Nitrogen in the Soil Environment

Flashcards covering principles of nitrogen in plants and soil, the nitrogen cycle (mineralisation, nitrification, denitrification, volatilisation), Carbon to Nitrogen ratios, and nitrogen fertiliser use in agroecosystems.

Nitrogen (N) in plants

A limiting nutrient for plant growth with concentrations ranging from $2 - 4\,\%$, acting as a major component of chlorophyll, amino acids (proteins), ATP, and nucleic acids.

Nitrogen deficiency symptoms

General yellowing of leaves (chlorosis) first visible in older leaves, as N is easily translocated via the phloem, leading to reduced biomass and crop yield.

Australian soil surface N

Typically ranges between $0.05 - 0.15\,\%$, which is lower than the general soil surface range of $0.1 - 0.6\,\%$.

Organic Nitrogen pool

The largest pool of nitrogen in soil, accounting for $80 - 95\,\%$, found in forms such as humus, proteins, and amino acids.

Mineral Nitrogen pool

Nitrogen in the form of ammonium ($NH_4^+$) and nitrate ($NO_3^-$), comprising less than $2\,\%$ of soil N.

Fixed N

$NH_4^+$ ions located in the interlayer of $2:1$ clay minerals like illite or vermiculite, accounting for $4 - 8\,\%$ of soil N.

Nitrogen mineralisation

The microbial release of $N$ from organic matter into the soil solution.

Nitrogen immobilisation

The microbial assimilation of $N$ from the soil solution into organic matter.

Net Mineralisation threshold

Occurs when decomposing organic material has a carbon to nitrogen ($C:N$) ratio less than $15/1$.

Net Immobilisation threshold

Occurs when decomposing organic material has a carbon to nitrogen ($C:N$) ratio greater than $30/1$.

Volatilisation

The loss of $NH_4^+$ as $NH_3\,(g)$ due to ammonium-ammonia equilibrium, which is heavily influenced by soil pH.

Nitrification

The microbial oxidation of $NH_4^+$ to nitrite ($NO_2^-$) and nitrate ($NO_3^-$) that requires warm, moist soils and oxygen.

Denitrification

The microbial reduction of $NO_3^-$ and $NO_2^-$ to gaseous forms in the absence of $O_2$, often occurring in waterlogged soils with available $C$ sources.

Nitrous Oxide ($N_2O$)

An intermediate gas of denitrification with $300$ times the warming effect of $CO_2$ and an atmospheric life of $150$ years.

Nitrate Leaching

The movement of $NO_3^-$ freely in the soil solution by mass flow, potentially adding $H^+$ to the soil and impacting waterways such as the Great Barrier Reef.

Anhydrous ammonia

A nitrogen fertiliser containing the highest nitrogen concentration at $82\,\%N$.

Urea

A nitrogen fertiliser containing $46\,\%N$, produced by combining ammonia and carbon dioxide.

Sugarcane nitrogen application

A commodity with one of the highest average nitrogen application rates in Australia at $229\,kg\,N\,ha^{-1}$.

Wheat Crop N demand (NSW)

The total amount of nitrogen needed for wheat in New South Wales including grain, straw, and rhizodeposition, averaging $74\,kg\,N\,ha^{-1}$.