草酸/草酸盐对森林暗棕壤的磷释放效应

EFFECTS OF OXALIC ACID/OXALATE ON PHOSPHORUS RELEASE OF DARK BROWN FOREST SOIL

模拟森林凋落物淋洗液中的草酸/草酸盐浓度范围,设计了不同浓度草酸/草酸盐溶液一次性浸提和多次连续浸提系列实验,其中的草酸(阴离子)载荷量为0～200 mmol kg-1.结果表明,草酸能显著促进暗棕壤A1层(腐殖质层)磷的释放,土壤磷溶出量随草酸溶液浓度升高而线性增加;但对B层土壤磷的释放效应相对较弱,草酸浓度低于5mmol L-1时B层磷的释放不明显.pH 5.16草酸钠溶液比相同浓度的草酸溶液具有更高的解磷效率,在设置二者浓度为0.5～20.0 mmol L-1时,前者的解磷量是后者的1.51～2.98倍,推断草酸盐溶液或凋落物淋洗液中草酸(盐)类物质促进暗棕壤磷释放的主要机理在于草酸阴离子(C2O2-4)配位反应.草酸盐对暗棕壤磷的释放效应具有一定累加性,土壤磷释放量主要由草酸阴离子累积载荷量决定,而与其加入方式(多次或一次性)关系不大;当以pH5.16草酸钠溶液加入时,土壤磷释放量Y(mgkg-1)与草酸阴离子累积载荷量X(mmol kg-1)间的回归方程为Y=-0.000 4X2 +0.176 6X+0.425 3,R2 =0.990 2.仅以凋落物层溶出的草酸(阴离子)量进行估计,由此增加的A1层土壤磷释放量达2.40 kg hm-2 a-1,大约相当于中龄林年吸收磷量的1/3～1/5,因此其实际作用是不可忽视的.

By simulating oxalic acid/oxalate concentrations in forest litter leachate, a series of experiments were conducted including one-time extraction and repeated sequential extraction, with the oxalate loading rate varying from 0 to 200 mmol kg^-1 soil. Oxalic acid loading substantially stimulated soil P release from A1 horizon, showing a linear relationship; but its effect on B horizon soil was relatively negligible, especially at low oxalic acid concentrations （ 〈 5 mmol L^-1 ）. Oxalate solutions （0.5 - 20.0 mmol L^-1 oxalic acid adjusted to pH 5.16 with 10.0 mmol L^- 1 NaOH） had a much higher （1.51 - 2.98 times） soil-P-release effect than oxalic acid solution the same in oxalate concentration. Therefore, the mechanism of oxalic acid/oxalate triggering release of soil phosphorus is assumed to be dominated by ligand-exchange or complexing reactions of oxalate anion（ C2O4^2-）, but not simply acid-dissolving effect. The effect of oxalate anion on soil phosphorus release was found to be somewhat cumulative, i.e. the amount of P released was primarily controlled by the cumulative oxalate loading rate, relatively independent of the manner of oxalate addition （one-time or sequential）. When oxalate anion was loaded as pH 5.16 sodium oxalate solution, the re- gression equation between A1 horizon soil P release （ mg kg^- 1 ） and cumulative oxalic loading rate （ mmol kg^- 1 ） was ： Y = - 0.000 4X^2 ＋ 0. 176 6X ＋ 0.425 3 ,R^2 = 0.990 2. Merely on a quantitative basis of oxalate anion leached from forest litter, the increased soil P release from A1 horizon （0 - 20 cm） was estimated to be 2.40 kg hm^-2a^-1, accounting for 1/3 - 1/5 of the annual P absorption in middle-aged forests. If other types of organic anion or other reactions producing organic anion were taken into account, the cumulative effect of organic anions on soil P release would be very conspicuous.