She told the Grassland Association conference in Tauranga last year that it was common to recommend applying nitrogen at the end of a drought.
However, she said, when a drought breaks late in the season (mid to late autumn), the natural release of soil nitrogen could compound fertiliser nitrogen loads at a time when pasture nitrogen demand was low.
A moderate drought of three weeks wasn’t enough to upset the soil-plant system, and pasture growth and nitrogen uptake recovered immediately on rewetting.
But in more extreme droughts, measurements indicated a mismatch between the flush of soil mineral nitrogen and pasture intake could increase nitrogen loss through leaching.
After a late-breaking drought, leaching could be made worse by fertiliser nitrogen additions, although outcomes depended on the severity and timing of a drought and whether rainfall was heavy or light.
Lucci said the 2008 drought was an example of rapid rewetting of the soil risking leaching before pasture could recover, but in other years rewetting was gradual and enabled good growing conditions and plant uptake of nitrogen.
In a paper co-authored with AgResearch land and environment colleague Dr Mark Shepherd and AgResearch systems modeller Dr Iris Vogeler-Cronin, Lucci said the flush of mineral nitrogen when rain arrived was from new sources of nitrogen and carbon, such as dead cell and plant material and excreted osmolytes (urea), becoming available.
A significant saving could be made on fertiliser at a time of pressing financial demands on farms, as well as avoiding an increase in nitrogen leaching over the following winter.
Data modelling based on measurements made on a Waikato dairy farm during the drought of 2007-8 predicted an accumulation of 20kg NO3-N (nitrate as nitrogen) a hectare in the top 50cm of soil post-drought and therefore any fertiliser nitrogen applied during the drought would have added to the available nitrogen pool.
Where a total of 70kg N/ha fertiliser was applied during the drought, 35 kg NO3-N/ha was leached during the following winter.
A glasshouse pot trial confirmed that a substantial amount of available N was released (40-60kg N/ha at 0-15cm) on rewetting after a “severe” drought, which might be sufficient to promote growth.
The conclusion was that more work was needed to refine advice and strategy for fertiliser management tailored to different properties and soil types in order to meet production and environmental goals.
In the same conference session Balance Agri-Nutrients technical adviser Murray Lane presented the case for no-tillage ground preparation for crops, which was unequivocally favourable in terms of soil conservation and economics, he said.
Lane’s paper, co-written with Bruce Willoughby of Ecometric Consulting, said the gains in conservation were in preserving soil biota, soil structure and the natural capital of the soil while retaining phosphorus within the paddock and away from waterways. Cost savings were from the reduced use of machinery and more efficient placement of fertiliser.
He said more research was needed to better define the relative advantages of differing amounts and forms of phosphorus applied with the seed for a range of soil types, fertility levels and crops.
In another paper co-authored by Lucci and Shepherd, the risk of nitrate leaching from autumn nitrogen fertiliser on pasture was shown to be less significant from an increase in pasture nitrogen concentration and more significant from the extra forage grown, consumed and waste nitrogen excreted.
Getting to the root of it all
Higher nitrogen and soil phosphorous fertility resulted in 20% lower root mass but 32% higher root production during September and November. There were no other significant differences in root mass or production between fertiliser treatments, results from Manawatu experiments by AgResearch lead scientists Dr Mike Dodd and Dr Alec MacKay showed.
They found soil compaction reduced root production by about 22% year-round and root growth was more influenced by nitrogen than phosphate under those conditions.
In a report presented by Ballance scientist Aaron Stafford and co-authored by his colleague Mohammad Zaman and Landcare Research principal scientist Professor Surinder Sagga, trial results suggest that 5 to 10mm of irrigation/rainfall is needed within eight hours after urea application to suppress nitrogen losses due to ammonia gas volatilisation, depending on initial soil moisture content.
