SOIL

Soils play a surprisingly important part in global carbon cycles.  Not only do microbes in soils respire and release CO2, but biogeochemical processes like weathering convert mineral carbon into bioavailable forms for respiration. Weathering can also chemically capture or release carbon to the atmosphere depending on the situation (Schlesinger et al 2013, Smith el al 2008, Fahey et al 2005). Deposition of organic carbon down to deeper soil layers can sequester carbon for long periods of time, while erosion or leaching can cause organic carbon to move from soils to waterways where it is likely to be respired by aquatic life forms (Schlesinger et al 2013, Kreutzweiser 2008). Different soils have varying properties and processes depending on latitude and other factors, so for the purpose of this summary it’s helpful to focus on a specific area.  This summary references the substrate of a New England temperate hardwood forest, specifically Hubbard Brook in New Hampshire.

The largest anthropogenic impact soil has on atmospheric carbon is through its reaction to disturbance: disturbed soils are susceptible to increased respiration of CO2 and loss of organic carbon and nutrients through runoff.  This nutrient loss decreases the soil’s potential for future plant growth and thereby slows future sequestration of organic carbon to the soil (Bowd et al 2019, Johnson 1995, Schlesinger et al 2013). In the Hubbard Brook Experimental Forest of New Hampshire, a clearcut watershed treated with herbicide for 3 years lost up to 28% of soil nitrogen.  After 3 years without plant cover, regrowth initially lagged but slowly caught up with untreated clearcut watersheds.  This resilience was likely due to organic compounds released by plant life that increased bioavailability of key nutrients despite significant leaching (Fahey et al 2005, Reiners et al 2012).  

            The balance between Net Primary Production and decomposition determines the total carbon storage in soils and plants over a given area. Net Primary Production or NPP represents the carbon harvested from the atmosphere through photosynthesis or Gross Primary Productivity, minus the carbon lost through plant respiration.  Plants take in CO2, use it to create carbohydrates in photosynthesis, respire about 50% back into the atmosphere, and deposit much of the retained carbon to the soil when they die. Animals, insects, fungi, and microbes in the soil decompose the detritus and in the process respire another portion of the carbon back into the atmosphere (Schlesinger et al 2013). Many factors influence net primary productivity, including moisture levels, nutrient availability, temperature, and sunlight.  Increases in atmospheric CO2 concentration can somewhat improve plants’ photosynthetic efficiency, and the most dramatic losses in primary production are caused by disturbance. Cool weather, nutrient shortages, and extreme moisture levels can likewise suppress decomposition, but both primary production and respiration are most highly impacted by large disturbances like logging (Schlesinger et al 2013). 

            Soils contain 60% of land-based carbon, so small changes in respiration or weathering could drastically change carbon dioxide emissions to the atmosphere. Traditionally most carbon cycle research has been limited to the organic horizon at the top of the soil profile, so investigations and carbon accounting of forestry activities have largely ignored activity in the mineral soils below.  The assumption was that carbon in the mineral soil is stable and unaffected by harvest disturbance and compaction (Bucholz, 2014).  Some research did address the mechanical soil mixing that can result from logging practices such as the use of tractors or skidders, but still concluded that carbon release from mineral soil was negligible (Yanai 2003).

            Recent research has found that timber harvests can not only increase decomposition in the surface layers of soil due to added solar warmth and input of debris when limbs and stumps remain, but some of the dissolved organic carbon that percolates down into the mineral soil during this decomposition boom may prime microbial activity at greater depths. Increased microbial activity can mineralize carbon at a faster pace, so could then respire assumedly stable mineral carbon previously sequestered at depths of more than 20cm.  This has been observed multiple times in spodsols, the sandy loam soils that underlie some hardwood forests in the northeastern United States (Nave et. al 2010, Diochon et al 2008). These effects seem to taper off around 30 years after the harvest, requiring a longer research window than usually used in the control studies investigating logging’s effects on soil carbon (Mushinski et al 2017, Bowd et al 2019, Zummo and Freidland 2011, Ussiri and Johnson 2007). If mineral soil contains more than 30% of land-based carbon (Bucholz et al 2014), and we normally assume that it’s stable, then substantial release to the atmosphere could have a large effect on the emissions associated with logging.  When calculating carbon emissions and lifecycle analysis of a wood product it would be important to account for a peak soil CO2 release later in the harvest rotation, and it may indicate a need to increase rotation length to allow soils to fully re-sequester the lost carbon (Diochon et al 2009, Diochon and Kelman 2009).

References: 

Bowd E. J., Banks S. C., Strong C. L., Lindenmayer D. B. (2019) Long-Term Impacts of Wildfire and Logging on Forest Soils. Nature Geoscience.12:113-118.DOI:10.1038/s41561-018-0294-2 

Bucholz T., Friedland A. J., Hornig C. E., Keeton L. S., Zanchi G., Nunery J. (2014) Mineral soil carbon fluxes in forests and implications for carbon balance assessments. Bioenergy. 6: 305-311. DOI: 10.1111/gcbb.12044

Diochon, Amanda & Kellman, Lisa. (2008) Natural abundance measurements of 13C indicate increased deep soil carbon mineralization after forest disturbance. Geophysical Research Letters. 35: L14402. DOI: 10.1029/2008GL034795

Diochon A., Kellman L., Beltrami H.  (2009) Looking deeper: An investigation of soil carbon losses following harvesting from a managed northeastern red spruce (Picea rubens Sarg.) forest chronosequence.  Forest Ecology and Management. 257:413-420. DOI:10.1016/j.foreco.2008.09.015.

Diochon A. C. and Kellman L. (2009) Physical fractionation of soil organic matter: Destabilization of deep soil carbon following harvesting of a temperate coniferous forest. Journal of Geophysical Research. 114: G01016. DOI:10.1029/2008JG000844 

Fahey T.J., Siccama T.G., Driscoll C.T., Likens G.E., Campbell J., Johnson C.E., Battles J.J., Aber J.D., Cole J.J., Fisk M.C., Groffman P.M., Hamburg S.P., Homes R.T, Schwarz P.A., and Yanai R.D. (2005) The Biogeochemistry of carbon at Hubbard Brook. Biogeochemistry.75:109-176. DOI 10.1007/s10533-004-6321-y 

Johnson, C.E. (1995) Soil nitrogen status 8 years after whole-tree clearcutting. Canadian Journal of Forest Research. 25, 1346–1355. 

Kreutzweiser D. P., Hazlett P. W., Gunn J. M. (2008) Logging impacts on the biogeochemistry of boreal forest soils and nutrient export to aquatic systems: a review. Environmental Reviews. 16: 157-179. DOI:10.1139/A08-066

Mushinski R. M., Gentry T. J., Dorosky R. J., and Boutton T. J. (2017) Forest harvest intensity and soil depth alter inorganic nitrogen pool sizes and ammonia oxidizer community composition. Soil Biology & Chemistry. 112: 216-227. DOI:10.1016/j.soilbio.2017.05.015

Nave L. E., Vance E. D., Swanston C. W., and Curtis P. S. (2010) Harvest impacts on soil carbon storage in temperate forests. Forest Ecology and Management.259: 857-866. DOI:10.1016/j.foreco.2009.12.009

Reiners, W. A., K. L. Driese, T. J. Fahey and K. G. Gerow. (2012) Effects of three years of regrowth inhibition on the resilience of a clear-cut northern hardwood forest. Ecosystems. 15:1351-1362, doi:10.1007/s10021-012-9589-0

Schlesinger W.H. and Bernhardt E.S. (2013) Biogeochemistry: An Analysis of Global Change, 3e. Academic Press.

Smith P., Changimng F., Dawson J. J. C.,  Moncreiff J. (2008) Impact of Global Warming on Soil Organic Carbon.  Advances in Agronomy. 97:1-43. DOI:10.1016/S0065-2113(07)00001-6

Ussiri D. A. N. & Johnson C. E. (2007) Organic matter composition and dynamics in a northern hardwood forest ecosystem 15 years after clear-cutting. Forest Ecology and Management.240:131-142. DOI:10.1016/j.foreco.2006.12.017

Yanai R. D., Currie W. S., Goodale C. L. (2003) Soil Carbon Dynamics after Forest Harvest: An Ecosystem Paradigm Reconsidered. Ecosystems. 6:197-212. DOI: 10.1007/s10021-002-0206-5 

Zummo L. M. and Friedland A. J. (2011) Soil carbon release along a gradient of physical disturbance in a harvested northern hardwood forest. Forest Ecology and Management. 262: 1016-1026.