Why do stubble-retained systems need more nitrogen?

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More than 70 per cent of the nitrogen contained in stubble is lost through burning, yet stubble-retained systems need more nitrogen than stubble-burnt systems to optimise yield.

 

Photo of man and tractor

Key points

  • Stubble can immobilise nitrogen and reduce yields by 0.3 to 0.5t/ha in modern systems, with cereal-on-cereal most at risk.
  • Reduce heavy stubble loads (bale, graze, late-burn).
  • Add more nitrogen (up to 5kg nitrogen per t/ha of cereal residue) and apply it early, deep banding if feasible.

Stubble-retained systems require extra applied nitrogen to overcome tie-up that can reduce yield by 0.3 to 0.5 t/ha.

Photo: CSIRO and Grassroots Agronomy

Stubble’s ability to protect the soil surface and increase the capture and storage of rainfall is why Australian growers are the highest adopters of stubble-retained systems worldwide. The benefits of stubble for soil and water conservation are clear, but the impact on nitrogen cycling is less well understood, especially in modern, no-till systems where the stubble remains on the soil surface.

Stubble-retained systems need more nitrogen than traditional burnt stubbles, partly due to tie-up of nitrogen by stubble, but also because the nitrogen contained in stubble is not a good source of nitrogen for crops.

Farming microbes

There are two crops growing in every paddock: the above-ground crop (wheat or canola, for instance) and the below-ground crop – microbes. Two-thirds of microbes live in the top 10 centimetres of soil and they can double in weight each growing season using carbon from crop residues and root exudates for energy.

Photo of tractor
Stubble-retained systems require extra applied nitrogen to overcome tie-up that can reduce yield by 0.3 to 0.5 t/ha.
PHOTO: CSIRO and Grassroots Agronomy

Cereal stubbles provide plenty of carbon, with a carbon-to-nitrogen ratio of about 90:1. But microbes have a 7:1 ratio, meaning they compete with the crop for nitrogen and temporarily ‘tie-up’ or immobilise the nitrogen in their bodies. As they die, they slowly release the nitrogen back into the soil.

A worst-case scenario for nitrogen tie-up is when large amounts of cereal residue are incorporated into the soil close to the time of sowing, as the competition between microbes and the crop for nitrogen is intense, and early nitrogen deficiency can limit crop yield potential in some circumstances.

But does nitrogen tie-up occur in modern farming systems where stubbles are left on the surface or standing? Recent CSIRO research within the GRDC Stubble Initiative has found that it does.

In two long-term field experiments CSIRO investigated where the nitrogen in retained stubble ends up, and how to avoid yield penalties from nitrogen tie-up.

Second wheat

At Temora, New South Wales, CSIRO compared nitrogen and crop growth in a nine-year, no-till, controlled-traffic, inter-row sowing trial with a canola/wheat/wheat sequence where weeds were strictly controlled.

Stubble was fully retained or reduced by either early crash grazing for seven to 10 days after harvest or late stubble burning about a month before sowing.

Stubble retention had little impact on crop yield, except in the second wheat crop, where yields were consistently reduced by an average of half a tonne per hectare when stubble was retained (Table 1). When stubble was reduced there was a significant increase in pre-sowing soil mineral nitrogen: 13 kilograms per hectare more in the burnt treatment and 33kg/ha more in the grazed.

Table 1. Effect of stubble burning or grazing on wheat (W) and canola (C) grain yields (t/ha) at Temora, NSW.

2009 W 2010 C 2011 W 2012 W 2013 C 2014 W 2015 W 2016 C 2017 W
Retain 1.7 4.2 4.6 4.4 0.7 3.8 4.1 3.2 3.7
Burn 1.7 4.0 4.6 5.0* 1.0 3.8 4.6* 3.2 3.2
Graze 1.7 4.3 4.5 4.8* 0.9 3.7 5.3* 3.3 3.3

* Yield was significantly different to stubble retained (P<0.05)

Nitrogen tie-up

In another long-term study at Harden, NSW (over 28 years), the average wheat yield in stubble-retained systems was 0.3 tonnes per hectare less than stubble burnt systems. These lower yields were mostly observed in wetter seasons (Figure 1).

Graphic showing that retained stubble was more likely to reduce yield compared with burn stubble
Figure 1. Retained stubble was more likely to reduce yield, when compared with burnt stubble, in wetter seasons at Harden, NSW (shown by the points below the line), suggesting that nitrogen tie-up may be involved.
SOURCE: John Kirkegaard

In 2017, to see whether reduced yield was due to nitrogen tie-up at the site, CSIRO measured the impact of doubling the nitrogen rate (from 50 to 100kg N/ha) on both burnt and stubble-retained plots. Nitrogen was applied by early broadcasting at sowing or by deep-banding to reduce the potential for microbes (found mainly in the soil surface) to immobilise the nitrogen.

The additional nitrogen improved crop growth, yield and protein in the stubble-retained treatments more than the burnt treatments, indicating that nitrogen was limiting via tie-up in the stubble. Deep banding also improved yield in both stubble-retained and burnt treatments but had more impact on the stubble-retained treatment (see top right for a link to the full GRDC Update paper).
CSIRO also tracked the fate of the nitrogen in retained wheat stubble using stubble grown and labelled with a stable radioisotope of nitrogen (N15). The wheat stubble was set up at three experimental sites at Karoonda, South Australia (Mallee sand), Horsham, Victoria (Wimmera vertosol), and Temora (acidic loam) in the summer fallow of 2014, and two successive wheat crops were grown in 2015 and 2016. The amount of nitrogen originally added in the stubble could be traced in the crops, the soil or remaining in undecomposed stubble over two seasons (Table 2).

For the two wheat crops grown in 2015 and 2016, only one to six per cent of their nitrogen requirement was provided by the nitrogen from the stubble, while more than 50 per cent of the original nitrogen remained locked-up in the stubble or the microbial pool. While stubble is a great source of carbon for microbes, it provides almost no nitrogen for crops.

Cereal stubble can tie-up nitrogen, with yield penalties of 0.3 to 0.5t/ha, mainly in successive cereal crops. Overcome yield penalties either by reducing the stubble load, or by applying more nitrogen (approximately 5kg nitrogen per t/ha of cereal residue) and applying it earlier to the following crop. Deep placement of nitrogen improves nitrogen capture by crops irrespective of stubble management but is more effective in stubble-retained situations.

Table 2. The fate of nitrogen contained in retained wheat residue after two years of cropping at three sites in southern Australia.

TemoraHorshamKaroonda
Stubble load in 2014 (t/ha) 7.5 4.0 2.5
N in stubble in 2014 (kg/ha) 55 32 12
After two wheat crops (2015-16)
% still in stubble 10 20 20
% in soil organic matter 35 40 45
% removed in crop 22 14 8.5
% unaccounted (lost) 33 26 26.5

SOURCE: CSIRO

GRDC Research Codes CSP00186, CSP00174, MSF00003, BWD00024

More information:

Dr John Kirkegaard, CSIRO
02 6246 5080
john.kirkegaard@csiro.au

GRDC Update paper The effects of stubble on nitrogen tie-up and supply