硫对秸秆还田下土壤镉有效性和稻米镉累积的影响机理

Crop straw incorporation is popular in oilseed rape (wheat)-rice rotation area. It was reported that straw incorporation increased soil fertility, also reduced the accumulation of cadmium (Cd) in rice, but results of a few experiments indicated straw incorporation increased soil Cd activity and promoted the accumulation of Cd in rice. The conflict was contributed to differences of soil pH, DOC due to straw incorporation, however, few knowledge is available about influence of sulfur on soil Cd available, and sulfur involved in migration and accumulation of Cd in rice under straw incorporation. A series of experiments (soil incubation experiments, pot experiments, field trials) were used to investigate the influence of straw incorporation of oilseed rape/wheat with different Cd accumulation ability to soils with different sulfur levels on the formation of S2-, SO42-. FeS, Cys. GSH in soil and soil pore water, and the formation of Cys、GSH、PC in rice, and sulfur-containing compounds involved in formation and activity of Cd (Cd2+, CdS) in soil, and root surface, plants (root, stem, shell, brown rice, aleurone layer, flag leaf), and subcellular (cell wall, cytoplasm, vacuole) of rice with different ability of Cd accumulation based on advance technologies, such as isotope tracer, FSEM+EDAX, GC/HPLC-MS, XAS etc., under two soil water managements (flooding during whole rice growth period; flooding before grain filling stage, subsenquent pre-harvest drainage). The results could help to understand sulfur involved straw incorporation on availability of Cd in soil, and accumulation of Cd in brown rice; and help to develop measures to reduce risk of Cd to food chain.

秸秆还田在油菜(小麦)-水稻轮作区普遍。秸秆还田可培肥土壤，减少稻镉累积；但镉污染区秸秆还田可提高稻镉累积，其与秸秆还田导致土壤pH、DOC变化有关；但硫对秸秆还田土壤镉活性及植株体镉运移影响的研究少。采用土壤培养、盆栽试验、田间试验，借助同位素示踪、扫描电镜、GC/HPLC-MS、XAS等技术，在二种水分管理(全生育期淹水、前期淹水+灌浆期排水)，前茬种植镉硫富集差异的油菜、小麦，将秸秆还田于不同硫水平的土壤上种植不同镉富集能力的水稻，探明土壤和孔隙水中S2-、SO42-、FeS、半胱氨酸、谷胱甘肽和稻植株体中半胱氨酸、谷胱甘肽、植物螯合肽形成规律，及其与土壤、根表、稻植株体（根、茎、壳、米、糊粉层、剑叶）和亚细胞（细胞壁、细胞质及液泡）中镉（Cd2+、CdS）含量的关联性；探明硫对秸秆还田土壤镉有效性和稻米镉累积的调控机制及影响机理，结果为减少污染稻田镉风险提供依据。

采用三个实验(土壤培养试验,硫与小麦/油菜秸秆单独施用或配合施用的水稻盆钵培养试验，前茬小麦/油菜施硫(四个水平)与二个镉水平的小麦/油菜-水稻轮作试验研究了小麦或油菜秸秆还田下硫对土壤镉有效性和水稻镉吸收的影响及机理。土壤培养试验结果表明，当土壤Eh保持较低值时，W+ S处理的土壤孔隙水中Cd浓度最低，这可能是由于Cd的沉淀和/或FeS2吸附Cd的共沉淀所致。单独施硫或硫与小麦秸秆配施处理，当土壤孔隙水Eh开始升高时，孔隙水Cd浓度也逐渐提高，其归于前期沉淀Cd或Cd-FeS2共沉淀在SOB作用下再溶解。盆栽水稻试验结果表明，在轻度镉污染的土壤 (土施5 mg Cd /kg) 中单独施用S可显著提高稻米Cd的浓度；而硫与小麦秸秆配施（S+ W）显著降低了糙米中Cd的浓度，但硫与油菜秸秆配施（S+R）确提高了稻米Cd浓度。单独施硫，和硫与小麦秸秆、硫与油菜秸秆配合施用于中度镉污染的土壤 (土施10 mg Cd /kg)种植的水稻 ，导致水稻糙米中Cd浓度显著下降。小麦/油菜-水稻轮作试验结果表明，在中度镉污染土壤上，前茬种植小麦时，前茬施硫显著增加后茬水稻稻米Cd浓度，最大值出现在60 mg S/kg水平；但前茬种植油菜时，前茬施硫对后茬水稻稻米Cd无影响。在低镉土壤（0.35 mg Cd/kg）上，前茬种植油菜，后茬水稻稻米Cd浓度随施硫水平增加而线性降低；但前茬种植小麦时，前茬施硫对后茬水稻稻米Cd无影响。秸秆在土壤中分解影响土壤DOC, Eh, pH, pe+pH性质；进而影响硫的还原-氧化(SO42- - S2-)、Fe的还原-氧化(Fe2+ - Fe3+)、Cd的沉淀-溶解（Cd2+ - CdS）、Cd与FeS/FeS2共沉淀-溶解的土壤化学过程，及其对镉从土壤溶液-土壤-根表胶膜界面迁移的过程，硫诱导的Cys 和 GSH形成而影响镉由根向稻米转移与分配生物化学过程是硫控制水稻吸收镉的关键机理，其可以解释本研究上述结果。可见，硫在小麦、油菜秸秆还田下是促进还是抑制水稻镉吸收、累积与土壤硫水平、镉污染水平有关。因此，制定镉污染土壤水稻稻米镉风险需要考虑土壤硫营状况、以及土壤镉污染程度，选择合适作为轮作系统。