Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, Minnesota, USA.
Odum School of Ecology, University of Georgia, Athens, Georgia, USA.
Glob Chang Biol. 2022 Aug;28(16):4819-4831. doi: 10.1111/gcb.16272. Epub 2022 Jun 2.
Changes in the biosphere carbon (C) sink are of utmost importance given rising atmospheric CO levels. Concurrent global changes, such as increasing nitrogen (N) deposition, are affecting how much C can be stored in terrestrial ecosystems. Understanding the extent of these impacts will help in predicting the fate of the biosphere C sink. However, most N addition experiments add N in rates that greatly exceed ambient rates of N deposition, making inference from current knowledge difficult. Here, we leveraged data from a 13-year N addition gradient experiment with addition rates spanning realistic rates of N deposition (0, 1, 5, and 10 g N m year ) to assess the rates of N addition at which C uptake and storage were stimulated in a temperate grassland. Very low rates of N addition stimulated gross primary productivity and plant biomass, but also stimulated ecosystem respiration such that there was no net change in C uptake or storage. Furthermore, we found consistent, nonlinear relationships between N addition rate and plant responses such that intermediate rates of N addition induced the greatest ecosystem responses. Soil pH and microbial biomass and respiration all declined with increasing N addition indicating that negative consequences of N addition have direct effects on belowground processes, which could then affect whole ecosystem C uptake and storage. Our work demonstrates that experiments that add large amounts of N may be underestimating the effect of low to intermediate rates of N deposition on grassland C cycling. Furthermore, we show that plant biomass does not reliably indicate rates of C uptake or soil C storage, and that measuring rates of C loss (i.e., ecosystem and soil respiration) in conjunction with rates of C uptake and C pools are crucial for accurately understanding grassland C storage.
鉴于大气中 CO 水平不断上升，生物圈碳（C）汇的变化至关重要。同时发生的全球性变化，如氮（N）沉降增加，正在影响陆地生态系统中可以储存多少 C。了解这些影响的程度将有助于预测生物圈 C 汇的命运。然而，大多数 N 添加实验以大大超过环境 N 沉降速率的速率添加 N，使得从现有知识中推断变得困难。在这里，我们利用了一项为期 13 年的 N 添加梯度实验的数据，该实验的添加速率跨越了实际的 N 沉降速率（0、1、5 和 10 g N m -2 年 -1 ），以评估在温带草原中刺激 C 吸收和储存的 N 添加速率。非常低的 N 添加速率刺激了总初级生产力和植物生物量，但也刺激了生态系统呼吸，使得 C 吸收或储存没有净变化。此外，我们发现 N 添加率与植物响应之间存在一致的非线性关系，即中间 N 添加率诱导了最大的生态系统响应。土壤 pH 值和微生物生物量和呼吸作用都随着 N 添加量的增加而下降，这表明 N 添加的负面后果对地下过程有直接影响，这可能会影响整个生态系统的 C 吸收和储存。我们的工作表明，添加大量 N 的实验可能低估了低至中等 N 沉降速率对草原 C 循环的影响。此外，我们表明植物生物量不能可靠地指示 C 吸收或土壤 C 储存的速率，并且测量 C 损失（即生态系统和土壤呼吸）的速率与 C 吸收和 C 库的速率相结合对于准确理解草原 C 储存至关重要。
