Improving stormwater infiltration and retention in compacted urban soils at impervious/pervious surface disconnections with biochar.

Urban development often results in compacted soils, impairing soil structure and reducing the infiltration and retention of stormwater runoff from impervious features. Biochar is a promising organic soil amendment to improve infiltration and retention of stormwater runoff. Soil at the disconnection between impervious and pervious surfaces represents a critical biochar application point for stormwater management from urban impervious features. This study tested the hypothesis that biochar would significantly improve water retention and transmission at four sites, where varying percentages (0%, 2%, and 4% w/w) of biochar were amended to soils between impervious pavement, and pervious grassed slopes. Field-saturated hydraulic conductivity (Ksat) and easily drainable water storage capacity were monitored at these sites for five months (two sites) and 15 months (two sites). At the end of the monitoring periods, the physical, chemical, and biological properties of each site's soil were assessed to understand the impact of biochar on soil aggregation, which is critical for improved soil structure and water infiltration. Results indicated that the field Ksat, drainable water storage capacity, and plant available water content (AWC) were 7.1 ± 3.6 SE, 2.0 ± 0.3 SE, and 2.1 ± 0.3 SE times higher in soils amended with 4% biochar, respectively, compared to the undisturbed soil. Factor analysis elucidated that biochar amendment increased the organic matter content, aggregate mean weight diameter, organo-mineral content, and fungal hyphal length while decreasing the bulk density. Across the 12 biochar/soil combinations, the multiple linear regression models derived from factor analysis described the changes in Ksat and AWC reasonably well with R2 values of 0.51 and 0.71, respectively. Using soil and biochar properties measured before biochar addition, two recent models, developed from laboratory investigations, were found helpful as screening tools to predict biochar's effect on Ksat and AWC at the four field sites. Overall, the findings illustrate that biochar amendment to compacted urban soils can significantly improve soil structure and hydraulic function at impervious/pervious surface disconnections, and screening models help to predict biochar's effectiveness in this context.

Understanding a wood-derived biochar's impact on stormwater quality, plant growth, and survivability in bioretention soil mixtures.

Bioretention systems are planted media filters used in stormwater infrastructure. Maintaining plant growth and survival is challenging because most designs require significant sand. Conventional bioretention soil media (BSM) might be augmented with biochar to make the BSM more favorable to plants, to improve nutrient removal efficiency, and enhance plant survivability during drought while replacing compost/mulch components that have been linked to excess nutrient export. Pots with BSMs representing high and moderate sand content were amended with wood biochar, planted with switchgrass, and subjected to weekly storms for 20 weeks, followed by a 10-week drought. After 20 weeks, 4% biochar amendment significantly increased stormwater infiltration (67%) and plant available water (52%) in the high sand content BSM (NC mix, which meets requirements for the state of North Carolina (US) and contains no compost/mulch), and these favorable hydraulic properties were not statistically different from a moderate sand content, biochar-free BSM with compost/mulch (DE mix, which meets requirements for state of Delaware (US)). While biochar amendment improved plant height (25%), the number of shoots (89%), and total biomass (70%) in the NC mix, these parameters were still less than those in the biochar-free DE mix containing compost/mulch. TN and NO3-1 removal were also improved (28-35%) by biochar amendment to NC mix, and the resulting TN and TP loadings to groundwater were 10 and 7 times less, respectively than biochar-free DE mix with compost/mulch. During the drought period, biochar amendment increased the time to switchgrass wilting by ∼8 days in the NC mix but remained 40% less than the biochar-free DE mix. A recalcitrant carbon-like biochar mitigates some of the deleterious effects of high sand content BSM on plants, and where nutrient pollution is a concern, replacement of compost/mulch with wood biochar in BSM may be desired.

Impact of wood-derived biochar on the hydrologic performance of bioretention media: Effects on aggregation, root growth, and water retention.

Bioretention systems are one example of green stormwater infrastructure that may mitigate the hydrologic impact of stormwater runoff. To improve water retention while maintaining rapid stormwater infiltration, conventional bioretention soil media (BSM) might be augmented with biochar. Biochar may improve the BSM's structure by increasing soil aggregation, which might improve water retention and increase stormwater infiltration while also improving root growth. Pots with BSMs representing high and moderate sand content media were amended with a wood-derived biochar, planted with switchgrass, and subjected to weekly storms for 20 weeks, followed by a 10-week drought. In the high sand content medium (NC mix), biochar amendment increased hydraulic conductivity (Ksat), and this effect increased with time. At 0 weeks, 2% and 4% (w/w) biochar increased Ksat by 4 ± 2% and 10 ± 4%, respectively, while at 30 weeks the increase was 30 ± 10 and 70 ± 20%, respectively, above biochar-free media. Similar improvements were seen in plant available water (PAW) in NC mix. However, minimal improvements in Ksat and PAW from biochar amendment were found in the moderate sand content BSM that contained compost and mulch (DE mix). Where biochar promoted Ksat, this was correlated with increased water-stable aggregate size (r = 0.86), fine root volume (r = 0.88), and below ground biomass (r = 0.83). Important factors affecting Ksat and aggregation in the NC mix were biochar's influence on organo-mineral association, fungal hyphae length, and plant roots. Wood-derived biochar amendment to BSM may obviate the need for compost/mulch since biochar has similar effects on improving BSM hydrology and root growth without the risk of undesired nutrient leaching.