“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
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
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
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