Silicon concentrations and stoichiometry in two agricultural watersheds: implications for management and downstream water quality

Agriculture alters the biogeochemical cycling of nutrients such as nitrogen (N), phosphorus (P), and silicon (Si) which contributes to the stoichiometric imbalance among these nutrients in aquatic systems. Limitation of Si relative to N and P can facilitate the growth of non-siliceous, potentially harmful, algal taxa which has severe environmental and economic impacts. Planting winter cover crops can retain N and P on the landscape, yet their effect on Si concentrations and stoichiometry is unknown. We analyzed three years of biweekly concentrations and loads of dissolved N, P, and Si from subsurface tile drains and stream water in two agricultural watersheds in northern Indiana. Intra-annual patterns in Si concentrations and stoichiometry showed that cover crop vegetation growth did not reduce in-stream Si concentrations as expected, although, compared to fallow conditions, winter cover crops increased Si:N ratios to conditions more favorable for diatom growth. To assess the risk of non-siliceous algal growth, we calculated a stoichiometric index to quantify biomass growth facilitated by excess N and P relative to Si. Index values showed a divergence between predicted algal growth and what we observed in the streams, indicating other factors influence algal community composition. The stoichiometric imbalance was more pronounced at high flows, suggesting increased risk of harmful blooms as climate change increases the frequency and intensity of precipitation in the midwestern U.S. Our data include some of the first published measurements of Si within small agricultural watersheds and provide the groundwork for understanding the role of agriculture on Si export and stoichiometry.

We analyzed three years of biweekly concentrations and loads of dissolved N, P, and Si from subsurface tile drains and stream water in two agricultural watersheds in northern Indiana. Intra-annual patterns in Si concentrations and stoichiometry showed that cover crop vegetation growth did not reduce in-stream Si concentrations as expected, although, compared to fallow conditions, winter cover crops increased Si:N ratios to conditions more favorable for diatom growth. To assess the risk of non-siliceous algal growth, we calculated a stoichiometric index to quantify biomass growth facilitated by excess N and P relative to Si. Index values showed a divergence between predicted algal growth and what we observed in the streams, indicating other factors influence algal community composition. The stoichiometric imbalance was more pronounced at high flows, suggesting increased risk of harmful blooms as climate change increases the frequency and intensity of precipitation in the midwestern U.S. Our data include some of the first published measurements of Si within small agricultural watersheds and provide the groundwork for understanding the role of agriculture on Si export and stoichiometry.

Introduction
Silicon (Si) is an essential nutrient in aquatic ecosystems and plays a critical role in the growth of freshwater diatoms. Diatoms are the predominant form of benthic algae in most streams (Lowe and LaLiberte 2007) and are an important food source for zooplankton and aquatic insects (Kilham 1971;Martin-Jezequel et al. 2000). Furthermore, diatoms are highly productive autotrophs, producing almost half of the total oxygen in our atmosphere and significantly affecting the global carbon cycle (Street-Perrott and Barker 2008;Smol and Stoermer 2010;Rousseaux and Gregg 2013). Unlike most other forms of algae, diatoms require dissolved silicon dioxide (SiO 2 , hereafter DSi) to build frustules (silicified cell walls). Diatoms often bloom until DSi becomes depleted relative to nitrogen (N) or phosphorus (P), at which time diatoms become Si-limited and non-siliceous taxa may become dominant (Officer and Ryther 1980;Teubner and Dokulil 2002;Frings et al. 2016). Thus, stoichiometric ratios between Si, N, and P that are imbalanced relative to the requirements of diatoms can favor the growth of non-siliceous and potentially harmful taxa, such as cyanobacteria (Schelske and Stoermer 1971;Conley et al. 1993;Turner et al. 2003).
Anthropogenically-driven modifications to the landscape can disturb the transport and fate of DSi in freshwaters. In particular, long-term agricultural land use, as is found across the midwestern United States (U.S.), can modify the biogeochemical cycling of Si through alteration of vegetative cover and hydrologic flow paths (Struyf et al. 2010;Carey and Fulweiler 2016). Most terrestrial plants will incorporate Si into biomass, creating phytoliths that aid in plant defense and structure (Epstein 2009). Crops such as corn and soybeans, which are the predominant crops in the midwestern U.S., have high Si concentrations, often above 1% of their dry weight (Epstein 1994;Guntzer et al. 2012). DSi weathered from soil minerals is incorporated into crop phytoliths, and then subject to loss from the system if grain or vegetative material is harvested and exported outside the watershed (Viaroli et al. 2013;Carey and Fulweiler 2016). Therefore, the long-term cultivation and harvest of crops can reduce Si availability in agricultural soils and the input of DSi to surface waters on century-long time scales (Struyf et al. 2010).
Hydrologic flow paths in agricultural systems often are modified to facilitate removal of water from fields and support soil conditions suitable for row-crop agriculture. In the midwestern U.S., this is accomplished through the use of subsurface tile drains, construction of drainage ditches, and the channelization of streams. Tile drains expedite the flow of water from the landscape to drainage ditches, essentially "short-circuiting" the flow of water through soil (Gentry et al. 2007;Vidon and Cuadra 2011;Williams et al. 2015). These modified hydrologic flow paths reduce rates of in-stream N and P processing and removal (Royer et al. 2006;King et al. 2015). Hydrologic modification also limits interactions between water and soil King et al. 2014), which could reduce silicate weathering and the flux of DSi from tile drains to surface waters; however, we are not aware of any data characterizing DSi export from tile drains. The channelization of agricultural streams alters hydrologic residence times and reduces groundwater-surface water interactions. These highly modified agricultural streams tend to lack the geomorphic features that promote hyporheic exchange, including meanders and sediment heterogeneity (Wörman et al. 2002;Boano et al. 2014). Thus, groundwater inputs, which can have DSi concentrations 2-3 × that of surface waters (Georg et al. 2009), are generally limited, potentially further altering DSi loads and downstream export of DSi.
Ultimately, row-crop agriculture reduces Si export while increasing export of N and P, thereby changing the stoichiometry of nutrient loads exported from agricultural watersheds (Turner et al. 2003;Leong et al. 2014;Downing et al. 2016). These changes in Si, N, and P stoichiometry can contribute to the potential DSi limitation of diatoms and the increasing frequency and severity of downstream non-siliceous and often harmful blooms of algae and coastal hypoxic zones (Dupas et al. 2015;Royer 2020). The factors affecting the magnitude and temporal patterns in DSi export from small agricultural watersheds are not well described, particularly for intensively farmed regions of the midwestern U.S. (but see Viaroli et al. 2013 andPinardi et al. 2018). The temporal patterns in stoichiometric imbalance in Si, N, and P are likewise poorly described, and this represents a knowledge gap in our understanding of how land management affects water quality in agricultural landscapes.
Vol.: (0123456789) Agricultural conservation practices, such as winter cover crops, can reduce excess N and P export by assimilating nutrients and reducing erosion (Strock et al. 2004;Kaspar et al. 2007;Blanco-Canqui et al. 2015;Speir et al. 2021b). Most cover crops are not harvested, and their biomass (after termination) is left on the fields, increasing soil organic matter and returning assimilated nutrients, including Si, to the soil. Studies have analyzed the effects of winter cover crops on total and dissolved N and P export at the tile drain (Kaspar et al. 2012;Trentman et al. 2020;Speir et al. 2021b) and watershed scale (Daryanto et al. 2018;Hanrahan et al. 2018;Hallama et al. 2019), but the effect of cover crops on DSi is unknown. To address this knowledge gap, we monitored dissolved inorganic N (nitrate and ammonium; hereafter DIN), soluble reactive P (SRP), and DSi loss at the field and watershed-scale from two agricultural watersheds in northern Indiana twice monthly for water years 2018-2020. The main objectives of this study were to: (1) characterize the seasonal pattern in DSi concentrations and stoichiometry at the watershed and field scale, (2) quantify the field-scale effects of winter cover crops on DSi loss and nutrient stoichiometry, and (3) document temporal patterns in stoichiometric imbalances between N, P, and Si loads and the potential for the nutrient loads to facilitate downstream non-siliceous algal blooms, including groups capable of toxin production (e.g., Cyanophycae, Dinophycae).
First, we hypothesized that seasonal patterns in DSi concentrations would relate to the growth of vegetation, including the cash crop (corn and soybeans) and the winter cover crop, as well as precipitation. Vegetative cover is a strong control on Si storage and export (Vandevenne et al. 2012;Carey and Fulweiler 2012b); therefore, we expected intra-annual variation in DSi concentrations at both the field and watershed scales. Specifically, we predicted (1a) lower tile water DSi concentrations from fields planted with cover crops, particularly during periods when cover crops were actively growing; and (1b) lower stream water DSi concentrations during the summer due to growth of the cash crop.
Second, we hypothesized cover crops would affect nutrient ratios in tile drains because cover crops affect N and P loss at the field scale. Specifically, cover crops at this study location reduce N loss (Hanrahan et al. 2018(Hanrahan et al. , 2021Speir et al. 2021b); however, the effect of cover crops on P loss to surface water is variable and depends, in part, on precipitation patterns (Trentman et al. 2020). Therefore, we predicted: (2a) tile water from cover cropped fields would have a higher Si:N molar ratio and that (2b) there would be no strong directionality in the response of Si:P molar ratios to winter cover crops.
Finally, we expected stoichiometric imbalance among N, P, and Si in these agricultural watersheds, and predicted this would: (3a) increase the potential for Si limitation of freshwater diatoms and the formation of cyanobacterial blooms in downstream waters; and (3b) the imbalance favoring cyanobacteria would be greatest in the summer and fall, when DSi concentrations are expected to be reduced relative to N and P.

Site description
The two study watersheds, Kirkpatrick Ditch Watershed (KDW; 26.3 km 2 ) and Shatto Ditch Watershed (SDW; 13.3 km 2 ), both located in northern Indiana ( Fig. 1), are predominantly planted with a corn (Zea mays L.) and soybean (Glycine max L.) rotation with approximately equal amounts of each crop each year. In KDW, soils are primarily Mollisols with a siltyclay texture while the SDW has soils that are primarily Alfisols with a texture that ranges from sandy loams to loams and muck (Christopher et al. 2021). The fields in each watershed are "working lands", managed by independent agricultural producers and are representative of typical agricultural practices in the midwestern U.S., including extensive subsurface tile drainage systems (Gökkaya et al. 2017;Trentman et al. 2020; Table 1) and N fertilization of corn at a rate of about 150 kg ha −1 yr −1 , most of which occurs in the early spring. Additional fertilization with inorganic P is common in KDW, and fall manure application is occasional in both watersheds (Speir et al. 2021b).
In both watersheds, farmers were reimbursed for costs associated with use of winter cover crops through the U.S. Department of Agriculture Regional Conservation Partnership Program. Farmer participation was voluntary and farmers made all decisions related to selection of cover crop species, timing of planting and termination, and all other management options. In KDW, cover crop planting began in 2015 and was maintained at 12-32% of the total tillable acres during our study period. In SDW, cover crop planting began in 2013 and coverage ranged from 22 to 68% during our study (Table 1). The most common cover crop species planted by producers in both KDW and SDW were annual rye-grass (Lolium multiflorum Lam.) and cereal rye (Secale cereale L.).

Sampling design and protocol
We monitored sixteen tile drains and the watershed outlet in KDW, and only the watershed outlet for SDW, and we sampled twice monthly throughout the 2018-2020 water years (Oct 1-Sept 30). For both tile drains and stream sites, we measured instantaneous discharge and sampled for DIN (nitrate and ammonium), SRP, and DSi. Additional results for DIN and SRP are reported in companion studies by Trentman et al. (2020Trentman et al. ( , 2021 and Speir et al. (2021a, b).
Instantaneous discharge was measured at tile drains using a timer and graduated receptacle; to ensure accurate volume and time values, discharge was measured multiple times until three consecutive measurements were all within 10%. For larger tile drains (diameter > 0.5 m) or when any tile flow was > 10 L s −1 , we used an electromagnetic water velocity meter (March-McBirney Model 2000 Flo-Mate) and a wading rod to calculate discharge (Q) as:  where = cos −1 1 − d r , r is the tile drain radius (m), d is the depth of water flow in the tile drain (m), and v is water velocity (m s −1 ). Stream discharge was monitored by U.S. Geological Survey gages at the KDW (station #05524546) and SDW (station #03331224) watershed outlets.
We collected water samples for nutrient analyses directly from the tile drains or stream sites using a 60-mL syringe that was rinsed with sample water before collection. We collected separate samples for DIN/SRP and DSi analysis and we filtered all DIN and SRP samples immediately upon collection using glass fiber filters (Whatman GF/F). For DSi, we used cellulose filters (0.45 µm pore size; Fisherbrand) to prevent contamination from glass. We transported samples on ice, froze them until analysis, and colorimetrically analyzed all samples using a Lachat QuikChem flow injection analyzer (Hach Company). We analyzed samples for SRP using the ascorbic acid method (Murphy and Riley 1962), for nitrate and nitrite (hereafter, nitrate) using the cadmium reduction method (APHA 2017), for ammonium using the phenol-hypochlorite method (Solórzano 1969), and for DSi using the heteropoly blue method (Sultan 2014). For all nutrient analyses, we ran a certified standard to validate the standard curve and routinely calculated the method detection limit. The dataset analyzed here includes 1108 individual tile drain measurements and 72 outlet measurements from KDW along with 75 outlet measurements from SDW. We monitored the same tile drains every year in KDW; however, due to changes in field management, the number of tiles draining fields with and without cover crops varied each year, with cover crop tiles ranging between 14 and 71% of the total monitored field tile drains. We found samples that were below detection were primarily from KDW tile drains; in total less than 1% of DIN, ~ 10% of SRP, and no DSi samples were below detection.

Data analysis
We conducted all data and statistical analyses using R (The R Foundation for Statistical Computing, Version 4.0.5, 2021). To characterize the seasonal pattern in DSi concentrations and stoichiometry, we divided data into four seasons: autumn (October-December), winter (January-March), spring (April-June), and summer (July-September) that correspond to crop planting and harvest and distinct temperature and hydrologic conditions affecting vegetation growth and nutrient transport in midwestern agricultural systems Hanrahan et al. 2018).
In order to evaluate the effect of tile drain and stream discharge on DSi concentrations, we modeled the relationship between DSi concentrations and discharge using a power-law equation, expressed as C = aQ b , where C is concentration, Q is discharge, and b is a constant representing the slope of the relationship. The sign of the slope indicates whether a solute exhibits enrichment (positive slope), dilution (negative slope), or chemostatic (zero slope) behavior with discharge (Godsey et al. 2009;Bieroza et al. 2018). The value of the slope is used to evaluate a solute's response to discharge, where values close to one represent a proportional change in concentration with discharge (Leong et al. 2014).
We analyzed the effects of winter cover crops on DSi concentrations, loss, and stoichiometry using data collected from fourteen unique tiles draining specific fields (field tile drains) and we removed two "county" drains which aggregate a larger drainage area across multiple fields because these drains incorporate multiple fields and cannot be classified on the basis of cover crop use. In all other analyses, we included the county drains.
Within each water year and season, we quantified the effect of winter cover crops on KDW field tile drain DSi concentrations, loss, and stoichiometry using a Hierarchical Regression Model (HRM) and pair-wise comparisons. The HRM tests for differences between cover crop and no cover crop treatments while accounting for the random effects associated with tile drain location and the fixed effects of cover crop treatment, year, and season. We then used a posthoc pairwise test to identify which years and seasons showed a significant (p < 0.05) response to cover crop planting. We conducted these tests using the lme4 (Bates et al. 2015), lmerTest (Kuznetsova et al. 2017), and emmeans (Lenth 2021) packages in R.
We calculated nutrient ratios between DIN, SRP, and DSi using molar concentrations of each sample value, and we based predictions for nutrient limitation on a modified Redfield ratio for freshwater diatoms (hereafter "freshwater Redfield ratio") of 106C:16N:1P:40Si (Redfield 1963;Brzezinski 1985). We also modeled daily loads of DIN, SRP, and DSi from each watershed outlet using Loadflex, an R package which estimates solute loads using a composite model that combines a regression model with a residuals correction for a more accurate estimation of nutrient loads (Appling et al. 2015); we then used estimated loads to calculate daily yields of nutrients based on the area of each watershed.
We used estimated daily yields to calculate the Indicator of Freshwater Eutrophication Potential (IFEP) which predicts the potential growth of nonsiliceous algae based on the amount of N or P in excess relative to the N:P:Si stoichiometric demand of diatoms expressed in the freshwater Redfield ratio (Garnier et al. 2010;Dupas et al. 2015). The IFEP is expressed as kg C area −1 time −1 and is calculated as follows: Positive IFEP values represent potential DSi limitation of diatoms, and thus the potential for growth of non-siliceous taxa, including cyanobacteria. Conversely, negative values indicate DSi is abundant relative to N and P, and thus conditions are favorable for diatom growth. When comparing N-and P-IFEP values, the nutrient (N or P) with the lower IFEP value will likely limit non-siliceous algal growth (Dupas et al. 2015). Using one-sample t-tests (x̄ = 0, α = 0.05), we assessed whether monthly averages of daily IFEP values were significantly greater than zero, indicating the stoichiometry of the nutrient loads favored non-siliceous, and possibly harmful, algal growth.

Seasonality of DSi concentrations and stoichiometry
In KDW, DSi concentrations in both field and county tile drains averaged 9.2 mg SiO 2 L −1 for the entire period of record (n = 613, SE = 0.06; Table 2), with a distinct seasonal pattern that corresponded with the growing season. Concentrations were generally highest in late summer through early fall, declined steadily over the fall to the lowest observed concentrations in winter, then increased in late spring (Fig. 2a). Average DSi concentrations were significantly different across all seasons in KDW tile drains, and also between fall and winter at the SDW outlet (Tukey Honestly Significant Differences, p < 0.05; Fig. S1a, c). There were no seasonal differences in DSi concentrations for the KDW outlet (one-way ANOVA; Fig.  S1b). At the KDW watershed outlet, DSi concentrations averaged 7.2 mg SiO 2 L −1 (n = 64, SE = 0.39) and similar values were observed in the SDW outlet, averaging 7.4 mg SiO 2 L −1 over the period of record (n = 53, SE = 0.39; Table 3). There was more interand intra-annual variation in DSi concentrations at the outlets than in the tile drains (Fig. 2b, c), suggesting in-stream processes influenced DSi concentrations between the field-and watershed-scales. Across the two watersheds, yields of DSi ranged from 1454 to 4969 kg km −2 yr −1 and the variation was strongly correlated to total annual runoff (Pearson's r = 0.89, p < 0.05; Table 3). At KDW, we used DSi concentrations from tile drains and watershed outlets, as well as discharge measurements, to model C-Q relationships at the field and watershed scales. For tile drains, the modeled DSi-Q relationship had a near-zero slope of 0.005 (power-law fit, p < 0.001; Fig. 3a) which suggested  chemostatic behavior at the field-scale. For the watershed outlets, DSi and discharge had a slightly positive slope of 0.114 (r 2 = 0.14, p < 0.001; Fig. 3b), suggesting DSi was transport-limited at the watershed scale (i.e., DSi concentrations increased with increasing discharge). Mean DSi concentration from the tile drains was significantly higher than at the outlet (Wilcoxon Sign-Rank test, p < 0.001), indicating the potential for dilution of DSi inputs from the watershed.
In KDW, we modeled the relationship between DSi loads from tile drains and the watershed outlet using measured DSi concentrations and discharge.
Outlet DSi loads were significantly higher than the total tile input (Wilcoxon Sign-Rank test, p < 0.001), indicating sources other than tile drains contributed to the watershed export of DSi. There was no linear relationship between tile drain and outlet DSi concentrations, and nearly all points were below the 1:1 line (Fig. 4a); however, there was a significant linear relationship between DSi loads from the outlet and the total input from all tile drains (p < 0.001, n = 55, R 2 = 0.40; Fig. 4b).
We calculated molar ratios between DSi:DIN and DSi:SRP concentrations (hereafter Si:N and Si:P, respectively) for all KDW tiles and analyzed variation Fig. 3 a All KDW tile drains (field and county) measured DSi concentrations (in mg SiO 2 L −1 ) plotted against measured instantaneous tile drain discharge. b Measured DSi concentrations plotted against mean daily discharge from both KDW and SDW outlets. Data is plotted on logarithmic axes and modeled with power-law fits (p < 0.001) through time and between seasons. The average Si:N in all KDW tiles over the monitoring period was 0.40 (n = 524, SE = 0.02) while Si:P averaged 576 (n = 526, SE = 0.002). For Si:N, nearly all sample means were lower than the freshwater Redfield ratio, whereas most Si:P values were higher than the Redfield ratio, indicating the potential for limitation by Si relative to N, but abundant Si relative to P throughout the year (Fig. 5).
There was little intra-annual variation in Si:P; however, Si:N ratios decreased from fall through spring and increased in the summer (Fig. 5). Neither Si:N nor Si:P ratios showed a significant linear relationship with instantaneous tile drain discharge; however we found variation in both Si:N and Si:P was more closely linked with variation in DIN (linear regression; p < 0.001, n = 524, R 2 = 0.89) and SRP concentrations (linear regression; p < 0.001, n = 466, R = 0.96), rather than DSi concentrations, indicating variation in stoichiometric ratios was primarily a function of variation in N and P rather than DSi (Fig.  S2).

Effect of winter cover crops on DSi concentrations, loads, and stoichiometry
We used nutrient concentration and discharge data from KDW tile drains to assess the effects of winter cover crops on DSi concentrations, loads, and stoichiometry. For each water year, tile drains were classified as either "cover crop" or "no cover crop" based on the presence or absence of a winter cover crop during the fallow period (October-April). Neither the water year nor season had significant differences in DSi concentrations or loads between cover crop and no cover crop conditions (HRM, α = 0.05); however, Si:N molar ratios were significantly higher in tiles draining cover crop fields during autumn and winter (HRM, p < 0.05; Fig. 6a), showing cover crops can influence Si:N ratios by reducing N loss from fields (as reported at these sites by Speir et al. 2021b). There was no difference in Si:P molar ratios between treatments during any season (Fig. 6b).

Indicator of freshwater eutrophication potential
We calculated daily N-and P-IFEP values using daily nutrient yields from both KDW and SDW stream outlets; there was a distinct intra-annual pattern in IFEP values indicating seasonal changes in the yields of DSi relative to N and P and the subsequent potential for non-siliceous algal growth. Daily N-IFEP values grouped by month were above zero in all months of the year, although values were highest during the winter and spring relative to late summer and fall conditions in both KDW and SDW (Fig. 7a). Conversely, P-IFEP values showed the opposite response, with values significantly less than zero in all months of the year and lower in the winter and spring relative DSi loads plotted against total daily tile inputs summed from all KDW tile drains (field and county). The solid lines indicate the 1:1 ratio between outlet and tile DSi concentrations and loads to summer and fall (Fig. 7b). P-IFEP values were consistently lower than N-IFEP values, indicating potential P limitation relative to N according to the freshwater Redfield ratio.
For both N-IFEP and P-IFEP, we modeled relationships with discharge using total daily discharge values recorded by the USGS gage in each watershed. On a daily timescale, N-IFEP values increased exponentially with total daily discharge while P-IFEP values decreased exponentially relative to total daily discharge (power-law fit; p < 0.001 for both N-and P-IFEP). The exponential relationships between IFEP values and discharge indicate a disproportionate increase in the stoichiometric imbalance between N, P, and Si with increasing flow. Mean values from both watersheds across all water years were 39.2 and -7.7 for N-and P-IFEP, respectively, and at the maximum recorded discharge (10 6 m 3 day −1 ), N-IFEP values were approximately 18-fold greater than the mean (Fig. 8a). In contrast, while the P-IFEP values also responded to changes in discharge, the values never exceeded zero across the range of observed discharge (Fig. 8b). Error bars extend to one standard deviation from the mean. Dashed lines denote the "freshwater Redfield ratio" of 16N:1P:40Si (Dupas et al. 2015). Data below the dashed line suggest Si limitation relative to either N or P according to the demands of freshwater diatomsconversely values above the line indicate Si in excess of either N or P

Discussion
Annual pattern of DSi concentrations at the field-and watershed-scale Contrary to our predictions, DSi concentrations increased in tile drains and watershed outlets during the summer growing season, indicating crop DSi demand did not draw down concentrations as expected. This may be a result of crop growth which facilitates mineral Si dissolution in soils and increased soil temperatures during the growing season (James and Drever 1994). Crops incorporate this Si into phytoliths, thereby increasing stores of biogenic Si (Van Cappellen 2003; Cornelis et al. 2010;Struyf and Conley 2012). Experimental studies have quantified the rate of phytolith dissolution to be an order of magnitude greater than Si mineral weathering, indicating biogenic Si may contribute significantly to the total bioavailable Si pool (Alexandre et al. 1997;Fraysse et al. 2009). Thus, crop growth facilitates increased silicate mineral weathering and production of phytoliths, and both processes may have contributed to the increase in DSi concentrations observed during the cash crop growing season.
We observed a chemostatic relationship between discharge and DSi concentrations in tile drains, whereas stream DSi concentrations exhibited transport limited behavior. The difference in DSi response to discharge between field-and watershedscale measurements might reflect the various biogeochemical processes affecting DSi from soil to tile drain and within streams. The dissolution rate of DSi from soils may have reached equilibrium with DSi export through tile drains at the field-scale (Maher 2010) while biotic processes and landscape inputs were integrated at the watershed-scale, resulting in a chemodynamic relationship between DSi concentrations and discharge. For example, as discharge increased, connectivity between the landscape and the stream channel expanded and likely mobilized DSi from new source areas within the watershed, thereby increasing stream water DSi concentrations with increasing discharge. Furthermore, the strong linear relationship between total DSi inputs from tile drains (both field and county drains) and the DSi loads at the KDW watershed outlet indicate total DSi loads at the watershed-scale were proportional to the sum of all tile inputs; however, stream DSi loads were significantly higher than tile loads suggesting other sources of DSi contributed to loads at the outlet, such as groundwater or overland flow paths.
Higher DSi loads at the watershed scale relative to the field scale can be an indicator of long-term decline in DSi availability within a watershed (Conley et al. 2008;Struyf et al. 2010). While DSi yields Within each plot, each box corresponds to the 25th and 75th percentiles, the solid line within each box denotes the median value, hinges extend to ± 1.5 interquartile range, and points indicate outliers in each group. Asterisks below box plots indicate seasonal differences in molar DSi:SIN between cover crop and no cover crop treatments (HRM, p < 0.05). Dashed lines denote the "freshwater Redfield ratio" of 16N:1P:40Si (Dupas et al. 2015) from both KDW and SDW were comparable to those measured in forested watersheds of New England (Carey and Fulweiler 2012a), intensive agriculture has been shown to reduce the export of DSi over long timescales (Struyf et al. 2010;Keller et al. 2012), and KDW (and fields in this region of the Midwest more generally) has been in continuous cultivation for > 100 years. Long-term decline in DSi availability due to agricultural land use has global impacts on the biogeochemical processing of Si, ecology of aquatic systems, and sequestration of atmospheric carbon dioxide (De La Rocha 2003;Beusen et al. 2009;Frings et al. 2016).
The annual pattern in DSi concentrations in tile drains and watershed outlets is similar to the pattern observed in forested and urban streams in New England (Carey and Fulweiler 2013) as well as agricultural rivers in Poland (Humborg et al. 2006), France (Abbott et al. 2018, and Italy (Viaroli et al. 2013). However, crop production and harvesting has undoubtedly altered the terrestrial Si cycle and significantly affected DSi inputs to aquatic systems (Van Cappellen 2003;Tubana et al. 2016). The mechanisms influencing seasonal variation in our study systems likely included temperature, soil mineral dissolution rates, stream discharge, and uptake by diatoms. Higher temperatures during the summer can contribute to increased dissolution rates and chemical weathering of silicate minerals in the soil. Conversely, DSi concentrations declined in the winter possibly as a result of increased demand for DSi by diatoms, which can outcompete other algal groups for nutrients at Fig. 7 a Daily N-IFEP values and b daily P-IFEP values from KDW outlet (white boxes) and SDW outlet (gray boxes). Each box represents daily IFEP values grouped into months and plots follow those described in Fig. 5. For the N-IFEP, the median value was above zero for all months (one sample t-test, p < 0.001, except for KDW in February p < 0.05) while the P-IFEP medians were below zero for all months (one sample t-test, p < 0.001). Dashed lines at zero denote stoichiometric balance between N, P, and Si according to IFEP equations described in the text. Positive values indicate non-siliceous algal biomass growth while negative values indicate diatom growth lower temperatures (MacIntyre et al. 2004;Anderson et al. 2008).

Impact of winter cover crops on DSi concentrations in tile water and soil
We also predicted cover crop growth would reduce DSi concentrations, but there was no difference in tile drain DSi between cover crop and no cover crop treatments in any season or water year. However, an analysis of water extractable DSi in soil from KDW showed significantly lower DSi concentrations from soils planted with winter cover crops compared with soils without winter cover crops (Hagemeier, unpublished data), suggesting the potential for uptake of DSi by cover crops. Termination of cover crop biomass in late spring should return Si to the soil, as cover crops are not harvested and the plant biomass remains in the fields. The biogenic Si contained within cover crop biomass is more soluble than mineral silicates; therefore, cover crops increase the bioavailability and transport of Si from soils to stream.
Although we have some evidence that cover crops affected DSi in soil, there was no difference in tile drain DSi concentrations. Similar to the time lags observed between nitrate mitigation practices and surface water nitrate reductions (Grimvall et al. 2000;Rabalais 2002;Meals et al. 2010), differences in soil DSi concentrations might take many years of continued cover crop planting to be expressed in tile drain Fig. 8 a Daily N-IFEP values and b daily P-IFEP values from KDW and SDW outlets plotted against total daily discharge. Note that only the x-axis is plotted on a logarithmic scale outflow. The lack of response between implemented conservation practices and field-scale observations highlights the "hydrological time lag" from field to stream and the importance of evaluating conservation practices over multi-year or decade long time-scales (Fenton et al. 2011).
Global crop uptake is estimated to be between 210 and 224 million Mg Si annually (Matichenkov and Bocharnikova 2001), which is of the same order of magnitude as the Si exported to the oceans from rivers globally (Meunier et al. 2008) and heavily contributes to the "desilication" of agricultural soils (Haynes 2014(Haynes , 2017a. Our results suggest the use of winter cover crops might enhance Si storage in soils which, over the long term, could increase the availability of biogenic Si to adjacent waters and decrease the risk of harmful algal blooms (Cornelis and Delvaux 2016;Haynes 2017b). It is well established that cover crops reduce loss of N (Hanrahan et al. 2018;Speir et al. 2021b) and P (Trentman et al. 2020) through tile drains at this site. Continuous and widespread use of cover crops might therefore facilitate a combination of increased Si availability and reduced inputs of N and P to freshwater systems-a situation that would shift stoichiometric ratios towards conditions more favorable to diatoms rather than cyanobacteria (Dupas et al. 2015;Royer 2020). In fact, at KDW, cover crops significantly increased Si:N during the fall and winter, the cover crop growing season, suggesting cover crops can affect stoichiometric ratios in addition to mass loss of nutrients.

Seasonality of nutrient limitation and eutrophication potential
Stoichiometric ratios between N, P, and Si consistently indicated abundant N relative to Si whereas P availability was limited relative to both N and Si; however, the seasonal patterns did not align with observations of in-stream algal communities. For example, low Si:N molar ratios indicate conditions favorable for growth of non-siliceous algae, yet we did not observe cyanobacterial blooms at any period during the 3-year study (L.R. Sethna, personal observation). Instead, during times of decreased Si:N, we observed diatom dominance and when Si:N increased in the summer months we observed blooms of filamentous, green algae. The deviation between theorized nutrient limitation and observed algal composition is not unexpected as stoichiometric ratios are but one driver of the seasonal shifts in algal community composition (e.g., Stevenson 1997). It is also possible that a bloom of diatoms preceded the nutrient sampling and contributed to low Si:N ratios in the stream water; however, explicit studies of nutrient limitation and uptake are necessary to quantify the role of diatoms and ambient nutrient stoichiometry.
Interestingly, in the upper Mississippi River system, stoichiometric ratios indicate Si-limitation relative to N and, at times, P while N is limited relative to P in both the mainstem of the river as well as its major tributaries (Carey et al. 2019). The change from P-limiting conditions in the headwater streams of KDW and SDW to N-limitation in the larger Mississippi River subbasins might explain why cyanobacterial blooms are rarely observed in headwater, agricultural streams while becoming increasingly common on larger rivers such as the Ohio (ORSANCO 2016) and Maumee (McKay et al. 2018) rivers.
Our data clearly show a disproportionate change in both N-and P-IFEP values with increasing discharge. At high flows, N-IFEP values exponentially increased while P-IFEP values exponentially decreased. The differences in response to high flows highlights the different mechanisms mobilizing N, P, and Si typical of agricultural systems across the Midwest, leading to the increased imbalance between these three nutrients at high flows. For example, N is rapidly mobilized in the late winter and spring, during high flows (Royer et al. 2006;David et al. 2010;Speir et al. 2021a), contributing to the exponential increase in the imbalance between N and Si. Conversely, in tile-drained landscapes P can be diluted at high flows, which explains why P-IFEP values exponentially decreased with discharge in our study watersheds. Finally, DSi concentrations have been shown to exhibit a chemostatic relationship with discharge, within larger Mississippi River subbasins (Leong et al. 2014) and in smaller, headwater streams (Godsey et al. 2009).
According to the U.S. Global Change Research Program (2018), the Midwest is expected to experience more frequent and intense precipitation events, leading to increased stream flow and nutrient export from agricultural landscapes (Rabalais et al. 2010;Raymond et al. 2012;Michalak et al. 2013;Grimm et al. 2013). Specifically, precipitation is expected to increase in the winter and spring, which are periods of high nutrient loss, thereby exacerbating nutrient loading and eutrophication in inland and coastal waters (Sinha et al. 2017;Bowling et al. 2020;Hamlet et al. 2020;Cherkauer et al. 2021). Given the varying response in N, P, and Si to high flow events, we expect a corresponding increase in nutrient imbalances which contribute to the cultural eutrophication of surface waters, particularly the proliferation of cyanobacterial blooms (Glibert and Burford 2017;Le Moal et al. 2021). Future work in these study sites could include an analysis of particulate Si, including amorphous and biogenic Si, in order to better quantify the transport and biogeochemical cycling of total Si within the watersheds. Further study on how warming and changing precipitation might affect mineralization of phytoliths in soils and, by extension, DSi inputs to freshwaters would also be valuable.
Studies on the transport and cycling of Si in aquatic systems are vastly underrepresented in the field of nutrient biogeochemistry, despite its importance in diatom productivity and the growth of many terrestrial and aquatic plants. Billions of dollars are directed to mitigating N and P export, while little attention has been paid to the role of Si in the determination of algal community composition and, more broadly, ecosystem function. This paper provides the groundwork for understanding the role of tile drainage and row-crop agriculture on Si export and stoichiometry and can further inform land management decisions that prioritize water quality and ecosystem function.