2.1.1. Nitrogen Removal Rate

Among a multitude of factors, it is primarily important to observe how SFWs respond to the N loads in tile drains that reach them. At this point, enlargement of the surface-flow medium makes the SFW area relevant when calculating the N load (Equation (2)). Nitrogen removal rate (Equation (3)) correlates positively to N load, which is normally a major explanatory factor [17,30,33,39,42], as N removal depends on the inputs of N. Therefore, N removal rate has a direct relationship to N concentration at the SFW inlet, and decreases as N is removed and its concentration is reduced through the system [46,47]. In line with this, a relevant correlation between N removal rate and load is observed when testing the relationship from a compilation of SFWs receiving agricultural subsurface drainage (Figure 2a). Therefore, increasing loads of N tend to enhance the removal rate, and SFWs receiving higher N loads tend to outperform others in a rate basis.

$$\text{N load} = \frac{\text{water flow} \left(\text{m}^3 \,\text{yr}^{-1}\right) \times \text{N concentration } \left(\text{g} \,\text{m}^{-3}\right)}{\text{SFW area} \left(\text{m}^2\right)}\tag{2}$$

$$\text{N removal rate = N load} \left(\text{g m}^{-2}\text{ yr}^{-1}\right) - \text{N export} \left(\text{g m}^{-2}\text{ yr}^{-1}\right) \tag{3}$$

Despite the strong correlation between N removal rate and load, Tolomio et al. [28] demonstrated through multiple linear regression models that N concentrations (flowweighted) at the inlet and outlet of a SFW were still strongly correlated (direct relationship) (R<sup>2</sup> = 0.60; regression coefficient = 0.67), especially for NO3 <sup>−</sup> (R<sup>2</sup> = 0.63; regression coefficient = 0.90). Thus, the study highlighted a relatively small contribution of the SFW to the reduction of N concentration. Similarly, Steidl et al. [26] found a strong negative correlation between N concentration at the inlet and the reduction of N concentration through the system (Kendall's τ coefficient with *p* < 0.001 = −0.30). This study indicated therefore an approximation of the N concentration at the outlet to that at the inlet as the latter increased. This study also displayed contrasting results to the above, whereby N removal rate strongly correlated negatively with N concentration (Kendall's τ coefficient with *p* < 0.001 = −0.13). The study found, however, a significant direct relationship between N concentration and water flow (*p* < 0.001; logarithmic function), thus suggesting that lower N removal rates given higher N concentrations occurred due to an associated shortening of HRT, taking

into account the effect of contact time for N removal (Section 2.2.2). Thereby, these studies suggest that the effect of N load on N removal, especially on reducing the N concentration, may be weakened if accompanied by significant increments in water flow (Section 2.2.1). The inverse relationship between N load and reduction of N concentration through the system is indeed supported by the first order model presented in Kadlec [46,47]; by Tanner and Sukias [41], who demonstrated relevant positive correlations between NO3 − load and concentration at the outlet (R<sup>2</sup> = 0.40–0.58; linear functions); and by Tanner et al. [42], in which this correlation for NO3 − was significant (*p* < 0.05; analysis of covariance). Hence, it can be expected that increasing N loads support higher N removal rates at the expense of lower reductions in N concentration through the system.

**Figure 1.** Annual variation in nitrogen (N) removal rate (**a**) and efficiency (**b**) from agricultural subsurface drainage within and between surface-flow constructed wetlands. Reference studies are indicated in square brackets. Wetlands in the same study are distinguished by different colors. \* Include a year with net N export (−38.9 g m<sup>−</sup>2). \*\* Average.

**Figure 2.** Simple regression between nitrogen (N) load and removal rate (data in red) and efficiency (data in blue) for total N (**a**) (data from Table 1) and the N forms nitrate (NO3 −) (**b**), ammonium (NH4 +) (**c**) and organic N (**d**) (data from Table 2). The numbers in brackets indicate outliers (x, y) removed from the analysis. Note that the scales for total N and NO3 − are equal.

Nitrogen load varies between SFWs depending on the N load in the tile drain (i.e., the water flow and N concentration) and the SFW area. Nitrogen concentration is sometimes reported to have a direct–yet inconsistent–relationship to water flow up to a certain threshold, above which a dilution effect occurs [22,26,44]—although this effect is not always obvious [42]. When examining a compilation of SFWs receiving agricultural subsurface drainage, it is observed that these parameters vary widely (e.g., 3.4–30.0, median 10.4 mg L−<sup>1</sup> in N concentration), thus explaining the large differences in N load (2–2338, median 181 g m−<sup>2</sup> yr<sup>−</sup>1), which subsequently contribute to the variation in N removal rate (1–452, median 55 g m−<sup>2</sup> yr<sup>−</sup>1) (Table 1), according to the strong correlation between N removal rate and load.

Although N load is a strong explanatory factor for N removal rate, it is still common to observe SFWs receiving comparable N loads with large differences in removal rate (Table 1), which probably results in differing correlation strengths between systems. In these cases, ascertaining the fractions of the different N forms at the SFW inlet, i.e., NO3 −, NH4 <sup>+</sup> and organic N, may be relevant, as these can affect differently the overall performance [20,48], and contribute to the variability. Specifically, it can be relevant to analyze whether the removal rate of each N form responds differently to its load. When testing the correlation strength between load and removal rate for the different N forms from a compilation of SFWs receiving agricultural subsurface drainage, it is observed that NO3 − removal rate clearly responds more promptly to the variation of its load (regression coefficient = 0.46) than the other N forms (regression coefficients = 0.24–0.42), whose simple linear regression models are rather weak (R2 < 0.20) (Figure 2b–d). Other studies also support and clearly state the major role of NO3 − for overall performance [20,48], thus indicating that higher fractions of NO3 − from total N at the SFW inlet are expected to enhance the N removal rate. Fortunately, this is generally the case for SFWs receiving agricultural subsurface drainage, i.e., NO3 − fractions normally higher than 70% (Table 1), which supports the use of SFWs in this context.

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**Table 1.** Characterization of surface-flow constructed wetlands (SFWs) in relation to the treatment of nitrogen (N) from tile-drained agricultural catchments (ACs). Included are the annual averages of hydraulic load, N load, nominal hydraulic residence time and N removal rate (g m−2 yr−1); as well as the average N concentration, input fractions of nitrate (NO3−), ammonium (NH4+) and organic N from total N, and N removal efficiency (%) for the total monitoring time.


In line with the above, it is clear that the loads, concentrations and removal rates of NO3 − are rather superior to the other N forms in most of the cases when examining a compilation of SFWs receiving agricultural subsurface drainage (Table 2). Moreover, a wide range of NO3 <sup>−</sup> loads (2–474, median 96 g m−<sup>2</sup> yr<sup>−</sup>1) and concentrations (1.4–15.4, median 8.6 mg L−1) can be observed between systems, thus largely contributing to explain the variation not only in NO3 <sup>−</sup> removal rate (1–277, median 35 g m−<sup>2</sup> yr−1), but also in the overall performance. The latter statement is clearly supported by the stronger correlation outputs of NO3 <sup>−</sup> (R<sup>2</sup> = 0.77) compared to total N (R<sup>2</sup> = 0.63), by which the regression coefficient of NO3 − (0.46) more than doubled that of total N (0.21) (Figure 2a,b). Furthermore, that statement is supported by the generally dominant NO3 − fractions (Table 1).

In relation to NH4 <sup>+</sup> and organic N, the latter generally reveals higher concentrations and loads at the inlet (Table 2), which leads to higher fractions of total N (Table 1). These two N forms, however, generally represent less than a quarter of the total N load (Table 1), and reveal low ranges of concentration (generally less than 0.5 and 2.0 mg L−<sup>1</sup> for NH4 + and organic N, respectively) and load (generally up to 10 g m−<sup>2</sup> yr−<sup>1</sup> for NH4 +) at the inlet, while organic N loads tend otherwise to highly vary between SFWs (0–360 g m−<sup>2</sup> yr−1) (Table 2). Suggestively, some studies demonstrated that transient pulses of organic N can be associated to highly pulsed water flows when the agricultural catchment soil undergoes significant mineralization [20,42]. Despite the weak correlation between organic N load and removal rate (R<sup>2</sup> = 0.11; Figure 2d), its removal rate was comparably variable (−75– 357 g m−<sup>2</sup> yr<sup>−</sup>1) to the load between systems (Table 2). This indicates a wide variance in the treatment performance of organic N, from little to highly effective SFWs, which is indeed verified when observing the large differences in its removal efficiency (Table 2). Removal rates of NH4 +, on the other hand, are rather mild (less than 5–8 g m−<sup>2</sup> yr−1), nearly zero or slightly negative in most of the cases (Table 2), thus normally negligible compared to those of NO3 − and organic N. The low NH4 <sup>+</sup> loads and removal rates, as well as the weak correlation between these parameters (R2 = 0.19; Figure 2c), indicate therefore that NH4 + plays a smaller role in the overall performance than the other N forms.

According to the discussed above, variation in the load of NH4 <sup>+</sup> and organic N appears to have a minor effect in the overall performance from the perspective of a mass balance analysis. Therefore, dominant fractions of these N forms at the SFW inlet may be undesirable when aiming to achieve relatively high and consistent N removal rates. This is probably due to deficient removal mechanisms for these N forms compared to those for NO3 −, or by the product of N transformation processes, which may generate NH4 <sup>+</sup> and organic N in situ (Section 2.3.1). The latter statement is indeed supported by the many cases, in which net removals of NH4 <sup>+</sup> and organic N are negative, commonly up to −8gm−<sup>2</sup> yr<sup>−</sup>1, although much larger exports may occur (Table 2). Given the above observations, the variation of N fractions at the inlet of SFWs receiving agricultural subsurface drainage (Table 1) consequently contributes to the variability in overall performance and hinders predictability. Thereby, it is important to acknowledge the incoming N fractions in order to better estimate the removal potential.

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**Table 2.** Treatment of the nitrogen (N) forms nitrate (NO3−), ammonium (NH4+) and organic N in surface-flow constructed wetlands receiving agricultural subsurface drainage. Included are the annual averages of load and removal rate (g m−2 yr−1) of the N forms, and nominal hydraulic residence time; as well as the average concentration and removal efficiency (%) of the N forms for the total monitoring time.


a Median;

 assuming a constant water depth of 0.3 m; c

flow-weighted

concentration.

### 2.1.2. Nitrogen Removal Efficiency

Nitrogen removal efficiency is useful to compare the treatment performance of SFWs, as it determines the fraction of N load that is removed in the system (Equation (4)). In the same way as N removal rate, the model of N removal efficiency implies that N load must be higher than N export to achieve positive net removal, and that higher N loads and exports increase and decrease, respectively, the removal efficiency. Moreover, the model implies that N removal efficiency varies according to the fraction of the N load leaving the system, i.e., the proportion of N export in relation to N load. As N export tends to increase under higher N loads (discussion below), N removal efficiency can only increase when the increment in N export is not sufficiently high to raise or stabilize the fraction of the N load leaving the system, thus in this case reducing that fraction. Therefore, N exports promptly responding to N loads can markedly suppress the removal efficiency. As a result, N removal efficiency depends on how N export responds to N load, i.e., the degree of change of the former in relation to the variation of the latter.

$$\text{N removal efficiency} = \left(1 - \frac{\text{N export} \left(\text{g m}^{-2} \text{yr}^{-1}\right)}{\text{N load} \left(\text{g m}^{-2} \text{yr}^{-1}\right)}\right) \times 100\tag{4}$$

Although N load accounts for the N inputs into the system, the relationship between N load and export normally weakens the effect of N load on removal efficiency differently from that for N removal rate (Section 2.1.1). Therefore, it is common to observe SFWs, in which N load plays a minor role in explaining the variation of N removal efficiency [33,41,49]. This is indeed observed when testing the relationship between N load and removal efficiency from a compilation of SFWs receiving agricultural subsurface drainage for both total N and the different N forms (R<sup>2</sup> < 0.10; Figure 2a–d). The weak correlations reflected in well distributed values of N removal efficiency on a scale of 0–100% with little influence of N load, as clearly observed for total N and NO3 − (3–90, median 38% and 9–84, median 45%, respectively; Tables 1 and 2) (Figure 2a,b). Ammonium and organic N, in turn, presented wider variations in removal efficiency, commonly including negative values (−334–70% and −263–99%, respectively; Table 2) (Figure 2c,d). Despite the above, N load was a major explanatory factor for N removal efficiency in a few studies, as observed in Tanner and Sukias [41] (R<sup>2</sup> = 0.66; linear function) and Strand and Weisner [30] (R<sup>2</sup> = 0.83; logarithmic function). Taking into account the variation in correlation strength between systems (including the regression coefficient), the relationship between N load and removal efficiency may be closely associated to the efficiency of N removal mechanisms, such as denitrification and biological uptake (Section 2.3.1).

Among the correlation tests between N load and removal efficiency described in this review, that for total N was the strongest and negative (R2 = 0.09; Figure 2a), suggesting that N removal efficiency may tend to have an inverse relationship to N load. This observation is indeed supported by many studies [30,33,37,41,49], indicating that increasing N loads tend to suppress the removal efficiency. According to the model for N removal efficiency (Equation (4)), the inverse relationship implies that higher N loads tend to raise N export above a certain threshold, which increases the fraction of the N load leaving the system and consequently makes the SFW less effective. As N removal depends on the inputs of N, the increasing fraction of the N load leaving the system as a function of higher N loads probably relates to the effect of hydrology, i.e., the load and movement of water through the system, which can promote passage of N without treatment. Therefore, in order to better understand the intra and inter-variability in N removal efficiency, factors unrelated to N inputs should be investigated.
