SOIL222 - Lecture 4: Nitrogen

Nitrogen in Plants

  • Nitrogen is essential for plant growth; deficiency leads to chlorosis and senescence.

  • 4th most abundant element; concentration in plants: 3% (young) to 0.5% (mature straw).

Nitrogen Deficiency Symptoms

  • Soybean: Chlorosis of older leaves.

  • Maize: Chlorosis observable in the field.

  • Deficiency affects older leaves first due to nitrogen's phloem mobility.

Nitrogen in Soil

  • Nitrogen accumulates through fixation of atmospheric N, not by weathering.

  • Soil nitrogen levels correlate closely with organic matter (5-6% N).

  • Forms:

    • Gaseous: N2, N2O, NO, NH3

    • Mineral: NH4+, NO2-, NO3- (<2%)

    • Fixed: NH4+ in vermiculite-like clays (4-8%)

    • Organic: Needs mineralization (80-95% of total soil N).

    • Australian soils: 0.05-0.15% N.

Nitrogen Cycle

  • Significant transformations in agricultural systems are crucial for nitrogen cycling.

  • Human impact: N fertilizer, fossil fuels, legume crops.

Nitrogen Mineralization/Immobilization

  • Mineralization: Release of NH4+ from organic matter breakdown.

  • Accompanied by immobilization (microbial uptake).

  • Influenced by temperature, moisture, pH, and microbial activity.

  • Net mineralization occurs if organic material has excess N; net immobilization occurs with insufficient N.

Nitrogen Recovery by Plants

  • Recovery rates influenced by C/N ratio and plant N%.

    • Net mineralization: C/N<15, N%>2.5%.

    • Net immobilization: C/N>33, N%<1.2%.

C/N Ratios of Organic Material

  • Typical ratios:

    • Microbial tissues: 6-12

    • Sewage sludge: 5-14

    • Cereal residues: 60-80

    • Decomposition of low C/N material results in net mineralization.

Soil N Changes and Cultivation

  • Cultivation can lead to rapid N losses in soil.

  • Net gains from fertilizers significant in the short term; organic N buildup is crucial long-term.

Nitrification

  • Occurs rapidly in well-aerated soils; affected by pH and environmental conditions.

  • Nitrate production plasticity impacted by temporary inhibitions due to pH changes.

Nitrate Reactions

  • Leaching of NO3- influenced by rainfall and soil characteristics; poses health risks (methaemoglobinaemia).

Denitrification

  • Conversion of NO3- to N2 facilitated by soil microbes in low O2 environments.

Assessing N Fertilizer Needs

  • Evaluate through symptoms, plant analysis, and soil testing (mineral N, organic release potential).

Nitrogen Fertilizers

  • Main source: Gaseous NH3 via Haber-Bosch process; critical for human protein supply.