Runoff Quantity
In our study, we found that lots that were amended with compost had a lower percent runoff produced and CN compared with tilled and null lots (Tables
1 and
2; Fig.
3). Similar patterns of compost reducing CN was found in other studies, e.g., Bean and Dukes (
2015) and Olson et al. (
2013), where CN and percent runoff produced were lowest in the compost-amended treatments compared with the other treatments, which were fly ash amendment and null, and tilled and null, respectively.
Overall, lots that were topdressed produced greater runoff percentages (Table
1) and CNs compared with lots that were not topdressed. Looking at the effects of topdressing on the individual treatments showed varied results. Due to limited replicates, the effects could not be statistically analyzed. However, null and tilled lots that were topdressed had increased runoff percentages and CNs compared to non-topdressed lots, while composted lots had increased CNs but lower runoff percentages. The mechanisms that caused greater runoff and increased CNs from topdressed null and tilled lots are not clear, although topdressing may somehow reduce storage within the vegetated layer above the soil or reduce the rate that rainfall is absorbed into the soil. These processes deserve further investigation because they are counter to expected runoff reductions due to increased soil organic matter and overall soil quality. As for the compost lots, increased CNs for topdressed lots correspond to null and tilled lots; however, overall runoff percentage was lower. While runoff percentages and CNs should typically increase or decrease together, CNs reflect the regressed runoff response to individual rainfall events, accounting for differences in rainfall amounts, while the runoff percentage reflects the cumulative runoff response regardless of rainfall characteristics. Future studies should focus on identifying and quantifying how topdressing applications affect mechanistic runoff processes from turfgrass landscapes.
Our study differed from that of Bean and Dukes (
2015) by having a higher CN for compost amended lots (74) than Bean and Dukes did (40, 44, and 62) for their two soil types in question, Arredondo with compost amendment and Orangeburg with compost amendment. We also saw a smaller increase in percent of runoff produced between null and composted soils, an increase of 4%, compared with 53% increase in the Olson et al. (
2013) study. Additionally, the Bean and Dukes (
2015) and Olson et al. (
2013) studies were carried out on sites that had different soils, which may be the reason for slight variation.
The main difference between the current study and previous studies analyzing amended soil and storm water runoff is that this study was carried out in the field, that is, in a full-scale residential development. Our study validates the extension of previous studies done on smaller scales to the catchment scale. This includes both pervious and impervious areas, whereas previous studies have only evaluated pervious areas. It demonstrates runoff reduction potential from amending urban soils with compost at the storm drain catchment scale.
Runoff Concentrations and Loadings
Overall, neither treatment nor topdressing had a significant effect on TN or TP concentrations in runoff. TN concentrations were log-transformed for analysis, and TP concentrations were square-root transformed for analysis. All but one value fell below
(Table
3; Fig.
4), and the majority of concentrations were below
(Table
4; Fig.
5). In comparison, the Florida Department of Environmental Protection (FDEP) uses
and
for estimating runoff loadings from medium density residential watersheds (
Harper and Baker 2007). Date was the only significant factor for both TN and TP concentrations (TN: df: 22, F: 7.87,
; TP: df: 22, F: 4.52,
), which reflects the event-to-event variability in runoff concentrations. Additionally, nitrogen species concentrations were not significantly different across treatment or topdressing (Table
3).
The TN EMC for null lots was
, which was 14% lower than composted lots and 40% greater than tilled lots (Table
5). This was lower than that found in Lusk et al. (
2020) for null lots (
). Yang and Toor (
2016), however, had a monthly mean concentration of
, which is equivalent to results from this study. Similar to our results, Lusk et al. (
2020) also showed that Org-N was the dominant form of nitrogen in stormwater runoff, making up about half of TN concentrations.
Neither treatment nor topdressing had a significant effect on event TN or TP loadings (kg/ha), which was expected given that concentrations were not significantly different (Tables
6 and
7). TN loadings were log-transformed and TP loadings were square-root transformed. Date was significant for both TN and TP event loading, which was likely because no two storm events or their antecedent conditions are the same. For TP loadings, the significant interaction of treatment and topdressing with date meant that date had an impact on how treatment and topdressing reacted in terms of TP loadings.
The highest annual loading rate for TN was produced by tilled lots (
) and least by compost amended lots (
) (Table
8; Fig.
6). In comparison, Obreza (
2004) states that for a single-family residential area, the typical loading in north Florida for annual TN loading is
, which is greater than the amount of TN loading from any treatments in the current study. For TP annual loading rates, tilled lots produced the highest, while null lots produced the least (Table
8; Fig.
7).
Overall, lots that were topdressed had higher TN and TP annual loading rates than lots that were not topdressed. However, topdressed tilled and null lots had lower annual loading rates than their non-topdressed counterparts. This was not the case for composted lots, which had higher rates from the topdressed lots compared with the non-topdressed lots. Thus, further research is needed to better understand the effects that topdressing has on TN and TP annual loading rates, and to potentially further examine the effect topdressing has on lots that were already composted.
A drawback of this study was the limited sample size (eight storm drains) and low replicates in each treatment area (one or two storm drains per treatment), which would increase confidence of significant differences. To improve the study, there could have been a greater replication of treatments to better understand the impacts of treatment type and topdressing. A longer time frame, in addition, may have yielded more consistent results, given that there was a sample collected for every storm drain for only four of the 16 sampled storm events. There were also challenges with coordinating during construction and with landscaping crews within the development process.