# Presentation: 477071

## Crop Residues:Soil Organic Matter, Biofuel, Both?

- Wally Wilhelm
- USDA-ARS
- Lincoln, NE

## Crop residues

**Crop residues**

## The good news

**Therefore, we have choices!**

**What is (are) the appropriate use (s) of crop residues?**

**We have technology to produce large quantities of residue.**

## Potential uses of crop residues

- Feed

## Potential uses of crop residues

- Feed
- Shelter

## Potential uses of crop residues

- Feed
- Shelter
- Industrial feedstock
- Fiber
- Building material
- _Biofuel_

## Potential uses of crop residues

- Feed
- Shelter
- Industrial feedstock
- Soil amendment

## The Concept

**The Concept**

**Massive amounts of crop residue produced in US Corn Belt**
**Available source of structural carbohydrate**
**Growers expend time and energy to eliminate residue (?)**
**Collecting residue as biofuel feedstock would save time and energy, as well as address US energy needs**

## How much residue is producedby a crop of corn?

**How much residue is produced****by a crop of corn?**

**Assumptions:**
**1. Bushel of corn weighs 56 lb bu****-1****.**
**2. Harvest index [grain mass/(grain mass + residue mass)] is 0.50.**

## Harvest index (HI)(rules-of-thumb)

- HI = grain yield / biological yield
- Biological yield = grain yield + stover yield

- Corn:	0.50 to 0.55
- Wheat:	0.40
- Soybean:	0.40 to 0.45
- Sorghum:	0.48

## How much residue is produced by the US corn crop?

**How much residue is produced by the US corn crop?**

_**2000**_**	****Grain production:****	****9.97 x 10****9**** bu**
**	****(x 56)****	****558 x 10****9**** lb**
**	****Residue production:****	****2.79 x 10****8**** ton**
**	****	****2.54 x 10****8**** MT**

**Assumptions:****	****Bushel (bu) = 56 lb**
**	****Harvest index (HI) = grain / (grain + residue) = 0.50**
**	****    ****(grain yield = residue yield)**

_**2001**_** (est.)****	****Grain production:****	****9.43 x 10****9**** bu**
**	****(x 56)****	****528 x 10****9**** lb**
**	****Residue production:****	****2.64 x 10****8**** ton**
**	****	****2.40 x 10****8**** MT**

## What happens to residue returned to the soil?

**Soil organic matter**

## What is residue good for?-as a soil amendment

- Soil structure
- Bulk density, aggregation
- Erosion control
- Soil tilth
- Aeration
- Water holding capacity
- Soil temperature

- Nutrient cycling
- Ion exchange
- Buffering capacity
- Insect/microbial activity
- Filtration
- Reducing evaporation

## Carbon cycle

**Where does it come from?**
_**Carbon cycle**_

## What is soil organic matter?

- Decomposed (decomposing) residue
- Plant, animal, and microbial
- Organisms
- Burrowing insects and animals
- Microorganisms
- Fungi, actinomycetes, and bacteria

## Where does it go?

- Erosion
- Wind
- Water
- Respiration
- CO2
- Decomposition
- CO2
- Returned to pools

## Albrecht. W.A. 1938. Loss of soil organic matter and its restoration. Soils and Man. Yearbook of Agriculture. USDA. Washington, DC.

**“****Attempting to hoard as much organic matter as possible in the soil, like a miser hoarding gold, is not the correct answer. Its (organic matter) value lies in its dynamic nature. A soil is more productive as more organic matter is regularly destroyed and its simpler constituents made usable during the growing season. ... The objective should be to have a steady supply of organic matter undergoing these processes (decomposition) for the benefit of the growing crop.”**

- Organic matter dynamics

## Where does it go?(Decomposition pools)

- Residues

- _Labile pool_
- Readily decomposed
- Low C:N ratio
- Sugar, protein, starch

- _Intermediate pool_
- Resists decomposition
- Intermediate C:N ratio
- Cellulose, hemicellulose

- _Resistant pool_
- Very resistant to decomposition
- High C:N ratio
- Lignin
- (Humus)

## Can we change it?

- Residue management
- Removal
- Additions
- Manure (animal and green manure crop)
- Tillage
- Mixing
- Aeration

## Can we change it? (continued)

- Crop selection
- Species
- Rotation or crop sequence
- Crop use (grain, forage, grazing)

- Management practices
- Irrigation
- Fertilization
- Pest control
- Planting time, planting density, etc.

## Can you change it?

**Can you change it?**

***(Cambardella and Elliot, 1992)***

- More disturbance

## Native Sod Site

**Native Sod Site**
**Tillage D, Sidney NE**

**Year**

**Organic C, Mg ha****-1**

**0 to 7.6 cm**

**Notes:**
A simplified depiction of the Century model, showing the main classes of driving variables (white boxes) and major model components for plant growth and development, water balance and the soil C and nutrient model which includes multiple organic matter pools.

## Model

- Balance
- Submodel

- Active
- SOM

- Residues

- Plant
- Growth
- Submodel

**CO****2**

**CO****2**

**CO****2**

**CO****2**

**CO****2**

**CO****2**

**SOM**
**Submodel**

- Climate

- Soils

- Topography

- Management

**Century**
**Model**

- Slow
- SOM

- Passive
- SOM

***after Paustian (2001)***

**Notes:**
A simplified depiction of the Century model, showing the main classes of driving variables (white boxes) and major model components for plant growth and development, water balance and the soil C and nutrient model which includes multiple organic matter pools.

## Modeling steps for soil C component of LCA

- Initialization based on previous Iowa C project
- - Simulation for major soil types (6-10) for each county
- - Historical management practices based on CSRA and ancillary data sets
- Yields calibrated to NASS county averages
- Corn-soybean with intensive tillage simulated as baseline system.
- Conversion to continuous corn with:
- no residue removal
- residue removal limited by allowable soil erosion rate
- Three tillage alternatives: 2 mulch tills, 1 no-till

***after Paustian (2001)***

**Notes:**
Synopsis of the modeling design to assess C stover removal effects on soil C for Iowa.  Note that the baseline scenario from which the simulation starts (1975) is a corn-soybean rotation under intensive tillage.  This baseline scenario was run for each of the major soil types within every county in Iowa (climate variables were averaged at the county level).  The biomass scenario involved a cropping change going to continuous corn (which was simulated for a 20 year ‘spinup’ time period) and then simulating over a 20 year period  a) no residue removal or b) residue removal constrained by the amount of residue needed to maintain soil erosion rates at the ‘acceptable’ levels as defined for that soil type, climate, management regime as computed by the RUSLE soil erosion model.  Each of these scenarios was run for 3 conservation tillage systems.

## after Paustian (2001)

***after Paustian (2001)***

**Notes:**
Example from a simulation for a soil type in Adair county.  Corn-soybean was converted to continuous corn in 1975.  From 1995, changes in soil C occur in response to the  tillage (no-till and 1 of the mulch till) and residue scenarios (in this case showing a 95% residue removal rate).

## after Paustian (2001)

***after Paustian (2001)***

**Notes:**
Composite diagram showing net C emissions as a function of residue removal from continuous corn rotations for each of the Iowa counties (within-county simulation outcomes were aggregated using area-weighting for different soil types).  For the two mulch tillage regimes, the results show that soil C levels are maintained or increased (net soil C emissions < 0) if stover removal rates are less than around 50%.  Under no-till removal rates of 60-80% of above ground residues are possible while still maintaining soil C levels.  Removal above these rates results in net losses of C to the atmosphere.  

Note that the potential for substantial residue removal while still maintaining or enhancing soil C levels ishighly dependent on the particular scenario used, that is, assuming intensively-tilled corn-soybean (the dominant rotation currently in Iowa) as the baseline and then changing to continuous corn under different conservation tillage systems.  This scenario results in a substantial increase in the annual rate of residue production (over the baseline) together with a reduction in tillage intensity (refer to previous data slides) – thus there is additional C in the form of residues that can be removed from the system without causing losses in soil C. If we instead simulated systems that were already at equilibrium, then removal of residues would be predicted to result in a loss of soil C in all cases (see slide #6).

## Residue Management Study(1978-1983)

**Residue Management Study****(1978-1983)**

- Background
- Response to energy crisis of the 70s
- Objective (question)
- What is the affect of removing crop residues from (and adding residue additional to) the soil surface
- On soil properties?
- On crop production?

## Residue Management Study

| Y Variable | Coefficient |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
|  | Intercept |  | Slope |  | r2 |
|  | b0 | SE | b1 | SE |  |
| Residue | 4.34 | 0.31 | 0.29 | 0.04 | 0.86 |
| Grain | 2.91 | 0.17 | 0.13 | 0.02 | 0.80 |

_***Over all years***_

***(Wilhelm et al., 1986)***

## Residue Management Study

| Dependent Variable | Coefficient |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
|  | Intercept |  | Slope |  | r2 |
|  | b0 | SE | b1 | SE |  |
| Soil water | 174 | 7 | 6 | 1 | 0.84 |

_***Soil Water***_

***(Wilhelm et al., 1986)***

## Residue Management StudySoil organic matter (0-30 cm)

**After 8 yr of treatment application (cont. corn, no tillage)**

***(Maskina et al., 1993)***

## Residue Management StudySummary (production)

- Greater plant stress
- Less available soil water
- Greater canopy and soil temperature
- Decreased yield (50% removal)
- 13% reduction in grain yield
- 17% reduction in residue yield
- Yield reductions more closely related to soil water than soil temperature

## Residue Management StudySummary (Soil organic matter)

- Soil organic matter
- Changed linearly with residue application
- Remove 100%	-5.7%
- 	Remove   50%	-3.4%
- 	No removal	 -----
- 	Add 50%	+4.6%

## Change in soil organic carbon (SOC) after 13 years of continuous corn (0-15 cm)

| Tillage | Residue | Year 0 (kg ha-1) | SOC change after Year 13 |  |
| --- | --- | --- | --- | --- |
|  |  |  | N rate (kg ha-1) |  |
|  |  |  | 0 | 200 |
| No till | Harvested | 56.2 |  |  |
|  | Returned | 53.1 |  |  |
| Moldboard | Harvested | 54.8 |  |  |
|  | Returned | 58.6 |  |  |
| Chisel | Harvested | 53.2 |  |  |
|  | Returned | 56.9 |  |  |

**Change in soil organic carbon (SOC) after 13 years of continuous corn (0-15 cm)**

***(Clapp et al., 2000)***

## Summary (reality?)

- Maintaining soil organic matter levels and yields with residue removal is VERY difficult.
- There are many _more_ ways to accelerate destruction of organic matter than building it.
- Field and simulation studies suggest cautious optimism for use of crop residues as biofuel.

## Clichés 

**“****Throwing rocks is easy!”**** (Pat Gruber, CargillDo  LLC, Sept. 6, 20001)**

**A bit of skepticism can be a good thing.**
**You can’t have your cake (soil organic matter) and eat (burn) it too.**
**If it sounds too good to be true, it probably is.**
**There are no free lunches.**

_**However**_

## Rules in the Real World

- Nature always bats last.
- Consequences of ignoring rules:
- “Today's solutions become tomorrow's problems.” ***(Tom Franzen, Iowa Farmer)***

## Crop Residues:Organic Matter, Biofuel, Both?

- The good news
- Large quantities of residue available
- Production technology exists
- The bad news
- Maintaining SOC levels with residue removal is difficult
- The question
- SOC or biofuel, both?

## Dynamics of soil organic matter

_**Dynamics**_** of soil organic matter**

- Secret to SOM’s pivotal role in soil (crop) productivity
- Key to its fragile (transient) nature

## Remember, grain pays the bills!

**Remember, **_**grain**_** pays the bills!**

## Slide 40