Carbon and Nitrogen Cycling in Soils Weathering represented processes that mainly deplete soils in elements relative to earth’s crust Biological processes differ from weathering in that they tend to enrich soils in certain elements, most importantly C and N (soil organic matter) Study of soil matter has always been important: –Organic N was main focus until 1950’s Maintenance of crop production (mainly N limited) until advent of commercial N production Still very important in countries lacking financial resources Soil C is now a focus: –Conversion of tropical forests to ag (and loss of SOM) is a major reason for increases in atm CO2 –Management of existing cropland in industrial countries a proposed way to reduce NET CO 2

Soil C Cycle Plants + O 2 = humus + CO 2 Plants are equivalent of parent material (primary minerals) Humus is equivalent of secondary minerals

Plant Organic Composition Plant chemistry varies greatly. Differences in lignin/N, ash content, etc determine how fast it is recycled by microbes will discuss decomposition more Ash can be bio-minerals

What is Soil Organic Matter? Contains everything from living microbes to humic compounds of great antiquity and degree of chemical alteration Determining exactly what soil organic matter is made of is one of the most challenging problems in all of soil science –Unlike secondary mineral classification, there is no analogous approach for organic matter Various methods of have devised to break total soil organic matter into different fractions represently what is in nature: –Chemical methods (different extractants) –Physical methods (density, size, …) –Combination of above Fractions have been chemically characterized in various ways –C/N ratios –Molecular structures – 14 C contents

Common Soil Organic Matter Classification Scheme SOM Microbe biomass plant partshumus (1-4%) non-humic substancehumic subs. humin humic acidfulvic acid

C/N: 11:1; 9 to 17:1; 7 to 21:1

Describing Soil C (and N) Cycling in Soils Except in very unusual situations, soil C and N storage (pools) are constantly be added to and subtracted from –Peat bogs (C loss minimal and C (peat) builds up) –Extreme deserts (N comes in but doesn’t leave) 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. The basics of this can be relatively easily described mathematically using a mass balance (accounting) approach:…..

CO 2 INPUTS= leaf litter, root death, root exudates LOSSES = CO 2, erosion, dissolved C

Change in soil organic matter vs time = inputs - losses Where K = decomposition constant (yr -1 ) Boundary condition for integration assumes no C at t=0

If no inputs occur (such as decomposition of a compost pile): dC/dt = L dC/dt = kC C(t) = C o e -kt where C o = starting amount

Visualization of Soil Organic Matter Buildup and Model Some important steady state relationships: k= I/C  = C/I= residence time Non-steady state (I>L) Steady state (I=L) Time

State Factors and Organic Matter Inputs Climate –MAP , I  (within limits) –MAT , I  (within limits) Biota –Controls way C is added to soil (leaves vs. roots) –Controls input quality (k) Topography –Aspect, etc affect available moisture, temp etc. Parent Material –Nutrients , I  Time –Time , I  (over very long time spans) Humans –Variable Decrease from crop removal Increase from irrigation, fertilization, etc.

State Factors and Losses (k) Climate –MAP and MAT , k  (within limits) Biota –Litter quality (lignin, C/N, etc.)affect k. –Possible that geographic distribution of microbes varies Topography –Can cause direct erosional loss of organic matter Parent Material –clay , k decreases (chemical and physical reasons) Time –Effect not well known - may cause decrease in k due to clay increase and nutrient declines Humans –cultivation , k  (!)

Soil organic C (to 1m), respiration=C inputs; decay rate vs. MAT dervied from global “Fluxnet” experiment (Sanderman et al., 2003)

State Factors and Losses (k) Climate –MAP and MAT , k  (within limits) Biota –Litter quality (lignin, C/N, etc.)affect k. –Possible that geographic distribution of microbes varies Topography –Can cause direct erosional loss of organic matter Parent Material –clay , k decreases (chemical and physical reasons) Time –Effect not well known - may cause decrease in k due to clay increase and nutrient declines Humans –cultivation , k  (!)

Soil C vs. Time Soil C commonly approaches steady state within 10 2 to 10 3 years Steady state value depends on array of other state factors…

Soil C vs. Climate Soil C increase with MAP and decreases with MAT ! Pattern is due to balance of inputs and losses and effect of climate on these

Measuring Inputs and Losses Inputs = litter (easy) + roots (difficult) Litter measured via ‘litter traps’ (mass/areatime) Roots not commonly measured directly except in grasslands - common to assume root=(litter)(x) where x=1-2 Losses = soil respiraiton (easy) - root respiration (very difficult) Soil respiration measured by surface chambers (and CO2 buildup) - Root respiration commonly assumed = (soil respiration)(x) where x ~ 0.5.

Soil C Concentrations vs. Soil Depth Discussion so far on total amounts (not how its distributed Inputs and in-soil redistribution processes vary greatly, resulting in 3 general depth trends: –Exponential C decrease vs. depth (e.g. grasslands) Inputs decline with depth Transport combined with decomposition move C downward –Erratic changes with depth (e.g. deserts) C inputs vary with root distribution (which is related to hydrology) Transport not so important (???) –Biomodal C maxima vs. depth (e.g. sandy forest soils in temp. climates) Large surface inputs Production and transport of dissolved C Precipitation of dissolved C via complexation with Fe/Al

Soil C Model vs. Depth (in reader #2)

Summary of Soil Carbon Cycle Soil C is controlled by inputs and losses Soil C strongly related to climate Soil C vs depth variable but somewhat predictable Some remaining questions: –How important is soil C globally (and what is global C cycle)? –How can humans affect global soil C budget? Cultivation Global warming –Role of soil C in international efforts to reduce atmospheric CO2