Nitrogen Losses in Sediments of the East China Sea: Spatiotemporal Variations, Controlling Factors, and Environmental Implications

Abstract

Global reactive nitrogen (N) has increased dramatically in coastal marine ecosystems over the past decades and caused numerous eco‐environmental problems. Coastal marine sediment plays a critical role in N losses via denitrification and anaerobic ammonium oxidation (anammox) and release of nitrous oxide (N2O). However, both the magnitude and contributions of denitrification, anammox, and N2O production in sediments still remain unclear, causing uncertainty in defining the N budget for coastal marine ecosystems. Here potential rates of N losses, and their contributions and controlling factors, were investigated in surface sediments during six cruises from 429 sites of the East China Sea. The potential rates of denitrification, anammox, and N2O production varied both spatially and seasonally, but the contribution of anammmox to total N2 production (%anammox) and N2O:N2 ratio only varied spatially. Both organic carbon and nitrate (NO3−) were important factors controlling N losses, N2O:N2 ratio, and %anammox. Our results also showed that marine organic carbon induced by eutrophication plays an important role in stimulating reactive N removal and increasing N2O production in warm seasons. The sediment N loss caused by denitrification, anammox, and N2O production in the study area were estimated at 2.2 × 106 t N yr−1, 4.6 × 105 t N yr−1, and 8 × 103 t N yr−1, respectively. Although sediments remove large quantities of reactive N, they act as an important source of N2O in this region influenced by NO3−‐laden rivers.

Incorporation of Microbial Functional Traits in Biogeochemistry Models Provides Better Estimations of Benthic Denitrification and Anammox Rates in Coastal Oceans

“…DNF was the dominant pathway for our benthic N 2 production, with contributions from 54 to 98%. Higher contributions of DNF relative to AMX have been reported in other coastal marine sediments (Lin et al, ; Rich et al, ; Trimmer & Nicholls, ). On the contrary, higher AMX contributions, accounting for more than 65% of N 2 production, were found in shelf sediments of the Celtic Sea (Kitidis et al, ), slope transect sediments in the North Atlantic (Trimmer & Nicholls, ), and Skagerrak sediments (Engström et al, ; Thamdrup & Dalsgaard, ).…”

“…Chl a in bottom waters is a proxy for the availability of labile organic matter (Franco et al, ; Kitidis et al, ) that is derived from phytoplankton cells sinking from upper waters, as well as phytobenthos and benthic algal debris in the shallow marginal system (average depth of less than 50 m, most in the euphotic zone). The correlation between bottom water Chl a and benthic DNF activity has received little attention previously because most prior investigations have only considered the importance of sedimentary organic matter pools (sedimentary TOC%) to the benthic DNF process (Dalsgaard et al, ; Kitidis et al, ; Lin et al, ; Trimmer & Nicholls, ), but ignored the effects of labile organic matter replenishment from the overlying water column. The high phytoplankton stock in May was associated with the high proportion of Chytridiomycota in the benthic fungal community in the studied area (Wang et al, ), suggesting that a large number of phytoplankton cells could be lysed to release labile DOC, which might support denitrification by diffusing down to anaerobic sediment layers.…”

“…Pearson's correlations were executed to reveal the associations among N 2 production, microbial gene variables, and the environmental parameters, which were the mean values of the triplicate samples at each sampling site. A stepwise regression approach was followed to further identify a subset of microbial and environmental variables that best explained the N‐loss activities (Lin et al, ; McCalley et al, ). The response variables were the total N 2 production rate, DNF rate, AMX rate, and ra %; the explanatory variables included the environmental factors (see Table ), abundances of individual marker genes, ratios of gene abundances, and alpha diversities (see Table ).…”

“…The seasonality of benthic N loss has been investigated in coastal sediments (Hietanen & Kuparinen, ; Kitidis et al, ; Lin et al, ) and shown to be generally related to seasonal variations of bottom water temperature, salinity, dissolved oxygen (DO) and NO 3 − , and sediment organic matter. Kitidis et al () found that benthic anammox rates peaked after the spring bloom, when organic matter (as represented by chlorophyll a ) fixed by phytoplankton reached the seafloor.…”

“…These results suggested that the concentration of chlorophyll a in bottom waters could be an important factor regulating benthic N‐loss activities. Additionally, a recent study in the East China Sea showed that the seasonality of ra % in N loss was tightly correlated with sediment metal ferrous iron contents (Lin et al, ). However, the majority of sedimentary denitrification and anammox measurements were from estuarine environments (Devol, ); thus, understanding of the seasonal dynamics of benthic N loss remains limited in coastal marine systems, especially at basin‐wide scales.…”