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.