Soil Warming Plus Nitrogen Addition Experiment at Harvard Forest since 2006

Climate warming and N deposition are occurring on a global scale with unknown
long-term effects on soil microbial communities and the biogeochemical processes they
perform. Few studies have examined the interactive effects of elevated temperatures
and N additions on soil microbial community structure and function. The overall
objective of this study is to investigate whether warming and N additions restructure
microbial communities and alter the response of soil C pools to these two stressors.
A related study is examining the interactive effects of warming and N additions on
plant and ant diversity. This research is being carried out at the Soil Warming x
Nitrogen Addition Study at the Harvard Forest which includes four treatments:
control, warming (heating to 5 deg C above ambient), warming x N, and N additions
only (addition of 50 kg N/ha/yr). Soil respiration measurements have been made monthly since the beginning of the
experiment in 2006. In 2010 and 2011, two different methods were compared: static
chamber measurements and instantaneous field IRGA assessments. Soil samples (~0-10
cm) have been sampled annually for total C and N, N mineralization, and microbial
community composition. Most recently, soils were collected in October 2011 from
across the entire profile (0-50 cm) to access potential changes in soil C and N pools
with depth. First, 20 x 20 cm forest floor samples were collected. Mineral soils were
then collected in 10 cm depth increments to ~50 cm. Samples are currently being
analyzed for total C and N, microbial biomass and community composition and fungal
gene expression (transcriptomics). Additionally, long-term incubations are being
conducted to measure labile and recalcitrant C fractions. Additional soil physical
(texture) and chemical (pH, inorganic N) are being measured. Field season measurements of soil respiration indicate that both warming and N
additions continue to stimulate CO2 flux, with warming treatments having a stronger
effect on respiration than N additions alone. Where warming x N occur together,
warming appears to moderate the negative effects of N additions alone on soil
respiration. Microbial biomass estimates show the greatest biomass within the warming
x N treatment. Declines in microbial biomass with warming or N additions alone
support similar findings from the same assessment in 2007 and 2009. Forest floor mass
show small declines under warmed conditions. Soil C storage in the mineral soil
showed modest changes with warming (-10% difference compared to control). Genomic and
transcriptomic pipelines optimized in the Frey Lab are being used to measure the
diversity, composition, and function (via gene expression) of the active fungal
community in response to warming and N additions. Recent transcriptomic work has
demonstrated that chronic N fertilization results in a decrease in expression of
several transcripts of fungal laccase and glycoside hydrolases which are enzymes
involved in lignocellulose degradation. Overall, our recent results suggest that
anthropogenic stressors and seasonal changes continue to interact to affect soil
microbial communities and biogeochemical cycles. Plant roots are primary drivers of soil organic matter dynamics, mediating belowground carbon (C) inputs, stabilization, and losses. Yet, how global changes such as rising temperatures and altered nitrogen (N) availability interact to affect these dynamics has rarely been tested empirically in the field. Here, we quantify how inputs to soil organic matter from fine root production, root exudates, and root-associated fungi respond to long-term (16 years) soil warming (+5°C), nitrogen (N) enrichment (+5 g N m⁻² yr⁻¹), and their combination in a temperate hardwood forest. Warming alone reduced root-derived C inputs by 21% and increased microbial respiration by 46%, resulting in a net soil C loss of 135 g C m⁻² yr⁻¹. In contrast, N enrichment increased root-derived SOC accumulation by 47% and reduced root respiration by 40%, contributing to a near neutral soil C balance. When combined, warming × N addition increased root-derived SOC fourfold (from 70 to 281 g C m⁻² yr⁻¹), fully offsetting warming-induced C losses and maintaining soil C stocks at control levels. Root-derived SOC accumulation was positively related to fine root production (r² = 0.42) and to maple:oak exudate ratios (r² = 0.31), highlighting species-specific control over C stabilization. These findings demonstrate that interacting global change factors can have balancing effects on root C allocation and microbial losses, highlighting soil N availability as a critical control determining whether warming accelerates SOC depletion or stabilizes new root-derived C.