The Effects of Nitrogen Fertilization

on Nutrient Cycling and Forest Productivity

By: Eric Sucre

Questions Regarding N Fertilization to Forests

•What are the pros and cons regarding different fertilizer types?

•Why is the amount of response so variable from site to site?

•How can we better predict the amount of response?

•How does N fertilization effect other essential plant nutrients?

•What are the long-term implications of repeated fertilization?

Background

•Regional Forest Nutrition Research Project-RFNRP

•Duration of response

•Nitrogen Deficiency and its indicators (TCV)

•>55,000 ha of forests fertilized annually

•Fertilizers Typically Used:

1) Urea ([NH2 ]2CO)

2) Ammonium Nitrate (NH4NO3)

3) Biosolids

Douglas-fir values of foliar concentrations for various nutrients

Background cont….

Effects of Urea and Ammonia Nitrate on Soil Solution pH

(Dangerfield & Brix, 1979)

Background cont……

•Nitrogen Pools in Pacific Northwest Soils

•Total Nitrogen vs. Available Nitrogen

•Mineralization Rates

•Potential Negative Effects of N Fertilization

•Response Time for Douglas-fir Varies

•Predictors for Douglas-fir Response to N Fertilization:

1) C:N ratio

2) Foliar SO4-S

3) Genotypes

4) Site Index

An Example in the PNW

•2000-4000 kg ha-1 of Total N

•1-2% Mineralization Rate

•20 to 80 kg ha-1 of Available N per year

• Fertilizer Rate of 200 to 400 kg ha-1

•Approximately 25% of total goes to Biomass Increment

•Typically 10% to 20% of Added Fertilizer Enters Trees

•Where does the remaining fertilizer go?

Forest Nitrogen Cycling Process Representing Major Fates and Effects of N Fertilization

(Nason and Myrold, 1992)

General Pattern of Fertilizer Distribution

(Fisher and Binkley,2000)

1) Less than a quarter of the fertilizer is taken up by the trees in the first few years.

2) Another quarter or so is immobilized in microbial biomass and soil organic matter.

3) A variable and difficult-to-measure amount is lost from the forest ecosystem through leaching and volatilization.

Factors Influencing Timing of Fertilization

1) Time of Year

2) Temperature

3) Wind Speeds

4) Precipitation Patterns

Fates Associated with Ammonium Nitrate and Urea Fertilization

Ammonia Volatilization-

• Factors Controlling Volatilization: pH, High Winds, Moisture, and Temperature

• Urea Has Higher Rates of Ammonia Volatilization Compared to Ammonium Nitrate

• Rainfall Occurring after Fertilization can Minimize Volatilization Rates Tremendously.

• What about in the PNW?

Fates Associated with Ammonium Nitrate and Urea Fertilization cont……

Immobilization-

• Concerns about small application rates

• Immobilization rates greater with urea than ammonium nitrate

• Strader and Binkley, 1989

Fates Associated with Ammonium Nitrate and Urea Fertilization cont……

Nitrification and Leaching-

• Factors Controlling Nitrification: acidic pH, warm soils, and moisture

• Ammonium Nitrate has higher rates of nitrification compared to Urea

• What about Potential Groundwater Contamination Associated with Fertilization?

Leaching Rates Associated with Urea from RFNRP Data

(Cole, 1995)

Stream water nitrate concentrations for Urea and AN fertilization.

(Fisher & Binkley, 2000)

Relationships that can predict N fertilization response?

•1) C:N Ratio and Site Index:

-What are good C:N Ratios in the PNW?

-Generally sites with higher C:N ratios have a larger %response to N fertilization.

-Productive Sites still respond to N fertilization, but not as much

-Site Index is inversely related to Response

(Edmonds & Hsiang, 1987)

Relationships that can predict N fertilization response?

•1) C:N Ratio and Site Index

•2) Foliar SO4-S-

•a) Has shown to be a better a way of estimating stand level response than foliar N

•b) Foliar SO4-S was a correct predictive index of response for 17 of 19 stands tested, whereas using foliage nitrogen concentration correct prediction was obtained in only 12 stands.

•c) Both boron and SO4-S were both positively correlated.

Relationships that can predict N fertilization response?

•1) C:N Ratio and Site Index

•2) Foliar SO4-S

•3) Genotypes-

a) Genotype X Environment Interactions

b) Genotype X Nutrient Interactions

1) nutrient use efficiencies

2) root systems and density

Potential long-term effects of repeated N fertilization on other nutrients

• Initial decrease in uptake of some nutrients, especially P, is common

• Initial increase in uptake of nutrients is common as well- K, Ca, Mg

• However, there’s concern about potential K deficiencies occurring after several rotations.

(Mitchell et al., 1996)

Conclusions

• Which fertilizer type to use?

•Do we really know what the long-term effects of repeated N-fertilization are?

• Evidence supports the claim that N fertilization doesn’t pose a threat to our ground and drinking water supply.

• There has to be some long-term consequences regarding N fertilization

• Increased harvesting due to increased volume and shorter rotations.

Thank You

????? Questions ?????