Evidence is emerging that the solubility of soil organic matter (SOM) in water is a key factor regulating the turnover of carbon (C) and nitrogen (N). We used data from a field trial with a wide range of treatments in a case study to: (1) examine the link between SOM solubility and bioavailability and (2) evaluate whether low water-solubility is a factor contributing to the persistence of refractory SOM. The trial was established in 2000 on a silt loam (Udic Dystocrept) at Lincoln, New Zealand to identify management practices that maintain SOM following the conversion of long-term pasture to arable cropping. The following land use treatments were sampled (0–7.5, 7.5–15 and 15–25 cm) in 2013: (1) long-term ryegrass-white clover pasture; (2) arable cropping rotation, managed using either intensive, minimum, or no tillage; and (3) continuous bare fallow (plots maintained plant-free using herbicide; not cultivated). The bioavailability of SOM was determined by measuring C and N mineralisation in a 98-day incubation at 25°C (soil maintained near field capacity) and water solubility was assessed by measuring hot-water-extractable C and N (16-h extraction at 80°C). After 13 years of arable cropping, C stocks (to 25 cm) were 11 t ha–1 less than in pasture soil (decrease of 14%). Tillage ‘intensity’ had no effect on C stocks in the top 25 cm. Large losses of C were observed in the bare fallow treatment (19 t C ha–1 less than pasture soil). The bioavailability of SOM (CO2-C mineralised as a proportion of soil C) also declined under arable cropping and bare fallow. The relationship between total C and mineralised C had a significant (P < 0.001) intercept, indicating that part of the organic matter (13 g C kg–1) did not contribute to C mineralisation (it was biologically inert). Across treatments and sampling depths, SOM mineralised in 98 days generally corresponded well with that extracted in hot water. A significant fraction of SOM (~9 g C kg–1) did not release C to hot water. Water-insoluble organic matter, including compounds that are strongly bonded to mineral surfaces, may comprise a significant part of the refractory SOM.

中文翻译：

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基于热水溶解度区分土壤有机质功能库

有证据表明，土壤有机质 (SOM) 在水中的溶解度是调节碳 (C) 和氮 (N) 周转的关键因素。我们在案例研究中使用了来自广泛治疗的现场试验数据来：(1) 检查 SOM 溶解度和生物利用度之间的联系，以及 (2) 评估低水溶性是否是导致难治性 SOM 持久性的一个因素. 该试验于 2000 年在新西兰林肯的粉砂壤土 (Udic Dystocrept) 上进行，目的是确定在将长期牧场转变为可耕作作物后维持 SOM 的管理做法。2013 年采样了以下土地利用处理（0-7.5、7.5-15 和 15-25 cm）：（1）长期黑麦草-白三叶草牧场；(2) 耕地轮作，采用精耕、少耕或免耕管理；(3) 连续裸休（使用除草剂保持无植物的地块；未耕种）。SOM 的生物利用度是通过在 25°C（土壤保持近场容量）培养 98 天中测量 C 和 N 矿化来确定的，并且通过测量热水可提取的 C 和 N（16 小时在80°C）。耕种 13 年后，碳库（至 25 厘米）比牧场土壤中的碳库少 11 t ha–1（减少 14%）。耕作“强度”对顶部 25 厘米的 C 库没有影响。在裸露的休耕处理中观察到大量 C 损失（比牧场土壤少 19 t C ha–1）。SOM 的生物利用度（CO2-C 矿化为土壤 C 的比例）在可耕作和裸休的情况下也下降。总碳与矿化碳之间的关系具有显着的（P < 0.001）截距，表明部分有机物（13 g C kg-1）对 C 矿化没有贡献（它是生物惰性的）。在不同的处理和采样深度，98 天内矿化的 SOM 与在热水中提取的 SOM 基本一致。很大一部分 SOM（~9 g C kg-1）不会将 C 释放到热水中。水不溶性有机物，包括与矿物表面紧密结合的化合物，可能构成难熔 SOM 的重要组成部分。