Deciphering crop adaptations to ammonium stress and soil acidification

Crop adaptations to ammonium stress and soil acidification 

Understanding soil acidification 

Soil acidification refers to the process where soil becomes more acidic characterized by a decrease in soil pH levels, typically falling below 7.0. It can occur naturally or be accelerated by human activity, particularly from extensive use of ammonium-based fertilizers.

Causes of soil acidification 

1. Natural processes: 

   - Rock weathering is a fundamental geological process in which rocks gradually disintegrate through chemical and physical interactions. Water, atmospheric gases, and biological agents trigger mineral transformations, releasing ions and creating new geological structures. 

   - Organic matter decomposition involves complex biological processes where microorganisms systematically break down dead plants and animal materials into simpler chemical compounds.

   - Rainfall leaching minerals describes water's capability to transport dissolved minerals through soil layers, fundamentally altering soil chemical composition and nutrient availability. 

2. Anthropogenic factors: 

   - Rain acid represents precipitation with pH levels below 5.6, resulting from atmospheric sulfur dioxide emissions, nitrogen oxide pollutants, chemical transformations in cloud layers, and hydrogen ion concentration increases. 

   - Industrial pollution includes chemical manufacturing emissions, heavy metal industrial releases, toxic atmospheric particulates, and waste stream contamination. 

   - Intensive agricultural practices, such as excessive use of ammonium-based fertilizers. 

Agricultural impacts of soil acidification 

Soil acidification affects agricultural productivity through multiple interconnected mechanisms: 

• Reduced nutrient availability- limiting essential mineral absorption by crops and disrupting fundamental plant growth processes. 

• Decreased microbial activity - soil microorganisms, which are essential for nutrient cycling and the decomposition of organic matter, become less effective in acidic environments. This decrease compromises overall soil health and the ecosystem's resilience.

• Altered soil structure - acidification reduces soil aggregation, impacting water retention, root penetration, and overall soil physical properties. These structural changes create less hospitable conditions for plant growth and development. 

• Limited crop productivity - reduced nutrient uptake, compromised microbial support, and structural soil degradation collectively result in diminished agricultural yields, posing substantial challenges for food production and agricultural sustainability. 

Key research findings 

Recent research by Wang et al. (2022) investigated how wheat seedlings respond to ammonium nitrogen (AN) stress and rhizosphere acidification. 

The research analyzed 31 wheat varieties, examining: 

• nitrogen assimilation capacity, 

• plant resistance mechanisms, 

• rhizosphere biochemical interactions. 

Results 

Wheat seedlings experienced significant inhibition under ammonium nitrogen conditions, with soil acidification intensifying these negative effects. The study revealed: 

• varying inhibition levels across different plant tissues- the roots and shoots biomass were reduced,

• nitrogen form as the primary driver of N and C metabolism variations, 

• pH levels critically affect plant height and root diameter- it strengthens the shoots and roots' biomass reduction,

• diverse plant tissues showed specific responses to ammonium nitrogen exposure, soil acidification, and nutrient assimilation challenges.

Under ammonium nitrogen conditions (pH=6,5) compared to normal growth conditions individual parts of the plant have been reduced by: 

• 14,7% - shoot, 

• 33,4% - root, 

• 19% - plant dry weights, 

• 16,2% - plant height, 

• 13% - leaf area. 

The various components of plants were adversely affected by changes in soil conditions. Specifically, elevated levels of ammonium nitrogen significantly reduced plant biomass and overall development.

Researchers concluded that the selection of the wheat cultivars with a greater capacity to adapt to changing AN levels and soil acidity, N assimilation rates in both shoot and root and the ability to maintain high levels of C shall be considered a key traits.

Implications for agricultural practices 

The study highlights the importance of understanding crop stress mechanisms, developing resilient wheat varieties, and reducing the impacts of soil acidification.

In modern agricultural practices, farmers can use strategic breeding considerations.  

Researchers recommend selecting wheat cultivars with the following: 

• high adaptability to changing ammonium nitrogen levels, 

• robust nitrogen assimilation rates in shoots and roots, 

• capacity to maintain high carbon metabolism. 

The results of this study are valuable for implementing effective strategies to reduce NH4+ stress in crops.

Source: Wang et al. 2022; Front. Plant Sci.