Particulate Nitrogen Concentrations Research Articles

Deciphering the impact of cascade reservoirs on nitrogen transport and nitrate transformation: Insights from multiple isotope analysis and machine learning

Construction of cascade reservoirs has altered nutrient dynamics and biogeochemical cycles, thereby influencing the composition and productivity of river ecosystems. The Lancang River (LCR), characterized by its cascade reservoir system, presents uncertainties in nitrogen transport and nitrate transformation mechanisms. Herein, we conducted monthly monitoring of hydrochemistry and multiple stable isotopes (δ15N-NO3-, δ18O-NO3-, δ18O-H2O, δD-H2O) throughout 2019 in both the natural river reach (NRR) and cascade reservoirs reach (CRR) of the LCR. Through the monthly detection of nitrogen forms and runoff in the import (M2) and export (M9) section, the average annual retention ratios for Total nitrogen (TN), Nitrate nitrogen (NO3--N), Particulate Nitrogen (PN) and Ammonium Nitrogen (NH4+-N) were about -35%, -53%, 48% and -65%, respectively. The retention rates were positively correlated with hydraulic retention time and negatively correlated with reservoir age, especially in the flood season. Compared to the NRR, the reservoir had significantly affected the nitrogen transport characteristics, especially for the large reservoirs (like Xiaowan and Nuozhadu), which enhanced phytoplankton uptake of NO3--N to form PN capabilities in the lentic environment and subsequently to precipitate or intercept it at the reservoir. This led to the overall decreasing trend of TN and PN concentrations along the CRR. The Bayesian stable isotope model quantified NO3--N sources from the NRR to the CRR. During this transition, soil nitrogen (SN) ratios decreased from 69.3% to 61.8%, while Manure & sewage (M&S) increased from 24.0% to 31.3%. Anthropogenic and natural factors, including urban sewage discharge, population density, and precipitation, were selected as key predictor variables. The eXtreme Gradient Boosting (XGBoost) model exhibited superior predictive performance for NO3--N concentrations, achieving an R2 of 0.70. These findings deepen our understanding of the impact of reservoirs on river ecology.

Response of runoff N and P concentrations, losses, and stoichiometry to rainfall amount category, multisource fertilization, and straw return in sloping croplands

Nitrogen (N) and phosphorus (P) losses via runoff from sloping croplands are a major threat to the degradations of soil fertility and water quality worldwide. Rainfall amount category and agricultural practices, such as multisource fertilization and straw return, are key factors regulating nutrient loss via runoff in sloping croplands. Yet, the impacts of these factors on N and P losses via runoff remain elusive. This study investigated the responses of N and P in runoff to rainfall amount category, multisource fertilization, and the combination of multisource fertilization and straw return in sloping cropland with a radish and maize rotation during multiple natural rainfall events between 2019 and 2021. Three agricultural treatments (three replications for each), i.e., local traditional fertilization (CK), multisource fertilization (T1), and a combination of T1 and straw return (T2), were applied on nine 8 m × 3 m plots. The concentrations and losses of total nitrogen (TN), total phosphorus (TP), dissolved nitrogen (DN), dissolved phosphorus (DP), particulate nitrogen (PN) and particulate phosphorus (PP) and their paired stoichiometries in runoff were determined. The runoff depth during rainstorms was similar to that in large rainstorms and was 191.1 % and 178.7 % higher than in moderate and heavy rains, respectively (p < 0.05). The PN concentration was the highest during moderate rain, and the highest concentrations of TP and DP were observed during large rainstorms (p < 0.05). Rainstorms and large rainstorms produced higher runoff losses for PN, TP, DP, and PP than for other rainfall amount categories (p < 0.05). Compared to CK, the runoff depth was marginally reduced by 4.2 % and 12.7 % in T1 and T2, respectively, and the concentrations and losses of N and P in runoff also slightly increased in T1 and T2. The DN and DP losses accounted for 70.5 % and 59.8 % of the TN and TP losses across all treatments, respectively, indicating that the dissolved forms played a dominant role in nutrient loss via runoff. TN:TP (77:1) in runoff was higher than the Redfield ratio (16:1) across all treatments, implying a stoichiometric potential for P depletion in the runoff. Our results highlight the potential risk of N and P losses in dissolved form via runoff regulated by rainfall, multisource fertilization and straw return in sloping croplands.

Runoff nitrogen losses under confluence and diverging drainage systems in the sloped plot scale: A comparative study

The drainage system is a key measure for regulating runoff nutrient losses on sloping farmlands. Confluence and diverging drainage systems are two drainage layouts representing natural water network systems and are widely distributed in sloping farmlands; however, the effects of these drainage systems on runoff nutrient losses in the sloped plots remain unclear. This study investigated the effects of different drainage systems on the characteristics of runoff nitrogen (N) losses in sloped plots using laboratory rainfall simulations. Three treatments, including bare slope (without drainage system, CK), confluence drainage system (T1), and diverging drainage system (T2), were used to compare the changes in concentrations and losses of total nitrogen (TN), dissolved nitrogen (DN), and particulate nitrogen (PN), and the DN:TN ratio in runoff under a combination of 1.8 mm min−1 rainfall intensity and three slope gradients (5°, 10°, and 15°). The results showed that the time to runoff was significantly delayed in T2 compared with that in CK and T1 across all slopes (p < 0.05). Accumulated runoff depth was considerably lower in T1 and T2 than in CK across all slopes (p < 0.05). The TN and PN concentrations in T1 were markedly lower than those in T2 on the 10° and 15° slopes (p < 0.05). The DN concentration in T1 was lowest at the 5° slope (p < 0.05). TN loss in T1 was 14.7–33.9% and 17.9–30.3% lower than those in CK and T2 across all slopes, respectively (p < 0.05). The PN loss in T1 was 56.7% and 53.3% lower than that in T2 on the 10° and 15° slopes, respectively (p < 0.05). DN loss in T1 was 39.3–72.5% lower than that in CK for all slopes (p < 0.05). DN:TN in T2 was lower than that in CK and T1 at the 10° and 15° slopes (p < 0.05). Our results confirm the effectiveness of drainage systems in reducing runoff nutrient losses in a sloped plot and demonstrate that the confluence drainage system is better at reducing N losses in runoff than diverging drainage systems.

Source, composition, and reactivity of particulate organic matter along the Changjiang Estuary salinity gradient and adjacent sea

The sources, distribution, and fate of particulate organic matter (POM) in estuaries are dynamic and complex, influenced by highly intensive human activities and high productivity. In this study, water samples were collected along the Changjiang Estuary salinity gradient and adjacent sea (CEAS) in February and May 2017. Particulate organic carbon (POC), particulate nitrogen (PN), the δ13C isotope values and major biochemical constituent (total particulate amino acids, TPAA) were measured. The concentrations of POC, PN, and TPAA showed an overall decreasing trend from the river end-member to the open sea; however, their maximum always occurred around the turbidity maximum zone (TMZ). Concentrations of POC and TPAA showed a negative correlation with salinity and a positive correlation with chlorophyll a, indicating that the variation in POM concentrations and composition was mainly controlled by both terrigenous input and in situ phytoplankton production. The δ13C values gradually increased from the river mouth to the open sea in both winter and spring, in contrast to the molar C/N, reflecting the transition from terrestrial POC to phytoplankton-derived fresh POC with increasing salinity. Major biochemical indicators of TPAA/POC (%) and the degradation index (DI), showed a gradual shift towards more bioactive POM with increasing salinity in spring, although low TPAA/POC (%) values appeared within the TMZ. In spring, POC reactivity was higher than in winter. The proportions of glycine (Gly) and serine (Ser) were higher in winter, indicating that POM had suffered extensive degradation. Based on a two end-member mixing model, the contribution of marine POC in spring (53 ± 14%) was significantly greater than in winter (39 ± 19%), indicating that phytoplankton-derived POM was dominant in spring, associated with the increase in phytoplankton biomass from winter to spring. Based on mass balance, a box model showed evidence of a net POC sink over the Changjiang estuary and its adjacent East China Sea shelf in both winter and spring, with a net POC budget of 20.49 ± 7.01 and 15.87 ± 6.57 kmol s−1, respectively. Results illustrate that the spatio-temporal distribution of POM varies distinctively and will further affect the variability in its composition and reactivity in the CEAS.

Phytoplankton dominates the suspended particulate nitrogen source in the Yangtze River

Globally, the large rivers export amounts of particulate nitrogen (PN) to the ocean, and studying the source and biogeochemical processes of the PN in the large river is important to regional/global N budget and cycle. However, the source apportionments and transformations of the PN along the Yangtze River remain unclear. Here, the elemental and isotopic compositions (N%, δ15N and δ13C value) of suspended particulate N (SPN) and surface sediment N (SSN) were determined in the wet and dry seasons along the Yangtze River, to investigate the seasonal and spatial variation in sources apportionments and transformations of the PN. In the wet season, the δ15N and PN content (%) of SPN increased from the headwater stream (2.3 ± 3.9 ‰ and 0.12 ± 0.04 %, respectively) to the lower stream (6.8 ± 2.4 ‰ and 0.47 ± 0.41 %, respectively) due to the imported anthropogenic N into the downriver section, while there was no significant change in the dry season. There were no distinctly spatial and seasonal variations in the source apportionment of SPN. Phytoplankton was identified as the dominant source of the SPN (42.3–56.9 %), which is probably the result of reducing soil erosion, reservoir construction, rapid urbanization and development of freshwater aquaculture. Terrestrial soil was the major source of SSN (34.5–49.8 %). Furthermore, the significant relationship between the δ15N-SPN values and NO3− concentrations, as well as the lower δ15N-SPN value compared to that of δ15N-NO3−, indicated a strong assimilation process of nitrate during phytoplankton growth. Meanwhile, most of the SSN showed lower C and N concentrations and higher δ13C and δ15N values, compared to those of the SPN samples at the corresponding sampling sites, which might be due to the SSN degradation. Our findings provided valuable information for the N cycle in the Yangtze River and underlined the importance of phytoplankton in the PN inputs.

Effects of tidal variations on total nitrogen concentration, speciation, and exchange flux in the Shuidong Bay coastal water, South China Sea

Nitrogen (N) plays an important role in marine ecosystems as a biogenic element for phytoplankton. The tidal cycle had major influence on various biogeochemical parameters of the bay and changed nutrients input with the ebb and flow of the tide. In this study, we collected samples from Shuidong Bay (SDB) in China, in August 2021, to explore the periodic variations in coastal waters during spring and neap tides. The effects of spring and neap tides on different speciation of nitrogen in the SDB and the exchange fluxes between the SDB and the South China Sea (SCS) were investigated. The results indicated that the concentrations of particulate nitrogen (PN) and N-NO2- were significantly different between the spring and neap tides (P &amp;lt; 0.05). The total nitrogen (TN) concentrations in SDB during the spring and neap tides were 258.12 ± 89.49 μmol/L and 231.77 ± 56.86 μmol/L. During the spring and neap tides, total dissolved nitrogen (TDN) accounted for 54.1% and 52.2% of TN, respectively. In addition, dissolved organic nitrogen (DON) accounted for 81.4% and 69.9% of the TDN during the spring and neap tides, respectively. Furthermore, the net exchange fluxes of different speciation of nitrogen showed that the net exchange fluxes of TN were transported from SDB to SCS during the spring and neap tide, with the net exchange fluxes of 37.7 t and 8.8 t, respectively. The net exchange flux of TN during spring tide was 4.3 times higher than that of neap tide. In addition, a significant negative correlation was observed between dissolved inorganic nitrogen (DIN) and salinity in SDB during the spring and neap tides (P &amp;lt; 0.001), indicated that DIN was mainly influenced by terrestrial sources inputs. A significant positive correlation (P &amp;lt; 0.01) was observed between PN and Chlorophyll-a in SDB during the spring and neap tides, implied that the assimilation of a large number of planktonic organisms promoted PN formation. The present results revealed that tidal variation played an important role in regulating N speciation and exchange flux in coastal waters, which had great implications for N biogeochemistry and water quality improvement in SDB.

Effects of tillage on soil nitrogen and its components from rice-wheat fields in subtropical regions of China

It is of great significance to explore the effects of different tillage practices on total nitrogen and its components in rice-wheat rotation farmland. The experiment was carried out in Jiangyan County, Jiangsu Province of China, and a total of four treatments were set up: minimum tillage (MT), rotary tillage (RT), conventional tillage (CT), and conventional tillage without straw retention (CT0). The total nitrogen (TN), light fraction nitrogen (LFN), heavy fraction nitrogen (HFN), particulate nitrogen (PN), and mineral-associated nitrogen (MN) in 0-20 cm soil were determined. The results show that MT increased TN concentration by2.26%-27.57% compared with the other treatments in 0-5 cm soil, but it lost this advantage in 5-10 cm and 10-20 cm soil. MT altered the concentration of LFN by 6.03%-95.86%, of HFN by 1.68%-20.75%, of PN by 12.58%-96.83%, and of MN by −1.73%-9.83% as compared to RT, CT, and CT0 in 0-5 cm soil, respectively. With the deepened of soil depth, the concentration of TN, LFN, HFN, PN, and MN decreased quickly in MT, which was lower than that in RT and CT at 10-20 cm soil depth. Straw return increased the concentration of TN and its components in 0-20 cm soil. The concentration of TN was extremely significantly positively correlated with that of LFN, HFN, PN, and MN (p<0.01). The variation of TN was significantly positively correlated with that of LFN, HFN, PN, and MN (p<0.01), and LFN showed the highest sensitivity to tillage practice. In general, minimum tillage combined with straw retention increased the concentration of soil TN and its components in topsoil. LFN was the best indicator to indicate the change in soil total nitrogen affected by tillage practice. Keywords: rice-wheat rotation, soil active nitrogen components, light fraction nitrogen, heavy fraction nitrogen, particulate nitrogen, mineral-associated nitrogen DOI: 10.25165/j.ijabe.20221503.5661 Citation: Cui S Y, Zhu X K, Cao G Q. Effects of tillage on soil nitrogen and its components from rice-wheat fields in subtropical regions of China. Int J Agric & Biol Eng, 2022; 15(3): 146–152.

Role of particle dynamics in processing of terrestrial nitrogen and phosphorus in the estuarine mixing zone

AbstractMultiple biogeochemical processes in estuaries modulate the flux of nutrients from land to sea, thus contributing to the coastal filter. The role of particle dynamics in regulating the fate of terrestrial nutrients in estuaries is poorly constrained. To address this issue, we resolved the particle size distribution of suspended material, and quantified size‐fractionated particulate nitrogen (PN) and phosphorus (PP), in a stratified mesotrophic estuary (Pojoviken, Finland). We also carried out a mixing experiment where the effects of salt‐induced flocculation on particle size distribution and concentrations of PN and PP were examined. The experimental results showed that salt‐induced flocculation at already very low salinities increases the total particle concentration and mean particle size, indicating transfer of dissolved material into particulates. Correspondingly, a significant increase in PP and particulate iron (Fe) was observed in the experiment results, suggesting coupled flocculation of P‐containing organic matter (OM) and ferrihydrite. Particle dynamics in the field data were dominated by processes occurring downstream of the flocculation zone. Primary production created a downward flux of autochthonous OM particles, promoting passive aggregation by random collisions with terrestrial material in the water column. Maximum particle concentrations were observed at and below the halocline. The highest PN and PP concentrations were observed in the subhalocline layer, 3.5 and 0.14 μmol L−1, respectively. Molar ratios of N:P in this material were &gt;40, consistent with typical marine snow in the early stages of microbial processing. Our study provides a mechanistic overview of the biogeochemical drivers of particulate nutrient dynamics in stratified estuarine environments.

Internal loop sustains cyanobacterial blooms in eutrophic lakes: Evidence from organic nitrogen and ammonium regeneration

Algal bloom species can live upon internal regenerated ammonium (NH4+) for growth during the nitrogen-limited period. However, the linkages between NH4+ regeneration and phytoplankton biomass and community composition dynamics remain largely unknown. To unravel the interactions between NH4+ regeneration and phytoplankton community, we measured water column NH4+ regeneration rates (REGs) during a continuous phytoplankton growing period and a contrast summer/winter turnover in eutrophic Lake Taihu. Measured REGs were higher in summer than in winter and significantly correlated to total phytoplankton biomass, Cyanophyta biomass and its biomass proportions, and the concentrations of particulate nitrogen and dissolved organic carbon as well as the relative abundance of labile components (proteins and lipids). Random forest regression analyses displayed that variation of REGs were mainly controlled by water temperature and algal-related parameters (including chlorophyll a, total phytoplankton biomass, and Cyanophyta biomass). Partial least squares path model further revealed that algal-related parameters were the direct and significant factors regulating REGs, and contributed to the largest effect of the variance in REGs. Of the algal community, Cyanophyta was the dominant phylum to accelerate REGs. Correspondingly, rapid internal NH4+ turnover may strongly support the persistence of cyanobacterial blooms, thus forming a positive feedback between cyanobacterial blooms and REGs during the nitrogen-limited summer months. We therefore deduced that the internal loop between cyanobacterial blooms and REGs during summer may be a key self-maintenance mechanism of continuous cyanobacterial blooms.

Transport process and source contribution of nitrogen in stormwater runoff from urban catchments

Nitrogen in urban stormwater has been widely studied, and effective management of nitrogen pollution is critical for improving urban stormwater and receiving water quality. This requires an in-depth understanding of the transport process and source contribution to both dissolved and particulate nitrogen in stormwater from urban catchments. In this study, 123 stormwater runoff samples were collected from an urban catchment during different rainfall events. Dissolved and particulate nitrogen concentrations in roof runoff, road runoff, and sewer flow were analyzed. The concentration of dissolved nitrogen was higher in roof runoff than in road runoff and sewer flow. However, the concentration of particulate nitrogen was lower in roof runoff than in road runoff and sewer flow. Isotopic analysis and Bayesian mixing models showed that road runoff was the largest source contributor of both nitrate and particulate organic nitrogen (PON) in sewer flow discharged from the study catchment. In addition, road runoff contributed the majority of PON associated with coarse particles (>105 μm), whereas PON associated with fine particles (<105 μm) was primarily washed-off of sewer sediments. The results provided several suggestions for the management of nitrogen pollution in urban catchments. This study could help to fully understand the transport and sources of nitrogen pollution in urban stormwater and provide recommendations to the government for implementing appropriate stormwater management strategies to minimize stormwater pollution.

Concentrations, ratios, and sinking fluxes of major bioelements at Ocean Station Papa

Fluxes of major bioelements associated with sinking particles were quantified in late summer 2018 as part of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign near Ocean Station Papa in the subarctic northeast Pacific. The thorium-234 method was used in conjunction with size-fractionated (1–5, 5–51, and &amp;gt;51 μm) concentrations of particulate nitrogen (PN), total particulate phosphorus (TPP), biogenic silica (bSi), and particulate inorganic carbon (PIC) collected using large volume filtration via in situ pumps. We build upon recent work quantifying POC fluxes during EXPORTS. Similar remineralization length scales were observed for both POC and PN across all particle size classes from depths of 50–500 m. Unlike bSi and PIC, the soft tissue–associated POC, PN, and TPP fluxes strongly attenuated from 50 m to the base of the euphotic zone (approximately 120 m). Cruise-average thorium-234-derived fluxes (mmol m–2 d–1) at 120 m were 1.7 ± 0.6 for POC, 0.22 ± 0.07 for PN, 0.019 ± 0.007 for TPP, 0.69 ± 0.26 for bSi, and 0.055 ± 0.022 for PIC. These bioelement fluxes were similar to previous observations at this site, with the exception of PIC, which was 1 to 2 orders of magnitude lower. Transfer efficiencies within the upper twilight zone (flux 220 m/flux 120 m) were highest for PIC (84%) and bSi (79%), followed by POC (61%), PN (58%), and TPP (49%). These differences indicate preferential remineralization of TPP relative to POC or PN and larger losses of soft tissue relative to biominerals in sinking particles below the euphotic zone. Comprehensive characterization of the particulate bioelement fluxes obtained here will support future efforts linking phytoplankton community composition and food-web dynamics to the composition, magnitude, and attenuation of material that sinks to deeper waters.

Seasonal-to-decadal scale variability in primary production and particulate matter export at Station ALOHA

Station ALOHA (A Long-term Oligotrophic Habitat Assessment) was established in the North Pacific Subtropical Gyre (22°45′N, 158°W) as an oligotrophic ocean benchmark to improve our understanding of processes that govern the fluxes of carbon (C) into and from the surface ocean. At approximately monthly intervals, measurements of the primary production of particulate C (PC) using the 14C method, and the export of PC and particulate nitrogen (PN) using surface-tethered sediment traps deployed at 150 m have been made along with a host of complementary physical, biological, and biogeochemical measurements. Euphotic zone depth-integrated (0–200 m) primary production ranged from 220.2 (standard deviation, SD, 10.8) mg C m−2 d−1 in Feb 2018 to 1136.5 (SD = 17.1) mg C m−2 d−1 in Jun 2000, with a 30-yr (1989–2018) mean of 536.8 (SD = 135.0) mg C m−2 d−1 (n = 271). Although the monthly primary production climatology was fairly well constrained, we observed substantial sub-decadal variability and a significant 0–125 m depth-integrated increasing trend of 4.0 (p < 0.01; 95% confidence interval, CI, 2.1–5.9) (mg C m−2 d−1) yr−1 since 1989, displaying a large relative increase of 37% (CI = 18–55%) in the lower portion (75–125 m) of the euphotic zone. Chlorophyll (Chl) a and suspended PC and PN concentrations also displayed significant (p < 0.01) increases in the 75–125 m region of the euphotic zone. PC export at 150 m exhibited both short-term (monthly) and longer-scale variability with a 30-yr mean of 27.9 (SD = 9.7, n = 265) mg C m−2 d−1. PC and PN export exhibited extended, multi-year periods of significantly lower or higher values compared to the 30-yr mean. These multi-year periods of anomalously low and high particle export, in the absence of contemporaneous variations in primary production, probably reflect periodic changes in remineralization efficiencies. The PC export ratio (e-ratio; PC export at 150 m ÷ 0–150 m depth-integrated 14C-based primary production) was low, with a 30-yr mean of 0.054 (SD = 0.021, n = 248), and exhibited a significant (p < 0.01) long-term decreasing trend over the 30-yr observation period. The 30-yr long-term increases in primary production (~37%), Chl a, and suspended PC and PN concentrations (~17%, 8%, and 8%, respectively) in the 75–125 m portion of the water column are hypothesized to result from an enhanced supply of nutrients to the lower portion of the water column over the past three decades.

Contrasting phytoplankton and biogeochemical functioning in the eastern Arabian Sea shelf waters recorded by carbon isotopes (SW monsoon)

This study examines the relationship between the isotopic signature of phytoplankton-derived particulate organic matter (POM) (δ13CPOC) and estimates of growth rate and community composition along a north-south gradient in the western Indian shelf waters which possess contrasting biogeochemistry and mixed layers depths. The Eastern Arabian Sea/western Indian shelf turns highly productive due to the SW monsoon induced upwelling at its southern region and experiences drastic seasonal reversal. We have characterized the POM pool by quantifying Chla, particulate organic carbon (POC) and particulate nitrogen (PN), C:N ratios, along with δ13CPOC values during the SW Monsoon. The prevailing physicochemical features contrasted between the southern (8°N to 12°N) and northern (13°N to 21°N) stations. Close couplings between POC, PN, and Chla contents indicated the autochthonous nature of POM. Low temperature, shallow mixed layer depths (MLDs), high concentrations of nutrients, POM, and Chla, marked the upwelling signature in the south. Conversely, relatively higher temperatures, salinity, deeper mixed layers (MLs), lower concentrations of nutrients, POM, and Chla were evident at the northern stations. Five times higher POC concentrations were noticed in the south (65.5 ± 22.0 μmol L−1) than in the north (12.8± 5.4 μmol L−1) within the MLs. Phytoplankton community shift (based on marker pigment analysis) and the δ13CPOC values were closely coupled. The nutrient replete microphytoplankton (diatoms) dominated southern stations were associated with higher values of δ13CPOC (−23.0 ± 2.3‰) which was attributed to faster growth rate; conversely, the oligotrophic nitrogen-limited waters in the north dominated by pico and nanophytoplankton (haptophytes and cyanobacteria), were characterized by distinctly lower δ13CPOC values (−26.6 ± 0.9‰). Phytoplankton growth rates based on a model were consistent with this trend. Our results show that the autochthonous POC is the primary source of organic matter, and the contrasted biogeochemistry induced phytoplankton communities and their growth rates mostly governed the δ13CPOC variability in this region. However, the lower values in the subsurface waters at the south were likely to be influenced by light limitation and heterotrophy.

Sources and features of particulate organic matter in tropical small mountainous rivers (SW China) under the effects of anthropogenic activities

Small mountainous rivers (SMRs), particularly those in the tropics, play a disproportionate role in global carbon cycling and have been heavily disturbed by anthropogenic activities during the Anthropocene. Identifying the sources of particulate organic matter (POM) in SMRs and their influencing factors have crucial scientific significance in terms of filling a cognitional gap concerning the fate of POM in tropical land–ocean ecosystems and practical values regarding environmental management and ecological evaluation. In this study, total suspended solids concentration, particulate organic carbon and particulate nitrogen contents, stable carbon isotope (δ13C), and C/N ratio of suspended particulates were investigated from five SMRs (Fangcheng, Maoling, Qin, Dafeng, and Nanliu) in Guangxi Province, southwest China. The soil (OMsoil), C3 plants (OMC3), and autochthonous production (OMauto) were identified as potential POM sources, and their relative contributions were quantified via the use of end-member mixing models that were based on the classic indicators of δ13C and C/N. The results showed that soil was the biggest POM contributor in Nanliu, Qin, and Maoling rivers, whereas OMC3 was more important in Fangcheng River. Compared to other watersheds, the OMauto accounted for a very minor proportion of the POM sources in Fangcheng River. Nanliu and Fangcheng, as typical representatives of agricultural and forest watersheds, respectively, exhibited remarkable differences in terms of POM sources. Land use patterns that were dominated by agricultural expansion, the soil erosion that resulted from the destruction of vegetation and engineering construction, and hydrological characteristics such as flow velocity and nutrient concentration were all found to vastly impact the sources of POM reaching the different types of SMR. These influencing factors are mainly derived from anthropogenic activities, whose effects on the tropical SMRs ecosystems and even the global biogeochemical cycle, may exceed our expectation.

Nitrogen isotopic fractionation of particulate organic matter production and remineralization in the Prydz Bay and its adjacent areas

During the 29th Chinese National Antarctic Research Expedition, spatial variations in nitrogen isotopic composition of particulate nitrogen (δ15NPN) and their controlling factors were examined in detail with regard to nitrate drawdown by phytoplankton and particulate nitrogen (PN) remineralization in the Prydz Bay and its adjacent areas. To better constrain the nitrogen transformations, the physical and chemical parameters, including temperature, salinity, nutrients, PN and δ15NPN in seawater column were measured from surface to bottom. In addition, the nitrogen isotopic fractionation factor of nitrate assimilation by phytoplankton in the mixed layer, and the nitrogen isotopic fractionation factor of PN remineralization below the mixed layer were estimated using Rayleigh model and Steady State model, respectively. Our results showed that suspended particles had its lowest δ15NPN in the surface layer, which was due to the preferential assimilation of 14N in nitrate by phytoplankton. The δ15NPN in the mixed layer of the Prydz Bay and its adjacent areas decreased from the inner shelf to the outer basin, ascribing to the effect of isotope fractionation during phytoplankton assimilation. In mixed layer, the spatial distribution of δ15NPN associated with particulate organic matter (POM) production can be well interpreted according to Rayleigh model and Steady State model. The nitrogen isotope fractionation factor during phytoplankton assimilating nitrate was estimated as 10.0%o by Steady State model, which was more reasonable than that calculated by Rayleigh model. These results validate the previous reports of fractionation factor during nitrate assimilation by phytoplankton. Increasing δ15NPN with depth below the euphotic zone correlated with the decreasing PN contents, and it was attributed to preferential remineralization of 14N in PN by bacteria. In subsurface and deep layer, the δ15NPN distributions also conformed to Rayleigh model and Steady State model during PN remineralization, with a fractionation factor of about 3.6% and 3.2%, respectively. It is the first time to estimate the fractionation factor during POM production and remineralization in the Prydz Bay and its adjacent areas. Such fractionation may provide a useful tool for the follow-up study of the nitrogen dynamics in the Southern Ocean.

Diurnal carbon dynamics in a mangrove-dominated tropical estuary (Sundarbans, India)

Based on a 24h of time-series study, we report the effects of a tidal cycle on carbon biogeochemistry of a mangrove dominated tropical estuary (the Sundarbans) located in the eastern part of India. Salinity, dissolved oxygen, and pH showed clear tidal variability with relatively higher values during high tide than low tide. Dissolved inorganic carbon (DIC) concentrations varied over a narrow range (1.92–2.19 mM) with relatively higher values during low tide; reverse trend, however, was noticed for δ13CDIC with significant variability (– 4.28 to – 2.21‰). During low tide, along with estuarine mixing, preliminary evidences for influences of biogeochemical (such as organic carbon mineralization, sulfate reduction, and denitrification) and hydrological processes (porewater exchange) were found on DIC dynamics. The δ13CDIC - DIC relationship suggested respiration of marine plankton to be one of the possible sources for DIC. Dissolved organic carbon showed tidal influence during high tide with a signal of porewater mediated addition during low tide. Both particulate organic carbon and particulate nitrogen concentrations reached the maximum during low tide with stable isotopic compositions showing predominantly marine signature along with the possibility of biogeochemical modifications within the estuary. Marine water contribution together with organic carbon mineralization and possible porewater influx resulted in ~214 μatm higher pCO2 and 1.13 times higher FCO2 during low tide than high tide. On diurnal basis, the estuary released ~1348 mg CO2 per m2 of surface area to the regional atmosphere.

Wind Field Influences on the Spatial Distribution of Cyanobacterial Blooms and Nutrients in Meiliang Bay of Lake Taihu, China

Wind field is a very important physical factor controlling the formation of cyanobacteria blooms. A surface particle tracking drift experiment was carried out to study the influence of wind field on the surface current in Meiliang Bay of Lake Taihu during the algal bloom season. For this, chlorophyll-a, nitrogen, phosphorus, the permanganate index, dissolved organic carbon (DOC), and dissolved oxygen (DO) were measured in surface, middle, and bottom waters of the Meiliang Bay during the cyanobacteria bloom period to test how wind field affects the temporal and spatial distribution of cyanobacterial blooms and biomass stock in the water column. The results showed that the average drift velocities of surface particles were 3.0 cm·s-1 and 5.0 cm·s-1 when wind speed averaged 1.9 m·s-1 and 2.3 m·s-1, respectively. The wind field determined the spatial distribution of cyanobacterial blooms in surface waters and led to a high spatial heterogeneity of cyanobacterial blooms. The spatial redistribution of cyanobacterial blooms exerted an important influence on water quality indexes such as particulate nitrogen, phosphorus, organic matter, and dissolved oxygen. The concentrations of particulate nitrogen, phosphorus, the permanganate index, and chlorophyll-a showed a similar vertical distribution pattern. Cyanobacterial blooms were less influenced by the distribution of dissolved nitrogen and dissolved organic carbon from external pollution, while long-term legacy loading played a more important role. This meant that the spatial distributions of dissolved nitrogen and dissolved organic carbon were different from that of chlorophyll-a. Because the redistribution of cyanobacterial blooms, as affected by wind fields, has a complex effect on the dissolved oxygen in the water column, the dissolved oxygen concentration decreased with depth, which may affect the release of soluble nutrients from the sediment. The cyanobacterial biomass stock in the surface water was estimated according to the survey of high-density sites. The dry matter of cyanobacteria in the surface 20 cm of Meiliang Bay was approximately 396 tons on the day of sampling. The results from the present study indicated that the factors influencing cyanobacterial blooms should be considered in sampling methods and the analysis of lake water quality due to the significant influence of wind fields on bloom drift. The collection of cyanobacteria has limited effect on the removal of the algal bloom biomass in whole lake, only being effective at prevention of the event of black spots in lake shore.

Temporal-spatial Distribution of Nitrogen and Phosphorus Nutrients in Lake Taihu Based on Geostatistical Analysis

It is of great significance to investigate the spatio-temporal variation in nitrogen and phosphorus nutrients as a mechanism for controlling the sudden increase of algae in eutrophic water. Based on the geostatistical analysis, we studied the sources and occurring forms of nitrogen and phosphorus in different areas of Lake Taihu-a large shallow and eutrophic lake in China. We also examined the spatial distribution of the type of nutrient restriction and its reason by monitoring the sites seasonally from August 2014 to May 2015. The results showed that:① The concentrations of soluble nitrogen and phosphorus were higher in the winter than in other seasons, and they gradually decreased spatially from the northwest to southeast. The concentrations of particulate nitrogen, phosphorus, and chlorophyll a were highest in the summer, and the high-concentration regions in the winter and other seasons were located in the southern and northwestern parts of the lake, respectively. ② The composition of nitrogen and phosphorus in algae-and macrophyte-dominated regions changed substantially with the seasonal change. The algae-dominated regions were dominated by nitrate nitrogen and organic phosphorus in the winter and particulate nitrogen and phosphorus during the other seasons. However, the macrophyte-dominated region was dominated by particulate nitrogen and phosphorus in the winter and ammonia, organic nitrogen, and phosphorus during the other seasons. ③ The ratios of total nitrogen to total phosphorus in the algae-dominated region were greater than 16:1 in the autumn and winter but less than 16:1 in the summer and spring. Meanwhile, the ratios of total nitrogen to total phosphorus in the macrophyte-dominated region increased from less than 16:1 in the autumn and winter to more than 16:1 in the summer and spring. In the algae-dominated region, the spatial variation in the dissolved total nitrogen to total phosphorus ratio was consistent with the ratio of total nitrogen to total phosphorus. In the macrophyte-dominated region, the dissolved total nitrogen to total phosphorus ratio increased from less than 16:1 in the autumn to more than 16:1 in the summer, winter, and spring. The temporal and spatial variation in the ratio of particulate nitrogen to phosphorus was not significant (P>0.05), and this ratio was less than and greater than 16 in algae-and macrophyte-dominated regions, respectively.

Spatial and temporal distribution characteristics of different forms of inorganic nitrogen in three types of rivers around Lake Taihu, China

In order to control nitrogen (N) pollution of Lake Taihu, China, we studied the spatial and temporal distribution characteristics of inorganic N in inflowing rivers polluted by industry, agriculture, and domestic sewage during low, moderate, and high flow periods. The results showed that dissolved total nitrogen (DTN) was the main fraction of total nitrogen (TN) input from these rivers. Inflowing rivers had distinct impacts on TN, DTN, ammonium N (NH4+), and nitrate N (NO3-) concentrations of Lake Taihu during the low flow period. Particulate nitrogen (PN) had an impact on Lake Taihu during the three flow periods and all the three types of rivers would increase PN concentration in the lake. Rivers polluted by agriculture had the greatest impact on Lake Taihu's TN, DTN, NO3-, and dissolved inorganic N (DIN) concentrations, while rivers polluted by industry had the greatest impact on NH4+ concentration. Therefore, agriculture and industry should be key targets for nutrient reductions. The in-lake N concentrations were higher than those of inflowing rivers during moderate and high flow periods.

A comprehensive biogeochemical record and annual flux estimates for the Sabaki River (Kenya)

Abstract. Inland waters impart considerable influence on nutrient cycling and budget estimates across local, regional and global scales, whilst anthropogenic pressures, such as rising populations and the appropriation of land and water resources, are undoubtedly modulating the flux of carbon (C), nitrogen (N) and phosphorus (P) between terrestrial biomes to inland waters, and the subsequent flux of these nutrients to the marine and atmospheric domains. Here, we present a 2-year biogeochemical record (October 2011–December 2013) at biweekly sampling resolution for the lower Sabaki River, Kenya, and provide estimates for suspended sediment and nutrient export fluxes from the lower Sabaki River under pre-dam conditions, and in light of the approved construction of the Thwake Multipurpose Dam on its upper reaches (Athi River). Erratic seasonal variation was typical for most parameters, with generally poor correlation between discharge and material concentrations, and stable isotope values of C (δ13C) and N (δ15N). Although high total suspended matter (TSM) concentrations are reported here (up to ∼ 3.8 g L−1), peak concentrations of TSM rarely coincided with peak discharge. The contribution of particulate organic C (POC) to the TSM pool indicates a wide biannual variation in suspended sediment load from OC poor (0.3 %) to OC rich (14.9 %), with the highest %POC occurring when discharge is &lt; 100 m3 s−1 and at lower TSM concentrations. The consistent 15N enrichment of the particulate nitrogen (PN) pool compared to other river systems indicates anthropogenic N loading is a year-round driver of N export from the Sabaki Basin. The lower Sabaki River was consistently oversaturated in dissolved methane (CH4; from 499 to 135 111 %) and nitrous oxide (N2O; 100 to 463 %) relative to atmospheric concentrations. Wet season flows (October–December and March–May) carried &gt; 80 % of the total load for TSM (∼ 86 %), POC (∼ 89 %), dissolved organic carbon (DOC; ∼ 81 %), PN (∼ 89 %) and particulate phosphorus (TPP; ∼ 82 %), with &gt; 50 % of each fraction exported during the long wet season (March–May). Our estimated sediment yield (85 Mg km−2 yr−1) is relatively low on the global scale and is considerably less than the recently reported average sediment yield of ∼ 630 Mg km−2 yr−1 for African river basins. Regardless, sediment and OC yields were all at least equivalent or greater than reported yields for the neighbouring dammed Tana River. Rapid pulses of heavily 13C-enriched POC coincided with peak concentrations of PN, ammonium, CH4 and low dissolved oxygen saturation, suggesting that large mammalian herbivores (e.g. hippopotami) may mediate the delivery of C4 organic matter to the river during the dry season. Given recent projections for increasing dissolved nutrient export from African rivers, as well as the planned damming of the Athi River, these first estimates of material fluxes from the Sabaki River provide base-line data for future research initiatives assessing anthropogenic perturbation of the Sabaki Basin.