Quantification and mitigation of nitrogen leaching in a maize silage cropping system

Abstract

Nitrogen (N) leaching is a major source of groundwater contamination. We used a series of lysimeters and suction cups to measure total N leaching losses in a high N maize crop system on a long-term maize paddock in the Waikato.

The goal of the study was to 1) quantify N leaching losses under maize silage, 2) determine the effectiveness of a cut-and-carry annual ryegrass catch crop (RG) in mitigating N leaching losses and 3) determine an appropriate depth for measuring N leaching losses under maize.

At least 200 kg/ha more fertiliser N than calculated plant requirements was applied to the maize crop to ensure surplus soil N after maize.

Nitrogen leaching losses were measured in a maize-RG and maize-fallow rotation over two seasons (October 2018 - September 2020). Less than 10 % of N leaching losses occurred during the maize growing season. This is likely due to evapotranspiration exceeding rainfall in summer.

The greatest leaching losses were observed in the maize-fallow rotation, averaging 60 kg N/ha and 88 kg N/ha for the 2018/19 and 2019/20 seasons. This was despite 2020 winter  being drier than 2019. The greater 2019/20 leaching losses can be attributed to a greater soil N concentration (+15 mg N/L) in 2020.

The importance of catch crops as a N leaching mitigation tool after maize was evident over the two years, resulting in leaching reductions >85% in the maize-RG than the maize–fallow plots. The catch crop removed >200 kg N/ha from the soil, significantly reducing the soil N concentration, which appear to be the main driver of leaching losses in our study. Additionally, RG also increased gravimetric soil water content by 7.3 % in the top 30 cm.

When N leaching was measured at 70 cm, losses in fallow plots were almost three times higher than the 41.6 kg N/ha recorded at the 120 cm soil depth, our default maize rooting depth. 

A similar amount of isotopic ¹⁵N (1.2 %) was observed in maize grain at maturity when ¹⁵N was inserted to 60 cm or 120 cm soil depth. This was greater than the background values obtained when standard urea was applied to the plots (0.37 %), indicating ¹⁵N interception at depth.  

It is speculated that provided fertiliser rates match crop N demand and catch crops are used, N leaching losses in maize crops should be much lower than values reported here, irrespective of soil type and season.