1 INTRODUCTION

The mineralisation of dissolved organic matter (DOM) by microbes is an important biogeochemical process in running waters, which strongly influences nutrient and carbon cycling at the landscape scale (Battin et al., 2008; Marx et al., 2017). Microbial DOM processing in streams and rivers controls the rates of fluvial carbon retention and carbon dioxide (CO₂) outgassing, and alters the quantity and quality of the organic matter transported downstream (Boulton et al., 1998). Despite a growing number of studies on the microbial mineralisation and degradation of DOM in running waters (e.g., Fasching et al., 2014; Pucher et al., 2021), there are still large uncertainties regarding the mechanisms and factors determining the rates of fluvial DOM processing by microbes.

An important site for microbial DOM processing in streams and rivers is the hyporheic zone (HZ). The HZ is the sediment interface between surface water and ground water (Krause et al., 2017), and has been described as a hotspot of biogeochemical processing (Fasching et al., 2016; McClain et al., 2003). This property arises from the intense interactions between dissolved substances and reactive sites (i.e., particle surfaces and microbial biofilms) in the HZ and the mixing of qualitatively different surface water and ground water, promoting diverse microbial communities, metabolic pathways and chemical reactions (Boano et al., 2014; Findlay & Sobczak, 2000; Hedin et al., 1998; Nogaro et al., 2013). Steep redox gradients can form as a result of the high activity in this zone, further promoting the diversity of metabolic pathways and microbial communities (Krause et al., 2017). Studies have shown that the HZ can act as a sink for dissolved organic carbon (DOC) with ≤70% removal of groundwater DOC passing through this zone (Boodoo et al., 2019; Findlay et al., 1993). Rasilo et al. (2017) suggested that a major part of the terrestrially-derived organic carbon is mineralised in the HZ before entering surface waters of streams or rivers. Indeed, mesocosm and reach-scale studies have shown decreased DOC concentrations along flow paths through the HZ (Findlay et al., 1993, 2002; Findlay & Sobczak, 1996; Schindler & Krabbenhoft, 1998).

The DOM occurring in the HZ consists of a broad spectrum of organic compounds of varying degradability and origin. In general, DOM refers to the total mass of the organic matter filtered through 0.45–0.7 μm filters, including carbon but also other elements present in the organic material, such as nitrogen, oxygen and hydrogen. DOM can be dissolved from the terrestrial watershed (allochthonous DOM; McDowell & Likens, 1988; Hornberger et al., 1994) but also may originate from aquatic primary producers such as macrophytes or benthic algae (autochthonous DOM; Kaplan & Bott, 1989). The main source of DOM in the HZ is soils (Billett et al., 2006; Caillon & Schelker, 2020; Marx et al., 2017), which is supplied mainly as ground water through the HZ via transient connection of hillslopes and riparian areas (McGlynn & McDonnell, 2003; Sawyer et al., 2014). The biodegradability of soil-derived DOM (DOM_soil) mostly has been described as low-to-moderate in various field and laboratory studies as a consequence of its aromatic molecular structures (Cincotta et al., 2019; Fellman et al., 2009; Hansen et al., 2016; Kalbitz et al., 2000), especially in comparison to DOM released from fresh leaf litter (Hongve et al., 2000). Furthermore, DOM_soil generally is more oxidised, indicating that the relative energy per unit of carbon gained is lower (Del Giorgio & Cole, 1998) and the breakdown of this material requires more energy gained via respiration for the production of specific enzymes such as phenoloxidases (Berggren et al., 2012). Benthic algae also can leach copious amounts of DOM (Kaplan & Bott, 1989), which can be transported from the stream channel into the HZ (Wong & Williams, 2010). In contrast to DOM_soil, algal DOM (DOM_algal) consists mainly of carbohydrates, amino acids and lipids with lower molecular weight compared to humic compounds (Wetzel, 2001). DOM_algal usually is easily degradable and thus more susceptible to microbial processing than DOM_soil (Hansen et al., 2016; Thorp & Delong, 2002).

In addition to DOM composition, enhanced inorganic nutrient concentrations in stream water, and here most notably the availability of inorganic phosphorus (P) (Elser et al., 2007), also may boost microbial activities and DOM degradation (Mutschlecner et al., 2018; Williams et al., 2010). The relevance of intrinsic properties of DOM defined by its molecular composition versus external factors (i.e., the physical, chemical and biological conditions, such as water temperature, redox state or externally supplied inorganic nutrients) for DOM degradation currently is widely debated (see, e.g., Catalán et al., 2021; Kothawala et al., 2021). The fundamental question of what is of overall greater relevance, the DOM molecular composition or the surrounding aquatic environment with its dissolved nutrients, has not been comprehensively answered for oxygen-rich stream ecosystems. An experimental study with a standardised microbial community suggested that DOM composition could be more relevant for DOM degradation by planktonic microorganisms than the environmental conditions (Catalán et al., 2021). However, the role of P already present in lake and stream water has been shown to be especially relevant for the degradation of DOM sources with low degradability such as DOM_soil (Fasching et al., 2014; Kragh et al., 2008; Williams et al., 2010).

Inorganic P can be supplied to an aquatic environment by anthropogenic sources, such as by fertiliser use from agriculture or by the release of treated or untreated wastewater. In nutrient-rich streams, the degradation of DOM_soil in the HZ may be enhanced compared to pristine systems, leading to potentially increased hyporheic microbial respiration and also stream CO₂ outgassing. However, substantial knowledge gaps remain regarding the microbial respiration of different DOM sources (DOM_soil and DOM_algal = intrinsic control) in the HZ and the role of enhanced P concentrations (inorganic P additions = environmental control).

In this study, we aimed to experimentally analyse and quantify the aerobic respiration of DOM_soil and DOM_algal alone and in mixtures by hyporheic microbial communities, and to clarify the role of P in stimulating the respiration of DOM_soil. For this purpose, closed model systems were used in laboratory experiments, which consisted of hyporheic sediment columns filled with colonised glass beads. In Experiment 1, we measured DOM degradation as microbial respiration rates under oxic conditions and the qualitative change of the DOM pool. We hypothesised that aerobic microbial respiration rates will be lower in DOM_soil than in DOM_algal and decrease with increasing fractions of DOM_soil. This lower DOM_soil availability will be compensated by P additions in our model systems. In Experiment 2, hyporheic sediments from streams along a P-pollution gradient were used to test whether any stimulating effects of P additions on the DOM_soil degradation depended on the respective ambient P loads of the streams under oxic conditions. This second step also enabled us to compare our homogeneous model systems with the naturally occurring heterogeneous hyporheic sediments. Here, we hypothesised that aerobic microbial respiration of DOM_soil will be higher at higher ambient P concentrations of the stream water. By contrast, we expected the effects of P additions on microbial respiration to be more pronounced in streams with low ambient P concentrations.

中文翻译：

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无机磷对河流低流带区陆地碳降解作用的实验评价

1 简介

微生物对溶解有机物（DOM）的矿化是流水中重要的生物地球化学过程，它强烈影响景观尺度上的养分和碳循环（Battin et al.,  2008 ; Marx et al.,  2017）。溪流和河流中的微生物 DOM 处理控制河流碳滞留和二氧化碳 (CO ₂ ) 释气的速率，并改变下游输送的有机物的数量和质量（Boulton 等人，  1998 年）。尽管关于自来水中 DOM 的微生物矿化和降解的研究越来越多（例如，Fasching 等人，  2014 年；Pucher 等人，  2021 年）)，关于微生物处理河流 DOM 速率的机制和因素仍然存在很大的不确定性。

溪流和河流中微生物 DOM 处理的一个重要场所是下流带 (HZ)。HZ是地表水和地下水之间的沉积物界面（Krause et al.,  2017），被描述为生物地球化学处理的热点（Fasching et al.,  2016 ; McClain et al.,  2003）。这种特性源于 HZ 中溶解物质和反应位点（即颗粒表面和微生物生物膜）之间的强烈相互作用，以及质量不同的地表水和地下水的混合，促进了不同的微生物群落、代谢途径和化学反应（Boano 等等人，  2014 年；Findlay 和 Sobczak，  2000 年；Hedin 等人，  1998 年; Nogaro 等人，  2013 年）。由于该区域的高活性，可以形成陡峭的氧化还原梯度，进一步促进代谢途径和微生物群落的多样性（Krause 等人，  2017 年）。研究表明，HZ 可以作为溶解有机碳 (DOC) 的汇，通过该区域的地下水 DOC 去除率≤70% (Boodoo et al.,  2019 ; Findlay et al.,  1993 )。拉西洛等人。( 2017 ) 提出，大部分来自陆地的有机碳在进入溪流或河流的地表水之前在 HZ 中矿化。事实上，中宇宙和河段规模研究表明，沿 HZ 的流动路径降低了 DOC 浓度（Findlay 等，  1993, 2002 ; 芬德利和索布扎克，  1996 年；辛德勒和克拉本霍夫，  1998 年）。

发生在 HZ 中的 DOM 由多种可降解性和来源不同的有机化合物组成。一般而言，DOM 是指通过 0.45–0.7 μm 过滤器过滤的有机物的总质量，包括碳以及有机材料中存在的其他元素，例如氮、氧和氢。DOM 可以从陆地流域溶解（异地 DOM；McDowell 和 Likens，  1988 年；Hornberger 等人，  1994 年），但也可能源自水生初级生产者，例如大型植物或底栖藻类（本土 DOM；Kaplan 和 Bott，  1989 年）。HZ 中 DOM 的主要来源是土壤（Billett 等人，  2006 年；Caillon & Schelker，  2020 年；Marx 等人，  2017 年）)，主要通过 HZ 通过山坡和河岸地区的瞬时连接作为地下水供应（McGlynn & McDonnell,  2003 ; Sawyer et al.,  2014）。由于其芳香分子结构，土壤衍生的 DOM（DOM_土壤）的生物降解性在各种现场和实验室研究中大多被描述为低到中等（Cincotta 等人，  2019 年；Fellman 等人，  2009 年； Hansen 等人，  2016 年；Kalbitz 等人，  2000 年），尤其是与新鲜落叶层释放的 DOM 相比（Hongve 等人，  2000 年）。此外，DOM_土壤通常氧化程度更高，表明每获得单位碳的相对能量较低（Del Giorgio & Cole，  1998 年），并且这种材料的分解需要通过呼吸获得更多的能量来产生特定的酶，例如酚氧化酶（Berggren 等人.,  2012 年）。底栖藻类还可以浸出大量 DOM（Kaplan 和 Bott，  1989 年），这些 DOM 可以从河道输送到 HZ（Wong 和 Williams，  2010 年）。与 DOM_土壤相比，藻类 DOM（DOM _algal）主要由碳水化合物、氨基酸和脂质组成，与腐殖质化合物相比，其分子量较低（Wetzel，  2001 年）。DOM_藻类通常很容易降解，因此比 DOM_土壤更容易受到微生物处理（Hansen 等人，  2016 年；Thorp & Delong，  2002 年）。

除了 DOM 组成，河流水中无机养分浓度的提高，以及最显着的无机磷 (P) 的可用性（Elser 等人，  2007 年），也可能促进微生物活动和 DOM 降解（Mutschlecner 等人，  2018 年） ；威廉姆斯等人，  2010 年）。DOM 的分子组成与外部因素（即物理、化学和生物条件，如水温、氧化还原状态或外部提供的无机养分）对 DOM 降解的相关性目前存在广泛争议（参见，例如, Catalán 等人，  2021 年；Kothawala 等人，  2021 年）。对于富氧溪流生态系统，DOM 分子组成或周围水生环境及其溶解的养分具有更大的相关性，这一基本问题尚未得到全面回答。一项具有标准化微生物群落的实验研究表明，与环境条件相比，DOM 组成可能与浮游微生物对 DOM 的降解更相关（Catalán 等人，  2021 年）。然而，已经证明已经存在于湖泊和溪水中的 P 的作用与降解性低的 DOM 来源（如 DOM_土壤）的降解特别相关（Fasching 等人，  2014 年；Kragh 等人，  2008 年；Williams 等人）等人，  2010 年）。

无机磷可以通过人为来源提供给水生环境，例如通过农业肥料的使用或通过处理或未经处理的废水的排放。在营养丰富的溪流中，与原始系统相比，HZ 中 DOM_土壤的降解可能会增强，从而导致潜在的水下微生物呼吸增加以及溪流 CO ₂脱气。_{然而，关于 HZ 中不同 DOM 来源（DOM土壤}和 DOM_藻类= 内在控制）的微生物呼吸以及 提高 P 浓度（无机 P 添加 = 环境控制）的作用，仍然存在巨大的知识差距。

在本研究中，我们旨在通过实验分析和量化 DOM_土壤和 DOM_藻类的单独和混合物中的低流微生物群落的有氧呼吸，并阐明 P 在刺激 DOM_土壤呼吸中的作用。为此，在实验室实验中使用了封闭模型系统，该系统由填充有定殖玻璃珠的次流沉积柱组成。在实验 1 中，我们将 DOM 降解测量为有氧条件下的微生物呼吸速率和 DOM 池的质变。_{我们假设 DOM土壤}中的好氧微生物呼吸速率将低于DOM_藻类，并且随着 DOM_{土壤分数的增加而降低}. 这种较低的 DOM_土壤可用性将通过我们模型系统中的 P 添加来补偿。在实验 2 中，沿 P 污染梯度的溪流中的低流沉积物被用来测试 P 添加对 DOM_土壤退化的任何刺激作用是否取决于有氧条件下溪流的相应环境 P 负荷。这第二步还使我们能够将我们的均质模型系统与天然存在的异质次流沉积物进行比较。在这里，我们假设在溪水中较高的环境 P 浓度下，DOM_{土壤的好氧微生物呼吸会更高。}相比之下，我们预计 P 添加对微生物呼吸的影响在环境 P 浓度较低的溪流中更为明显。