Biol Fertil Soils (1991) 11:279-284 Biology anti

of S O f t S �9 Springer-Verlag 1991

Effects of some environmental factors on ammonia volatilization from simulated livestock urine applied to soil

D.C. Whitehead and N. Raistrick

Institute of Grassland and Environmental Research, Hurley, Maidenhead, Berks. SL6 5LR, UK

Received January 21, 1991

Summary. The proportion of the N that was volatilized as ammonia during 8 days, following the application of simulated livestock urine to soil, increased from 25 to 38 ~ as the temperature of incubation was increased from 4 o to 20 ~ in a system with a continuous flow of air at 70O7o relative humidity. However, volatilization was re- duced if the application was followed by simulated rain; the reduction was greater as the amount of rain increased (up to at least 16 mm) and became less with an increasing length of time (up to 2 - 3 days) after the application of the urine. The effects of the soil water content before ap- plication of the urine, and of the relative humidity of the air, were generally small but volatilization was reduced by a combination of air-dry soil with a low relative humidity. Volatilization was slight (7O7o) when the flow of air was re- stricted to 0.5 h in every 12 h but, with an air flow for 12 h in every 24 h, the volatilization was much closer to that with a continuous flow for the whole 8-day period. When cool or dry conditions were imposed for 8 days and then more favourable conditions were instituted for a second period of 8 days, there was a substantial increase in vola- tilization following the change.

Key words: Ammonia volatilization - Nitrogen - Soil enzymes - Urea - Urine

Ammonia is volatilized from the urine of grazing live- stock following urea hydrolysis by urease, an enzyme that is widespread in soils (Bremner and Mulvaney 1978) and on plants and plant litter (Freney et al. 1983). With cattle and sheep grazing in temperate regions, such as the UK, the Netherlands and New Zealand, the ammonia volatilized usually amounts to 5-30~ of the urinary N (Ball et al. 1979; Carran et al. 1982; Vertregt and Rutgers 1987; Lockyer and Whitehead 1990) though as much as 66~ of the urinary N has been reported to have been

Offprint requests to: D.C. Whitehead

volatilized during warm dry conditions in New Zealand (Ball and Ryden 1984). This volatilization removes poten- tially plant-available N from the area being grazed and transfers it to the atmosphere. In the atmosphere, ammo- nia reacts with acidic compounds to produce ammonium aerosols, and both gaseous and aerosol forms are subject to the processes of wet and dry deposition (Haynes and Sherlock 1986). The extent of volatilization from urine is influenced by environmental conditions, but there is little information on the effects of the various factors studied individually. Factors that are likely to be important in- clude temperature, soil water status, the amount and tim- ing of rainfall after the application of urine, and the rela- tive humidity and rate of flow of the air. The aim of the present investigation was to examine the effects of these factors on ammonia volatilization from a simulated urine solution applied to the surface of small columns of soil. It was not feasible to examine all possible combinations of the various factors, but the interaction between soil water content and relative humidity of the air was as- sessed. Although the values for percentage volatilization of urinary N are not directly applicable to the field situa- tion, a comparison with other results, obtained using nat- ural urine applied in the field during the spring/summer period (Lockyer and Whitehead 1990), suggests that vola- tilization under the controlled environment conditions of the present investigation, with a constant temperature of 20~ was approximately twice that occurring in the field.

Materials and methods

Soil

The soil, in an area of grassland grazed by dairy cattle, was sampled to a depth of about 10 cm. It was a sandy loam of the Sonning series with a pH (0.01 M CaC12) of 6.5. The soil had an organic C content of 2.4%, determined as described by Kalembasa and Jenkinson (1973), and a water-holding capacity of 39~ of dry soil weight, determined as described by Allen et al. (1974). The field-moist soil was sieved (< 4 ram) and stored at 4 ~ until required.

280

Simulated urine

The composition of the simulated urine solution was that proposed by Whitehead et ai. (1989), viz. urea plus hippuric acid (10.0+0.25 g N li- tre-a, respectively) adjusted to pH 8.0 with KOH. A fresh solution was prepared for each experiment.

Assessment of ammonia volatilization

Measurements were made of the ammonia volatilized from 4.0 ml of simulated urine applied to columns of soil containing 210 g dry soil weight; the measurements were continued for 8 days. The columns were prepared by packing the appropriate weight of moist soil into 100-mm lengths of rigid plastic tubing of 48-mm internal diameter. The base of each column was covered by muslin held by an elastic band. Each col- umn was packed with soil to a depth of 90 mm, and then placed in a 1.0-1itre glass incubation jar (approximately 90 mm diameter, 160 mm depth) which had a polyacetyl lid fitted with two tubes to provide an inlet and an outlet for a stream of air. The inlet tube terminated near the base of the jar. Normally, the air entering the inlet tube was passed first through a column (420 • 16 mm) of silica gel (at a moisture content in equilibrium with the air in the controlled environment room) to re- move any ammonia from the incoming air. The outlet tube from the jar was connected by a short length of polyethylene tubing to an absorption flask fitted with a sintered glass distribution tube and containing sul- phuric acid (30 ml 10 mM) to absorb ammonia. Following the applica- tion of the simulated urine, a continuous flow of air was maintained through each jar by a diaphragm pump which was connected to the out- let tube from the absorption flask. The air flow rate was adjusted by a valve, normally to 3 litres min-1, equivalent to four air changes rain-1. The absorption flasks containing sulphuric acid were changed 24 h after the application of simulated urine (day 1) and then on days 2, 4, 6, and 8. The ammonia absorbed during each interval was determined col- orimetrically as indophenol blue (Weatherburn 1967).

In order to maintain constant conditions during each experiment, the equipment was housed in a controlled-environment room which was normally operated at 20 ~ and with a relative humidity in the air that could be controlled within the range 30-95070.

Influence of amount and timing of simulated rainfall

Simulated rainfall was applied, either 2 or 24 h after the application of urine, to columns of soil at 25070 water-holding capacity. There were 4 - 8 replicate columns for each of six treatments, providing the equiva- lent of the following amounts of rainfall: 0.0, 2.0, 4.0, 8.0, 12.0, and 16.0 mm. The temperature was 20 ~ C and the air was at 70070 relative hu- midity.

Influence of air flow rate

Five air flow rates were compared. Two rates were intermittent, both providing 3 litres min -1, one for 0.5 h in each period of 12 h, and the other for 12 h continuously in each period of 24 h. The remaining three rates were continuous throughout the 8 days at 1, 3, and 5 litres min -1, representing about 1.3, 4.0, and 6.7 air changes min -1. In this experi- ment, there were five replicate columns per treatment, the soil was ad- justed to 50070 water-holding capacity, the temperature was 20 ~ and the relative humidity of the air was 47070.

Influence of changing conditions after 8 days

The results of the experiments described above had shown that, when conditions were otherwise favourable, volatilization was reduced by a low temperature and by a combination of air-dry soil with low air hu- midity. The aim of this experiment was to assess the effect of imposing these conditions for 8 days and then changing to conditions more fa- vourable for volatilization. Three treatments, each with six replicate soil columns, were imposed during the first 8 days: (1) soil at 35070 water- holding capacity, air at 70070 relative humidity, 4 ~ (2) soil at 8070 wa- ter-holding capacity, air at 1070 relative humidity, 20~ (3) soil at 35070 water-holding capacity, air at 70070 relative humidity, 20 ~ At the end of 8 days, all the columns in treatment I were transferred to 20 ~ while those of treatment 2 were given sufficient water (22 ml) to reach 35~ water-holding capacity and the relative humidity was increased to 70070. The measurement of ammonia volatization was then continued for a further 8 days.

Influence of temperature

Volatilization from simulated urine applied to soil at 35% water-hold- ing capacity was measured at four temperatures, 4 ~ 10 ~ 20 ~ and 30~ with eight replicate columns per temperature. In practice, the mean temperatures over the 8-day periods differed slightly from the des- ignated values, being 4.3, 9.6, 19.9, and 30.2~ At each temperature, the relative humidity of the air was maintained as closely as possible to 70%, but actual values were 74, 71, 70 and 68%, respectively, with the four temperature regimes.

Influence of initial soil water content and relative humidity of the air

Volatilization was measured at three soil moisture contents (8~ 50070 and 100070 water-holding capacity) in combination with four values of relative humidity of the air entering the incubation jars (1, 45, 83, and 95070) at a uniform temperature of 20~ For the relative humidity of 1 070, the air stream was passed through a column of dry silica gel (200 g) which was replaced daily. For the relative humidity of 45070, the air stream was passed through a column of silica gel equilibrated with the air in the controlled environment room which was adjusted to 45070. For the relative humidity of 83070, the air stream was passed through water in a Drechsel bottle fitted with a sintered glass distribution tube. The relative humidity of 95070 was obtained by adjusting the air in the con- trolled environment room to this humidity. Checks on the relative hu- midity of the air entering the incubation jars were made at intervals us- ing a Vaisala HM 34 humidity meter (Vaisala, Helsinki, Finland). Mea- sured values for the four treatments ranged from 0.3 to 2.4070, 44 to 47070, 81 to 87070 and 95 to 96070 relative humidity. In this part of the investigation, there were six replicate columns per treatment and three treatments were carried out simultaneously in one run.

Results and discussion

Influence of temperature

T h e i n f l u e n c e o f t e m p e r a t u r e o n a m m o n i a v o l a t i l i z a t i o n

was p a r t i c u l a r l y m a r k e d d u r i n g t h e f i r s t 2 days a f t e r a p - p l i c a t i o n o f t h e u r ine , b u t f o r e a c h s a m p l i n g i n t e rva l , cu - m u l a t i v e v o l a t i l i z a t i o n was s i g n i f i c a n t l y g r e a t e r a t 10 ~ t h a n a t 4 ~ a n d s i g n i f i c a n t l y g r e a t e r a t 20 ~ t h a n a t 1 0 ~ (Fig . 1). Howeve r , t h e d i f f e r e n c e b e t w e e n 20 ~ a n d 3 0 ~ was s m a l l a n d n o t s i g n i f i c a n t . A m a j o r e f f ec t o f t h e low t e m p e r a t u r e s was to r e t a r d t h e v o l a t i l i z a t i o n . T h u s b y t h e e n d o f d a y 1, v o l a t i l i z a t i o n a t 4 ~ was o n l y 1107o o f t h a t a t 20 ~ b u t b y d a y 2 t h i s p r o p o r t i o n h a d i n c r e a s e d t o 37070, b y d a y 4 to 58070, a n d b y d a y 8 t o 65070. B e t w e e n d a y 6 a n d d a y 8, v o l a t i l i z a t i o n was g r e a t e s t in a b s o l u t e t e r m s a t 4 ~ a n d l ea s t a t 30 ~ , i n d i c a t i n g t h a t i f t h e m e a s u r e m e n t s h a d b e e n c o n t i n u e d fo r a p e r i o d l o n g e r t h a n 8 days , t h e dif- f e r e n c e in t h e e x t e n t o f v o l a t i l i z a t i o n b e t w e e n 4 ~ a n d 2 0 - 30 ~ w o u l d h a v e b e c o m e smal le r . A s w o u l d b e expec t - ed , i n c r e a s i n g t h e t e m p e r a t u r e c a u s e d a n i n c r e a s e d e v a p o - r a t i o n o f w a t e r f r o m t h e soi l c o l u m n s . A t 4 ~ 1607o o f t h e t o t a l v o l u m e o f soi l w a t e r p lu s u r i n e e v a p o r a t e d ; a t 10 ~ t h e p r o p o r t i o n was 28070, a t 20 ~ it was 37070, a n d a t 30 ~ , 51070.

T h e a b o v e r e su l t s a re c o n s i s t e n t w i t h t h o s e o f M c G a r r y et al. (1987) , w h o f o u n d , w i t h a m o r e r e s t r i c t e d r a n g e o f t e m p e r a t u r e s , t h a t v o l a t i l i z a t i o n f r o m u r e a ap -

84

45

1

Temp ~

40 30.2 19.9

z 9.6 |

30

4.3

~= 20

z

z 10

0 2 4 6 8 Days

Fig. 1. Cumulative volatilization of ammonia from simulated urine ap- plied to soil (35o70 water-holding capacity) at 22.1 g urea N m -2 and maintained at four constant temperatures (Temp) for 8 days with air at approximately 7007o relative humidity (means of eight replicates with SD on days 1 and 8)

plied to soils generally increased with temperatures be- tween 8 ~ and 18 ~ and that maximum rates of volatil- ization occurred during days 1 - 3 at 13 ~ and 18 ~ and during days 4 - 6 at 8 ~ The absence of a significant dif- ference in volatilization between temperatures of 20 ~ and 30 ~ (Fig. 1) is unexpected, since Vlek and Carter (1983) found that the rate of urea hydrolysis in soil was linearly related to temperature over the range 10 ~ 40 ~ Howev- er, for ammonia volatilization, an important factor is likely to be the response to temperature of other process- es, particularly nitrification, which remove ammonium from the soil. Low temperatures restrict both urea hydro- lysis and nitrification, but the restriction at 5 ~ compared with 20~ ~ appears to be greater with nitrification than with urea hydrolysis (Low and Piper 1961). If this is so generally, ammonium derived from urea hydrolysis would remain susceptible to volatilization for longer at lower temperatures.

Influence of soil water content and relative humidity o f the air

There was an interaction between the soil water content and the relative humidity of the air, as shown by the re- sults for three values of soil water content in combination with three values of relative humidity (Fig. 2). At each value of relative humidity, an increase in the soil water content increased the volatilization, the effect being greater the lower the relative humidity. Although the data in Fig. 2 were obtained in three experimental runs in which relative humidity was varied at a constant soil wa- ter content, closely similar results were obtained in a sin- gIe run with the same three contents of soil water and a constant relative humidity of 45~ volatilization over 8 days amounted to 32.4~ at 8070 water-holding capacity, 38.407o at 50~ water-holding capacity and 41.8070 at 10007o water-holding capacity (mean SD 0.89).

Z

o~

z

,'r" z

30

20

10

(a)

8 % W H C % RH of air

z i

+6 o~ 0~

z

-1- z

2 4 6 8

Days

Z |

L :3

"6 o~

Z

"1"

Z

40~

(b) 50% WHC

3O

20

10 0 . . . .

0 2 4 6 8 Days

(c) 100% WHC -,

30

20

10

281

45 84 1

84 1

45

0 2 4 6 8

Days

Fig. 2 a - c . Cumulative volatilization of ammonia from simulated urine applied to soil at (a) 8~ of water-holding capacity (WHC), (b) 50o70 of WHC, and (e) 100~ of WHC, and with air stream at 1, 45, or 83o70 rel- ative humidity (RH); all at 20 ~ (means of six replicates with SD on day 8)

40 -(b)

The relatively small difference in ammonia volatiliza- tion between soil at 50 and 100% water-holding capacity (Fig. 2) is consistent with the report of Vlek and Carter (1983) that the rate of hydrolysis of urea, thoroughly mixed into three soils, was little affected by differences in soil water content above permanent wilting point (1.5 MPa) though it decreased rapidly with further dry- ing. At low soil water contents, there is inevitably a differ- ence between urine and solid urea, reflecting the addition of water in the urine. With solid urea, a low initial soil water content reduces ammonia volatilization by restrict- ing the rate at which the urea dissolves, and also the rates of diffusion and hydrolysis (Vlek and Carter 1983; Black et al. 1987; Reynolds and Wolf 1987). The present results show that, even with a urine solution, a low soil water content reduces volatilization, but the effect is small, especially at a high relative humidity.

Increasing the relative humidity of the air produced an increase in volatilization from simulated urine applied to a dry soil (8% of water-holding capacity) but it had no significant effect with soits of 50 or 10007o water-holding capacity (Fig. 2). Even with the dry soil, there was no sig- nificant effect on volatilization during the 1st day, pre- sumably because the water added in the simulated urine ensured that moisture did not limit the hydrolysis of urea over this period. Volatilization with a relative humidity of 95% (not shown in Fig. 2) was close to that with 83% rel- ative humidity, for each soil water content. These results are consistent with the finding that when solid urea was applied to the surface of a dry soil (< 1.5 MPa), a high humidity increased the volatilization whereas, with a wet soil ( -0 .033 MPa), differences in relative humidity over the range of 2 5 - 85% had a negligible effect provided the soil water content was maintained (Reynolds and Wolf 1987).

It has been suggested that a combination of wet soil with only slight evaporation would reduce the volatiliza- tion of ammonia compared with more drying conditions (Haynes and Sherlock 1986). However, in the present in- vestigation, there was no significant effect on volatiliza- tion from a wet soil (100% water-holding capacity) of varying humidity between 1 and 95% relative humidity. For the 12 combinations of soil water content and relative humidity, the proportion of urinary N volatilized during 8 days and the loss of water by evaporation during this period were not closely correlated (r = 0.53).

Influence of amount and timing of simulated rainfall

Simulated rainfall after the application of urine reduced ammonia volatilization, and the effect was greater with a 2-h interval than a 24-h interval between the urine appli- cation and the rainfall (Fig. 3). With both time intervals, the effect increased with increasing amounts of water, al- though the effect of 16 mm was little different from that of 12 mm. When applied 2 h after the urine application, the equivalent of 2 mm rain reduced the volatilization by 15%, whereas the equivalent of 12 mm rain reduced it by 81%. When the rain was applied 24 h after the urine ap- plication, the corresponding reductions were 6 and 33%. Rather similar effects were reported with fertilizer urea in

simulated 40 -(a) rain (mm)

0.0

i 2.0 ' 30 4.0

~ 20

~ 8.0

z 10 �9 ~ 12.0 = -----42_16.0

0- 0 2 6 8

i

4 Days

0.0 2.0 4.0

8.0 ~2:o 16.0

Z

30

20

Z 10

282

2 4 6 8 Days

Fig. 3 a, b. Cumulative volatilization of ammonia from simulated urine applied to soil (25% water-holding capacity) at 22.1 g urea N m -2 fol- lowed by the application of various amounts of water, expressed in mm, (a) after 2 h, (b) after 24 h; all at 20~ with air at 70% relative humidity (means of four to eight replicates with SD on day 8)

field experiments (Fox and Hoffman 1981; Black et al. 1987).

Influence of air flow rate

The intermittent air flow of 0.5 h in each 12 h allowed much less volatilization than the flow for 12 h in each 24 h and the three rates of continuous flow (Fig. 4). How- ever, with a flow for 12 h in each 24h, the cumulative volatilization over 8 days was about 83~ of that with a continuous flow at the same rate. Of the three continuous rates, 1 and 3 litres min-1 resulted in a little more volatil- ization than did 5 litres min -1 (Fig. 4), possibly reflect- ing differences in the extent of soil surface drying.

The present results are compatible with the finding of Watkins et al. (1972) that volatilization from solid urea applied to soils increased with increasing air flow rate at rates of less than one air change min -I . On the other hand, they appear to differ from the finding of Kissel et al. (1977) that volatilization increased with air flow rate up to about 15 air changes min -1. However, the volatil-

40 - air flow

1L min -1 . ~ I f ~ . _ - - - ~ _ aL min-1

z / " ~ . . _ - - - - - J ~ ~ 5L min -1 30 / / . . ~ -'e~ ~ 3L min -1

~ -(12h in 24h)

"6

20

Z

z 10

3L rain 1 ~[(0.S h in 12h)

0 v i i i

0 2 4 6 8 Days

Fig. 4. Cumulative volatilization of ammonia from simulated urine ap- plied to soil (50070 water-holding capacity) and subjected to an air stream of 47070 relative humidity at five rates of flow at 20~ (means of five replicates with SD on day 8)

ization chambers used by Kissel et al. had a much smaller air volume per unit of soil surface than those of the pre- sent investigation, and this probably accounts for the dif- ference in results. When high rates of air flow dry soil rapidly, they may curtail volatilization by restricting the hydrolysis o f urea. This effect was reported by Reynolds and Wolf (1987) with solid urea applied to shallow layers of soil (1.3 cm) in volatilization chambers. It may account for the slightly reduced volatilization, referred to above, from a continuous air flow rate of 5 litres min -1 com- pared with 1 or 3 litres min - t .

Influence of changing conditions after 8 days

When columns of moist soil (35~ water-holding capaci- ty), treated with simulated urine and incubated at 4 ~ for 8 days, were subjected to an increased temperature of 20 ~ there was a marked increase in the rate of ammonia volatilization (Fig. 5). At the end of a further 8 days at

283

20 ~ values for cumulative volatilization from this treat- ment, and from the treatment with continuous incuba- tion at 20 ~ for 16 days, were not significantly different. Nevertheless, the amount of ammonia volatilized during each of the sampling intervals in the second 8-day period was significantly greater from the treatment with the change in temperature. Similarly, when columns of dry soil, treated with simulated urine and incubated at a low relative humidity, were moistened and subjected to 70~ relative humidity, there was a marked increase in the rate of volatilization (Fig. 5). This occurred despite the addi- tion of water equivalent to 12 m m rain which, as shown above, would have tended to reduce volatilization.

C o n c l u s i o n

In the context of grazed swards in temperate regions, the results indicate that two important environmental factors influencing the extent of ammonia volatilization from livestock urine are temperature and the frequency of heavy rainfall. However, in addition to its effect on the rate of urea hydrolysis, an increasing temperature also in- creases the rate of processes that compete with ammonia volatilization, these being nitrification, the uptake of am- monium by plants, and immobilization by microorgan- isms and soil organic matter. The effect of an increasing temperature on the balance between these various pro- cesses is likely to vary between different soils, and an in- crease in temperature will not necessarily increase volatil- ization. Stimulation of nitrification may explain why, in some investigations including the present, increasing the temperature above 20 ~ produced no increase in ammo- nia volatilization. Certainly, the relative rates of urea hy- drolysis and nitrification will vary, as large differences be- tween soils have been reported, both in urease activity (Bremner and Mulvaney 1978) and in rates of nitrifica- tion (Stevens et al. 1989).

As would be expected, the soil water content and the humidity of the air appear to have less effect on ammonia volatilization from urine than from solid fertilizer urea.

4o

Z = 30

*6

20

Z

z 10

0 v

o

(b)

(e)

, i i i i i i J

2 4 6 8 10 12 14 16 Days

Fig. 5. Cumulative volatilization of ammonia from simulated urine applied to soil and incu- bated for 8 days (a) at 20~ with soil at 35~ water-holding capacity and air at 70~ relative humidity, (b) at 4~ with soil at 35070 wa- ter-holding capacity and air at 70070 relative humidity, (c) at 20~ with soil at 8070 wa- ter-holding capacity and air at 1070 relative hu- midity; and then all incubated as (a) for a fur- ther 8 days (means of six replicates with SD on days 8 and 16)

284

Nevertheless, the da ta in Fig. 2 show that a combina t i on o f a dry soil with a low humid i ty at the t ime o f the ur ine appl ica t ion will reduce vola t i l iza t ion compared with more mois t condi t ions . Increasing the wind speed might be expected to increase vola t i l iza t ion by moving a m m o n i a more rapidly f rom the soi l /a i r interface but, i f it accentu- ates drying o f the soil surface, it may reduce urea hydroly- sis and a m m o n i a volat i l izat ion, as noted above.

The results o f changing condi t ions after 8 days (Fig. 5) show that when a m m o n i a vola t i l iza t ion f rom soil t rea ted with ur ine was restricted by cool or dry condi- t ions, the t reated soil retained the potent ia l for enhanced vola t i l iza t ion subsequently. The increase in vola t i l iza t ion fol lowing the change in condi t ions (Fig. 5) suggests tha t the restr ict ion dur ing the cool or dry per iod was due to a reduced rate o f urea hydrolysis, rather than a shift in fa- vour o f the processes compe t ing with vola t i l iza t ion for a m m o n i u m in the soil.

Acknowledgment. The Institute of Grassland and Environmental Re- search is financed through the Agricultural and Food Research Council, Swindon.

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