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Forest ecosystems are important global soil carbon (C) reservoirs, but their capacity to sequester C is susceptible to climate change factors that alter the quantity and quality of C inputs. To better understand forest soil C responses to altered C inputs, we integrated three molecular composition published data sets of soil organic matter (SOM) and soil microbial communities for mineral soils after 20 years of detrital input and removal treatments in two deciduous forests: Bousson Forest (BF), Harvard Forest (HF), and a coniferous forest: H.J. Andrews Forest (HJA). Soil C turnover times were estimated from radiocarbon measurements and compared with the molecular‐level data (based on nuclear magnetic resonance and specific analysis of plant‐ and microbial‐derived compounds) to better understand how ecosystem properties control soil C biogeochemistry and dynamics. Doubled aboveground litter additions did not increase soil C for any of the forests studied likely due to long‐term soil priming. The degree of SOM decomposition was higher for bacteria‐dominated sites with higher nitrogen (N) availability while lower for the N‐poor coniferous forest. Litter exclusions significantly decreased soil C, increased SOM decomposition state, and led to the adaptation of the microbial communities to changes in available substrates. Finally, although aboveground litter determined soil C dynamics and its molecular composition in the coniferous forest (HJA), belowground litter appeared to be more influential in broadleaf deciduous forests (BH and HF). This synthesis demonstrates that inherent ecosystem properties regulate how soil C dynamics change with litter manipulations at the molecular‐level. Across the forests studied, 20 years of litter additions did not enhance soil C content, whereas litter reductions negatively impacted soil C concentrations. These results indicate that soil C biogeochemistry at these temperate forests is highly sensitive to changes in litter deposition, which are a product of environmental change drivers.
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Retention of Nitrate-N in Mineral Soil Organic Matter in Different Forest Age Classes
Conceptual models of nutrient retention in ecosystems suggest that mature forests receiving chronically elevated atmospheric nitrogen (N) deposition should experience increased nitrate (NO3-) losses to streams. However, at the Hubbard Brook Experimental Forest (New Hampshire, USA), recent stream NO3- concentrations have been unexpectedly low in mature watersheds. Poorly understood retention of NO3 matter (SOM) may explain this discrepancy. The relative availability of C and N in SOM influences NO3--N retention and may vary during succession due to processes of N mining and reaccumulation. To evaluate the strength of the SOM sink for NO3--N, we applied a 15NO3- tracer to the mineral soil in eight stands spanning a forest chronosequence from about 20 years to old growth ( 200 years). We tracked 15N recovery in SOM fractions in the upper 10 cm of B horizon over 5 weeks. Overall, forest age did not directly control the 5-week recovery of 15N, but it had an indirect effect via its influence on SOM properties such as C/N. Old-growth forest soils had the lowest C/N, implying closer proximity to effective N saturation. Across sites, both the particulate- and mineral-associated SOM fractions rapidly incorporated 15N, but recovery in each fraction generally declined with time, reflecting the dynamic nature of SOM. These results indicate that mineral horizons can provide an important N sink through the short term in forests of all ages, but that SOM-N remains subject to active cycling and potential loss from the soil pool over the longer term.
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- Award ID(s):
- 1637685
- PAR ID:
- 10085814
- Date Published:
- Journal Name:
- Ecosystems
- ISSN:
- 1432-9840
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s10021-018-0328-z
