Nitrogen mineralization in O horizon soils during 27 years of nitrogen enrichment at the Bear Brook Watershed in Maine, USA

The characters of dissolved organic matters from litter-mimic with the different humification states and their effects on drinking water treatment processes

Dissolved organic matter (DOM) is one kind of the main pollutant in surface water that will cause many problems during drinking water treatment processes. In this study, a simulated humification process of forest litter-mimic was investigated for eight weeks continuously to study the variations in chemical properties such as DOM composition, polysaccharide/protein ratio, average molecular weight, oxidation degree, hydrophilicity, etc., as well as the impact of these variations on the coagulation, ultra-/nanofiltration (UF/NF). Results showed that the removal rate of coagulation (from 67.5 % to 37.0 %) and UF (from 14.4 % to 5.8 %) decreased significantly during the humification process as a function of time, while the removal rate of NF increased from 40.0 % to 72.9 % at first, and then decreased to 47.4 %. This study gave a deep insight into the effect of DOM with different humification ages on the drinking water treatment process with the influence of seasons and vegetation around the water source, which finally aimed to improve drinking water treatment.

Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems

The productivity of ecosystems and their capacity to support life depends on access to reactive nitrogen (N). Over the past century, humans have more than doubled the global supply of reactive N through industrial and agricultural activities. However, long-term records demonstrate that N availability is declining in many regions of the world. Reactive N inputs are not evenly distributed, and global changes-including elevated atmospheric carbon dioxide (CO₂) levels and rising temperatures-are affecting ecosystem N supply relative to demand. Declining N availability is constraining primary productivity, contributing to lower leaf N concentrations, and reducing the quality of herbivore diets in many ecosystems. We outline the current state of knowledge about declining N availability and propose actions aimed at characterizing and responding to this emerging challenge.

Snowmelt periods as hot moments for soil N dynamics: a case study in Maine, USA

The vernal transition represents the seasonal transition to spring, occurring as temperatures rise at the end of winter. With rapid snowmelt, microbial community turnover, and accelerated nutrient cycling, this is a critical but relatively under-studied period of ecosystem function. We conducted a study over two consecutive winters (2015-2016) at the Bear Brook Watershed in Maine to examine how changing winter conditions (warming winters, reduced snow accumulation) altered soil nitrogen availability and stream N export during winter and the vernal transition, and how these patterns were influenced by ecosystem N status (N-enriched vs. N-limited). Of the two study years, 2016 had a warmer winter with substantially less snow accumulation and a discontinuous snowpack-and as a result, had a longer vernal transition and a snowpack that thawed before the vernal transition began. Across both years, snowmelt triggered a transition, signaled by increased ammonium concentrations in soil, decreased soil nitrate concentrations due to flushing by meltwater, and increased stream nitrate exports. Despite the contrasting winter conditions, both years showed similar patterns in N availability and export, differing only in the timing of these transitions. The vernal transition has conventionally been considered a critical period for biogeochemical cycling, because the associated snowmelt event triggers physicochemical and biochemical changes in soil systems. This was consistent with our results in 2015, but our data for 2016 show that this may not always hold true, and instead, that warmer, low-snow winters may demonstrate a temporal asynchrony between snowmelt and the vernal transition. We also show that ecosystem N status is a strong driver of the seasonal N pattern, and the interaction of N status and changing climate must be further investigated to understand ecosystem function under our current predicted trajectory of warming winters, declining snowfall, and winter thaw events.