Nitrate leaching losses and mitigation on dairy winter forage grazing systems

Abstract

Dairy winter forage grazing is a common practice within the Canterbury region of New Zealand. However, until recently there was a lack of knowledge about the concentration and amount of nitrate-N (NO₃⁻-N) leached from dairy winter forage grazing systems. In addition, there was a lack of knowledge around what ammonia oxidising microbes mediated the nitrification process in intensively grazed winter forage systems where the soils are usually wet, cold, and anaerobic. Furthermore, it was not known whether the nitrification inhibitor dicyandiamide (DCD) or biochar could be successful mitigation tools in reducing nitrate (NO₃⁻-N) leaching from these systems. Thus the objectives of this research were to: 1) quantify the concentration and the amount of NO₃⁻-N leached from cow urine patches deposited during dairy winter forage grazing; 2) quantify the effects of trampling on NO3- leaching losses under dairy winter forage grazing conditions; 3) improve knowledge and understanding of the effectiveness of the mitigation tools DCD and biochar, to reduce NO₃⁻-N leaching losses under dairy winter forage grazing conditions; and 4) improve knowledge and understanding on the effect of cow urine, animal trampling, DCD, and biochar, on the abundance and activity of AOB and AOA under dairy winter forage grazing conditions.

Two lysimeter trials were carried out using a Balmoral stony silt loam under a kale forage crop to cover these objectives by measuring the NO₃⁻-N leaching losses, the effect of soil trampling, and the use of DCD and biochar as mitigation tools. Companion plots were used to measure the soil nitrification rates and ammonia oxidising community abundance and activity under dairy winter forage grazing conditions. The first trial had a urine application rate of 700 kg N ha-1. However, the second trial had a lower urine application rate of 400 kg N ha-1 due to recent research identifying a lower urine-N concentration from kale-fed dairy cows. Both lysimeter trials identified that the dominant source of NO₃⁻ leaching loss was from the cow urine. The lower application rate of urine (400 kg N ha-1) had significantly lower NO₃⁻-N leaching losses compared to urine applied at 700 kg N ha-1. In addition, the first lysimeter trial identified that DCD reduced NO₃⁻-N leaching losses by 38% and DCD combined with biochar resulted in a reduction of 46%. However, biochar alone did not significantly reduce NO₃⁻-N leaching losses. The second lysimeter trial found that trampling the soil reduced NO₃⁻-N leaching losses by 34% (urine only). DCD reduced NO₃⁻-N leaching losses by 61% when soil was trampled and by 40% when un-trampled. Both companion soil plots showed that ammonia oxidiser communities were affected by the application of urine: the AOB amoA gene abundance and AOB amoA transcript abundance significantly increased, whereas AOA growth was inhibited.

A companion incubation study was carried out to determine the effect of urine application rates, DCD application rates, and biochar presence on nitrification, ammonia oxidising community abundance and activity under simulated dairy winter forage grazing conditions. The application of urine increased soil nitrification rates. However, there was no difference between the two urine application rates. DCD reduced the soil NO₃⁻-N concentration by 86 - 91%. The application rate of 20 kg DCD ha-1 had a significantly lower soil NO₃⁻-N concentration compared to the application rate of 10 kg DCD ha-1. DCD combined with biochar reduced the soil NO₃⁻-N concentration by 86%. However, biochar alone did not affect the soil NO₃⁻-N concentration. The AOB amoA gene abundance and AOB amoA transcript abundance significantly increased with the application of urine, whereas the AOA growth was suppressed.