Forages for Reduced Nitrate Leaching, Brent and Maryn Austin (Canterbury)

The Simple Crop Resource Uptake Model operating within the Agricultural Production Systems sIMulator (SCRUM-APSIM) was used to simulate N balances at Brent and Maryn’s farm. APSIM predictions indicated that paddocks with high N leaching (≥30 kg N/ha) had greater residual soil N after the summer harvest.

Modelling outcomes of two seasons on the farm identified the following factors that increased the amount of leachable soil N:

• application of N fertiliser in excess of crop N requirements,
• mineralisation of N-rich crop residues,
• paddocks remaining fallow during the high N leaching risk period (autumn-winter), and
• “leaky” soils e.g. the free-draining Eyre and Lismore soils.

Leaching was greater when at least two of these factors were in play. For example, excessive N application to spring-sown crops resulted in high residual soil N at risk of leaching if no crops were sown after the summer harvest to take up the N. Similarly, mineralised N from residues retained in paddocks was available for leaching if there was no vegetation in autumn/winter to take up water and N to reduce drainage and N leaching.

N leaching mitigation options that reduced N leaching and increased gross margins included:

• growing oats as a cover/catch crop on paddocks that remained fallow after summer harvest
• using lower rates of fertiliser N for high soil residual N paddocks

When fertiliser N applications were modelled to match crop N demand (taking into account soil mineral N levels) without affecting yield and/or growing of catch crops, this resulted in a reduction in N leaching. These findings encouraged Brent to modify management in subsequent seasons. Demonstration paddock results supported the modelling of lower N fertiliser inputs and greater N use efficiency, and this translated into greater farm profitability and reduced N footprint.

Overall, results have shown that leaching is mainly influenced by rainfall through its impact on drainage, but farm management practices determine the amount of soil N at risk of leaching. Strategies that can reduce the amount of soil N available for leaching include: sowing catch crops immediately after the summer harvest to mop up residual soil N or N mineralised from soil organic matter and crop residues, and reduced N fertiliser use by calculating requirements with a recommendation system that accounts for soil mineralisation.

Crops sown in autumn, following summer cropping, or sown in winter, following grazing of forage crops, can be very useful in mopping up residual mineral N from the soil and therefore reducing the risk of N leaching during winter and spring. The key to maximising the benefit is to plant a catch crop as early as practical and to select a winter-active species.

A grazed fodder beet paddock had oats and faba beans sown as catch crops on 26 May 2017 and these were compared to a fallow in an unreplicated demonstration. The cover crops were in the ground for three months, with dry matter cuts taken on 7 September 2017 prior to the sowing of a spring barley. Due to the wet winter conditions the catch crops were slow to establish. Even though overall yields were low, soil mineral N levels in September were higher in the fallow and indicated that around 25 kg/ha of N was mineralised (see below table). Changes in mineral N for the two catch crop treatments are a net result of N mineralisation (increasing mineral N), crop N uptake and immobilisation of N during the decomposition of the residual fodder beet. These numbers suggest that there was greater immobilisation in the presence of catch crops, despite the low yields.

Catch crop yields (t/ha), N uptake (kg N/ha), baseline and harvest profile soil mineral N (kg/ha); soil sampling dates 30/5/17 and 7/9/17 respectively.

Autumn sown barley was compared to leaving the paddock fallow (i.e. prior to sowing spring barley) to see how much N the autumn sown barley takes up prior to fertiliser being applied. Barley (Tavern) was sown 10 May 2018.

Autumn sown barley was shown to utilise N which would otherwise be at risk of leaching, compared with fallow and spring sown barley. Autumn barley also has the potential to decrease spring N leaching losses as it will be well established compared to an August or September sown crop and able to take up more N.

Note: Brent’s experience of sowing barley earlier, instead of having a winter fallow, is that this can be risky as there are no true winter barleys and if it does not get established well due to unfavourable establishment conditions the spring sown crops will yield better.

Baseline (1 June 18), 11 July and 22 November soil mineral N (kg/ha) from the fallow and autumn sown barley (0-60 cm).