低分子有机酸/盐对森林暗棕壤铁的释放效应

模拟森林凋落物淋洗液中草酸/草酸盐和柠檬酸/柠檬酸盐的浓度范围,设计了不同浓度草酸/草酸盐和柠檬酸/柠檬酸盐溶液一次性浸提和多次连续浸提等系列实验,结果表明,草酸/草酸盐和柠檬酸/柠檬酸盐对暗棕壤铁具有活化作用,活化作用大小因有机酸/盐种类、浓度和土壤性质而异。随着低分子有机酸/盐浓度的增加,A1层和B层暗棕壤活化释放的铁也逐渐增多。pH 5.16的低分子有机盐溶液对A1层和B层暗棕壤铁的活化作用强于同浓度的相应低分子有机酸,即柠檬酸盐>柠檬酸,草酸盐>草酸,因此推断草酸盐和柠檬酸盐溶液促进暗棕壤铁活化释放的机理主要在于低分子有机酸阴离子的配位反应。柠檬酸/柠檬酸盐对A1层和B层暗棕壤铁的活化作用强于同浓度的草酸/草酸盐,即柠檬酸>草酸,柠檬酸盐>草酸盐,这主要与柠檬酸对铁较强的络合能力及较大的离解常数有关。A1层与B层相比,草酸/草酸盐和柠檬酸/柠檬酸盐均对B层暗棕壤释放的铁量较高。另外,低分子有机酸对暗棕壤铁的释放并不是一次就能完成的,第一次以后的连续4次浸提仍源源不断释放出数量可观的铁。

Effects of Low Molecular Weight Organic Acids/salts on Iron Release of Dark Brown Forest Soil

By simulating oxalic acid/oxalate and citric/citrate concentrations in forest litter leachates,a series of batch experiments were conducted through one-time extraction and repeated sequential extraction procedures.It was resulted that oxalic acid/oxalate and citric acid/citrate substantially stimulated soil Fe release of dark brown forest soil,and the release capacity varied with the kinds and concentrations of low molecular weight organic acids/salts and soil properties.The effects of low molecular weight organic acids/salts on Fe release of A1 and B horizons would be strengthen with increasing of their concentrations.To A1 and B horizons,oxalate and citrate solutions(oxalic and citric adjusted to pH 5.16 with 10.0mmolL-1 NaOH) both had a much higher effects than oxalic acid and citric acid solutions the same in concentration,i.e.citrate>citric acid,oxalate>oxalic acid.Therefore,the mechanism of oxalic acid/oxalate and citric acid/citrate triggering release of soil Fe was assumed to be dominated by complexation reactions of organic anions.Similarly,citric acid/citrate solutions also had much higher effect than oxalic acid/oxalate at same concentrations to A1 and B horizons,i.e.citrate>oxalate,citric acid>oxalic acid,which was primarily related with the greater complexing capacities and dissociation constants of citric acid.The contents of Fe released by low molecular weight organic acids/salts from B horizon soil were higher than that from A1 horizon.Otherwise,the effects of citric acid on soil Fe release from A1 horizon were found to be somewhat cumulative,i.e.conspicuous Fe could be released from A1 horizon soil following four sequential extractions after one-time addition by citric acid solution.