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You are here: Agriculture » About Agriculture » Newsletters and Updates » Ag in Focus » Ag in Focus - Spring 2011 » Trade off revealed in carbon, nitrogen study

Trade off revealed in carbon, nitrogen study

Nick O’Halloran measures water infiltration rates on soils treated with different organic amendments

At a glance

• Three year study of soil health in vegetable crops.
• Organic soil amendments were applied to gauge impact on crop nutrition, soil organic carbon levels and crop yields.
• Chicken litter and green organic compost broke down quickly, increasing yields.
• Seventy per cent of applied lignite remained in the soil (0–10cm) at the end of the study, but yields were reduced.
• Better organic amendment management to improve environmental outcomes and maximise production responses are being investigated.

Some organic soil amendments are better at improving soil organic carbon levels, while others supply more plant available nitrogen, a recently study has found.

The Horticulture Australia and DPI funded study investigated the impact of various organic soil amendments on soil nutrition, soil organic carbon levels and vegetable crop yields at Boneo on the Mornington Peninsula.

“Experiments compared repeated applications of organic amendments ranging in decomposability — or ‘lability’ — from chicken litter, which is very labile, to more stable green organic compost and highly biologically resistant lignite (brown coal)”, project scientist Nick O’Halloran said.

“We were interested in how these different organic amendments affected crop production and soil organic carbon in the longer term, knowing some would break down quickly, and the consequence of this breakdown for plant nutrition,” he said.

The experiments were carried out on sandy Tenosol type soils as classified by Raymond Isbell’s Australian Soil Classification.

Full standard grower practice (SGP) fertiliser rate was 55kg nitrogen per hectare (N/ha) at transplanting + 35kg/N/ha midcrop + 39kg/N/ha at budding, each applied as calcium nitrate (CaNO₃).

The results

In the first year of experiments, chicken litter was applied at 17.5 tonnes of organic carbon per hectare. In the second and third years, this application was reduced to five tonnes annually (see Table 1).

TABLE 1 - Total dry weight, organic carbon and organic nitrogen application rates (t/ha) for each of three applications for green organic compost, chicken litter and lignite

	1st broccoli crop			2nd broccoli crop			3rd broccoli crop
	DM1	OC2	ON3	DM1	OC2	ON3	DM1	OC2	ON3
1 total dry matter application rate; 2 Organic carbon application rate; 3 Organic nitrogen application rate
Treatment	t / ha
Green organic compost	64	14.4	1.09	19	5.3	0.32	19	5.3	0.32
Chicken litter	46	17.5	1.47	13	4.9	0.42	13	4.9	0.42
Lignite	–	–	–	7	4.8	0.03	7	4.8	0.03

Mr O’Halloran said the initial large application of chicken litter was able to significantly increase soil organic carbon, but this wasn’t maintained with the smaller subsequent applications.

Similarly, an initial large application of green organic compost (14.4t/ha) significantly increased soil organic carbon, and the increase was bigger than for chicken litter at the end of the first crop despite less carbon being applied.

“Compost wasn’t able to maintain that higher organic carbon level with subsequent smaller applications (5t/ha annually) either,” Mr O’Halloran said.

So, why did organic carbon levels decline so rapidly?

FIGURE 1 - Proportion of applied organic carbon (OC) measured in the soil (0–10cm) following three applications of organic amendments (green organic compost, chicken litter and lignite)

“The reason for the decline in organic carbon (see Figure 1) following the application of these more labile organic amendments was that they stimulated microbial activity,” Mr O’Halloran said.

“Soil microbes thrive on the readily available energy and nutrient source, but their activity decomposes organic carbon releasing some of it as CO2.

“This breakdown shouldn’t be seen as a bad thing, it is actually the increased microbial activity that is responsible for many of the benefits associated with higher soil organic carbon levels.”

It is this microbial activity that is responsible for the release of plant nutrients, the building and stabilisation of soil structure and the suppression of diseases, Mr O’Halloran explained.

“In these experiments, the decomposition of chicken litter released large quantities of plant-available nitrogen resulting in significantly higher broccoli yields — even with reduced nitrogen fertiliser inputs (see Figure 2).

But Mr O’Halloran said there were potential environmental drawbacks that must also be considered.

“Chicken litter provides a lot of nitrogen to the crop, but this can also lead to high rates of nitrogen leaching and emission of powerful greenhouse gasses like nitrous oxide.

“Further work is required to quantify the potential environmental impacts and to develop management strategies to minimise these losses.”

Mr O’Halloran said that lignite ended up with similar organic carbon levels after three years to the other organic amendments despite just five tonnes of organic carbon being applied each time, but that it had no benefit for soil fertility.

“We actually saw a significant decrease in yield (see Figure 2), but couldn’t be sure of the cause,” he said.

FIGURE 2 - Yield of three broccoli crops following the application of organic amendments (green organic compost, chicken litter and lignite) prior to transplanting of each crop combined with nitrogen fertiliser (calcium nitrate) applied and 100% and 50% of standard grower practice. Vertical bars represent LSD (p=0.05) for each crop for comparison between both organic amendment and fertiliser rate treatments

The bottom line

Project team applying organic amendments

Incorporation of organic amendments seven days before planting a broccoli crop

So, what effect did organic amendment application have on economic returns?

Mr O’Halloran said that to determine the net economic benefit of each treatment, purchase and application costs, and yield benefit need to be considered.

But he said the economics of applying organic amendments is most sensitive to changes in yield, as shown in 2009 DPI research carried out by Kevin Wilkinson.

A small increase or decrease in yield caused by applying an organic amendment can easily offset amendment purchase and application costs.

“A simple economic analysis for this study found that chicken litter was cheapest to purchase, but more importantly gave the biggest yield responses, and therefore had the best gross margins.” Mr O’Halloran said.

“However, the potential offsite impacts and environmental costs need to be considered.”

More work is also needed around application.

“Scientists still have considerable work to do around application methods, timing and rates to get the best production and environmental outcomes from the application of organic amendments,” Mr O’Halloran said.

“These losses are costing the farmer money because that nitrogen is no longer available to the plant and reduces nitrogen use efficiency.”

Research is also investigating applying combinations of different organic amendments with inorganic fertilisers and the addition of additives such as nitrification inhibitors, which slow the release of nitrogen.

“All of which should enable growers to better match nitrogen supply to crop nitrogen demand and further increase the benefits from the application of organic amendments,” Mr O’Halloran said.