Department of Marine and Environmental Sciences, Marine Science Center, Northeastern University, Nahant, Massachusetts.
The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts.
Glob Chang Biol. 2019 Oct;25(10):3224-3241. doi: 10.1111/gcb.14726. Epub 2019 Jul 17.
Salt marshes sequester carbon at rates more than an order of magnitude greater than their terrestrial counterparts, helping to mitigate climate change. As nitrogen loading to coastal waters continues, primarily in the form of nitrate, it is unclear what effect it will have on carbon storage capacity of these highly productive systems. This uncertainty is largely driven by the dual role nitrate can play in biological processes, where it can serve as a nutrient-stimulating primary production or a thermodynamically favorable electron acceptor fueling heterotrophic metabolism. Here, we used a controlled flow-through reactor experiment to test the role of nitrate as an electron acceptor, and its effect on organic matter decomposition and the associated microbial community in salt marsh sediments. Organic matter decomposition significantly increased in response to nitrate, even at sediment depths typically considered resistant to decomposition. The use of isotope tracers suggests that this pattern was largely driven by stimulated denitrification. Nitrate addition also significantly altered the microbial community and decreased alpha diversity, selecting for taxa belonging to groups known to reduce nitrate and oxidize more complex forms of organic matter. Fourier Transform-Infrared Spectroscopy further supported these results, suggesting that nitrate facilitated decomposition of complex organic matter compounds into more bioavailable forms. Taken together, these results suggest the existence of organic matter pools that only become accessible with nitrate and would otherwise remain stabilized in the sediment. The existence of such pools could have important implications for carbon storage, since greater decomposition rates as N loading increases may result in less overall burial of organic-rich sediment. Given the extent of nitrogen loading along our coastlines, it is imperative that we better understand the resilience of salt marsh systems to nutrient enrichment, especially if we hope to rely on salt marshes, and other blue carbon systems, for long-term carbon storage.
盐沼固定碳的速率比其陆地对应物高出一个数量级以上，有助于缓解气候变化。随着氮素不断向沿海水域输入，主要以硝酸盐的形式，其对这些高生产力系统的碳存储能力的影响尚不清楚。这种不确定性在很大程度上是由硝酸盐在生物过程中可以发挥的双重作用驱动的，它可以作为一种营养物质刺激初级生产，也可以作为一种热力学上有利的电子受体，为异养代谢提供燃料。在这里，我们使用受控的流动通过反应器实验来测试硝酸盐作为电子受体的作用，以及它对盐沼沉积物中有机质分解和相关微生物群落的影响。即使在通常被认为难以分解的沉积物深度，硝酸盐的添加也显著促进了有机质的分解。同位素示踪剂的使用表明，这种模式主要是由刺激反硝化作用驱动的。硝酸盐的添加还显著改变了微生物群落，降低了 alpha 多样性，选择了属于已知还原硝酸盐和氧化更复杂形式有机质的类群。傅里叶变换红外光谱进一步支持了这些结果，表明硝酸盐促进了复杂有机质化合物分解为更具生物可利用性的形式。总之，这些结果表明存在只有在硝酸盐存在时才变得可利用的有机质库，否则这些有机质库将在沉积物中保持稳定。这些库的存在可能对碳存储具有重要意义，因为随着氮负荷的增加，分解速率的增加可能导致富含有机质的沉积物的总体埋藏量减少。考虑到我们沿海地区氮素的输入程度，我们必须更好地了解盐沼系统对养分富化的恢复能力，特别是如果我们希望依靠盐沼和其他蓝碳系统来实现长期的碳存储。
