Major sources of particulate organic matter in the Godavari River, India: Role of synthetic fertilizers

doi:10.21203/rs.3.rs-3409240/v1

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Major sources of particulate organic matter in the Godavari River, India: Role of synthetic fertilizers

https://doi.org/10.21203/rs.3.rs-3409240/v1

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In this study, we examined major sources of particulate organic matter (POM) in Godavari River during high flow and low flow periods, to understand the impact of excess N-fertilizers used in agricultural fields. δ¹³C_POC and δ¹⁵N_PN, elemental C:N and POC:Chl-a ratios indicated that in-situ sources predominantly contributed (~ 60%) during low flow period, whereas, terrestrial sources largely contributed during high flow period (75–80%). This is attributed to prevailing conducive conditions for phytoplankton growth during former, and increased transport of particulate and dissolved materials from land during latter period. δ¹⁵N_PN during low flow (7.4 ± 2.9‰) and high flow (9.4 ± 2.1‰) periods demonstrate that contribution of POM produced from N-fertilizers (δ¹⁵N_PN: 0 ± 1‰) is not significant, rather than hitherto hypothesized. It could be due to seepage of excess N-fertilizers used in agricultural fields into groundwaters rather than transporting to rivers and/or transformation to another from through nitrification/denitrification processes within soils.

stable isotopes

organic carbon

nitrogen

particulate organic matter

freshwater algae

Godavari

Rivers are the major means of transport of dissolved and particulate material from land to sea via estuaries (Meybeck 1993; Hedges et al. 1997). Rivers carry significant amounts of particulate organic matter (~ 150 to 200 Tg yr^-1; Ludwig et al. 1996; Hedges et al. 1997; Schlunz and Schneider 2000) and suspended sediments (Beusen et al. 2005) along with dissolved inorganic nutrients (Seitzinger et al., 2006; Krishna et al. 2016), dissolved organic matter (Krishna et al. 2015; Yan et al. 2023) and various types of pollutants (Van Vliet et al. 2017; Strokal et al., 2021) to the global ocean. Riverine export of land-driven nutrients in large quantities, mainly due to anthropogenic activities, such as the use of excess synthetic fertilizers in agricultural and aquacultural practices, domestic and industrial sewage, often causes ‘eutrophication’, in estuaries and coastal waters (Nixon 1995; Howarth et al. 2011). Production of excess organic matter load supported by anthropogenic nutrients results in formation of eutrophication, and ultimately it leads to the development of ‘dead zones’ in coastal waters. Maure et al. (2021) reported that about 1.15 million km² area of global coastal waters (depth: <200m) are eutrophic potential.

India is the second largest consumer of synthetic fertilizers in the world, with an annual consumption of 26.5 Tg, after China (48.8 Tg), with a global share of 15.3% in N-fertilizer consumption (Jaga and Patel, 2012). Excess fertilizers used in agriculture and aquaculture activities are expected to reach rivers, estuaries and coastal oceans. Impact of these fertilizer nutrients on ecosystem in Indian rivers, estuaries and coastal waters remain poorly understood. Occurrence of oxygen minimum zone (OMZ) due to seasonal upwelling along west coast of India is reported to be intensified, and leads to eutrophication due to transport of fertilizer nutrients to coastal ocean (Naqvi et al. 2000). Bristow et al (2017) reported that OMZ in the Bay of Bengal is at its tipping point, and it will become dead zone if primary production in the overlying water column increases by transport of anthropogenic nutrients and/or intensification of summer monsoon. Expansion of OMZ (both horizontally and vertically) in the Arabian Sea was attributed to increased organic matter loading to deeper depths. This increase of organic matter could be due to either wind-driven coastal upwelling or supply of anthropogenic nutrients from coastal cities along west coast of India (Gomes et al. 2014). Although many studies attributed various biogeochemical processes in the Bay of Bengal and Arabian Sea to anthropogenic nutrients (agricultural, industrial and domestic sewage) from the Indian subcontinent, quantitative studies on anthropogenic nutrients budget and identification of nutrient sources are very limited. Krishna et al. (2016) reported that Indian monsoonal rivers annually export only 0.22 ± 0.05 Tg (1Tg = 10¹² g) of dissolved inorganic nitrogenous nutrients to the northern Indian Ocean, and such a low fluxes were attributed to efficient (~ 91%) elimination/retention of nutrients before reaching the coastal ocean.

In this study, we investigated major sources of particulate organic carbon (POC) and particulate nitrogen (PN) in the Godavari river from the place of its origin (Nasik) to the place at which it confluences with the Bay of Bengal (Yanam), covering a stretch of ~ 1600 km, during both low flow and high flow periods. The Godavari is the largest monsoonal river that exports highest amount of POC (2.81*10⁶ tons yr^-1), PN (0.29*10⁶ tons yr^-1), DOC (0.45*10⁶ tons yr^-1), DON (0.085*10⁶ tons yr^-1) and suspended load (~ 170x10⁶ tons) (Gupta et al. 1997; Gaillardet et al. 1999; Krishna et al. 2015a) among the monsoonal rivers in India. It accounts for ~ 1.7% of the total POC export by major rivers in the world (Ludwig et al. 1996).

Elemental ratios of carbon to nitrogen (C:N ratio), POC:Chl-a ratios and stable isotopic signatures of POC (δ¹³C_POC) and PN (δ¹⁵N_PN) are well established tracers to delineate the major sources of organic matter in rivers (Kendall et al. 2001; Balakrishna and Probst 2005; Wu et al. 2007; Bouillon et al. 2012; Tamooh et al. 2012), estuaries (Savoye et al. 2003), coastal (Gearing 1988) and open ocean regions (Sherin et al. 2018; Krishna et al. 2019). However, a caution is required while using these tracers for identification of POM sources because decomposition of POM by heterotrophic organisms modifies signatures of residual POM (Ittekkot and Laane 1991; Smith et al. 1992; Thornton and McManus 1994). The δ¹³C_POC values indicate source of inorganic carbon used for production of POC, and therefore have distinctly different δ¹³C_POC values for freshwater phytoplankton (-33.2‰ to -27.5‰; Kao and Liu 1999) and terrestrial POC (-28‰ to -26‰, Thornton and McManus 1994). Similarly, δ¹⁵N_PN depends on δ¹⁵N of nitrogenous nutrients used for production of PN. However, various internal processes such as nitrification, denitrification, N₂ fixation influence δ¹⁵N_PN values due to preferential uptake of lighter isotope (¹⁴N) over heavier isotope (¹⁵N) in these processes (Altabet et al. 1994; Brandes et al. 1998). Nutrients derived from diverse sources have distinctly different δ¹⁵N values. For instance, PN produced from sewage nutrients has relatively enriched δ¹⁵N (~ 14±4‰; Sarma et al. 2020) from that PN supported by synthetic N-fertilizers (0 ± 1‰; Cole et al. 2004; Dailer et al. 2010; Fernandes et al. 2012) and N₂ fixation (-2‰ to 1‰; Carpenter et al. 1997; Montoya et al. 2002). The δ¹⁵N_PN values are thus indicate nutrient sources used for primary production. The main objectives of our study are (i) to investigate major sources of POC and PN, (ii) seasonal variability in major sources of POC and PN, and (iii) to understand the impact of synthetic N-fertilizers on particulate organic matter (POM) in the Godavari River.

2.1 Study region

Godavari River is the 3rd largest river (after Ganges and Brahmaputra) in India that originates in the Western Ghats at Triambak near Nasik City at an altitude of ca. 1600 m. It flows eastward for about 1600 km across the Indian peninsula before it drains into the Bay of Bengal. The Godavari River discharges ~ 110 km³ yr^− 1 of fresh water and 170 million tons yr^− 1 of suspended sediment to the Bay of Bengal (Ludwig et al. 1996; Gaillardet et al. 1999). The Godavari River basin extends from 73^o26'E to 83^o07'E and from 16^o16'N to 22^o36' N (Fig. 1), with an area of ~ 3.1 x 10⁵ km² (Gupta et al. 1997). The river drains mainly Deccan Trap basalts and metamorphic rocks (Pre-Cambrian) in upper and lower reaches of the river, respectively. Due to strong seasonal variability in rainfall pattern over the catchment, with > 80% of the annual rainfall during SW monsoon, the high flow of water in the river confine only to SW monsoon season (Fig. 2). Lower basin of the river receives intense rainfall (1600 to > 3200 mm yr^− 1) than upper basin (< 800 to 1200 mm yr^− 1) within during SW monsoon (Giosan et al. 2017). Semi-arid and monsoonal climate prevails over the basin. Maximum and minimum temperatures are higher in lower basin than in upper basin (CWC 2017). Dominance of vegetation in the Godavari basin changes from grassland/cropland in upper basin to forest and shrub in lower basin of the river. Along the entire stretch, about 25 tributaries join the Godavari River and create an extensive delta on east coast of India (Rao 1975). A total of 921 dams of different sizes were constructed so far on the Godavari River and its tributaries mostly for irrigation purposes, and only a few for hydro-electric power generation (water resources information system of India, http://india-wris.nrsc.gov.in). Population density in the basin ranges from as low as 25–50 persons km^− 2 to 500–1000 persons km^− 2, with a mean value of 194 persons km^− 2 (CWC 2014).

2.2 Sample collection

Surface water samples were collected using 5L Niskin samplers (Tomczak 2000) at 39 locations along entire stretch (~ 1600 km) of the Godavari River from its headland waters to mouth (Fig. 1) during low flow (non-monsoon) period (Fig. 2; November-December 2011). However, samples were collected only at 18 locations along entire stretch of the river during high flow (southwest monsoon) period (Fig. 2) because many of the locations were not accessible for sample collection during this period. Surface water samples were collected for dissolved inorganic nutrients, dissolved oxygen (DO), chlorophyll-a (Chl-a), total suspended matter (TSM), particulate organic carbon (POC) and particulate nitrogen (PN). Surface water samples were collected in middle of the river-channel using mechanized boats to minimize contamination from river banks. Samples for freshwater phytoplankton were collected in the lower reaches of the river (Fig. 1). Different parts (leaf, part of stem and part of root) of the abundant C₃ plants (rice, cotton, turmeric and wheat) and C₄ plants (sugarcane, maize, bajra and sorghum) and surface soils (0-10cm) were collected in drainage basin of the river. Location details, δ¹³C and δ¹⁵N values of soils, C₃ and C₄ plants collected in drainage basin of the river were given elsewhere (Krishna et al. 2015b). Rainfall and temperature data in the Godavari basin was taken from Central Water Commission, Ministry of Earth Sciences, Government of India (CWC 2017). Study region was divided into two parts for discussion based on geological characteristics and spatial variability in rainfall intensity and environmental conditions of the river basin. Upper reaches of the river extend up to ~ 1100 km from origin of the river, and lower reaches extend from ~ 1100 km to mouth of the river.

About 500 ml of water sample was filtered through a non-combusted GF/F filter (nominal pore size: 0.7 µm; Whatman) under vacuum, and the material retained on filter was extracted into N,N, dimethyl formamide (DMF). Concentration of Chl-a in the extract was measured using spectrofluorophotometer (Carry Eclipse, Varian, USA) following Suzuki and Ishimaru (1990). Analytical precision for the method, expressed as standard deviation, was ± 4%. DO was determined by Winkler’s titration method of Carritt and Carpenter (1966) using potentiometric end point detection (Metrohm, Switzerland) at a temporary field laboratory on the same day of collection. Analytical precision, in terms of standard deviation, was ± 0.07%. Total suspended matter (TSM) was determined by the weight of material retained on filter (0.22 µm, polycarbonate) after filtering 300–500 ml of water sample followed by drying in an oven (50 ^oC, 48 h). Dissolved inorganic nutrients were analysed by the standard colorimetric method following Grashoff et al. (1992) using an auto analyzer (SAN⁺⁺, Skalar, The Netherlands). Based on repeated analysis of standards and samples, precision of the method was found to be ± 0.02, 0.02, 0.01 and 0.02 µM for ammonium, nitrite + nitrate, phosphate and silicate, respectively.

3.1 Content and isotopes of POC and PN

For POC analysis, water sample (~ 500 ml) was passed through a pre-combusted (300 ^oC; 6 h) GF/F filter (nominal pore size: 0.7 µM; diameter: 47 mm; Whatman) under vacuum at a field laboratory, and preserved in ice for further processing at shore laboratory. After drying at 60 ^oC (24 h) in an oven at institute’s laboratory, part of the filter was used directly for determination of concentration and isotopic composition of PN using an elemental analyzer (Flash EA, Thermo) coupled to isotope ratio mass spectrometer (IRMS, Delta V Plus, Thermo Electron, Germany) via Conflo IV. Part of the filters was acid fumigated to remove traces of inorganic carbon, and analyzed for concentrations and stable isotope ratios of POC using EA-IRMS instrument. The international atomic energy agency (IAEA) standards and laboratory standards such as glutamic acid, alanine, organic analytical standard (OAS) were used for calibration. Long term precision of the instrument was ± 0.2‰ for both C and N isotopes. Concentrations of POC and PON were reported in mg L^− 1 while their stable isotope ratios were expressed in per mil deviation from VPDB (Vienna Pee Dee Beleminte) for carbon, and atmospheric air for nitrogen as given below.

$${\delta }^{13}C \left(or {\delta }^{15}N\right)= \left[\frac{{\text{X}}_{\text{s}\text{a}\text{m}\text{p}}-{\text{X}}_{\text{s}\text{t}\text{d}}}{{\text{X}}_{\text{s}\text{t}\text{d}}}\right]\text{x} 1000\text{‰}$$

where, X_samp stands for ¹³C/¹²C and ¹⁵N/¹⁴N of a sample for carbon and nitrogen, respectively. The X_std represents ¹³C/¹²C and ¹⁵N/¹⁴N of standard, i. e., VPDB and atmospheric air, respectively. Measurement of δ¹³C, δ¹⁵N and C:N ratios of freshwater phytoplankton (group level) was described elsewhere (Krishna et al. 2015b). Software program “Grapher” (version 5) was used for graphical representation of data. Two tailed homoscedastic t-test was used to examine statistical significance of difference for various parameters between upper and lower reaches of the river.

3.2 Quantification of proportional contributions from major sources

Bayesian isotope mixing model SIAR (Stable Isotope Analysis in R) was used to estimate proportional contributions of OM from major sources to POM in the Godavari River. SIAR is an open source R package and available at Comprehensive R Archive Network (CRAN) site (http://cran.r-project.org). SIAR model uses Markov Chain Monte Carlo (MCMC) method for fitting algorithms (Parnell et al. 2008, 2010). Inclusion of the residual error term is an additional advantage of this model compared to the other Bayesian mixing models (Parnell et al. 2010). The model has already been used for quantification of proportional contributions of OM from major sources to POM in estuaries (Sarma et al. 2014; Krishna et al. 2015b) and coastal systems (Dubois et al. 2012; Krishna et al. 2018). Since 'SIAR' is a dual isotope mixing model that uses both δ¹³C and δ¹⁵N, it gives a better quantification of source apportionment of OM than single isotope based models. Selection of end member values is crucial for isotope mixing models because output of the model mainly depends on δ¹³C and δ¹⁵N values of end members.

Measured δ¹³C and δ¹⁵N values of microscopically separated freshwater phytoplankton groups collected from lower reaches of the river (-31.2 ± 0.6‰ and 6.2 ± 0.5‰, respectively) were used as freshwater phytoplankton end member values. Mean δ¹³C and δ¹⁵N values of abundant terrestrial C₃ plants (rice, cotton, turmeric and wheat) (-25.9 ± 1.2‰ and 5.1 ± 2.1‰, respectively) and C₄ plants (sugarcane, maize, bajra and sorghum) (-13.1 ± 1.2‰ and 4.4 ± 2.1‰, respectively), and surface soils (-19.1 ± 2.4 and 10.1 ± 2.7‰, respectively) in the Godavari basin (Krishna et al. 2015b) were used as end members for terrestrial C₃ and C₄ plants and soil derived OM, respectively. δ¹⁵N of sewage is typically enriched, and ranges between 12‰ to 25‰ (Connolly et al. 2013; Corbett et al. 2015). Mean δ¹³C and δ¹⁵N values of the sewage collected from Bombay (-31.5 ± 1.0‰ and 15.8 ± 1.6‰, respectively; Sarma et al. 2019), the major city near upper reaches of the river were used as sewage end member values. Similar δ¹⁵N values are also reported for sewage from Visakhapatnam City near the lower reaches of the river (~ 14 ± 4‰, Sarma et al. 2020)

4.1 Seasonal variability (low flow and high flow periods)

Dissolved inorganic nutrients such as ammonium, nitrite + nitrate and silicate concentrations were higher during high flow period (3.7 ± 5.1, 53.2 ± 20.9 and 49.7 ± 14.5 µM, respectively) compared to low flow period (0.4 ± 0.2, 15.5 ± 18.6, and 40.6 ± 11.7 µM, respectively) (Figs. 3a&b). However, phosphate concentrations were nearly equal (p = 0.72) during low flow (12.0 ± 7.6 µM) and high flow (12.3 ± 8.2 µM) periods (Figs. 3a&b; Table 1). Chl-a distribution was relatively less heterogeneous during low flow period (mean 11 ± 6 mg m^− 3) compared to high flow period (20 ± 19 mg m^− 3’ Table 1). TSM concentrations varied between 2.0 and 38.6 mg L^− 1 (except higher concentration of 72 mg L^− 1) during low flow period, whereas it varied widely between 1 and 186 mg L^− 1 during high flow period (Table 1). Percent of organic carbon in TSM (%OC) was higher (p = 0.04) during low flow period (mean 11.5 ± 8.4%, range: 0.9–35.7%) compared to high flow period (6.3 ± 6.3% and 0.5–20.3%, respectively) (Figs. 4a&b).

Table 1
Parameter	Low flow period		High flow period
Parameter	Range	Mean (± SD)	Range	Mean(± SD)
Ammonium (µM)	n.d – 5.3	0.4 ± 0.2	0.7–22.1	3.7 ± 5.1
Nitrate + Nitrite (µM)	1.9–71.6	15.5 ± 18.6	12.5–83.6	53.2 ± 20.9
Phosphate (µM)	2.9–33.0	12.0 ± 7.6	5.2–42.8	12.3 ± 8.2
Silicate (µM)	5.8–88.9	40.6 ± 11.7	24.1–74.4	49.7 ± 14.5
Chlorophyll-a (mg m^− 3)	1.7–24.3	10.4 ± 6.3	1.4–46.9	20.6 ± 19.1
TSM (mg l^− 1)	2.0–72.0	12.4 ± 14.2	1-186	59 ± 65
%OC in TSM	0.9–35.7	11.5 ± 8.4	0.5–20.3	6.3 ± 6.3
POC (mg l^− 1)	0.4–2.1	0.8 ± 0.3	0.2–1.8	0.9 ± 0.5
PON (mg l^− 1)	0.05–0.35	0.1 ± 0.05	0.01–0.11	0.06 ± 0.03
POC: Chl-a ratio	32–275	106 ± 65	13–677	155 ± 225
Elemental C:N ratio	3–22	9 ± 4	10–29	18 ± 4
δ¹³C_POC (‰)	-32.8 to -20.7	-28.9 ± 2.6	-30.9 to -22.8	-25.8 ± 2.6
δ¹⁵N_PN (‰)	1.2–15.0	7.4 ± 2.9	5.7–13.1	9.4 ± 2.1

POC concentrations ranged from 0.4–2.1 mg L^− 1 and 0.2–1.8 mg L^− 1 during low flow and high flow periods, respectively (Table 1). Concentrations of POC found in this study are similar with those reported earlier in the Godavari River (Gupta et al. 1997; Balakrishna and Probst 2005). Relatively higher range of PN concentrations were found during low flow period (0.05–0.35 mg L^− 1) compared to high flow period (0.01–0.11 mg L^− 1). Elemental C:N ratios ranged from 3 to 22 and 10 to 29 during low flow and high flow periods, respectively (Table 1), with lower mean ratios during former than latter period (Figs. 5a&b) (t-test; p < 0.0001). POC:Chl-a ratio, an indicator of contribution from phytoplankton biomass to total POC, varied from 32 to 275 during low flow period, whereas it varied widely between 13 and 677 during high flow period (Table 1). The δ¹³C_POC varied from − 32.8‰ to -20.7‰ and from − 30.9‰ to -22.8‰ during low flow and high flow periods, respectively (Table 1). Relatively depleted mean δ¹³C_POC values were observed during low flow (-28.9 ± 2.6‰) compared to high flow period (-25.8 ± 2.6‰) (t-test; p < 0.001). The δ¹³C_POC values observed in this study during low flow (mean − 28.9 ± 2.6‰) and high flow (-25.8 ± 2.6‰) periods are similar to those reported earlier in the Godavari River (Pradhan et al., 2014) and elsewhere in the world, for example, Danube (-32.3 to -24.76‰; Hein et al., 2003), Congo (-27.8 to -26.3‰; Mariotti et al., 1991), Mekong (-29‰; Ellis et al., 2012) and Orinoco (-29.5 to -24.4‰; Tan and Edmond, 1993). δ¹⁵N_PN values broadly varied from 4.8‰ to 15.0‰ and from 5.7‰ to 13.1‰ during low flow and high flow periods, respectively, except three stations near origin of the river where depleted δ¹⁵N_PN (1.2–2.9‰) were observed during low flow period

4.2 Spatial variability (upper and lower reaches of the river)

Both nitrite + nitrate and phosphate concentrations were higher in upper than lower reaches of the river during low flow period (Figs. 3a&b), however, this variation is not statistically significant (p = 0.22 and p = 0.18, respectively). During both low flow and high flow periods, upper reaches of the river recorded relatively high mean Chl-a (27.5 and 12.2 mg m^− 3, respectively) compared to lower reaches of the river (6.8 and 6.3 mg m^− 3, respectively) (t-test, p < 0.05 and p < 0.05, respectively) (Fig. 4a&b). Higher mean TSM concentrations were found in upper (14.4 ± 15.5 mg L^− 1) than lower reaches of the river (5.1 ± 2.1 mg L^− 1) during low flow period, but the difference is not statistically significant. Contrastingly, lower TSM concentrations were observed in upper (21 ± 20 mg L^− 1) than lower reaches of the river (133 ± 53 mg L^− 1) (t-test; p < 0.001) during high flow period (Fig. 4a&b). Percent organic carbon in TSM (%OC) was low in upper (10 ± 8%) compared to lower reaches (17 ± 5%) of the river during low flow period (p = 0.05), whereas an opposite pattern was found during high flow period, with higher values in upper (9.6 ± 5.9%) compared to lower reaches of the river (0.9 ± 0.3%) (p = 0.002) (Figs. 4a&b). Mean POC concentrations were similar in upper and lower reaches of the river during low flow period (Fig. 4a). Whereas, lower reaches of the river recorded high POC concentrations compared to upper reaches during high flow period (Fig. 4b), however, it is not statistically significant (t-test; p = 0.12). Upper and lower reaches of the river recorded similar PN concentrations during both low and high flow periods (Figs. 4a&b).

Mean C:N ratios were similar in upper and lower reaches of the river during both low flow and high flow periods (p = 0.54 and p = 0.82, respectively) (Fig. 5a&b). Mean POC:Chl-a ratios were marginally lower in upper (98 ± 67) compared to lower reaches of the river (146 ± 36) during low flow period (Fig. 5a), however, it is significantly lower during high flow period (53 ± 64 and 318 ± 300, respectively) as shown in Fig. 5b (t-test, p < 0.05). Mean δ¹³C_POC values were similar in upper (-28.6±‰) and lower reaches (-29.9±‰) of the river during low flow period (Fig. 5a). Whereas, depleted δ¹³C_POC values were found in upper (-26.8±‰) compared to lower reaches of the river (-23.8±‰) during high flow period (Fig. 5b). δ¹⁵N_PN showed relatively less spatial variability during low period, however, relatively enriched δ¹⁵N_PN were found in lower reaches (10.5 ± 2.0‰) than upper reaches (8.7 ± 1.8‰) of the river during high flow period (Fig. 5b).

5.1 Seasonal variations in the sources of POC and PN

Relatively higher dissolved inorganic nutrient concentrations during high flow period are primarily due to transport of nutrients from land to river by the surface runoff during southwest (SW) monsoon. Drainage basin of the Godavari River receives > 80% of its annual rainfall only during four months period of SW monsoon (June-September; Finch 1994, Gupta et al. 1997). Intense rainfall during this period leaches nutrients, POC, PN and other dissolved materials from soils, leaf litter, debris of land plants and agricultural crops, excess fertilizers used in agricultural fields and sewage etc., and transports to rivers. Intense rainfall also enhances soil erosion in the drainage basin, leading to enhanced transport of particulate material to rivers (Woodward and Foster 1997). Relatively higher TSM concentrations during high flow period (59 ± 65 mg L^− 1) compared to low flow period (12.4 ± 14.2 mg L^− 1) in this study indicate that significant input of terrestrial material to rivers during former period. However, such a difference was not noticed in POC concentrations which showed similar concentrations during high flow (0.8 ± 0.3 mg L^− 1) and low flow (0.9 ± 0.5 mg L^− 1) periods. This is attributed to significant amount of POC contribution from in-situ sources during low flow period because of prevailing conducive conditions for phytoplankton growth such as better light availability (low TSM) and stable water column (low flow) during this period. Relatively high PN concentration during low flow (mean: 0.11 mg L^− 1) than high flow period (0.06 mg L^− 1) also suggest that major contribution of POM is from N-rich in-situ sources during former period, and N-poor terrestrial sources during latter period.

Mean C:N ratios found during low flow period (9 ± 4) are close to the range of C:N ratios of POM produced by phytoplankton (6–10, Montagnes et al., 1994; Savoye et al., 2003) (6.7, Redfield, 1963) (6–8, Onstad et al., 2000; Jennerjahn et al., 2004), indicating that predominant contribution of POM is possible from freshwater phytoplankton. On the other hand, C:N ratios during high flow period (18 ± 4) are close to typical C:N ratios of terrestrial POM (> 12, Thornton and McManus 1994; Hedges et al. 1997; Lamb et al. 2006), and measured C:N ratios of soils in the Godavari basin (19 ± 4), but lower than measured C:N ratios of dominant C₃ plants (41 ± 32) and C₄ plants (53 ± 41) in drainage basin of the Godavari river. These results indicate that soil OM may be one of the major sources of POM in the river during high flow period. Although soils contain debris of C₃ and C₄ plant material, the lower C:N ratios of soils than C₃ and C₄ plants are due to bacterial colonization (Thornton and McManus, 1994), and humification and mineralization processes (Schmidt et al., 2000) during decomposition of terrestrial POM in soils that result in C:N ratios of soils in the range of 8–20 (Meyers, 1994). The POC:Chl-a ratios of < 200 and > 200 were used to characterize POC of phytoplankton and terrestrial origin, respectively (Cifuentes et al., 1988; Bentaleb et al., 1998). Broad range of POC:Chl-a ratios during low flow period (32–275, mean 106 ± 65) and high flow period (13–677, 155 ± 225) indicate that both freshwater phytoplankton and terrestrial sources contribute to POM, with a predominance of the former during low flow period and the latter during high flow period.

The range of δ¹³C_POC observed during low flow period (-32.8 to -20.7‰) is close to δ¹³C of phytoplankton in the Godavari River (-31.2 ± 0.6‰), typical range of freshwater phytoplankton (-33.2 to -27.5‰; Kao and Liu 1999), sewage (-31.5 ± 1.0‰; Sarma et al. 2019) and terrestrial POM (-28 to -26‰; Thornton and McManus 1994; Middelburg and Nieuwenhuize 1998), and measured δ¹³C of C₃ plants (-27.4 to -24‰) and soils (-23.5 to -15.3‰) in the Godavari basin (Fig. 6). These results suggest that both in-situ and terrestrial sources, including sewage, contribute to POM during low flow period, with a predominance of the former than the latter. On the other hand, δ¹³C_POC values during high flow period (-30.0 to -22.8‰) are close to δ¹³C of typical terrestrial POM and measured terrestrial C₃ plants of the Godavari basin, and slightly different from that of δ¹³C of freshwater phytoplankton and sewage (Fig. 6), suggesting that POM is contributed by both terrestrial and in-situ sources, with a predominance of the former than the latter.

Broad range of δ¹⁵N_PN during low flow period (4.8 to 15.0‰; mean 7.8 ± 2.6‰) indicates that predominant contribution of POM is from phytoplankton (5 to 8‰), with minor contribution from terrestrial sources, such as C₃ plants (5.1 ± 2.1‰), C₄ plants (4.4 ± 2.1‰) and soils (10.1 ± 2.7‰) (Fig. 6), corroborating with δ¹³C_POC results. However, mean δ¹⁵N_PN values during high flow period (9.4 ± 2.1‰; range: 5.7–13.1‰,) are very close to δ¹⁵N of soils (10.1 ± 2.7‰) in the drainage basin, and significantly enriched than δ¹⁵N of C₃ plants (5.2 ± 2.1‰) and C₄ plants (4.4 ± 2.1‰) plants. These results indicate that terrestrial sources, mainly soil derived POM, contribute to POM with minor contribution from in-situ sources during high flow period. Contrary to this, Chl-a concentrations were lower during low flow (11 ± 6 mg m^− 3) than high flow period (21 ± 19 mg m^− 3). It could be due to significant spatial variability, and variable contributions from both terrestrial and in-situ sources during low flow and high flow periods. No significant relationship of POC with either TSM or Chl-a confirm that both terrestrial and in-situ sources are contributing to POM (Meybeck 1993; Tamooh et al. 2012).

Elemental C:N and POC:Chl-a ratios, and δ¹³C_POC and δ¹⁵N_PN values clearly indicated that in-situ phytoplankton and terrestrial sources contribute to POM in the Godavari River, with a predominance of the former during low flow period and the latter during high flow period. SIAR model results showed that freshwater phytoplankton contributes up to 60% during low period while contribution from terrestrial sources is predominant (~ 75–80%) during high flow period (Fig. 7). Prevailing conducive conditions, such as nutrient availability, stable water column (low flow) and better light availability (low TSM) to phytoplankton are attributed for major contribution from phytoplankton during low flow period. Consistent with our observation, predominance of phytoplankton derived POM during low flow periods was also reported in the Godavari River (Gupta et al., 1997; Balakrishna and Probst, 2005) and some of the major rivers in the world, for example, Mississippi, Colorado, Rio Grande and Columbia (Kendall et al., 2001), Oubangui (Bouillon et al., 2012), Umpqua (Goni et al., 2013), Alsea (Hatten et al., 2012), Danube (Besemer et al., 2009), Garonne, Loire and Rhone (Kempe et al., 1991). On the other hand, predominant (~ 75–80%) contribution of POM from terrestrial sources, mainly sewage (~ 25–30%), soils (~ 20%), C₃ plants (~ 20%) and C₄ plants (~ 10%), during high flow period is attributed to export of POM from soils, debris of C₃ and C₄ plants and sewage in drainage basin to the river by intense runoff during this period (CPCB, 1995). Also, less contribution from phytoplankton (20–25%) due to high TSM (67 ± 66 mg L^− 1) that limits light availability to phytoplankton.

5.2 Spatial variations in the sources of POC and PN

Both POC and PN concentrations were similar in upper and lower reaches of the river during low flow period (Fig. 5a). However, significant spatial variability was observed during high flow period, with higher POC concentrations in lower reaches (1.2 mg L^− 1) than upper reaches (0.8 mg L^− 1) of the river. This pattern is consistent with TSM (133 ± 53 mg L^− 1 and 21 ± 20 mg L^− 1, respectively), but contrasting to that of Chl-a distribution (6.8 ± mg m^− 3 and 27.5 ± mg m^− 3, respectively). These results suggest that terrestrial sources predominantly contribute to POM pool in lower reaches of the river during high flow period. Intense rainfall (1600 to > 3200 mm yr^− 1; Giosan et al., 2017), dominance of sandy clay loam soils (FAO, 2003) and higher soil OC in northern part of the basin which drains into lower reaches of the river through the major tributaries, Wardha, Pranahita and Indtravati (Fig. 8) are attributed for the observed high POC and TSM concentrations in lower reaches of the river. Sandy clay loam soils are more susceptible to erosion by intense rainfall during SW monsoon due to higher soil detachment capacity (Li et al., 2019) and restrict infiltration of water (Hengade and Eldho, 2019). In addition, high temperatures (CWC, 2014), anthropogenic activities such as deforestation and increased agricultural activities in lower basin (Silveira, 1993) also support erosion of soils. Balakrishna and Probst (2005) reported high concentrations of TSM in tributaries of the Godavari River in lower basin, Wardha (129 mg L^− 1), Pranahita (113 mg L^− 1) and Indravati (79 mg L^− 1) than tributaries in upper basin of the river during high flow period. Although, upper basin is dominated by black soils and grasslands, lower TSM concentrations in upper reaches of the river could be due to less intensity of rainfall (< 800 to1600 mm yr^− 1) than lower basin of the river (1600 to > 3200 mm yr^− 1) (Giosan et al., 2017). On the other hand, high TSM concentrations (133 ± 53 mg L^− 1) limit light availability to phytoplankton leading to low primary production in lower reaches (Chl-a: 6.8 mg m^− 3) than upper reaches of the river (Chl-a: 27.5 mg m^− 3), where TSM concentrations were low (21 ± 20 mg L^− 1). These results demonstrate that in-situ sources predominantly contributes to POM pool in upper reaches while terrestrial sources in lower reaches of the river during high flow period. It is also evidenced from significantly high %OC in TSM in upper (9.6 ± 5.9%) than lower reaches of the river (0.9 ± 0.3%). However, based on δ¹³C (-24.5 ± 0.3‰) and low yield of lignin phenols (Λ8), Pradhan et al. (2014) concluded that algal POM is predominant in lower reaches of the river during high flow period. Even though, lignin phenols (Λ8) are specific markers for terrestrial sources of POM, they cannot differentiate contribution from lignin free algal POM and lignin degraded soil OM. Hence, lower yield of lignin phenols (Λ8) in lower reaches of the Godavari River during high flow period (Pradhan et al., 2014) could be due to significant contribution from soil OM. SIAR model estimated that contribution from soil OM is ~ 30% and it is higher than freshwater phytoplankton contribution (~ 10%). Spatial variability in sources of POM in the Godavari River is therefore mainly controlled by spatial variations in the intensity of rainfall, basin geology and anthropogenic activities.

5.3 Impact of excess fertilizers use in agricultural activities

The δ¹⁵N_PN signatures have not shown any evidence for significant contribution of POM produced from synthetic N-fertilizers in the Godavari River. The δ¹⁵N of synthetic fertilizer nitrogen was reported to be depleted (~ 0 ± 1‰; Cole et al. 2004; Bateman and Kelly 2007; Fernandes et al. 2012) because they are manufactured from atmospheric N₂. The δ¹⁵N of N-fertilizers such as KNO₃, NH₄NO₃, Urea, (NH₄)₂SO₄ and NPK (20:10:10) etc., manufactured by different companies were found to be in the range of -5.9 to 2.6‰, with a mean δ¹⁵N of -0.2‰ (Bateman and Kelly 2007). Even though, India consumes 26.5 Tg yr^− 1 of synthetic fertilizers, and it accounts for 15.3% of the global N-fertilizer consumption (Jaga and Patel 2012), δ¹⁵N_PN values of this study (< 4.8‰) are enriched compared to typical δ¹⁵N of POM derived from fertilizer nitrogen (0 ± 1‰). These results demonstrate that impact of synthetic N-fertilizer usage in agricultural fields in drainage basin may be minor on ecosystem in the Godavari River. It could be due to retention and/or elimination of fertilizer nutrients due to their low utilization efficiency; only ~ 30 to 35% of the fertiliser nutrients applied is taken up by plant (Xiaoyu et al. 2013; Versino et al. 2019). For instance, most commonly used nitrogen fertilizer, Urea, have the utilization efficiency of ~ 50% and the remaining 50% lost in different pathways; 2–20% through volatization, 15–25% reacting with organic compound in soils and 2–10% leaching into water (Savci, 2012). Central Ground Water Board of India (CGWB, 2014) reported that concentrations of nitrate in ground waters of many Indian states are higher than 45 mg L^− 1, and attributed it to seepage of excess N-fertilizer used in agricultural activities into ground waters (Foster 2000; Kendall et al. 2007). Rahman et al. (2021) reported elevated NO₃⁻ concentrations in ground waters of Rajasthan state, India and attributed to anthropogenic sources. Based on land use map and NO₃⁻ concentrations in ground waters (1 to 415 mg L^− 1) of Tamilnadu state, India, Jayarajan and Kuriachan (2021) confirmed that use of excess fertilizers and sewage are the major sources of elevated NO₃⁻ concentrations in ground waters. Several other studies also reported elevated NO₃⁻ concentrations in ground waters of different river basins and attributed to N-fertilizer usage (Shukla and Saxena, 2020; Sarkar et al., 2021). Based on multiple stable and radioactive isotopes tracers, recently, Harris et al. (2022) demonstrated that ground water NO₃⁻ is originated from fertilizers in the western and eastern banks of the Nogoa River, Queensland, Australia. Although ground water δ¹⁵N_NO3− data is not available from the Godavari drainage basin, δ¹⁵N_PN values found in study demonstrate that impact of N-fertilizer on POM in the Godavari River is not significant, rather than hitherto hypothesized.

Major sources of POM, and their spatial and seasonal variations were investigated in the Godavari River using isotopic signatures of carbon and nitrogen in suspended particulate matter. Elemental C:N and POC:Chl-a ratios, and δ¹³C_POC and δ¹⁵N_PN of suspended particulate matter clearly indicated that POM is predominantly contributed by water column phytoplankton (~ 60%) and terrestrial sources (75–80%) during low flow and high flow periods, respectively. Predominant contribution from phytoplankton is attributed to prevailing conducive conditions for phytoplankton growth during low flow period. On the other hand, major contribution from terrestrial sources is attributed to export of soils, debris of land plants, agricultural crops and sewage from drainage basin by the intense runoff during high flow period. Spatial variations in rainfall intensity, soil characteristics, land use change and agricultural activities in drainage basin of the river govern spatial variability in major POM sources. Relatively enriched δ¹⁵N_PN ratios suggest that impact of N-fertilizers usage in agricultural fields of drainage basin is not significant on POM in the Godavari River. It is attributed to seepage of excess N-fertilizers into ground waters rather than their transportation to rivers.

Ethical approval, consent to participate: Not applicable

Consent for publication: Not applicable

Competing interests: The authors declare no competing interests

Availability of data: The data used in this study is available with the corresponding author (M.S. Krishna) and will be provided on reasonable request by e-mail ([email protected])

Funding: The work is part of the Council of Scientific and Industrial Research (CSIR), Government of India, funded research project (OLP 2011). It is already mentioned in the acknowledgement section

Author’s contributions: MS Krishna and NPC Reddy conceptualized the work and planned for field campaign. SA Naidu, MHK Prasad and Ch.V. Subbaiah were participated in the field campaign for sample collection and analysis of samples. Original draft of the manuscript was prepared by MS Krishna. and N.Srinivasa Rao provided land use change and soil OC maps of Godavari basin. All authors read and approved the final manuscript.

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Major sources of particulate organic matter in the Godavari River, India: Role of synthetic fertilizers

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Abstract

Figures

1. Introduction

2. Study region and sample collection

2.1 Study region

2.2 Sample collection

3. Methodology

3.1 Content and isotopes of POC and PN

3.2 Quantification of proportional contributions from major sources

4. Results

4.1 Seasonal variability (low flow and high flow periods)

4.2 Spatial variability (upper and lower reaches of the river)

5. Discussion

5.1 Seasonal variations in the sources of POC and PN

5.2 Spatial variations in the sources of POC and PN

5.3 Impact of excess fertilizers use in agricultural activities

6. Conclusions

Declarations

References

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