Contrasting stream nitrate and sulfate response to recovery from experimental watershed acidification 1988 - 2018

This is the data archive for the corresponding publication with the same title (doi: 10.1007/s10533-020-00711-5). Improvements in air quality have led to ecosystem recovery from acidic deposition, but the mechanisms and trajectories of this recovery are not fully understood. Here, we present long-term stream response and recovery data for paired watersheds at the Bear Brook Watershed in Maine (BBWM) during declining ambient SO4 and NO3 in precipitation. East Bear (EB) received ambient deposition from 1989 to 2018; West Bear (WB) received artificially elevated N+S from 1989 to 2016. The WB treatment was discontinued after 2016, the beginning of the recovery from both the experimental N+S and ambient decline. Stream SO4 in WB gradually declined after the treatment ended, from ~147 μeq L-1 in 2010-16 to ~126 μeq L-1 in 2017-18. The declining S inputs induced desorption of SO4 from soil phase surfaces, with stream loss far exceeding precipitation input. At the current rate of recovery, it will be many decades before the WB stream returns to pre-treatment SO4 concentrations. In contrast, NO3 is only weakly adsorbed in soil, and WB stream NO3 concentrations rapidly declined from ~39 μeq L-1 in 2010-16 to ~5 μeq L-1 in 2017-18, comparable to the N-limited EB stream. The acid anions are strongly coupled to base cation chemistry in streams, and there was a distinct hysteretic response of Ca and Mg to the chronic acidification, as (Ca+Mg) increased rapidly during the initial years, followed by declining values due to depletion of the soil exchange complex. This 30-year monitoring study (1989-2019) provides insights into recovery mechanisms from acidic deposition and highlights the role of abiotic processes in soil that mediate nutrient cycling and retention. Documenting the rapid response of N alongside the slower recovery for S identifies the temporal resolution necessary for other whole-watershed recovery studies.

本数据集为同名学术出版物（DOI：10.1007/s10533-020-00711-5）配套的数据存档文件。空气质量改善已推动生态系统从酸性沉降影响中逐步恢复，但目前学界对该恢复过程的机制与轨迹尚未完全明晰。本数据集呈现了美国缅因州贝尔溪流域（Bear Brook Watershed in Maine, BBWM）配对集水区在降水硫酸盐（SO4）与硝酸盐（NO3）沉降逐步降低期间的长期溪流响应与恢复数据。东贝尔溪集水区（East Bear, EB）于1989年至2018年间接受自然沉降；西贝尔溪集水区（West Bear, WB）则在1989年至2016年间接受人工提升的氮（N）与硫（S）沉降。2016年停止对WB集水区的人工处理，标志着其同时开启了从实验性氮硫沉降与自然沉降降低的双重影响中恢复的进程。人工处理终止后，WB集水区溪流中的硫酸盐浓度逐步下降，从2010-2016年的约147 μeq·L⁻¹降至2017-2018年的约126 μeq·L⁻¹。硫沉降输入的减少引发了土壤固相表面硫酸盐的解吸过程，此时溪流中硫酸盐的流失量远高于降水输入量。以当前恢复速率计算，WB集水区溪流的硫酸盐浓度需数十年才能恢复至处理前水平。与之形成对比的是，硝酸盐在土壤中仅发生弱吸附，因此WB集水区溪流的硝酸盐浓度从2010-2016年的约39 μeq·L⁻¹快速降至2017-2018年的约5 μeq·L⁻¹，与受氮限制的EB集水区溪流浓度水平相当。溪流中的酸性阴离子与盐基阳离子的化学特征紧密耦合；钙（Ca）与镁（Mg）对长期酸化过程呈现出显著的滞后响应：酸化初期钙镁总量快速上升，随后因土壤交换复合体被耗竭而逐步下降。这项长达30年（1989-2019年）的定位监测研究，为阐明酸性沉降影响下的生态系统恢复机制提供了重要见解，并凸显了土壤中非生物过程在调控养分循环与固持过程中的关键作用。本研究同时记录了氮的快速响应与硫的缓慢恢复过程，明确了其他全集水区恢复研究所需的时间分辨率要求。