How do soil microbes respond to urea fertiliser?


In contradiction to the authors’ hypothesis, regular exposure to urea in the form of urine from grazing animals did not encourage a more vigorous microbial response in the soil sample from the dairy farm compared to samples from the other sites. Interestingly, however, the microbial communities in the samples taken from the set-aside farmland and from the dairy farm responded more vigorously to the second application than they did to the first application of urea. Further illumination regarding this ‘priming effect’, however, will have to wait for future investigation.

Research findings

Over the past decade, urea has become a very common form of fertilizer – providing not only an economic source of nitrogen, but also readily available organic matter and increased soil microbial activity (Rutgers NJ Agricultural Experiment Station 2006).

The currently wide ranging use of urea as a fertilizer has raised many questions regarding its influence on soil biology and plant growth (Overdahl et al. 1991). In particular, there is still a degree of uncertainty regarding microbial response to urea.

When urea is added to soil in combination with water (either through the urine of animals or as a fertilizer) it breaks down to produce ammonia and carbon dioxide. This reaction can happen very quickly, especially in soils with a high pH. Ammonia hydrolyzes during the process of dissolution in water – producing ammonium (NH4+) that can be used by plants and hydroxide ions (OH-) that increase the pH of soil and the solubility of organic matter. Soil microbial activity increases due to the increased availability of organic matter (Overdahl et al. 1991).

It has previously been observed (De Nobili et al. 2001 in Kelliher et al. 2005) that microbial communities develop a ‘priming effect’ after repeated exposure to urea. During this previous study it was noted that microbial communities exposed to urea demonstrate more activity in response to repeated application. A recent paper published in the Australian Journal of Soil Research (Kelliher et al. 2005) has investigated this ‘priming effect’ in three soils with varying histories of exposure to urea from the urine of grazing animals.

In their study, Kelliher et al. postulated that microbial communities in soils adapted to frequent exposure to urine from grazing animals would be most responsive to the application of urea in the laboratory. The authors investigated the response of microbial communities to application of urea in soil samples from three sites with different grazing histories near Lincoln in New Zealand: a dairy farm, a former dairy-farm taken out of production 20 years ago, and site that had never been grazed.

A pair of samples was taken from each of the sites and incubated in the laboratory before urea was applied to one sample of each pair. As an indicator of microbial activity, Kelliher et al. calculated carbon dioxide produced by soil microbial respiration. In order to investigate the relationship between microbial activity, pH and the availability of soil organic matter, the authors also measured soil pH and water soluble carbon after the application of urea to the samples.

As predicted, the pH of samples from all three sites increased after the addition of urea. At a distance of between 0 to 20mm beneath the surface, the pH of all the samples increased by approximately four units. The samples reached their maximum pH of approximately 9 by day 1 or 2, after which the pH of samples from all three sites gradually declined. At a similar depth from the surface, carbon availability also increased dramatically one day after the application of urea. For example, water soluble carbon in the sample taken from the dairy farm increased 10-fold from 0.26 to 2.6 grams per kilogram after application of urea, while water soluble carbon in the sample taken from the un-grazed site increased 21-fold from 0.17 to 3.5 grams per kilogram. After this peak, the proportion of water soluble carbon gradually decreased over the period of the investigation.

Before the addition of urea, all of the samples had similar soil carbon dioxide production rates (FCO2), measured in micrograms of carbon dioxide produced per kilogram of soil over one second. One day after the application of urea, FCO2 for the sample taken from the dairy farm was five times greater than the control sample, while FCO2 for the sample taken from the un-grazed site was seven times greater than the control. Five days after application of urea, FCO2 had declined to pre-application levels for all the samples.

Addition of urea to soil produces carbon dioxide not only through increased microbial respiration, but also through hydrolization of urea itself. By subtracting the carbon dioxide produced through hydrolization from the total carbon dioxide produced after the application of urea, the authors calculated that the three samples produced 1.39 g CO2 / kg soil (dairy farm), 1.67 g CO2 / kg soil (set-aside) and 1.85 g C02 / kg soil (un- grazed) solely from microbial respiration. Expressed as carbon per unit of soil, microbial respiration increased 0.13 (dairy farm), 0.15 (set aside) and 0.2 (un-grazed) after application of urea. After statistical analysis, the authors found no significant difference between the responses to urea for the samples from the three sites – contradicting their hypothesis that the microbial communities in the sample from the dairy farm would respond more vigorously.

The authors applied a second round of urea to the sam- ples in order to test the affect of repeated application. After the second application, the pH of samples from the three sites all increased to a similar level observed at the previous application, before gradually decreasing. In the same way, after the second application of urea, water soluble carbon was replenished in the dairy farm sample to almost the same level observed after the first application. In the un-grazed site sample, water soluble carbon levels were replenished to a level exceeding that observed after the first application.

The carbon dioxide efflux rates for the samples after the second application of urea reflected the increased availability of carbon for microbial respiration. Again, all the samples registered a significant response to the second application of urea, but no significant variance was recorded between the three samples. The amount of carbon dioxide produced solely by bacterial respiration over a period of nine days after the second application of urea was calculated as 1.58 g CO2 /kg (dairy farm), 2.26 g CO2 /kg (set aside), and 1.43 g CO2 /kg (un-grazed). Expressed as a carbon per unit of soil basis, microbial respiration was found to have increased 0.26 (dairy farm), 0.41 (set-aside) and 0.2 (un-grazed) after the second application.

As expected, soil pH, availability of carbon, and soil microbial activity all increased after the application of urea. Results from this study indicate that soil microbial response was not directly proportional to the amount of water soluble carbon available in the soil.

Kelliher FM, Sedcole JR, Minchin RF, Wan Y, Condron LM, Clough TJ, Bol R (2005) Soil microbial respiration responses to repeated urea application in three grasslands. Australian Journal of Soil Research 43: 905-13. Read Abstract.

Overdahl CJ, Rehm GW and Meredith HL (1991). Nutriet management - Fertiliser urea.


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