Soil and Plant Processes Causing Soil Acidification

Introduction

Review of simulations on soil acidification in the West River catchment.
- Patricia Hill's bonus project.
- Analyzed historical farming systems and projected future acidification.
- Used acidification formula based on pH difference, buffering capacity, and soil properties.
Catchment size: Approximately 7-8 km across and 15 km north-south.
Modest acidification rates measured by the amount of lime needed to neutralize acidity.

Uneven pH changes observed over time.
Green to red color change indicates strong acidification.
- Areas at the top and southeast corner of the catchment show significant acidification.
- Up to four kilos of lime equivalent of acidity per hectare per year.
Higher acidification rates lead to more extensive acidification.
Tools developed to aid farmers in deciding when and where to invest in lime.
After 50 years, most of the catchment is red, indicating pH levels unsuitable for agriculture.

Farmers can alter farming systems to manage soil acidification.
Limited financial rewards may restrict these options.
The simulations demonstrate the potential implications of not addressing soil acidification.
Implications vary across different parts of the catchment.

Plants photosynthesize, using CO_2 to produce organic compounds (initially sugars).
Sugars are converted to organic acids through metabolic steps.
Organic acids dissociate, releasing hydrogen ions (H^+).
Hydrogen ions are pumped out of cells via membrane transport processes.
Cation uptake is fueled by oxidation of hydrogen ions.
Cations neutralize organic acid anions, balancing charges.
Anion uptake is coupled with hydrogen ion consumption.
Cation uptake increases soil acidity, while anion uptake decreases it.

Plant material dies and decomposes.
Hydrogen ions are used up during decomposition due to the production of organic acid anions.
Complete decomposition releases CO_2 and cations.
In a closed system: No net acidification because hydrogen ions are neutralized during decomposition.
Natural ecosystems maintain a balance between temporary soil acidification during plant growth and neutralization during decomposition.

Agriculture disrupts the closed carbon cycle by removing organic matter.
Harvested material is not allowed to decompose in situ.
A portion of the hydrogen ions initially left in soil is not neutralized.
This incomplete decomposition leads to an imbalance in the carbon cycle and soil acidification.

Mineralization: Production of ammonium (NH_4^+) from organic matter (also called ammonification).
Nitrification: Conversion of ammonium to nitrate (NO_3^-).
Intensive mineralization and nitrification occur in the top 2.5 cm of soil.
Organic matter and microorganisms are most abundant in the topsoil.
Microbial abundance decreases significantly with soil depth.

During mineralization, one equivalent of hydrogen is consumed, leading to temporary alkalinization.
During nitrification, two equivalents of hydrogen are released.
Overall: From dead organic material to nitrate, one hydrogen ion is effectively left in the soil, potentially causing acidification.

Nitrate uptake by plants is accompanied by hydrogen ion consumption.
Complete Nitrogen Cycle: Proteins in organic material -> amino acids -> deamination -> ammonium -> nitrification -> nitrate uptake by plants.
Consumption of one H^+ during mineralization, release of two during nitrification, and consumption of one during nitrate uptake.
If complete, there is no acidification.

Incomplete cycle due to nitrate leaching.
Nitrate is easily displaced from the topsoil by water.
Leaching separates the mineralization/nitrification processes from nitrate uptake.
Nitrate moved deeper into the soil can still be taken up by roots, consuming H^+ at a deeper level than where mineralization/nitrification previously occurred.
Imbalanced nitrogen cycling: Production and consumption of hydrogen ions are spatially separated.

Nitrate leaching below the root zone leads to loss of nitrate and environmental damage.
Groundwater contamination: Nitrate concentrations > 50 mg/L unsuitable for drinking.
Eutrophication of surface water: Leads to algal blooms (e.g., Swan River).
Imbalanced nitrogen cycle leaves one extra, non-neutralized hydrogen ion in the topsoil (acidification).

Extrapolation of the above situation to fertilizer nitrate is incorrect.
Statement: Leaching of nitrate causes soil acidification (correct only for nitrate from mineralization & nitrification).
Adding nitrate as fertilizer nitrate bypasses mineralization and nitrification.
No imbalance in hydrogen ions being consumed and produced when fertilizer nitrate is added (alkalinization instead).
Plants take up fertilizer nitrate accompanied by consumption of hydrogen ions.
Leaching of fertilizer nitrate: No soil acidification occurs; alkalinization may occur where nitrate is taken up.
Environmental problems related to nitrate leaching remain (groundwater contamination).

Hydrogen ion oxidation (H+ ATPase) is primary ion transport in plant roots resulting in acidification.
Acidification follows the parts of the soil profile where most roots are present.
Differences in anion vs. cation uptake impact acidification.
Excess cation uptake leads to acidification, especially in legumes.
Imbalance in the carbon cycle prevents neutralization of all acidity generated by plant growth.
Leaching of nitrate from natural cycle (mineralization and nitrification) differs from leaching of fertilizer nitrate regarding acidification effects.

Excess cation uptake by plants results in acidification, mostly in the topsoil.
Residue decomposition leads to alkalinization at the very topsoil surface.
Nitrogen cycle: Formation and leaching of nitrate result in acidity in the topsoil and potential alkalinization deeper down.
Net acid production typically occurs around 20 cm depth in soils.
Lime application to the soil surface does little to ameliorate this bulge of acidity.
High lime application rates (5-7 tons) may provide some movement of alkalinity down the profile over several years.
Soil profile disturbance is being considered to deposit lime deeper in soil to counteract the bulge of acidity, despite being a dirty word for no-tillage farming advocates.

Leaching of nitrate leads to topographical acidification and sub-soil pH reduction. This occurs, however, relative to other nitrogen cycle processes.
Use of ammonium fertilizers (without completing mineralization and nitrification) can worsen acidification, particularly if uptake is inhibited.
Ellenbrook's excessive fertilization of gardens and lawns contributes to urban acidification.