Ode to the O Horizon The Uniqueness of

Ode to the O Horizon: The Uniqueness of Forest Soils Dan Binkley Colorado State University Presented as the Sergei Wilde Lecture in Forest Soils at the 2014 SSSA meeting

Let’s let Sergei Wilde have the first words: Photo from Chuck Davey The geologic substratum is dead and sterile; it cannot be “parent” to anything, least of all to the animated entity called soil.

Let’s let Sergei Wilde have the first words: Photo from Chuck Davey …when pedologists brushed off the “dead litter” they removed with it not only the practical interests of silviculture but also the scientific foundation of soil genesis.

Let’s let Sergei Wilde have the first words: Photo from Chuck Davey …the surface layer of a forested soil presents a dynamic entity in which the energy of dead remains is transferred to new organisms and new compounds.

The unique horizon of forest soils:

The O horizon is only skin deep, but that’s where we find the most Beauty Photo: Jean-Paul Laclau

But first: Words have great power, both intentional and unintentional. What are “fossil fuels”? 47% of Americans think fossil fuels come from decaying dinosaur bodies Savvy experts now say “coal, oil and gas” – never using the words “fossil fuel”

Which of these animals has greater mass? 2 years old 8 years old The hippo has no trunk, so a fair comparison should overlook the mass of the elephant’s trunk, right?

This is a forest floor Scott Mutter

This is an O horizon, an integral part of a whole forest soil

How much wouldon the C in this soil change if we planted >90% of papers trees and theychanges grew impressively large in 30 years? afforestation in soil left out the O horizon! Berthrong et al. (2009) Ecological Applications: A global meta-analysis of soil exchangeable cations, p. H, carbon, and nitrogen with afforestation. Cook 2012

Let’s start with some obvious beauty of O horizons buffer temperature extremes (and swings) in underlying mineral horizons O horizon after Cochrane 1969

O horizons are the sponges that turn rainfall deluges into life-supporting soil moisture Beetle-killed trees: retained O horizon Fire: Lost O horizon

When beetles kill trees, the O horizon remains, and hydrology doesn’t change: Fraser Experimental Forest, CO Beetles: Before After From Chuck Rhoades and RMRS colleagues

4 -mile fire Fires that consume O horizons dramatically increase floods Elevation (m) From Stephanie Kampf, 2014

Fires that consume O horizons dramatically increase erosion XXX

Beautiful variations in O horizons across space

Coefficient of variation among random samples in this plot: 0. 51 Coefficient of variation among 0. 3 ha plots: 0. 12 Andy Scott’s MS

N mineralization (kg ha-1 yr-1) Beauty of O horizon variation across mountainsides: A tale of 10 spruce-fir forests Nitrogen supply is mostly about the O Stump and Binkley 1993

N mineralization (kg ha-1 yr-1) Beauty of O horizon variation across mountainsides: A tale of 10 aspen forests Consistent proportion from O Same total, Difference is all different O about the O Stump and Binkley 1993

Beauty of O horizon variation across regions: Loblolly pine across the SE US 0 Why do O horizon masses vary by >5 -fold? 10 20 30 40 50 60 O horizon mass (Mg/ha) Binkley 2002

Seeking beautiful explanations for patterns of O horizons O horizon C (Mg/ha) O horizons increase with temp and precip in Scandinavia Why? Just higher Sequestration inputs? processes? 5 0 40 30 20 10 0 Callesen et al. 2003 60 50 40 30 20 10 0

O horizons show fascinating changes over time:

But actual changes may not (or may? ) match the chronosequence: Yanai et al. 2003

O horizon changes over time: reforestation (beautiful tests of hypotheses within sites with low variance) Planted to loblolly pine forest in 1958 Photo from Calhoun Experimental Forest website

50 years later, where did C accumulate? 0 -60 cm mineral soil O horizon 0. 03 Mg C ha-1 yr-1 1. 0 Mg C ha-1 yr-1 Trees 4. 2 Mg C ha-1 yr-1 Richter et al. 1999, etc. Richter et al. 1999

Glimpses into deeper time: O horizons over millenia Photos from Island chronosequence, Lakes Uddjaure & Hornavan, August 2009, by Karina Clemmensen and Adam Bahr

Lightning starts fires more often on large islands in lakes than on small islands

Large island, last burned 205 yr ago; O horizon is 15 cm deep, biomass ~120 Mg/ha) Medium island, last burned 1950 yr ago; O is 25 cm deep, biomass ~230 Mg/ha) Small island, last burned 3185 yr ago; O is 40 cm deep, biomass ~600 Mg/ha) Without fire, decomposition doesn’t keep up with inputs Karina Clemmensen, Adam Bahr, David Wardle, pers. comm.

A beautiful conundrum about O horizons: If decomposition is relatively rapid (a decade or so), but the age of material in O horizons can be decades or even centuries. How can this be? Vesterdal et al. 2012

How soil organic matter can be old:

Manipulating O horizons: What would happen if the normal inputs of organic matter changed? 2 X aboveground litter inputs? Prevention of root inputs? Photo: David Foster

Aboveground inputs of litter influence O horizon mass – but root inputs matter much more Detritus Input/Removal Treatments (DIRT) experiment, Harvard Forest, Lajtha, Bowden, Nadelhoffer 2014

At a single site, tree species lead to wildly different O (and A) horizons: 28 years in a common garden in Ontario

A common garden in Siberia Larch Siberian pine Aspen Scots pine Birch Norway spruce Photo: Oleg Menyailo

Fascinating differences in C accumulation in O horizon vs. A horizon at one site: Shugalei 2005

Of course interactions are important: tree species and soil animals have feedbacks, and interactions aren’t constant over time: Bezkorovaynaya 2005

Beautiful answers tell us the typical story. And also typical exceptions, and factors driving variation.

Gahagan et al. 2015 Berger et al. 2002 Comparing 8 sites in Michigan, the big difference between pine and maple was mostly in the O, not mineral soil. Comparing 9 sites in Austria, the bigger relative difference between spruce and beech was in the O.

Can O horizon taxonomy be beautiful? Mull and Mor: Mulls turnover faster, right? Recycle nutrients faster too? Is there value in going into more detail? Klinka et al. (1981) applied a species-taxonomy approach to classification of O horizons: 3 Orders: mor, moder, and mull, 19 Groups: such as hemimor and hemihumimor 69 Subgroups: such as amphihemihumimor and microvelomoder

A European foray into O horizon classification of six major types: Anmoor, Mull, Moder, Mor, Amphi, and Tangel. Proposed classification depends on O structure, and on the rest of the soil: the same sort of mull horizon would be a Terromull if it occurred on a well-drained soil; an Entimull if lying directly on bedrock; or a Paramull if plant roots were particularly dominant. ? y a w e m o s n i l u f e s u s i h t ll a s i … m Hmm Zanella et al. 2011

“If the O horizon is in steady state, nutrient release from decomposition will equal litterfall inputs whether the O horizon is a mull or a mor. ” Carl Olaf Tamm 1977 I’m not sure taxonomy without clear power is beautiful

Taxonomy -- Mull and Mor: Mulls turnover faster, right? So nutrient supply should be higher for mulls? Mixing with mineral soil lets Fe bind P in Mulls… Paré and Bernier 1989

Circularity of cause and effect for O horizons Reich et al. 2005

Changes in O horizons in response to treatments

Long-term Site Productivity Study Ponder et al. 2012

Taking off the living “skin” of the upper soil entails not just removing organic matter – it’s ripping the epidermis off the soil Powers et al. 2005

Removing slash is like shaving off a beard (dead material) Slash+O O Retained Removing O horizon is like shaving off skin! Powers et al. 2005

Effects of prescribed fire on O horizon in ponderosa pine 4 Mg/ha loss 108 kg/ha loss Covington and Sackett 1979

Effects of prescribed fire on O horizon in loblolly pine 7 Mg/ha loss 63 kg/ha loss Schoch and Binkley 1986

New horizons in O horizons Saprophytes Mycorrhizae Vertical structure in a Scots pine soil also has strong pattern of fungal communities Lindahl et al. 2007

New horizons in O horizons Add 15 N to plot O horizon: Does it make into the mycorrhizae? Do the mycorrhizae “give it up” to trees? Days since labelling At low N, mycorrhizae feed on tree-C, but give no N to trees! Näsholm et al. 2013

O horizons are alive Photo © Kjell Olofsson “Although fragmentation of (…soil/plant) systems is sometimes justified, we should recognize that soils are connected to plants almost as integrally as arteries are to veins in animals, and that our investigations need to embrace these connections rather than sever them in the name of convenience. ”

This is an O horizon, the characteristic horizon of forest soils Scott Mutter This is a forest floor

“Ode to Forest Soils” by Sergei Wilde …Aye, bury me deep in the mouldy earth And sprinkle me well with lime, Where I can rest in deep sweet peace Until the endpoint of time… Photo from Chuck Davey