半干旱森林下非饱和带大气二氧化碳作为无机碳的隔离
Sequestration of atmospheric carbon dioxide as inorganic carbon in the unsaturated zone under semi-arid forests
摘要：
半干旱土壤中无机碳以异源（迁移）和成土（土壤）碳酸盐的形式存在，可能构成重要的碳汇。二氧化碳，CO2，源自大气并由树根呼出到土壤中，可以在半干旱土壤的非饱和带（USZ）内被土壤水水合以产生碳酸（H2 CO 3）溶质HCO 3-碳酸氢根和H+氢离子。然后，这种H+可以溶解残留土壤中的CaCO 3碳酸盐（方解石），释放出Ca+2钙阳离子和更多的HCO 3-碳酸氢盐。当条件允许时，1摩尔Ca+2和2摩尔HCO 3-结合，沉淀出1摩尔方解石，并释放出1摩尔CO 2：Ca+2 + 2 HCO 3-> CaCO 3 + CO 2 + H 2 O。然而，有人声称，这种碳酸盐不会螯合大量的当今大气CO2。给出的理由是，它们部分来自预先存在的石灰石;每摩尔方解石沉淀，就有一摩尔二氧化碳释放到大气中。有人认为，只有当Ca+2阳离子来自非碳酸盐来源时，才能假定螯合作用。我们已经在以色列的两个半干旱地区的实地条件下测试了这些假设。我们发现，碳酸氢盐，源自根呼气，耗尽，并纳入USZ内的碳酸盐沉淀。因此，在半干旱森林下确实发生了大气CO2的净固存。此外，在沉淀反应中释放的大部分CO2可能保留在土壤中。沉积物中的Ca+2也可能来自方解石以外的来源。因此，植树造林可以增加成土碳酸盐的形成。通过对我们的全球数据进行外推，我们认为，全球范围内的半干旱森林（现有的和将要种植的）可能会将目前每年人为增加的大气二氧化碳中的5-20%作为成壤碳酸盐封存。
英文摘要：
Inorganic carbon, in the form of allogenic (transported) and pedogenic (soil) carbonates in semi-arid soils, may comprise an important carbon sink. Carbon dioxide, CO2, originating from the atmosphere and exhaled by tree roots into the soil, may be hydrated by soil water within the unsaturated zone (USZ) of semi-arid soils to produce the carbonic acid (H2CO3) solutes HCO3- bicarbonate and H+ Hydrogen ion. This H+ may then dissolve relict soil CaCO3 carbonate (calcite), to release Ca+2 calcium cations and more HCO3- bicarbonate. When conditions allow, one mole of Ca+2 and two moles of HCO3- combine to precipitate one mole of calcite, and to release one mole of CO2: Ca+2 + 2HCO3- --> CaCO3 + CO2 + H2O. However, it has been claimed that such carbonates do not sequester significant amounts of present day atmospheric CO2. The reasons given were that they originate in part from the pre-existing limestone; and that for every mole of calcite precipitated, one mole of CO2 may be liberated to the atmosphere. It was argued that only if the Ca+2 cation is derived from a non-carbonate source can sequestration be assumed. We have tested these assumptions under field conditions at two semi-arid sites in Israel. We found that bicarbonate, originating from root exhalation, is depleted and is incorporated within the USZ as carbonates precipitate. Thus, a net sequestration of atmospheric CO2 does occur under semi-arid forests. Moreover, most of the CO2 liberated in the precipitation reaction may remain in the soil. And Ca+2 in the sediment may also be supplied from sources other than pre-existing calcite. Forestation can therefore augment pedogenic carbonate formation. By extrapolating our data globally, we suggest that worldwide semi-arid forests (existing and to be planted) may sequester 5-20% of the current annual anthropogenic increase of atmospheric carbon dioxide as pedogenic carbonate.
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