How do I manage nitrogen in a stubble-retained system

Stubble retention is associated with a temporary nitrogen lag during early crop growth stages, as microbes ‘borrow’ soil nitrogen to break down the stubble from the previous crop. This is known as nitrogen tie-up or immobilisation.

Nitrogen tie-up can leave the current crop short on available nitrogen during the initial stages of plant growth if adequate fertiliser nitrogen is not supplied. A rough rule of thumb is that every tonne per hectare of cereal or canola stubble will tie-up 5 kg/ha of nitrogen.

The type of stubble, in addition to the amount of stubble, also influences the extent and duration of nitrogen tie-up. Nitrogen tied-up in microbes will be freed up later in the season (4-8 weeks) and could contribute to later season growth depending on seasonal conditions.

Additional nitrogen at sowing helps to provide the nutrition required to break down the previous stubble and also makes sure the germinating crop has sufficient nitrogen to establish, which ultimately helps maintain yield potential.

Retaining stubble is unlikely to have a major effect on the availability of other nutrients (phosphorus, sulphur, potassium) in the short term, but does have a positive impact in the longer term as stubbles decompose. Stubble retention only has a minimal impact on maintaining soil organic carbon levels in low-rainfall farming systems.

How does nitrogen tie-up work?

Schematic of nitrogen pathways

Figure 1. Nitrogen pathways: mineral N (NH4 and NO3) is immobilised by microbes and released through mineralisation when microbes die. Source: M. Unkovich et al (2016) Nitrogen mineralisation and N decision support tools in cropping systems of south eastern Australia (GRDC Report).

Immobilisation is the conversion of inorganic nitrogen to organic nitrogen by microbes or plants (Figure 1). As soil microbes break down stubble, they extract available nitrogen from the soil as a source of energy to fuel the stubble decomposition process. This limits the amount of nitrogen available to growing crops, often resulting in nitrogen deficiency.

Crop residues with a large carbon to nitrogen (C:N) ratio (more than 22:1) will result in immobilisation, while lower ratios will result in mineralisation. Wheat stubbles tend to have C:N ratios of around 80:1, whereas legume stubbles are more commonly around 35:1 and therefore tie-up less nitrogen (Table 1).

Table 1. C to N ratio of crop shoot residues remaining after grain harvest from a survey of the published literature. Source: M. Unkovich et al (2016) Nitrogen mineralisation and N decision support tools in cropping systems of south eastern Australia (GRDC Report).

Crop species

Number of reports in the literature

C:N ratio reported (mean value)

C:N ratio reported (range: lowest to highest)

Barley

4

66

39 - 129

Wheat

9

79

35 - 143

Canola

7

79

23 - 179

Chickpea

2

32

19 - 44

Faba bean

3

28

23 - 34

Field pea

12

32

14 - 83

Lentil

2

25

17 - 37

Narrow-leaf lupin

6

42

15 - 67

Mineralisation Is the reverse process of immobilisation. It is the microbial decomposition of organic nitrogen from manure, organic matter and crop residues to plant available inorganic forms. The amount of mineralisation depends on the organic matter content of the soil, soil pH and soil water and temperature. Mineralisation will occur during the growing season where soil temperatures are greater than 5°C, and the rate of mineralisation increases with increasing soil temperatures. The optimal temperature for mineralisation is 15-28°C.

How does stubble management influence nitrogen tie-up?

Nitrogen immobilisation will increase with increasing stubble load, which means less nitrogen is available to the crop at the start of the season especially following a dry summer.

How stubble is managed affects the rate of nitrogen immobilisation and mineralisation.

Incorporation of cereal stubble will increase immobilisation quickly and is dependent upon background soil mineral nitrogen status, soil type and rainfall events. Incorporated wheat stubble can immobilise 5–13kg/ha of nitrogen for each tonne of stubble, while standing stubble has a slower rate of stubble decomposition and will generally immobilise less than 5kg/ha. If soil nitrogen levels are low, applying nutrients at the time of incorporation may assist in reducing the amount of nitrogen that is immobilised.

Burning heavy stubble loads can reduce nitrogen tie-up, but burning increases the risk of wind erosion, lowers soil fertility and can exacerbate moisture loss through increased soil evaporation. While burning will make some nitrogen immediately available for plant uptake, up to 80 per cent of the total nitrogen and a significant amount of carbon, sulphur, phosphorus and potassium contained in the stubble is lost as a result of the burn (Table 2).

Table 2. Nutrient content in stubble and percentage nutrient loss from burning. Source: DPIRD via EPARF Stubble management and weed control (PDF 923kb)

Nutrient

Amount of nutrients in one tonne of cereal stubble (kg/ha)

Loss from a hot burn (%)

N

5.0

82

P

0.5

44

S

0.5

80

K

10.0

40

Grazing both stubbles and crops increases the availability of soil mineral nitrogen to subsequent crops, which can increase grain yield and protein in some seasons.

How do I budget for nitrogen in a stubble retained system?

Creating a nitrogen budget can help you track and estimate how much nitrogen is needed for a growing crop.

  1. Know soil nitrogen available at sowing (as measured with a deep-N soil test)
    1. At the minimum segregate into two depths (0-10cm, 10-60cm). For Yield Prophet® the depths need to be 0-10, 10-40, 40-70 and 70-100cm.
    2. Convert to kg N/ha by using estimated (or measured) bulk density
  2. Establish net mineralisation in-crop (mineralisation less immobilisation)
    1. If incorporating cereal or canola stubble, some extra nitrogen may be needed to ensure soil levels are above 60kg N/ha to reduce the impact of tie-up early in the crop
    2. Mineralisation will provide additional nitrogen to the crop later in the season, particularly in spring when soil temperatures warm up. The amount will depend on soil type and crop rotation history (e.g. pulses in the rotation will generally result in more nitrogen being mineralised when compared with a canola-wheat rotation)
  3. Applied fertiliser nitrogen - the additional amount of nitrogen required (above soil N and mineralised N) to provide crops with the amount required to achieve target yields
    1. Wheat (11% protein) - 40 kg/ha N per tonne of grain
    2. Canola - 80 kg/ha N per tonne of grain
    3. Barley - 35 kg/ha N per tonne of grain

When should I apply nitrogen fertiliser?

Pre-sowing or at sowing

For wheat when soil tests are below 40kg N/ha in the top 60cm then you should apply 20kg N/ha at sowing to make sure the crops get through to the beginning of stem elongation. If there is more than 40kg/ha mineral N then wheat and barley will make it to Zadoks stage Z30 (start of stem elongation) and canola will get to 6-leaf without losing yield potential.

The amount of nitrogen applied at sowing may be increased where:

  • the crop is following a non-legume (e.g. cereal or canola)
  • soil organic carbon levels are low (<0.8 per cent)
  • stored soil moisture is above average
  • stubble loads are high (>3t/ha)
  • the target yield is high

Post-sowing (in-crop)

Post sowing (in crop) applications should be determined throughout the season based on seasonal conditions, soil tests, expected (and desired) yield and entire crop nutrition budget including the expected contribution of mineralisation. These will all be influenced by the previous crop grown and the management of that stubble (i.e. standing, mulched, burnt, incorporated, etc)

Split application (pre-sowing and post-sowing)

Splitting applications is perhaps the most common and sensible technique. This involves an application of 30-70 per cent of nitrogen at sowing, followed by an in-crop topdress of 30-70 per cent. This technique allows the option to increase or decrease the in-crop nitrogen rate based on seasonal conditions, without compromising plant health in the early growth stages.

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