The effects of urine nitrogen application rate on nitrogen transformations in grassland soils

Abstract

Urine is a critical nitrogen (N) input in temperate grazed grasslands and can drive substantial nitrous oxide (N2O) production in soils. However, it remains unclear how differences in the N input rate affect N2O fluxes and vary between different grassland soils. The effect of increasing urine N application on ammonium (NH4+), nitrite (NO2−) and nitrate (NO3−) concentrations and N2O production was tested in two grassland soils, a free-draining loam and an imperfectly drained sandy-loam. It was hypothesized that high-urine N application rates would lead to ammonia/ammonium (NH3/NH4+) accumulation influencing N transformation rates and N2O production which differ between grassland soils. Fresh cattle urine was applied at rates equivalent to 300 and 1000 kg N/ha in an aerobic incubation experiment. Soils were destructively sampled over 80 days to measure changes in inorganic-N and pH. The higher N addition rate was associated with elevated NH3 concentrations up to day 35 in soils, probably inhibiting NO2− to NO3− reduction. In contrast, there was no inhibition of nitrification in the 300 kg N/ha treatment. Cumulative N2O fluxes were greatest from the 300 kg N/ha treatment for the loam soil, but were greater for the sandy-loam under the 1000 kg N/ha treatment. The results also show that differences in soil properties, in particular carbon availability, can be important in regulating N transformation and N2O production. Collectively, these results demonstrate the proposed mechanism of nitrification inhibition at high-N input rates, driven by either high NH3/NH4 and/or increased levels of NH4HCO3 from urea hydrolysis.

Effects of Irrigation and Nitrogen Application on Soil Nutrients in Triploid Populus tomentosa Stands

“…This finding was consistent with the research result of Huang et al on the Huang-Huai-Hai Plain [52]. However, C. Somerset al argued that nitrate-nitrogen accumulated significantly when the urea concentration was 300 kg•ha −1 , while ammonianitrogen accumulated most significantly when the urea concentration was 1000 kg•ha −1 [63]. This result may be due to the inhibition of NO 3 − formation by high concentration urea, which led to the accumulation of NO 2 − , followed by an increase in NH 4 + caused by NO 2 − denitrification [64].…”

Effect of Cow Urine Nitrogen Rates and Moisture Conditions on Nitrogen Mineralization in Andisol from Southern Chile

“…Similar to other studies carried out with bovine urine [22,33,34], our study showed higher pH levels from the beginning of the incubation period that followed the same trend as the N rate applied, with a similar trend in NH4 + concentrations and both variables declining over time. Accumulated NH4 + was reported to nitrify to NO2 − and NO3 − , generally within a few weeks after urine application to the soil [35], with longer periods also having been reported [36,37].…”

“…The oxidation of NH 3 has been identified as a limiting step in nitrification [18], which occurs through two steps of mutualistic symbiosis between ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) [19]. Several studies have shown that urinary N rates equivalent to 1000 kg N ha −1 inhibit nitrification, and this inhibition can be explained by the accumulation of NH 3 [14, [20][21][22]. However, the duration of this inhibition is variable [23].…”

The effect of metal oxide coating of urea on mineralization in two contrasting soils

“…Soil mineralization was studied over a period of 30 days in the current study. However, it should be noted that N transformations beyond the time examined (30 days) have also been reported (Somers et al, 2019); hence, the possibility to extend the incubation time should be considered in follow-up experiments. It would have been beneficial to measure soil urea concentration directly, although, for these two soils, the loss of urea was probably well reflected in the increase of mineral N through the early part of the incubation.…”