1 INTRODUCTION

Headwater streams are tightly coupled to adjacent riparian zones via the input of organic matter from land to water (Fisher & Likens, 1973; Tank et al., 2010). Leaf litter is an important fraction of this input, and its decomposition fuels a range of stream biogeochemical processes (Tank et al., 2010,2018; Webster et al., 2009) as well as headwater food webs (Wallace et al., 1997). Decomposition of leaf litter in streams is regulated by the cumulative action of aquatic microbes, including fungi and bacteria, with their enzymatic inventory and respiration (Romaní et al., 2006; Sinsabaugh et al., 1994). Microbial activity, in turn, is influenced by multiple physical and chemical factors (Ferreira & Chauvet, 2011; Sinsabaugh & Follstad Shah, 2012). These factors are dynamic in time and sensitive to a range of catchment processes, which ultimately regulate the microbial efficiency of leaf litter mineralisation (Ely et al., 2010).

Temperature is typically considered an overarching constraint on microbial activity and, thus, leaf litter decomposition in streams (Follstad Shah et al., 2017; Petersen & Cummins, 1974). Indeed, the energy needed to activate biological reactions, especially those related to degrading recalcitrant polymers (e.g. cellulose), increases under low temperature (Sierra, 2012; Wang et al., 2012). However, even at 0°C, fungal growth and microbial respiration have been documented (Bärlocher & Kendrick, 1974; Buttimore et al., 1984), and microbial enzymes related to cellulose decomposition have been shown to retain about 30% of their activity measured at 25°C (Sinsabaugh et al., 1981). These observations suggest that microbial assemblages can sustain leaf litter decomposition even at low water temperatures (Follstad Shah et al., 2017).

In addition to thermal constraints, multiple water chemistry parameters regulate the activity of leaf-associated microbes. For example, nitrogen (N) and phosphorus (P) dissolved in the water column are essential to leaf-associated microbes (e.g. Ferreira et al., 2015; Suberkropp & Chauvet, 1995), and therefore partly explain spatial variability in decomposition rates (i.e., k; Rosemond et al., 2015; Woodward et al., 2012). In addition, dissolved organic carbon (DOC) may affect the activity of microbial decomposers by providing an additional carbon (C) source (Abril et al., 2019; Miller, 1987; Pastor et al., 2014). In this way, DOC represents a potential, but largely overlooked, energy source to leaf-associated microbes that could either facilitate decomposition (e.g. via priming; Kuehn et al., 2014) or decouple microbial enzyme activity from the mineralisation of particulate organic matter (Halvorson et al., 2019). The latter may be of particular relevance when leaf litter quality progressively (e.g. seasonally) decreases (Chauvet, 1987), so that DOC becomes a comparatively better energy source to microbes on leaf litter surfaces. However, microbial activity can also be constrained by increased stream acidity from organic acids contained in DOC, thus reducing leaf litter k (e.g. McKie et al., 2006). Ultimately, to understand the fate of leaf litter in headwater streams, it is important to disentangle the influences of nutrients, DOC, and other environmental conditions on the enzyme activity and respiration by leaf-associated microbes.

While these various factors that influence leaf litter decomposition are relatively well studied in tropical and temperate streams (e.g. Boyero et al., 2016; Ferreira et al., 2012; Follstad Shah et al., 2017), we know less about such influences in boreal streams. Northern boreal streams are characterised by extended periods of low water temperatures (c. 0°C; Burrows et al., 2017), snowmelt-driven hydrology that shapes allochthonous DOC supply and stream acidity (Ågren et al., 2008; Emilson et al., 2017; Laudon et al., 2011), and strong seasonality in concentrations of inorganic N (Creed et al., 1996; Sponseller et al., 2014) and bioavailable P (Jansson et al., 2012). Superimposed on these temporal patterns is spatial heterogeneity in stream properties that arises from the variable influence of upstream lakes and wetlands (mires), which are notably abundant at high latitudes. In this context, headwater mires play particularly important roles as sources of DOC (Laudon et al., 2011), dissolved organic N, ammonium (NH₄⁺; Sponseller et al., 2014), and P (Dillon & Molot, 1997) to boreal streams. Greater mire cover in catchments is also linked to higher specific discharge during summer (Karlsen et al., 2016), whereas headwater lakes can have strong thermal effects downstream (Mellina et al., 2002). Collectively, this spatial and temporal template in environmental conditions may exert strong influences over leaf-associated microbes and the extent to which their activity is coupled to decomposition rates.

Here, we ask how spatial and temporal variation in environmental conditions characteristic of boreal streams (i.e., cold, acidic, DOC-rich waters) influence microbial-mediated degradation of leaf litter. We incubated leaf litter from early October (leaf senescence) to late June (after spring flood) and quantified rates of decomposition, fungal biomass, microbial extracellular enzyme activity, and microbial respiration in 11 streams within the same drainage network. We tested how variation in decomposition and microbial variables relates to physical and chemical variables and, in turn, to the relative cover of upstream mires. Among streams, we predicted that microbial variables would be positively correlated with water temperature and nutrient concentrations. We further expected that microbial variables would be negatively correlated with DOC if organic acidity represents as an important constraint, whereas positive correlations with DOC may emerge if this C pool acts as a key energy source for leaf-associated microbes. Overall, we predicted that the relative strength and direction of the relationship with DOC would determine the extent to which variation in microbial processes is closely coupled (or not) to rates of litter decomposition. Finally, we anticipated that variation in mire cover would act as a network-scale organiser of microbial variables and decomposition, as a result of previously well-documented effects of mires on stream N, P, and DOC concentrations.

中文翻译：

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北方河流网络中微生物活动与落叶分解耦合的季节性变化

1 简介

源头水流通过从陆地到水的有机物输入与相邻的河岸带紧密耦合（Fisher & Likens，  1973 年；Tank 等人，  2010 年）。凋落物是这种输入的重要组成部分，其分解促进了一系列河流生物地球化学过程（Tank 等人，2010 年，2018 年；Webster 等人，  2009 年）以及源头食物网（Wallace 等人，  1997 年） ）。溪流中落叶的分解受水生微生物（包括真菌和细菌）的累积作用及其酶库存和呼吸作用的调节（Romaní 等人，  2006 年；Sinsabaugh 等人，  1994 年））。反过来，微生物活动受多种物理和化学因素的影响（Ferreira & Chauvet,  2011 ; Sinsabaugh & Follstad Shah,  2012）。这些因素在时间上是动态的，并且对一系列集水过程敏感，最终调节落叶矿化的微生物效率（Ely 等，  2010）。

温度通常被认为是对微生物活动的总体限制，因此，河流中的落叶分解（Follstad Shah 等人，  2017 年；彼得森和康明斯，  1974 年）。事实上，激活生物反应所需的能量，尤其是那些与降解顽固聚合物（例如纤维素）相关的反应，在低温下会增加（Sierra，  2012 年；Wang 等人，  2012 年）。然而，即使在 0°C 下，真菌生长和微生物呼吸也已记录在案 (Bärlocher & Kendrick,  1974 ; Buttimore et al.,  1984 )，并且与纤维素分解相关的微生物酶已显示保留其测量的约 30% 的活性在 25°C（Sinsabaugh 等人， 1981 年）。这些观察结果表明，即使在低水温下，微生物组合也可以维持落叶分解（Follstad Shah 等人，  2017 年）。

除了热限制之外，多种水化学参数还调节叶片相关微生物的活性。例如，溶解在水柱中的氮 (N) 和磷 (P) 对叶片相关微生物至关重要（例如 Ferreira 等，  2015；Suberkropp & Chauvet，  1995），因此部分解释了分解速率的空间变异性（即，k；Rosemond 等人，2015 年；伍德沃德等人，  2012 年）。此外，溶解的有机碳 (DOC) 可能会通过提供额外的碳 (C) 源来影响微生物分解器的活性（Abril 等人，2019；Miller，  1987；Pastor 等人，  2014）。通过这种方式，DOC 代表了叶相关微生物的潜在但很大程度上被忽视的能量来源，它可以促进分解（例如通过引发；Kuehn 等人，  2014 年）或将微生物酶活性与颗粒有机物的矿化解耦（ Halvorson 等人，  2019 年）。当凋落物质量逐渐（例如季节性）下降时，后者可能特别相关（Chauvet，  1987 年），因此 DOC 成为落叶表面微生物的相对更好的能源。然而，微生物活性也可能受到 DOC 中有机酸增加的流酸度的限制，从而减少落叶k (eg McKie et al.,  2006）。最终，要了解源头溪流中落叶的命运，重要的是解开养分、DOC 和其他环境条件对叶片相关微生物的酶活性和呼吸作用的影响。

虽然这些影响落叶分解的各种因素在热带和温带溪流中得到了较好的研究（例如 Boyero 等人，  2016 年；Ferreira 等人，  2012 年；Follstad Shah 等人，  2017 年），但我们对这些影响知之甚少北方溪流。北部北方溪流的特点是长时间的低水温（c . 0°C；Burrows 等人，  2017 年）、融雪驱动的水文，塑造了异地 DOC 供应和溪流酸度（Ågren 等人，  2008 年；Emilson 等人.,  2017 ; Laudon et al.,  2011 )，以及无机氮浓度的强烈季节性（Creed et al., 1996; Sponseller 等人，  2014 年）和生物可利用 P（Jansson 等人，  2012 年）。叠加在这些时间模式上的是河流特性的空间异质性，这是由上游湖泊和湿地（沼泽）的可变影响引起的，这些在高纬度地区尤为丰富。在这种情况下，源头水淤泥作为 DOC（Laudon 等人，  2011 年）、溶解有机 N、铵（NH ₄ ⁺；Sponseller 等人，  2014 年）和 P（Dillon 和 Molot，  1997 年）的来源发挥着特别重要的作用到北方溪流。流域中更大的泥浆覆盖也与夏季更高的特定排放量有关（Karlsen et al.,  2016)，而源头湖泊可能对下游产生强烈的热效应 (Mellina et al.,  2002 )。总的来说，环境条件下的这种空间和时间模板可能对叶片相关微生物及其活性与分解速率的耦合程度产生强烈影响。

在这里，我们询问北方溪流（即冷、酸性、富含 DOC 的水）的环境条件特征的时空变化如何影响微生物介导的落叶降解。我们从 10 月初（叶子衰老）到 6 月下旬（春季洪水之后）培育落叶，并量化了同一排水网络内 11 条溪流中的分解速率、真菌生物量、微生物细胞外酶活性和微生物呼吸。我们测试了分解和微生物变量的变化如何与物理和化学变量相关，进而与上游泥沼的相对覆盖率相关。在溪流中，我们预测微生物变量将与水温和养分浓度呈正相关。我们进一步预计，如果有机酸度作为一个重要的约束条件，微生物变量将与 DOC 呈负相关，而如果该 C 池作为叶片相关微生物的关键能源，则可能与 DOC 呈正相关。总体而言，我们预测与 DOC 关系的相对强度和方向将决定微生物过程的变化与垃圾分解率密切相关（或不相关）的程度。最后，我们预计，由于之前有充分记录的泥沼对流 N、P 和 DOC 浓度的影响，泥沼覆盖的变化将成为微生物变量和分解的网络规模组织者。而如果这个 C 池作为叶片相关微生物的关键能源，则可能会出现与 DOC 的正相关。总体而言，我们预测与 DOC 关系的相对强度和方向将决定微生物过程的变化与垃圾分解率密切相关（或不相关）的程度。最后，我们预计，由于之前有充分记录的泥沼对流 N、P 和 DOC 浓度的影响，泥沼覆盖的变化将成为微生物变量和分解的网络规模组织者。而如果这个 C 池作为叶片相关微生物的关键能源，则可能会出现与 DOC 的正相关。总体而言，我们预测与 DOC 关系的相对强度和方向将决定微生物过程的变化与垃圾分解率密切相关（或不相关）的程度。最后，我们预计，由于之前有充分记录的泥沼对流 N、P 和 DOC 浓度的影响，泥沼覆盖的变化将成为微生物变量和分解的网络规模组织者。