Hot water is believed to extract bio-available soil organic matter (SOM), including organic compounds from the biomass of soil microbes. The role of soil physico-chemical factors in relation to extractability of SOM in hot water is not well understood. We evaluated the influence of exchangeable sodium (Na) on the quantity and quality of organic matter extracted in hot water from soils with a range of total and extractable C (total C 19–60 g kg–1; hot water extractable C (HWC) 659–3292 mg kg–1). The soils were pre-treated with different rates of Na (0–156 cmolc kg–1, as NaCl) to establish a range of exchangeable Na percentages (ESP), and then extracted with hot water (80°C) for 16 h. Hot water extractable C increased linearly as ESP increased, but the rate of increase differed between soils (the increase in HWC per unit increase in ESP ranged within 19–71 mg kg–1). At ESP 15, a threshold used to separate sodic and non-sodic soils, HWC was 30–60% greater than that measured without added Na. Ultraviolet absorbance (260 nm) data indicated that aromatic organic matter was preferentially released following Na pre-treatment. The proportion of HWC in phenolic form was generally little affected by Na treatment but there was a consistent increase in protein in response to Na (hot water extractable organic N in protein form increased from an average of 5.5 ± 2.2% without added Na to 11.0 ± 3.6% at the highest Na rate). The Na-induced increases in UV absorbance may be largely attributable to release of proteins containing aromatic amino acids (tryptophan and tyrosine). Our results suggest that organic matter desorbed from mineral surfaces may be an important contributor to hot water extractable C and N, and factors that affect the adsorption–desorption process may significantly influence organic matter extractability in hot water.

中文翻译：

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可交换阳离子对农业土壤中热水可提取碳和氮的影响

热水被认为可以提取生物可利用的土壤有机质 (SOM)，包括来自土壤微生物生物量的有机化合物。土壤理化因素与 SOM 在热水中的可提取性有关的作用尚不清楚。我们评估了可交换钠 (Na) 对从土壤中提取的热水中有机质的数量和质量的影响，其中的总碳和可提取碳范围为 19–60 g kg–1；热水可提取碳 (HWC) 659–3292 毫克千克–1)。土壤用不同比例的 Na（0-156 cmolc kg-1，作为 NaCl）预处理，以建立一系列可交换的 Na 百分比（ESP），然后用热水（80°C）提取 16 小时。随着 ESP 的增加，热水可萃取 C 线性增加，但是不同土壤的增加率不同（ESP 中每单位增加的 HWC 增加范围在 19-71 mg kg-1 之间）。在 ESP 15（用于分离钠和非钠土壤的阈值）处，HWC 比未添加 Na 时测量的值高 30-60%。紫外吸光度（260 nm）数据表明，Na 预处理后优先释放芳香族有机物。酚类形式的 HWC 的比例通常受 Na 处理的影响很小，但蛋白质对 Na 的响应持续增加（蛋白质形式的热水可提取有机 N 从平均 5.5 ± 2.2% 增加到 11.0 ± 3.6%（最高 Na 比率）。Na 诱导的紫外线吸收增加可能主要归因于含有芳香族氨基酸（色氨酸和酪氨酸）的蛋白质的释放。