School of Forest Resources, University of Maine, 5755 Nutting Hall, Orono, ME, 04469, USA.
Biological Sciences Division, Pacific Northwest National Laboratory, Richland, WA, 99352, USA.
Environ Monit Assess. 2020 Nov 21;192(12):777. doi: 10.1007/s10661-020-08733-0.
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.
春季过渡代表着冬季结束时气温升高导致的季节性向春季的过渡。随着快速的融雪、微生物群落更替和加速的养分循环，这是一个关键但相对研究较少的生态系统功能时期。我们在缅因州的 Bear Brook 流域进行了一项为期两个连续冬季（2015-2016 年）的研究，以研究不断变化的冬季条件（变暖的冬季、减少的积雪积累）如何在冬季和春季过渡期间改变土壤氮素有效性和溪流 N 输出，以及这些模式如何受到生态系统氮状态（富氮与氮限制）的影响。在这两个研究年份中，2016 年的冬季更温暖，积雪积累量大大减少，积雪层不连续——因此，春季过渡时间更长，积雪在春季过渡开始前就已经融化。在这两年中，融雪引发了一个转变，土壤中铵浓度的增加、由于融水冲刷导致土壤硝酸盐浓度降低以及溪流硝酸盐输出增加都标志着这个转变。尽管冬季条件截然不同，但这两年的氮素供应和输出模式都相似，只是这些转变的时间不同。春季过渡通常被认为是生物地球化学循环的关键时期，因为相关的融雪事件会引发土壤系统的物理化学和生物化学变化。这与我们在 2015 年的结果一致，但我们在 2016 年的数据表明，情况可能并非总是如此，而是温暖、少雪的冬季可能会导致融雪和春季过渡之间出现时间上的不同步。我们还表明，生态系统氮状态是季节性氮模式的一个重要驱动因素，必须进一步研究氮状态和气候变化的相互作用，以了解在我们目前预测的变暖冬季、降雪减少和冬季融雪事件的轨迹下，生态系统功能的情况。
