Land use changes are known to alter terrestrial silicon cycling and the export of dissolved silicon from soil to fluvial systems, but the impact of such changes on groundwater systems remain unclear. In order to identify the processes responsible for groundwater geochemistry and to assess the impact of agricultural processes, we examined multiple isotopic tracers (δ30Si, oxygen (δ18O) and hydrogen (δ2H) isotopes) in groundwater, soil porewater and surface water from two contrasted watersheds having the same gneissic lithology, one forested (Mule Hole) and one intensely cultivated (Berambadi) in the Kabini basin in South India. In the cultivated watershed, groundwater exhibits high Cl− and NO3− concentrations indicative of fertilizer inputs and solute enrichment from evapotranspiration due to multiple groundwater pumping/recharge cycles. The DSi concentration in groundwater is significantly higher in the cultivated watershed (980 ± 313 μM) than in the forested one (711 ± 154 μM), indicating more intense evapotranspiration due to irrigation cycles. The groundwater δ30Si values ranged from 0.6 ‰ to 3.4 ‰ and exhibit no significant differences between cultivated (1.2 ± 0.5 ‰) and forested (1.0 ± 0.2 ‰) watersheds, indicating limited impact of land use and land cover. Groundwater also shows no significant seasonal differences in DSi and δ30Si within watersheds, indicating a buffer to seasonal recharge during wet season. The δ30Si of a majority of groundwater samples fits a steady-state open flow through system, with an isotopic fractionation factor (30ε) between precipitating phase and groundwater ranging from −1.0 ‰ and − 2.0 ‰, consistent with precipitation of kaolinite-type clays, dominant in the study area. The steady-state flow through system in groundwater can be interpreted as a continuous DSi input from mineral weathering reactions with a dynamic equilibrium between Si supply and precipitation of secondary phases. We also observe, in both watersheds, similar DSi and δ30Si values in local surface water that includes small streams and a river (406 ± 194 μM, 1.6 ± 0.3 ‰) and in soil porewater (514 ± 119 μM, 1.6 ± 0.2 ‰). Compared to soil porewater, groundwater exhibits significantly lower δ30Si signatures and higher DSi, reflecting the contribution of an isotopically light silicon source, resulting from water-rock interaction during percolation through the unsaturated zone. We assign this steady input of DSi to the weathering of primary silicate minerals in the regolith, such as Na-plagioclase, biotite and chlorite, with formation of kaolinite and smectites type clays. A simple isotopic mass balance suggests that deep regolith weathering can contribute to almost half of the DSi in groundwater. We conclude that silicon cycling in soil porewaters, and surface waters are directly impacted by land use, while the isotopic composition of groundwater remains unaffected. Our results indicate that Si isotopic signatures of weathering, adsorption, and plant uptake occurring in the shallow soil and saprolite horizons are partly overprinted and homogenized by the regolith weathering in the deep critical zone, irrespective of land use and seasonality.

中文翻译：

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热带流域土地利用对比下深层风化层控制地下水δ30Si组成


众所周知，土地利用变化会改变陆地硅循环以及溶解硅从土壤到河流系统的输出，但这种变化对地下水系统的影响仍不清楚。为了确定地下水地球化学的过程并评估农业过程的影响，我们检查了两个对比流域的地下水、土壤孔隙水和地表水中的多种同位素示踪剂（δ30Si、氧 (δ18O) 和氢 (δ2H) 同位素）印度南部的卡比尼盆地具有相同的片麻岩岩性，一处为森林覆盖（Mule Hole），一处为密集耕种（Berambadi）。在耕作流域，地下水呈现出较高的 Cl− 和 NO3− 浓度，表明由于多次地下水抽水/补给循环，肥料投入和蒸发蒸腾导致溶质富集。耕作流域地下水中的 DSi 浓度 (980 ± 313 μM) 显着高于森林流域 (711 ± 154 μM)，表明灌溉周期导致的蒸散量更强。地下水δ30Si值范围为0.6‰至3.4‰，耕地流域（1.2±0.5‰）和森林流域（1.0±0.2‰）之间没有显着差异，表明土地利用和土地覆盖的影响有限。流域内地下水的 DSi 和 δ30Si 也没有表现出显着的季节性差异，这表明雨季期间季节性补给存在缓冲。大多数地下水样品的 δ30Si 符合稳态开放流过系统，沉淀相与地下水之间的同位素分馏因子 (30ε) 范围为 -1.0 ‰ 至 - 2.0 ‰，与高岭石型粘土的沉淀一致，在研究领域占据主导地位。 地下水中流经系统的稳态流量可以解释为矿物风化反应的连续 DSi 输入，其中 Si 供应和次生相沉淀之间存在动态平衡。我们还观察到，在两个流域中，包括小溪和河流在内的当地地表水（406 ± 194 μM，1.6 ± 0.3 ‰）和土壤孔隙水（514 ± 119 μM，1.6 ± 0.2 ‰）中具有相似的 DSi 和 δ30Si 值。 。与土壤孔隙水相比，地下水表现出明显较低的 δ30Si 特征和较高的 DSi，反映了同位素轻硅源的贡献，这是由非饱和带渗透过程中水-岩石相互作用产生的。我们将这种稳定的 DSi 输入归因于风化层中原生硅酸盐矿物的风化，例如钠斜长石、黑云母和绿泥石，并形成高岭石和蒙脱石型粘土。简单的同位素质量平衡表明，深层风化层风化可造成地下水中近一半的 DSi。我们得出的结论是，土壤孔隙水和地表水中的硅循环直接受到土地利用的影响，而地下水的同位素组成不受影响。我们的结果表明，无论土地利用和季节性如何，浅层土壤和腐泥土层中发生的风化、吸附和植物吸收的硅同位素特征部分被深层关键区域的风化层覆盖和均匀化。