Friday, March 29, 2024

Green tools are not available

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Farmers need to increase and retain more carbon in soil and reduce greenhouse gas emissions but don’t have the tools to do it, Landcare Research scientist David Whitehead says. “We have got to find a way for farmers to retain more soil carbon but it’s very difficult to do.
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“This is the problem, farmers are under pressure to do something about greenhouse gases but we don’t yet have the technologies available using current widespread, high-intensity management practices without negative trade-offs for productivity.

“We can’t expect farmers to do anything when we haven’t provided them with management options,” Whitehead said.

Results from a four-year investigation testing practical changes to farming management reveal greater than expected carbon losses.

High soil water content from irrigation was found to enhance carbon losses while irrigating lucerne with water and dairy shed effluent on stony soils really is not good practice, programme leader Whitehead said.

The programme tested practical changes to farming management to reduce carbon and nitrogen losses on a dairy farm on stony soils at Lincoln University’s Ashley Dene Research and Development Station.

The agriculture sector, particularly dairy farming, is facing increasing demands to reduce its greenhouse gas emissions and nitrogen leaching, more so in the eastern region of the South Island where soils are stony and nitrogen is lost through nitrate leaching and through nitrous oxide emissions, Whitehead said.

“We examined farm management to reduce nitrogen and phosphorous losses by manipulating irrigation and carbon inputs from fodder species.

“We are also looking into the soil microbiological processes that regulate nitrogen losses to gain a better understanding of how and why carbon and nitrogen are either lost or immobilised.”

Carbon can be lost through decreases in soil carbon mainly caused by the sensitive balance between carbon inputs, harvest intensity and removal of water constraints to decomposition of soil organic matter.

Presenting the research to a workshop at Lincoln Whitehead said he’s surprised to find systems are losing so much carbon.

“We didn’t expect to find we are losing so much carbon.”

The next stage of the project is to change the sward to a mix with red and white clover, plantain and Italian ryegrass.

“The prospect is that this will be much more effective in reducing nitrate leaching.

“Careful management of the sward will help with the harvest versus grazing aspect and will impact on potential changes in carbon stocks that can be negative or positive depending on the grazing regime.”

While plantain is showing promise to reduce nitrate leaching it has the disadvantage of having to be re-sown and is less effective in cold conditions.

Meantime, four years of seasonal and annual estimates of water, carbon, nitrogen and phosphorus leaching losses under both water and effluent irrigated and non-irrigated lucerne growing on excessively drained, stony soils – typical of much of the land converted from dryland farming to irrigated in Canterbury have been gathered.

Results of trials show the largest carbon losses occur in the later part of the season when photosynthesis declines faster than respiration.

High soil content from irrigation enhances carbon losses but keeping soil water close to minimum crop requirement minimises respiration losses.

For non-irrigated lucerne leaching losses were 7kg to 30kg of nitrogen a hectare a year with larger losses occurring in grazing relative to cut-and-carry management.

In contrast leaching losses from irrigated lucerne are higher, 39-102kg N/ha/yr with summer leaching having the highest losses.

Inclusion of plantain with perennial ryegrass/clover can decrease nitrogen leaching by 14% and with Italian ryegrass/clover by 24%.

Well managed irrigation minimises nitrogen leaching from August to December while a winter active crop such as Italian ryegrass following grazing of fodder beet crops is critical to minimise winter nitrogen leaching.

Irrigating lucerne with effluent on stony soils leads to high nitrogen leaching and carbon loss.

“So, this is not recommended practice.”

Grazing rather than cutting non-irrigated lucerne retains more soil carbon, attributable to excreta returns and lower biomass removal rates, but the trade-off is a threefold increase in nitrogen leaching, Whitehead said.

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