Soil Carbon and Nitrogen Linkages Along an Urban Elevational Gradient in Humid Tropical Forests

Abstract

Tropical forests in and around urban centers face a suite of
anthropogenic disturbances that may be muted or absent in more remote
forests. In particular, urban-proximate forests are likely to have
elevated soil nitrogen (N) levels because of local N deposition. High
background N availability common in humid tropical soils may result in
soil carbon (C) cycling responses that differ from those observed in
N-poor Northern soils. Furthermore, N availability and temperature have
been shown to have interacting effects on soil C cycling, such that
responses may vary over elevational gradients. We used fragments of
secondary forest along an urban elevational gradient in Puerto Rico to
address the following questions: (a) Is there evidence that increased N
availability in urban-proximate tropical forests alters soil C cycling?
(b) Do effects of N availability vary over elevational gradients? To
address these questions, we measured soil C and N content, soil
respiration, mineral N pools, total dissolved N (TDN), dissolved organic
C (DOC), pH, microbial biomass, and decomposer enzyme activities. Data
from the urban gradient were compared with results for rural and remote
Puerto Rican forests. Forest soils along the urban gradient had elevated
levels of soil nitrate (NO3-) relative to rural and remote forests,
whereas extractable DOC and TDN were both lower in the urban forest
soils. Soil pH was significantly higher and more variable in urban
forests, ranging from 4.5 to 8.5 across nine forest stands, whereas
remote forests had soil pH ranging from 4.4 to 5.2. Dissolved organic C
and TDN declined with increases in pH across all sites (R2 = 0.64 and
0.48 respectively, n = 48, p < 0.05). Microbial biomass was not
different among the study areas, but several microbial enzyme activities
were lower in urban forest soils relative to the remote forests,
including phosphorous-acquiring phosphatase, N-acquiring NAGase, and
oxidative enzymes that degrade complex C compounds. Soil moisture was a
strong predictor of soil enzyme activities in general, and average soil
moisture was significantly lower in the urban soils than in the remote
sites. Together, these data indicate a significant effect of proximity
to urban development on forest soil C and N cycling. Also, the effects
of N availability on soil C cycling may vary in relation to changes in
soil moisture, both over elevational gradients and with proximity to
urban areas.