Decomposition of labeled litter

The objective of this experiment is to measure the response of below-snow organisms and soil organic matter decomposition to differences in winter snowpacks. Snowpacks insulate soils from winter temperature extremes and supply liquid water during snowmelt and these effects facilitate the activity of microbial communities below the snow. These communities reach high biomass levels by the onset of snowmelt and play an active role in organic matter decomposition and biogeochemical cycling. This experiment measures rates of ^13^CO~2~ efflux and decomposition of labeled needle litter under low, and normal (control) snowpack treatments. We expect that winter snowpack size is a key driver of interannual variability in soil temperature and moisture, and that this variability has consequences for biological C (and N) cycling in soils. See Schmidt and Lipson, 2004((Schmidt SK, Lipson DA (2004) Microbial growth under the snow: implications for nutrient and alleochemical availability in temperate soils. Plant Soil 259:1–7. )), Schmidt et al, 2008((Schmidt SK, Wilson KL, Gebauer MM, Meyer AF, King AJ (2008a) Phylogeny and ecophysiology of opportunistic ‘‘snow molds’’ from a sub-alpine forest ecosystem. Microb Ecol . )), Schmidt et al, 2007((Schmidt SK, Costello EK, Nemergut DR, Cleveland CC, Reed SC, Weintraub MN et al (2007) Biogeochemical consequences of rapid microbial turnover and seasonal succession in soil. Ecology 88:1379–1385. )), Monson et al, 2006((Monson, R. K., D. A. Lipson, S. P. Burns, A. A. Turnipseed, A. C. Delany, M. W. Williams, and S. K. Schmidt (2006), Winter forest soil respiration controlled by climate and microbial community composition, Nature, 439, 711 – 714))


  • Winter decomposition and associated soil ^13^CO~2~ efflux are reduced in low snowpack years (simulated by snow removal), and this effect is correlated with lower soil temperatures and a shorter snow-covered period.
  • Control snowpack plots will have longer periods of below-snow decomposition, and shorter periods of growing-season soil drought, leading to greater cumulative decomposition and soil ^13^CO~2~ during the experiment.
  • Labeled carbon compounds will be more rapidly added to stable fractions of the soil organic matter in control snowpack treatments.

Experimental design

To test these hypotheses we will make continuous measurements of respiration from ^13^C labeled organic matter added to soils, and measure the enrichment of SOM constituents at intervals after this addition. These measurements will occur under snow removal treatments and an undisturbed snow control. There will also be an undisturbed control with no added needles. Efflux of ^13^CO~2~ from labeled needle litter additions indicates active periods of label decomposition. ^13^C enrichment of soil organic matter fractions can be used to measure the transformation of fresh litter into other soil organic matter fractions.

If microcosms are used in the experiment, decomposition may be measured in a more quantitative way.

Labeled litter addition

^13^C and ^15^N labeled needles from //Pinus ponderosa// will be obtained from Jeff Bird. These have a δ^13^C of 2487 per mil and a ^15^N enrichment of 5.5 atom %. See the Bird and Torn, 2006 paper for details on this material((Bird, J.A., Torn, M.S. Fine roots vs. needles: A comparison of 13C and 15N dynamics in a ponderosa pine forest soil (2006) Biogeochemistry, 79 (3), pp. 361-382. doi: 10.1007/s10533-005-5632-y)). 10 grams of labeled litter will be placed on the forest floor over a 20cm^2^ area and secured with plastic netting. Gas sampling inlets will be placed directly over this netting in the center of the label.

Labeled root litter is also available and can be placed under a separate set of inlets. 10g of root litter would be placed at 5-10cm depth at the interface of the organic and mineral horizons. Inlets would either be placed directly above this root litter and then be covered with organic horizon soil, or at the surface as in the needle litter arrangement above. Deployment of litter would require some reduction in the replication of inlets.

Label and gas sampling inlets, plus any other measurement activities will be replicated (n = 3-5) under these treatments:

  • Snow removal: Snow shoveled to a constant 20cm depth for the entire winter.
  • Control: Undisturbed snowpack
  • No-Label control: Undisturbed snowpack, no label addition

The experimental design without and with roots is summarized in these tables:

Design with needles only

\^ \^ Snowpack treatment \^\^\^ \^ No. of inlets | Removal | Control | Control (no label) | \^ Needle litter | 5 | 5 | 5 | //15 sample inlets total//

Design with needles and roots

\^ \^ Snowpack treatment \^\^\^ \^ No. of inlets | Removal | Control | Control (no label) | \^ Needle litter | 3 | 3 | 3 | \^ Root litter | 3 | 3 | 3 | //18 sample inlets total//

Alternative microcosm litter addition method

Microcosms similar to those used in Bird and Torn, 2006 will be installed in each treatment plot (n=9) with inlets placed at the top of one microcosm per treatment. The advantage of this design is that smaller amounts of litter might be used (~1.25 per microcosm in Bird 06), so the potential for replication is higher, and the addition of 13C to soil organic matter pools can be measured in a more quantitative way. These microcosms will be installed for equilibration in July or August of 2010. Labeled litter will be mixed in to the soil at the top of the microcosm in mid-late October. Three microcosms will be collected at three discrete time intervals for measurement of 13C enrichment of SOM pools. These time intervals might be following snowmelt (May 2011), the next fall (Oct, 2011) and the second snowmelt (May 2012) after installation.

Microcosm design including roots

\^ \^ Snowpack treatment \^\^\^ \^ No. of microcosms(# inlets) | Removal | Control | Control (no label) | \^ Needle litter | 9(3) | 9(3) | 9(3) | \^ Root litter | 9(3) | 9(3) | 9(3) | //18 sample inlets total//

Three microcosms to be collected during the first spring (~ May 2011), first fall (Oct 2011), and second spring (May 2012) after installation.

Efflux of ^13^CO~2~ from inlets will be measured for two full winters (2010/11, 2011/12) if possible.

Measurement of labeled carbon dioxide efflux

Inlets are placed directly above the labeled litter and CO~2~ concentration and δ^13^CO~2~ will be measured every 3 hours using a Campbell Tunable Diode Laser that has been operating at Niwot Ridge for several years. There are 20 inlets to this device currently available. CO~2~ flux rates can be measured with measurements of the concentration gradient (below snow sample vs an above snow sample) and Fick's law. These measurements will require frequent sampling of snowpack density, and flux measurements may only be feasable for the control snow treatment.

Replicated measurements of soil moisture and temperature will be continuously taken in each treatment. Soil temperature will be measured using either thermistors or iButtons. Soil moisture will be measured using sensors such as Decagon EC-5s or Campbell CS-616s. In addition, there is a soil moisture and temperature profile operating 10 m away.

Soil organic matter sampling

At the end of this experiment, enrichment of soil organic matter fractions will be assessed by coring beneath the label and measuring the carbon isotope ratio (δ^13^C) of selected SOM fractions. Coring will occur immediately after snowmelt in 2012. SOM fractions to be analyzed include dissolved organic carbon from both O and A fractions, microbial carbon (by fumigation extraction) from O and A horizons, bulk organic layer samples, and light (particulate) and heavy (mineral associated) organic matter from the A horizon. These samples will be measured for carbon isotope ratios using an EA-IRMS system at the SIRFER lab (University of Utah).

If microcosms are used, similar measurements and methods will be employed with each set of microcosms that are returned to the lab.

See the measurements page for more detail.

Issues to address

  • What are the best ways to measure the addition of the labels (13C and 15N) to the soil organic matter, including the microbial community, over time?
  • The activity of many organisms is being observed in this experiment. Can we separate them into phylogenetic groups and identify what communities are actively decomposing/respiring during different times of the year? What measurements would be useful to do this?
  • If we are identifying actual organisms, enzymes, etc, what are they telling us about C cycling in terms of what is being decomposed, when, and how quickly?
  • Using microcosms, would it be possible to have a true mass balance of the label? Will the mass of 13C respired, and added to SOM pools at the harvest of the microcosms all add up to what was originally added as labeled litter?
  • There are problems with calculating flux rates from soil beneath a disturbed snowpack.
  • What parts of the SOM are we going to measure for changes in enrichment? What do these SOM fractions represent?
  • What is the overall significance? Climate variability (in the form of snow depth) leads to biologically mediated changes in C processing and storage? Snow removal may decrease respiration, but does this lead to increases in stored soil C, or could a reduced rate of processing litter into longer-lived C compounds lead to less soil C storage?