Nutrient The Cycle
Atmospheric pool Precipitation Canopy, wood, and root Litter fall Organic material
SOIL
2
Soil and rock minerals cations
1 2 3
5
Soil solution storage
4
Groundwater level
4 4 Leaching
Channel
BEDROCK
1 Cation exchange 2 H+ and exudates 3 Nutrients
5 Acids, chelates, nutrients
Nutrients Distribution in Soils
Source: Jobbagy, EG, and RB Jackson. 2001. The distribution of soil nutrients with depth: Global patterns and the inprint of plants. Biogeochemistry 53: 51-77.
Litter Production
Above ground production is easy to measure. Below ground production is very difficult to assess because of the:
Short life and rapid root turnover (duration of root growth). Most roots live less than one year. Difficulty in quantifying rates of root exudation.
Nutrient Cycling in Soils and Vegetation
Biome
Total biomass Mineral elements in biomass Net primary production Net mineral uptake Total litter fall Minerals returned in litter
(kg ha-1 )
(kg ha-1 y-1)
Tropical rainforest Forest (Central Europe) Northern taiga Dry savanna Artic tundra
517,000 370,000 260,000 26,800 5,000
11,081 4,196 970 978 159
34,200 13,000 7,000 7,300 1,000
2,029 492 118 319 38
27,500 9,000 5,000 7,200 1,000
1,540 (76%) 352 (71%) 100 (85%) 312 (98%) 37 (97%)
Source: Chorley, RJ, SA Schumm, DE Sugden. 1984. Geomorphology. Methuen
Vertical Distribution of Nutrients
Source: Jobbagy, EG, and RB Jackson. 2001. The distribution of soil nutrients with depth: Global patterns and the inprint of plants. Biogeochemistry 53: 51-77.
Figure 11.17 Distribution of organic carbon in four soil profiles, two well drained and two poorly drained. Poor drainage results in higher organic carbon content, particularly in the surface horizon.
Soil Organic Matter in Soil
Amount in mineral soil (w/w)
range in A horizon for North Jersey for South Jersey subsoils have less: 0.5 - 7.0 % 1.5 - 3.0 % 0.75 - 2.0 % 0.25 - 1.75 %
Organic Soils
> 20%
Classification of SOM
Soil Organic Matter Living Organisms: BIOMASS Indentifiable dead tissue: DETRITUS Non-living, Non-tissue: HUMUS
Humic Substances
NonHumic substances
Humus
Complex organic substances in soil not identifiable as organic tissue
Amorphous, colloidal
non-humic compounds
identifiable biomolecules
humic compounds
product of decomposition + synthesis (polymerization) relatively stable, resistant to further breakdown
A Model of SOM
Source: modified from http://www.soils.wisc.edu/virtual_museum/som/index.html
Functional Groups in SOM
Carboxyl groups: -COOH Phenolic groups: -ArOH Proteinaceous material
Alcoholic groups: -ROH
Saccharides (sugars)
Water
Source: modified from http://www.soils.wisc.edu/virtual_museum/som/index.html
Humic Substances
Random, complex polymers resistant to breakdown Resulting composition
C N P S 50-60% 5% 0.6-1.2% 0.5% C:N ratio=10:1
Solubility classification
Humin, Humic Acids, and Fulvic Acids
From Schlesinger, W.H. 1997. Biogeochemistry. Academic Press.
Humin and Fulvic and Humic Acids
Component Fulvic Acid (FA) Humic Acid (HA) Humin Residence time, y 1,800 - 4,300 1,900 5,400 2,900 3,500 C,% 43 - 52 50 - 60 >60 N, % 0.5 0.2 2.4 5.0 5.0 8.0 Molecular mass low high high Climate Cool, temperate warm
The long residence time of FA and HA contributes to the memory of a soil. Fulvic acids are soluble in water and are transported to deeper horizons along plant roots. Humic acids form strong complexes with Al, Ca, Mg, and Fe.
Chatsworth, NJ
Simplified Carbon Cycle
Photosynthesis:
Numbers represent Pg (1015 g) of carbon stored in the respective pools.
Plants + CO2 = Organic Molecules + O2 Respiration: Organic Molecules + O2 = Humus + CO2
Estimates of Active N Pools
Medium Air N2 N2O Land Plant Animals SOM Sea Plant Animals Solution or suspension Dissolved N2 0.3 0.2 1,200 22,000 15 0.2 1,500 3,900,000 1.4 Form Pg N
The Nitrogen Cycle
Figure 12.1
Carbon and Nitrogen Balances
Soil C and N storage (pools) are in general balanced. Exceptions to this are: Peat bogs (accumulation of carbon). Northern peatlands contain ~30% of the global storage of SOC. Extreme deserts (accumulation of nitrogen) The result is that the amounts change rapidly over limited spans of time and then stabilize (steady state) at levels characteristic of climate, topography, etc.
Soil Carbon vs. Climate
Soil C increases with Mean Annual Precipitation (MAP) and decreases with Mean Annual Temperature (MAT). Pattern is due to balance of inputs and losses as influenced by climate.
Source: Amundson, R. 2001. The Carbon Budget of Soils. Annu. Rev. Earth Planet. Sci. 2001. 29:53562
US Carbon Budget: Land Use Change
Annual net sources and sinks of carbon resulting from different types of land use in the United States. Source: Houghton, RA, JL Hackler, and KT Lawrence. 1999. The U.S. Carbon Budget: Contributions from Land-Use change. Science 285: 574-578.
In the News: April 2008
Carbon Losses
Carbon was lost from soils across England and Wales over the period 19782003 at a mean rate of 0.6% yr-1 (relative to the existing soil carbon content), reaching 2% yr-1 in soils with a carbon content greater than 10%.
Source: Bellamy, P. H., P. J. Loveland, R. I. Bradley, R. M. Lark, and G. J. D. Kirk. 2005. Carbon losses from all soils across England and Wales 19782003. Nature 437: doi:10.1038/nature04038.
Carbon Content of Different Soil Groups
Soil Group C mass virgin (Pg C)
24.4 49.8 47.3 21.4 17.7 10.6 43.6 7.3 222
C mass cultivated (Pg C)
18.1 36.9 35.1 15.9 13.1 7.8 35.6 5.4 168
Historic loss (1700-2000)
6.3 12.9 12.2 5.5 4.6 2.8 8.0 1.9 54
----------------------- Pg C -----------------------Temperate Forest Temperate Grassland Tropical Forest Tropical Grassland Shallow/saline/arid Wetlands/paddy Histosols Andosols TOTAL
Anthropogenic Alteration to the N Cycle
The main source of alteration to the N cycle derives from the application of fertilizers, which shows the most dramatic rate of increase over the last 40 years.
Source: Vitousek, PM et al. 1997. Human alteration of the global nitrogen cycle: Sources and consequences. Ecological Applications 7: 737-750.
Trends in Fertilizer Use
Source: Tilman et al., 2001. Forecasting agriculturally driven global environmental change. Science 292: 281-284.
Consequences of N Cycle Alteration
According to Vitousek et al. (1996) human alteration of the N cycle resulted in:
Approximately doubled the rate of input to the terrestrial N cycle, Increased the concentration of NxO gases in the atmosphere, Contributed to the acidification of soils, stream and lakes Increased the quantity of organic C.
What is the Effect of Increased N in the Ecosystems?
Terrestrial ecosystems:
Accumulation in degraded soils. Influences the accumulation (storage) of carbon. Loss in biodiversity In N-saturated systems, N is lost to groundwater (problem in southern NJ), streams, estuaries (eutrophication) and to the atmosphere.
Nitrogen Input/Output Relationship in Large Watersheds
Organic Carbon in Urban Soils
A
B
A. In the presence of non-native earthworms and higher temperatures, the layer of litter (O horizon) is thinner in urban forest soils, but the amount of C in the soil is greater than in suburban/rural forest soils. Without earthworms the O horizon in urban forest soil is thicker than in suburban/rural soils (lower quality litter in urban B. stands). Well maintained laws (low density residential and institutional land) contained SOC densities similar to forest soils. In the city of Baltimore, Organic matter content in urban soils was negatively correlated with bulk density. Caution: these observations should not be generalized before other similar studies in cities located in different climates.
Source: Pouyat, R, P Groffman, I Yesilonisc, and L Hernandez. 2002. Soil carbon pools and fluxes in urban ecosystems. Environmental Pollution: S107-S118.
Decomposition
Decomposition and mineralization reactions are microbial enzymatic oxidation and reduction reactions. Enzymes are catalyst that aid decomposition or building of organic material. During decomposition the material is broken down into their organic constituents and finally into CO2 and H2O. Decomposition processes can take place in aerobic or anaerobic conditions.
Rate of Decomposition of Organic Materials
Organic compound Sugar, starches and single proteins Hemicellulose Cellulose Fats, waxes Lignins and phenolic compounds Slow Rate of decomposition Rapid
An Example
A mixture of residues from Pinus nigra, P. sylvestry, and Quercus robur
Original litter Sugars Cellulose Hemicellulose Lignins Waxes Phenols % of total 15 20 15 40 5 5 %age lost by decomposition by 1st year 2nd year 5th year 10th year 99 100 --90 75 50 25 10 100 92 74 43 20 -100 97 77 43 --100 95 75
Decomposition in Aerobic Soils
In the presence of oxygen, the general reaction is:
Organic C + O2 CO2 + H2O + energy
First, the cell constituents (aminoacids, proteins, lipids, etc) are released. Then decomposition of the most resistant material occurs in stages. Final products: NH4+, SO42-, NO3-, and H20.
Decomposition in Anaerobic Soils
Without oxygen, decomposition proceeds very slowly. Under anaerobic conditions organic matter tends to accumulate. The final products are a variety of partially oxidized compounds: organic acids, alcohol, and methane gas.
Sequence of Redox Reactions
Factors Controlling Decomposition and Mineralization
Environmental conditions promoting mineralization and decompositions are:
Temperature: 25-35 C is optimum. Water content: extreme (dry and waterlogged) conditions reduce plant growth and microbial activity (~60% of porosity filled with water is optimum). Soil texture: other conditions being equal, clay tend to retain more humus. Near-neutral pH.
The Priming Effect
Fresh residue addition fuels microbial activity. Microbial population and metabolic capacity increases Native, stable soil humus is attacked. Slow pool of SOM is depleted, while total SOM has increased.
Figure 11.3 Diagram of the general changes that take place when fresh plant residues are added to a soil. The arrows indicate transfers of carbon among compartments. The time required for the process will depend on the nature of the residues and the soil. Most of the carbon released during the initial rapid breakdown of the residues is converted to carbon dioxide, but the smaller amounts of carbon converted into microbially synthesized compounds (biomass) and, eventually, into soil humus should not be overlooked. Although the peak of microbial activity appears to accelerate the decay of the original humus, a phenomenon known as the priming effect, the humus level is increased by the end of the process. Where vegetation, environment, and management remain stable for a long time, the soil humus content will reach an equilibrium level in which, the carbon added to the humus pool through the decomposition of plant residues each year is balanced by carbon lost through the decomposition of existing soil humus.
Typical C/N Ratios for Soil-Related Organic Material
Young legumes 12-20:1 Young grasses 20-40:1 Manure 20-50:1 Corn stalks 60:1 Oat/wheat straw 80-90:1 Tree leaves 60-100:1 Pine needles 200-250:1 Woody material 250-400:1
A C/N ratio depends on the biochemical composition of a tissue, i.e. relative amounts of protein, cellulose, lignin, etc.
Figure 11.4 Changes in microbial activity, in soluble nitrogen level, and in residual C/N ratio following the addition of either high (a) or low (b) C/N ratio organic materials. Where the C/N ratio of added residues is above 25, microbes digesting the residues must supplement the nitrogen contained in the residues with soluble nitrogen from the soil. During the resulting nitrate depression period, competition betwee...