Soil biogeochemical consequences of a 'sustainable' urban grassland to shrubland transition
Climate uncertainty and attention to urban sustainability is prompting US cities to promote household water conservation. Because ~30% of household water is used outdoors, municipalities are incentivizing replacement of water-intensive turfgrass lawns with water-efficient landscapes composed of shrubs and trees with mulch groundcover (e.g. xeriscapes, rain gardens, ornamental gardens). This change from grassland to shrubland reduces water and often fertilizer use, but can have consequences for water quality. Studies show that transitioning from grassland, by desertification or woody encroachment, leads to loss of nitrogen (N) from the system through erosion, runoff, and leaching. Managed urban grasslands contain as much N as agricultural soils in organic form. However, when an urban grassland is removed, the fate of this nutrient pool is uncertain. We explored the soil biogeochemical consequences of this ecosystem change across a chronosequence of landscape conversion from lawns (grassland) to 'xeriscapes' (shrubland) in Tempe, AZ. To observe difference between yard type and over time, we compared soil nutrient pools and availability in turfgrass yards and xeriscaped yards of homes that received a financial rebate to convert between 1991-2014 (n=46). Within yards, we stratified soil sampling to capture heterogeneity in ecosystem structure (under/between shrubs and three depths to 45 cm).
Soil inorganic N pools were significantly higher in shrub-dominated xeriscapes compared to turfgrass lawns. On average xeriscapes contained 4.2 kg NO3-N/m2 in the first 45 cm of soil, compared to 1.3 kg NO3-N/m2 in lawns. As predicted, soil organic matter declined from 6% to 4% following conversion from turfgrass to xeriscape, while NO3- production initially increased significantly after conversion. Nitrate decreased significantly as xeriscapes aged (p<.01), suggesting that there is a pulse of decomposition within 5 years of turfgrass removal with subsequent N leaching or uptake into maturing plants. Patterns in soil N availability and landscape age are highly influenced by management practices. Total shrub cover is negatively related to soil NO3- pools (p<.01), and highly irrigated yards contain less NO3- than yards irrigated infrequently (p=.001). This research is the first field exploration of the soil ecological dynamics of an increasingly common, ‘sustainable’ land use practice that is promoted in many US cities. Our findings show that soil NO3- production and potential export is large following conversion from residential grassland to shrubland. These results highlight possible trade-offs and unintended consequences of water-saving landscape changes, especially for cities in temperate and coastal environments where precipitation and leaching losses are more likely.