Disturbance
Disturbance in the urban environment is practically unavoidable, especially during the initial land conversion. As land is developed, soil layers are removed, mixed, and replaced with backfill soil that often comes from other locations (Craul, 1985). This disturbance regime can result in altered soil horizons and chemistry compared with less-disturbed soils (Huot et al. , 2017). To our knowledge, no one has yet attempted to track the changes in soil microbial communities on the short-term time scale of pre- and post-development in order to determine the initial impacts. However, with a chronosequence of sites at different ages since development, we can assess how the soil and microbial communities may respond over time after the initial disturbance.
Yao et al. (2006) analyzed a chronosequence of turfgrass lawns ranging from 1-95 years of age. They found that microbial diversity was similar across all turfgrass ages and microbial function remained relatively consistent aside from some differences in preferred carbon substrate. This study indicates that microbial communities may be highly resilient and able to return to steady state rapidly after a major disturbance.
Other research, however, has shown that it may take 25 years or more for soil carbon and nitrogen storage to recover to pre-development levels (Golubiewski et al, 2006). Scharenbroch et al. (2005) found that older urban soils have more abundant and active microbial communities and higher rates of carbon and nitrogen mineralization than new urban soils. The above-mentioned studies focused on differences in microbial communities based on urban soil age. Crucially, because few studies have compared microbial communities pre- and post-development, it is difficult to determine whether these communities have truly “recovered” or if they might be novel in composition and functioning. Thus, it is unclear how quickly microbial communities recover after disturbance to urban soils. Even if microbial communities bounce back quickly, there may be a substantial lag for the recovery of soil geochemical properties.
Soil bulk density may be one important factor driving response to disturbance. Bulk density of recently developed residential soils is significantly higher than old residential and park soils (Scharenbrochet al. , 2005). Edmondson et al. (2011) found urban soils to be least compacted under trees and most compacted under lawns. Dense soils limit the flow of oxygen, water, and nutrients through the soil matrix which in turn changes the resources to which microbes have access. Higher density soils may favor anaerobic bacteria, which correlate with higher denitrification potential (Linn & Doran, 1984). However, compacted soil has also been correlated with generally lower microbial abundance, enzyme activity, organic carbon and total nitrogen (Li et al. , 2002; Dick et al. , 1988). Therefore, bulk density may be important in explaining the reduced microbial abundance and activity observed in recently developed urban soils.
The initial development period is the most intense disturbance that urban soils likely experience, but many soils continue to experience smaller repeated disturbances. For example, athletic fields undergo frequent surface restoration which has been shown to inhibit carbon sequestration (Townsend-Small & Czimczik, 2010). This result is consistent with a study by Chen et al. (2013) which found that disturbance of urban soils resulted in greater carbon loss. Interestingly, their study found that microbial biomass in the top 10 centimeters did not differ between soil disturbance and rehabilitation treatments. It may be possible that although microbial biomass did not change, community functioning may be impacted by the disturbance. This possibility should be investigated. Likewise, Townsend-Small and Czimczik’s (2010) study left open the question of whether the microbial community continues to be abundant and active with turfgrass management, and what role microbes might play in patterns of carbon storage and loss.