“Soil appeals to my senses. Warm brownish colors characterize fields and roofs in Cézanne’s landscape paintings of

southern France.” Hans Jenny (1984)

Paul Cézanne, Environs de Gardanne (detail), 1886-90

Now give we place to the genius of soils, the strength of each, its hue, its native power for bearing. Vergil, Georgics, Book II

Sustaining “the Genius of Soils”

Garrison Sposito University of California

at Berkeley

The human exploitation of soil foragriculture has been an enormoussuccess. But the modern practice offarming has greatly acceleratedrates of soil erosion, with soil beinglost at a global rate that is ordersof magnitude greater than that ofproduction. Farming also greatlyaltered the natural soil C cycle,thus beginning the ignition of thelargest surficial C reservoir, which,under anthropogenic warming, iscapable of driving large positivefeedbacks that will further increasethe emission of greenhouse gases,exacerbating climate change.[Amundson et al., Science 348, 1261071 (2015)]

Agricultural land occupies 38 % of the ice-free Earth surface, having taken over 70 %of the grassland, 50 % of the savannah, 45 %of the temperate deciduous forest, and 27 %of the tropical forest. [Foley et al., Nature 478,337 (2011)]

12 %

26 %

Soil Horizons

Symbols for horizons

O organic horizon containing litter and decomposed organic matter

A mineral horizon darkened by humus accumulation

E mineral horizon lighter in color than an A or O horizon and depleted in clay minerals

AB or EB transitional horizon more like A or E than B

BA or BE transitional horizon more like B than A or E

B accumulated clay and humus below the A or E horizon

BC or CB transitional horizon from B to C

C unconsolidated earth material below the A or B horizon

R consolidated rock

Soil Profile

Hans Jenny

The Five Factors of

SoilFormation

Soil Orders are classes of soils with similar characteristics as determined mainly by

climate

ALFISOLS

SPODOSOLS

Global Distribution of Soil Orders

9.5

1.23.7

0.8

8.02.6

12.6

8.67.4

10.6

16.0

19.0

Percent of Global Land Area

15.2

23.5

10.1

13.59.1

10.6

The natural capital of soils derives from three fundamental soil properties: texture, mineralogy, and humus

Natural Capital: The stock of assets that permits soils to function beneficially

[Palm et al., Annu. Rev. Environ. Resour. 32, 99 (2007)]

Texture is defined by the percentages of sand-, silt-, & clay-sized particles:

Sand: 2.0 – 0.05 mmSilt: 0.05 – 0.002 mm

Clay: < 0.002 mm

Texture determines the nature of the soilpore space and soil aggregate formation,thus affecting aeration, water-holdingcapacity, transport of water and solutes,as well as the life cycles of the soil biota.

Ecosystem Services•Storage & flow of green water•Runoff of blue water•Nutrient transport•Contaminant transport•Habitat for the soil biota

TEXTURE

Mineralogy refers to rock-forming(primary) and secondary minerals insoil. These minerals are reservoirs ofmetal nutrients and mediators ofnutrient cycling. Secondary mineralsexert major controls on contaminanttransport.

Mineralogy

Mineralogy determines the capacity of soil to provide nutrient elements to

the biota and retain them against loss by leaching. It determines the

potential for indigenous metal toxicity.

Ecosystem Services•Nutrient storage (metals)•Nutrient cycling•Carbon sequestration•Water purification•Waste attenuation

[Chorover et al., Elements 3, 321 (2007)]

[Amundson et al., Elements 3, 327 (2007)]

Humus is the dark-colored mixture

of organic materials

in soil produced

by microbes

HUMUS

Ecosystem Services•Nonmetal nutrient storage •Nutrient cycling•Carbon sequestration•Waste transformation[Scharlemann et al., Carbon Management 5, 81 (2014)]

topsoilsubsoil471.4 Pg C

454.0 Pg C

490.3 Pg C

IPCC Climatic RegionsSoil organic C density to 1 m depth

Example: Low Nutrient Capital & Carbon DebtLow nutrient capitalmeans a low content of primary minerals (metal nutrients) and

humus (nonmentalnutrients) in soil.

Carbon debt is the change in carbon stock resulting from land conversion (carbon stock in prior natural vegetation minus that in crop) divided by the crop yield.

[West et al., PNAS 157, 19645 (2010); Foley et al., Nature 478, 337 (2011)]

Percentage of soils with < 10 %primary minerals in sand and siltfractions

Understanding Green & Blue Water

green water

blue waterflow

blue water Blue Water is the water in rivers, lakes, and aquifers. Annual river flow: 45,900 ±

4,400 km3

Green Water is the water in soil originating from rainfall and

accessible to plants. Annual ET flow: 70,600 ± 5,000 km3

green water flowgreen water flow

61 ± 15 %

[Coenders-Gerrits et al., Nature 506, E1 (2014)Schlesinger & Jasechko, Agric. For. Meteor. 189-190, 115 (2014)Rodell et al., J. Climate 28, 8289 (2015)]

[Falkenmark, Environment, March/April (2008)]

[Rost et al., WRR 44, W09405 (2008)]

Most of the water consumed by global croplands is green water

Green water fraction of total cropland consumptive use

Green water supports about 80 % of the global cropland and accounts for about 90 % of the consumptive use of water by croplands, rainfed or irrigated. Even irrigated croplands have a large green water footprint (more than half of their consumptive use). [Rost et al., WRR 44, W09405 (2008); Mekonnen & Hoekstra, Hydrol. Earth Syst. Sci. 15, 1577 (2011); PNAS 109, 3232 (2012)]

Addressing the Hydrologic Challenge for AgricultureG

reen

Wat

er A

vaila

bilit

y (%

)

Productive Green Water Flow (%)

At current yields, about 2/3 of thegreen water resource is lost via soilevaporation. But when yields rise from 1 to 3 t/ha, the crop canopycloses and about 2/3 of the green water flow is productive. [Sánchez, Nature Geoscience 3, 299 (2010)]

The basic hydrologic challenge is to increase green water availability and productive green water flow (T/ET),

thus increasing crop yield.

Example: Maize yields in sub-SaharanAfrica average 1 t/ha (as compared to3 t/ha in Latin America & South Asia)because ca. half of the rainfall is lost torunoff and deep percolation, while ca.60 % of the green water flow is lost tosoil evaporation. “If all the green waterresource could be used productively,i.e., without evaporative loss andnutrient deficiency, the maize yieldcould reach 3 t/ha.” [Rockström &Falkenmark, Crit. Rev. Plant Sci. 19,310 (2000); Rockström et al., PNAS 104,6253 (2007)]

Rain input

Green waterAvailable

Crop residues reduce the evaporation ofwater from soil by shading, causing a lowersurface soil temperature and reducing windeffects. A number of studies from irrigatedand rainfed croplands in the United Stateswhere no-tillage is used have reportedsignificant water savings from residues.Irrigation converts blue water into greenwater, which flows by evapotranspiration.Transpiration is productive green waterflow essential for crop production, whereasevaporation is generally not useful for cropproduction, although it does cool the cropcanopy environment.

The Sánchez Strategy

Green water dynamics in the rhizosphere are affectedby the root exudate, mucilage (mainly polysaccharides),which has a high water-holding capacity. Neutrontomography studies show that, during soil drying, thewater content in the rhizosphere is greater than in thebulk soil. The high water adsorption capacity ofmucilage affects the water retention curve by increasingthe water content of the rhizosphere for a given matrichead. In turn, the increased water content results in ahigher hydraulic conductivity in the rhizosphere.

Neutron radiography of the green water distribution around lupine roots during soil drying. Gray value is proportional to water

content (dark means wet).

Green Water Dynamics in the Rhizosphere

[Kroener et al., WRR 50, 6479 (2014)Ahmed et al., Funct. Plant Biol. 41, 1129 (2014)Schwartz et al., WRR 52, 264 (2016)Oburger & Schmidt, Trends Plant Sci. 21, 243 (2016)]

water retention curve

Separation by wet-sieving, humus (HOC) defined

operationally by size < silt. Chemical properties by elemental

analysis and NMR/IR/X-ray spectroscopy.

[Baldock et al., Soil Res. 51, 561 (2013)]

Biomass Region

van KrevelenDiagram

Humification traces a downwardpath in the van Krevelen diagram,from upper right to lower left, fromBiomass through Lignins towardCondensed Aromatics.

[Ohno et al., Environ. Sci. Technol. 48, 7229 (2014)]

Understanding Humus

P = particulate R = recalcitrant = char

aliphatic

polar

[Luo et al., Global Biogeochem. Cycles 30, 40 (2016)]

HumificationMineral-Humus

corn-oat-alfalfa rotationmoldboard plow tillage

corn-soybean intense tillage

Urea N

corn-soybean conservation tillage

NH3 N

erosionalzone

depositional zone

Coupled biogeochemical-erosion modelsto assess farmland soil C management donot consider humus stabilization throughmineral-humus interactions. [Papanicolau etal., J. Geophys. Res. Biogeosci. 120, 2375 (2015)]

Current ESMs differ widely asto predicting soil C change over21st century, as well as currentNPP and soil C stock. Only twomodels meet the benchmark forNPP and soil C, but they differgreatly as to their predictions ofsoil C change. [Todd-Brown et al.,Biogeosciences 11, 2341 (2014)]

Modeling Soil C Changes

Jeff Mitchell Department of Plant SciencesUniversity of California, Davis