Understanding the Nitrogen cycle in your compost

SESL Australia
When it comes time to having your compost analysed against the AS4454 for Composts, Soil Conditioners and Mulches at SESL Australia you will receive test results for the many forms of nitrogen: total nitrogen, nitrate (NO3–) in mg/L and mg/kg, ammonium (NH4+) in mg/L and mg/kg and nitrogen drawdown index or NDI. This article takes a look at the different forms of nitrogen that occur in the nitrogen cycle and why they are important to your compost.
 
Firstly, we will look at why nitrogen is an essential nutrient to plant growth. A healthy plant is characteristically a dark or vivid green in colour whereas a nitrogen poor plant will typically show chlorosis (yellowing) starting in the older leaves but eventually progressing to the whole plant. Nitrogen is responsible for this vivid green colour since its main role is involved in chlorophyll synthesis which is involved in photosynthesis. Lack of nitrogen and chlorophyll will mean that photosynthesis process will be compromised since the green pigment in chlorophyll will be low and it’s this pigment which transforms energy from the sun into sugars. 
 
A plant lacking nitrogen will be a paler green/yellow, stunted with slow growth.
 
Nitrogen makes up 78% of our atmosphere however in its gaseous form (predominantly N2) is unusable to plants. Only once it’s been through the nitrogen cycle and fixation process will it be suitable for plant uptake. Atmospheric nitrogen is fixed by a range of soil microbes including Azobacter and Rhizobium in some plant roots and converted into ammonium (NH4+). Nitrifying bacteria will then convert this into nitrites (NO2–) and then into nitrates (NO3–). Once in its nitrate (NO3–) form it is available for plant uptake.

The AS4454 for Composts, Soil Conditioners and Mulches measures the total nitrogen, nitrate (NO3–) and ammonium (NH4+) forms of N. In fresh composting substrates N is mainly present in ammonium form or relatives of ammonium such as amines in protein, urea and uric acid in manures. Proteins from plant material and wastes such as food scraps and animal wastes rapidly degrade to ammonium. So, in immature compost we often see high ammonium levels. Ammonium is not that friendly to plant growth so high ammonium can often be associated with poor growth of seedlings and shows up as a low or failing bioassay result in the AS 4454 bioassay test. Ammonium usually peaks at around 1-3 weeks into the composting cycle.

After further composting this ammonium is used by the composting microorganisms to make their own protein so ammonium levels start to drop. As the readily available energy sources get used up these microbes start to die off and nitrifying organisms start to turn the N into nitrate, just like in the soil. So, in well matured compost we see little or no ammonium and a predominance of nitrate.

Knowing the total nitrogen content of a compost gives an insight into the potential ability of the product to supply plants with nitrate. Above around 0.8% total N there is an increasing likelihood the compost will actually supply N. This is usually associated with a C/N ratio of less than about 20.

In N poor composts with total N less than 0.8% and C/N ratios over 20 it is more likely the compost will actually consume N and this is measured using the NDI test where you incubate the compost with added nitrate and recover less nitrate after 4 days than what you started with.

 Depending on the ingredients of the compost the total nitrogen values will differ. Table 1 outlines the main sources of composts/manures and their typical total nitrogen values. The total nitrogen value will give a rough indication of whether the available nitrogen will be high or low. For example poultry manure once fully composted will give higher source of available nitrate than green waste compost.

Table 1.  Total nitrogen values in varying compost and manure sources. 

Source

Total Nitrogen (%)

Feedlot manure

2 – 3

Poultry manure

2 – 4

Dairy manure

1 – 2

Urban green waste

0.8 – 1.5

Crop residue

1.5 – 2.5

The NO3–, NH4+ level and NDI value will give insight into the maturity and N supplying ability of the compost. If the NH4+ level is high and the NO3– is low then this indicates that the compost nitrogen cycle is not complete, the compost is still immature and may need a few more weeks for the microbial activity to convert NH4+ into NO3–. Of course, if you add immature compost to soil the N cycle will be completed in the soil but sensitive seeds and seedling may be affected by ammonium toxicity.

If the NDI is > 0.2 then this product is considered composted and if > 0.5 then it is considered mature. If below < 0.2 and the NH4+ is also low, then total N is likely to be too low to provide N to plants, the opposite in fact, it may cause N draw down. An application of high N waste like manure or urea at around 300g/m3 may be required to improve the NDI. However, if the NH4+ is high then this is an indication that the compost is still immature. N deficiency caused by NDI is commonly seen in gardens where uncomposted woody mulch has been applied and microbes consume all the available soil nitrogen before the plants have a chance to get it. In this instance an application or urea at 20g/m2 will give enough nitrogen to satisfy both the plants and microbes. We always recommend urea be applied over uncomposted woody mulches.

A question we sometimes receive is why we report NH4+ and NO3– results in both mg/L and mg/kg. Aside from this being a requirement of the AS4454, these results differ in the way they were initially assessed. Simply results reported in mg/L are measured by volume of the extract solution whereas results reported in mg/kg are measured by weight. This allows an agronomist to calculate how much to apply to provide a certain amount of N to a crop.

In summary, if your compost has a high total nitrogen value (>0.8%) an NDI result > 0.5, and a ratio of NH4+ to nitrate of <0.5 (ie twice as much nitrate as ammonium) then your compost has reached maturity and will likely supply plants with nitrogen rather than consume it.