soil.Previous studies indicated that the application of freshly exogenous organic materials with a high proportion of water-soluble organic matter could lead to the formation of Zn complexes with soluble organic matter such as dissolved organic matter and fulvic acids,and thus enhance Zn availability and mobilization.Currently,crop straw returning has become an important method to reduce the environmental pollution,maintain and improve the farmland soil quality and production,and increase the energy conversion efficiency in China.However,little information is available for the response of exogenous Zn availability and transformation,Zn diffusion,Zn uptake by wheat,and Zn nutritional quality in wheat grain to the incorporation of crop straw.Consequently,the present study was conducted to investigate the effect of crop straw returning on Zn availability,movement,transformation,and uptake by winter wheat and Zn bioavailability in wheat grain by using four incubation and a 2-yr field experiments in calcareous soil.The main results were as follows:(1)The incubation experiment was conducted in the field to evaluate the effect of straw returning on Zn availability and ageing of exogenous Zn in calcareous soil.Results indicated that straw returning alone did not affect available Zn(DTPA-Zn)concentration and Zn fractions,but exogenous Zn alone did.Particularly,the Zn fraction loosely bound to organic matter(Lom-Zn)increased by 4-fold in comparison with that in CK treatment.Although straw returning plus Zn application had the similar results of DTPA-Zn,Zn fractions,and ineffective rate of Zn fertilizer with exogenous Zn alone,straw returning obviously increased the apparent diffusion coefficient of Zn and ageing rate of exogenous Zn in calcareous soil.(2)The incubation experiment using the half-cell device was carried out to assess the straw returning and Zn fertilizer on Zn diffusion in calcareous soil.Results showed that straw returning alone did not cause the change of soil DTPA-Zn concentration and diffusion.Exogenous Zn alone increased the soil DTPA-Zn concentration and DTPA-Zn diffusion,but over 45-d incubation,the DTPA-Zn was only limited to 0~15 mm away from the fertilized layer and the total DTPA-Zn recovery of Zn fertilizer was less than 40% in the device.When applying Zn fertilizer together with crop straw,the diffused DTPA-Zn could be detected at the 15~20 mm layer from the fertilized point and the total DTPA-Zn recovery reached to 45% over 45 d.Furthermore,compared with the Zn alone,straw returning plus Zn fertilizer enhanced the readily available Zn fraction bound to fulvic acids,which may be the main cause of the increase in soil DTPA-Zn diffusion and recovery in calcareous soil.(3)Two soils with a wide range of organic matter content were incubated without amendment,with Zn fertilization alone,or with Zn fertilization plus maize straw to investigate the response of Zn availability and transformation to straw returning in calcareous soils with the different soil organic matter content.This incubation experiment revealed that the combust of calcareous soil at 375°C was an effective method to remove soil organic matter with little variation in other properties in calcareous soil.75% removal of soil organic matter significantly reduced Lom-Zn concentration and consequently DTPA-Zn,revealing that soil organic matter(especially humic substances)controlled the DTPA-Zn level in calcareous soil.Exogenous Zn increased soil DTPA-Zn and Lom-Zn concentration,and the mobility factor of Zn,but this positive effectiveness gradually decreased with the passage of incubation time,due to the participate of exogenous Zn from carbonate and adsorption from clay minerals(i.e.montmorillonite).Additionally,when the exogenous Zn was applied into calcareous soil,straw returning could enhance the mobility factor of Zn and reduce the distribution index of Zn,suggesting that straw application effectively promoted the Zn transformation from the less available fractions to the readily available fractions,and thus,counteracted the process of exogenous Zn fixation and adsorption bycalcareous soil.Particularly,this effectiveness from straw returning was reinforced with the increased soil organic matter content.(4)Soils with the addition of combining Zn with organic materials were incubated in thefield to investigate the response of the availability and transformation of Zn fertilizer to exogenous organic matter in calcareous soil.Results indicated that when Zn fertilizer was applied into calcareous soil,the exogenous organic materials obviously increased soil DTPA-Zn,Lom-Zn,and mobility factor of Zn,and reduced the distribution index of Zn.However,although the positive responses of exogenous Zn availability to both manure and biofertilizer were stronger than those to maize straw and fulvic acids,maize straw plus exogenous Zn may be a better choice to improve the Zn availability in the potentially Zn-deficient calcareous soils when comprehensively considering the economic and environmental benefits.(5)The 2-yr field experiment assessed the effect of straw returning on soil Zn availability and Zn uptake by winter wheat in calcareous soil.Results found that there was no strong response of soil Zn availability and Zn uptake by winter wheat to maize straw amendment alone,but the 1-fold increases were recorded in DTPA-Zn,Lom-Zn,and the mobility factor of Zn after the exogenous Zn was added into calcareous soil.However,the Zn uptake by winter wheat only increased by 5%,and the use efficiency and DTPA-Zn conversion rate of Zn fertilizer were less than 0.1% and 20%,respectively.Straw plus exogenous Zn showed the similar results of soil indictors with those of Zn alone,but the use efficiency of Zn fertilizer reached to 0.15% and the Zn uptake by winter wheat increased by 10%.Moreover,the correlation analysis revealed that both the Zn concentration and Zn uptake by winter wheat were positively associated with DTPA-Zn,Lom-Zn,and the mobility factor of Zn.Accordingly,in addition to DTPA-Zn concentration,both Lom-Zn concentration and the mobility factor of Zn can be also used to evaluate the Zn availability in calcareous soil.(6)Analyzing the wheat grain from the 2-yr field experiment found that maize straw amendment alone did not change the grain Zn concentration and bioavailability of winter wheat,but treatments with Zn alone or Zn plus maize straw significantly did.Particularly,the Zn concentration in wheat grain reached to 39.5 mg kg-1 with the amendment of exogenous Zn plus maize straw.Furthermore,the ratio of phyate/Zn and TAZ were used to assess the Zn bioavailability of wheat grain.The results showed in all the treatments,the ratio of phyate/Zn were higher than 20 and TAZ were less than 3 mg Zn d-1.Compared with the control treatment,Zn plus maize straw had a lower ratio of phyate/Zn and a higher TAZ than other treatments(maize straw or Zn alone).Furthermore,when Zn plus maize straw was adopted,the improvement of grain Zn concentration and bioavailability of winter wheat did not cause the reduction in the grain Fe and Mn concentration and bioavailability.In sum,some of conclusions can be made as follows:(1)Crop straw returning combined with Zn fertilizer not only increases the soil available Zn(DTPA-Zn)concentration,but also enhances the DTPA-Zn diffusion in the potentially Zn-deficient calcareous soil;(2)Soil native organic matter determines the DTPA-Zn concentration in calcareous soil,and exogenous Zn organic materials(i.e.crop straw)can effectively limit the adsorption and fixation of exogenous Zn by calcareous soil and consequently improve Zn availability;(3)Crop straw returning combined with Zn fertilizer improves Zn uptake and grain Zn concentration and bioavailability of winter wheat.Although the Zn concentration of winter wheat when combining crop straw returning with the application of Zn fertilizer does not reach to the target concentration of wheat grain biofortification,soil Zn fertilization is a promising practice to improve Zn uptake and grain Zn concentration and bioavailability of winter wheat in potentially Zn-deficient calcareous soil in China where crop straw returning has been extensively adopted. |
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