## 4 Discussion
### Vegetation  
The presence of vegetation significantly increased gross primary production, ecosystem respiration, as well as rates of N and P uptake across all hydraulic residence times (Table 1). Increasing vegetative nutrient content combined with decreasing C:N and C:P ratios indicates effective short-term nutrient removal occurred within ditches via luxury uptake [66; 67; 68]. As no significant belowground tissue changes were observed, it appears _L. oryzae_ roots are able to effectively transfer nutrients to aboveground tissues in this environment [69] Plant nutrient uptake and storage may be underestimated by our data because we rinsed plant material, potentially dislodging epiphytic algae, bacteria, and associated nutrient storage. Unaccounted for uptake and storage may help to explain strong patterns of enhanced GPP and ER coupled with weaker nutrient storage patterns within vegetated systems. However, macrophyre cover has been shown to enhance GPP and ER rates and contribute proportionally more to ecosystem metabolism than other plant forms; moreover, plant biomass and chlorophyll a have been found to explain the majority of variation in daily GPP (80%) and ER (63%) in agricultural streams [70; 71]. Additionally, C assimilation and processing via metabolic processes require nutrient inputs; plant and associated microbial uptake can be a significant nutrient removal mechanism in vegetated aquatic systems [42]. In smaller-scale studies, mesocoms planted with _L. oryzae_s removed >90% of the retained NO3\({}^{-}\)-N and >70% of PO4\({}^{-3}\)-P [36; 39; 72]. \begin{table}
\begin{tabular}{c c c c} \hline
**Nutrient** & **Pre** & **Post** & **P** \\ \hline Percent C & 30.23 \(\pm\) 2.32 & 34.92 \(\pm\) 1.63 & 0.12 \\ Percent N & 0.75 \(\pm\) 0.09 & 0.90 \(\pm\) 0.04 & 0.18 \\ Percent P & 0.17 \(\pm\) 0.01 & 0.18 \(\pm\) 0.01 & 0.40 \\ C:N & 51.64 \(\pm\) 5.49 & 45.98 \(\pm\) 2.62 & 0.37 \\ C:P & 475.92 \(\pm\) 34.90 & 506.43 \(\pm\) 23.70 & 0.48 \\ N:P & 10.02 \(\pm\) 1.27 & 11.29 \(\pm\) 0.81 & 0.41 \\ \hline \end{tabular}
\end{table} 
Table 2: Percent nutrient concentrations and molar ratios (\(\mu\)\(\pm\) se) of belowground vegetation for pre- and post-experimental sampling periods. Higher GPP observed in systems vegetated with _L. oryzaeides_ likely fuels higher ER rates, which may enhance conditions favoring denitrification by reducing O\({}_{2}\) and increasing C [43; 73]. Previous investigations have found that models that predict rates based on diel N\({}_{2}\) patterns were best fit by including terms for both N\({}_{2}\) consumption (i.e., N fixation, ebullition) and production (i.e., denitrification), particularly in highly productive, lower flow reaches; estimates of denitrification among reaches of various flows were further improved by using a two-station approach [49]. The use of a single-station approach, which introduces the potential for upstream influence on rates, necessitates denitrification rate values reported herein be used with caution. However, the observation that reach-scale denitrification increased in ditches planted with _L. oryzaeides_ is reasonable, as all estimates were equally influenced by the upstream water source, yet rates were higher in vegetated ditches. Additionally, a previous comparison of one and two-station models in larger experimental ditches found minimal differences in estimated denitrification rates, suggesting that biological activity within short reaches can have a larger effect on N metabolism than potential upstream influences, providing some support for our estimates [49]. Further, our previous smaller scale mesoccosm studies have demonstrated that a significant proportion of N retention can be attributed to high denitrification efficiencies in _L. oryzaeides_ beds [36; 37; 39]. While the idea that aquatic vegetation increases metabolism and nutrient uptake in streams and ditches is not necessarily novel, this study fills a critical intermediate gap between small-scale experiments and field observations.