Abstract
Sediment particle size and heterogeneity play an important role in sediment denitrification through direct and indirect effects on, for example, the material exchange rate, environmental gradients, microbial biomass, and grazing pressure. However, these effects have mostly been observed in impermeable sediments. On the other hand, the material exchange of permeable sediments is dominated by advection instead of diffusion, with the exchange or transport rates exceeding those of diffusion by two orders of magnitude relative to impermeable sediments. The impact of permeable sediment particle size and heterogeneity on denitrification remains poorly understood, especially at the millimeter scale. Here, we conducted an in situ control experiment in which we sorted sand sediment into four homogeneous-particle-sizes treatments and four heterogeneous treatments. Each treatment was deployed, in replicate, within the riffle in three different river reaches with contrasting physicochemical characteristics. After incubating for three months, sediment denitrifier communities (nirS, nirK, nosZ), denitrification gene abundances (nirS, nirK, nosZ), and denitrification rates in all treatments were measured. We found that most of the denitrifying microbes in permeable sediments were unclassified denitrifying microbes, and particle size and heterogeneity were not significantly correlated with the functional gene abundances or denitrification rates. Water chemistry was the key controlling factor for the denitrification of permeable sediments. Water NO3−-N directly regulated the denitrification rate of permeable sediments, instead of indirectly regulating the denitrification rate of sediments by affecting the chemical characteristics of the sediments. Our study fills a knowledge gap of denitrification in permeable sediment in a headwater river and highlights that particle size and heterogeneity are less important for permeable sediment denitrification.
Highlights
Significant negative correlations were found between water electrical conductivity (EC)
Sediment Total organic carbon (TOC) and total phosphorus (TP) were positively correlated with sediment water content, but there were no significant correlations between sediment NH4 + -N and total nitrogen (TN) and sediment water content (Figure S3)
The TOC, TN, and TP of the sediments increased significantly as particle size decreased, while NH4 + -N and NO3 − -N did not change as particle size changed
Summary
River ecosystems play an important role in the global N cycle [1]. It is estimated that about 40% of the nitrogen input into rivers is permanently removed by the denitrification of rivers at a global scale [1]. The amount of organic matter and microbial biomass in river sediments is thousands or hundreds of thousands of times that of the upper water column and is one of the hotspots for denitrification [2]. River sediments can be classified according to their particle sizes, such as silt, clay, and sand [3]. Sediments can be classified as being permeable or impermeable [4]. As the median particle size (Dg) increases, the water permeability of sediments gradually increases [5].
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