Plant and Soil (2005) 273: 15–27 DOI 10.1007/s11104-004-6261-2
Ó Springer 2005
Seasonal variation in N2O emissions from urine patches: Effects of urine concentration, soil compaction and dung Jan Willem van Groenigen1,3, Gerard L. Velthof1, Frank J.E. van der Bolt2, An Vos1 & Peter J. Kuikman1 1
Soil Science Center, Alterra, Wageningen University and Research Center, P.O.Box 47, 6700 AA Wageningen, The Netherlands. 2Water and Climate Center, Alterra, Wageningen University and Research Center, P.O. Box 47, 6700 AA Wageningen, The Netherlands. 3Corresponding author* Received 15 October 2004. Accepted in revised form 15 November 2004
Key words: dung, greenhouse gas emissions, nitrous oxide, soil compaction, urine patches
Abstract Urine patches in pastures rank among the highest sources of the greenhouse gas nitrous oxide (N2O) from animal production systems. Previous laboratory studies indicate that N2O emissions for urine-N in pastures may increase with a factor five or eight in combination with soil compaction and dung, respectively. These combinations of urine, compaction and dung occur regularly in pastures, especially in socalled camping areas. The aims of this study were (i) to experimentally quantify the effect of compaction and dung on emission factors of N2O from urine patches under field conditions; (ii) to detect any seasonal changes in emission from urine patches; and (iii) to quantify possible effects of urine concentration and -volume. A series of experiments on the effects of compaction, dung, urine-N concentration and urine volume was set up at a pasture on a sandy soil (typic Endoaquoll) in Wageningen, the Netherlands. Artificial urine was applied 8 times in the period August 2000–November 2001, and N2O emissions were monitored for a minimum of 1 month after each application. The average emission factor for urine-only treatments was 1.55%. Over the whole period, only soil compaction had a clear significant effect, raising the average N2O emissions from urine patches from 1.30% to 2.92% of the applied N. Dung had no consistent effect; although it increased the average emissions from 1.60% to 2.82%, this was clearly significant (P < 0.01) for only one application date and marginally significant (P ¼ 0.054) for the whole experiment. Both compaction and dung increased water-filled pore space (WFPS) of the topsoil for a more prolonged time than high urine volumes. No effect of amount of urine-N or urine volume on N2O emissions relative to added N was detected for the whole experiment. There were clear differences between application dates, with highest emissions for urine-only treatments of 4.25% in October, 2000, and lowest of )0.11% in June, 2001. Emissions peaked at 60–70% WFPS, and decreased rapidly with both higher and lower WFPS. We conclude that compaction leads to a considerable increase in the N2O emissions under field conditions, mainly through higher WFPS. Dung addition may have the same effect, although this was not consistent throughout our experiment. Seasonal variations seemed mainly driven by differences in WFPS. Based on this study, mitigation strategies should focus on minimizing the grazing period with wet conditions leading to WFPS > 50%, avoiding camping areas in pastures, and on avoiding grazing under moist soil conditions. Greenhouse gas budgets for grazing conditions should include the effects of soil compaction and dung to represent actual emissions.
* FAX No: +31(0)317 419000, e-mail:
[email protected]
16 Introduction Anthropogenic emissions of the greenhouse gas nitrous oxide (N2O) are to a large extent derived from animal production systems. Mosier et al. (1998) estimated that between 30 and 50% of the total N2O emissions from agriculture originates from animal production systems. Oenema et al. (in press) showed that projected increases in the number of animals in production systems in the period 2000–2030 (Bruinsma, 2003) are unlikely to be offset by any mitigation strategies aimed at higher nutrient use efficiency. Therefore, N2O emissions from animal production systems are likely to rise further in the future. Approximately 60% of the global N2O emissions and N excretions from animal production systems originates from cattle (Bouwman et al., 1997; Oenema et al., in press). Emissions are highest for dairy cattle in the developed world, where intensive farming system with high inputs of N result in N excretions averaging 70–100 kg N per head per year (Mosier et al., 1998; Smil, 1999). Van Groenigen et al. (2005) gives a detailed review of the effect of urine patches on microbial processes in the soil and subsequent losses of N2O. The combination of high concentrations of nitrogen (in urine and dung) and easily available carbon (in dung) result in a relatively high default N2O emission factor for excreted N of 2.0% of the applied N (IPCC, 1996). Emissions from urine patches (0.1–3.8% of applied N) are thought to be higher than from dung patches (0.1–0.7% of applied N), mainly due to higher N concentrations, salinity prohibiting N uptake by grass, and high pH following urea hydrolysis (Haynes and Williams, 1992; Lantinga et al., 1987). The effect of urea on N2O emissions is not clear: whereas denitrification rates may increase with higher soil pH, the N2O/ N2 ratio may actually decrease significantly (Weier and Gilliam, 1986), especially in the presence of large amounts of NO 3 (Firestone et al., 1980). For nitrification, N2O emissions tend to increase at higher pH (Martikainen and De Boer, 1993). Possible increases in the N2O emission factor from urine patches due to concurrence with dung patches and/or soil compaction are a concern, and currently not addressed in IPCC guidelines. Van Groenigen et al. (2005) showed in an incubation study that urine-derived N2O emissions
could increase fivefold after soil compaction, and up to eightfold after combination with dung. Both dung and soil compaction are associated with higher soil anaerobicity through an increase in respiration and a decrease in gas diffusivity, respectively (Ball et al., 1999; Simojoki et al., 1991). However, there is little data to indicate to what extent these increases in N2O emissions will occur under field conditions. Ball et al. (2000) could only detect a weak relationship between cone resistance and N2O emissions in the field. Hansen et al. (1993) reported an increase in N2O emissions for fertilizer from 3.9% to 5.3% due to compaction. There are several indications for a seasonal effect in N2O emission from urine patches (Anger et al., 2003). Such a seasonal effect could be driven by changes in susceptibility to soil compaction (Naeth et al., 1990) or other factors affecting denitrification such as temperature, rainfall, N uptake by grass or soil water-filled pore space (Luo et al., 1999; Velthof et al., 1996). Seasonal effects could be an important aspect of mitigation strategies, since different grazing schedules can be practiced (lower stocking rates, unlimited grazing, seasonal grazing, zero grazing, etc.). It has been suggested that variations in urine concentration or -volume due to differences in fodder may control emissions from urine patches (Oenema et al., 1997). A recent incubation study did not show a clear effect of the amount of applied urine N (in equal volumes of urine) on N2O emissions, but reported a significant effect of the volume of applied urine (containing equal amounts of urine-N) (Van Groenigen et al., 2005). However, different conditions with respect to N and moisture dynamics in the field (especially N uptake by plants and draining of the urine-affected area) warrant more research of the effect of urine volume and concentration under field conditions. The purpose of this study was to quantify to what extent season, dung, compaction, urine-N concentration and volume control N2O emissions from urine patches, in the practice of dairy farming in the Netherlands. The aims of the present field experiment where therefore (i) to experimentally quantify to what extent the effects of compaction and dung patches occur under field conditions; (ii) to detect any seasonal changes in these effects; and (iii) to quantify possible
17 effects of urine concentration and volume on N2O emissions.
polyvinylchloride ring and urine was applied. The first application of urine was made on August 8, 2000, and other applications were planned for the grazing seasons of 2000 and 2001. Unfortunately, due to the foot- and mouth disease crisis in the Netherlands, access to experimental fields was prohibited in March and April 2001. Due to practical constraints, no measurements were possible in July 2001, leaving 8 application dates between August 2000 and October 2001. Table 1 lists the different treatments, which were all replicated three times. The artificial urine was composed following Van Groenigen et al. (in press), with the nitrogen content composed of urea (88.6% of total urine N), hippuric acid (6.2%), creatine (0.8%), allantoin (1.5%), ureic acid (0.4%) and NH4Cl (2.5%). The concentration of total N was set at 9.33 g N L)1, but for a few treatments different concentrations of 4.66, 6.99 and 18.65 g N L)1 were created (Table 1). All artificial urine contained 14.20 g L)1 KHCO3 and 10.50 g L)1 KCl. A standard urine application was set at 4 mm or 4 L m)2. For a few treatments, applications of 2 mm and 8 mm were made, treatment 2 received only water (Table 1). Fresh cattle dung (without urine) was collected and mixed, and 10 kg dung m)2 was
Materials and methods Experimental setup The experiment was set up at an experimental farm near Wageningen, the Netherlands, on a 1 year old pasture on a typic Endoaquoll. The soil contained 2% clay, 23% silt and 75% sand. Total C and N contents were 40.2 and 2.01 g kg)1 dry soil, respectively. Before the experiment was started in 2000, the field was fertilized using calcium ammonium nitrate (CAN) on March 29 (81 kg N ha)1), May 10 (81 kg N ha)1), June 16 (67.5 kg N ha)1), July 26 (67.5 kg N ha)1) and August 7 (27 kg N ha)1). In 2001, a similar fertilizer application schedule was followed. The whole experiment was laid out as a different randomized block subexperiment for every urine application date, with 14 treatments and three replicates. Two weeks before a urine application, the plots were mown. For each urine application, a previously unvisited circle with a diameter of 70 cm was delineated with a
Table 1. Treatments for the field study. All treatments were replicated three times; the experiment was laid out as a different randomized block design for every urine application date Treatment
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Extra1
Urine concentration/application N concentration, g N L)1
Urine volume, L m)2
N appl., g N m)2
–
0.00 0.00 4.00 4.00 4.00 4.00 8.00 2.00 4.00 4.00 4.00 4.00 0.00 4.00
0.00 0.00 18.64 27.96 37.32 74.60 37.32 37.32 37.32 35.40 18.64 37.32 0.00 37.32
0.00 4.66 6.99 9.33 18.65 4.66 18.65 9.33 8.85 4.66 9.33 – 9.33
w
u f c c d d
1 c = compaction; d = 10 kg fresh dung m)2 ; f = fresh urine instead of artificial urine; u = urea solution instead of artificial urine; w = 4 L water m)2 was applied instead of urine.
18 superficially added to treatments 13 and 14, directly followed by the urine application. The average composition of the fresh dung was 439.8 g total C kg)1, 4.00 g total N kg)1 and 0.20 g NH4–N kg)1. For treatment 10, fresh urine was collected from the same cows, at 4 am in the morning, for all 8 application dates. The average composition of urine was 200.3 g C total L)1, 8.00 g total N L)1 and 367 mg NH4–N L)1. Both dung and fresh urine were collected at the day of application. Artificial urine was prepared at most one day before application, and stored at 4 °C before use. Compaction was simulated by manually pounding the surface of the experimental plots using a wooden fore hammer, directly before urine application. Although compaction varied somewhat due to the soil moisture content and other seasonal differences, measurements showed an average increase in bulk density from 1.54 (±0.01) to 1.60 (±0.02) g cm)3 soil in the upper 10 cm. No effect was detected below 10 cm from surface. N2O measurements and soil analyses Fluxes of N2O were measured for 4 weeks after application for each application date, at which point fluxes generally returned to background levels. Measurements were taken three times during the first week, and twice a week after that, for all 8 application dates, totaling 72 measurement dates. Fluxes were measured using vented closed flux chambers and a photo-acoustic infrared gas analyzer, following Van Groenigen et al. (2004). The flux chamber had a diameter of 30 cm, and was placed in the middle of the experimental plots. Fluxes were measured using differences between ambient N2O concentrations before closing of the flux chamber, and concentrations in the flux chambers after closing for approximately 1 h, assuming linear increases over time based both on previous experiments (e.g. Velthof and Oenema 1995) and checks during our experiment. More details on N2O flux measurements are reported in Van Groenigen et al. (2004). Cumulative fluxes were calculated by linear interpolation between daily fluxes. Emissions for urine-derived N2O, expressed as the percentage of urine-N emitted as N2O, were calculated after correction for background
emissions (treatment 1). Treatment 14, which received both urine and dung, was corrected using treatment 13. Simultaneously with N2O flux measurements, the volumetric soil moisture content was measured using a Trase system 1 Time Domain Reflectometry Moisture Measuring Equipment (6050X1, Soilmoisture Equipment Co.) next to each flux chamber. From May 8th 2001, a Trime FM-3 Field Portable TDR Moisture Meter (Mesa Systems) was used. Both TDR setups used 3-rod probes of 10 cm length. Water-filled pore space (WFPS) was subsequently calculated from the bulk density and soil moisture content at each sampling date, assuming a particle density of 2.60 g cm)3. Soil temperature was measured next to each flux chamber using Pico PT-100 temperature probes with a 12 cm probe. Statistical analyses The effect of a number of different treatments on N2O emissions was established using a variety of statistical tests, all based upon a subset of the randomized block design of the experiment. Each test was performed for each individual application date using the analysis of variance module of Genstat 7 (VSN International Ltd.). Subsequently, the effect over the whole experiment was tested with application date as additional blocking. The effect of dung addition was tested with dung as factor (treatments 5 and 14). The effect of soil compaction was tested with compaction and amount of urine as factors (treatments 3, 5, 11 and 12). The effect of urine-N rate at a constant urine volume of 4 L m)2 was tested with amount of urine-N as a factor (treatments 3, 4, 5 and 6). The effect of urine volume at a constant amount of urine-N of 37.32 g N m)2 was tested with urine volume as a factor (treatments 5, 7 and 8). Finally, the effect of N compound was tested with N compound as a factor (treatments 5, 9 and 10).
Results Figure 1 shows the pattern of N2O fluxes, WFPS and temperature during the whole experimental period. For reasons of clarity, only 3 of 14 treatments are shown, but the general emission
19 N2O flux, mg N2O-N m-2 d-1
Water-filled pore space, % 100.0
87.5
75.0
62.5
50.0
37.5
25.0
12.5
0.0
84
72
60
48
36
24
12
0 01/08/2000
Urine application
1/09/2000
Urine application
01/10/2000
Urine application
1/11/2000 1/05/2001
Urine application
Soil water-filled pore space Temperature
detection limit
1/07/2001
-2 4 L urine m (Tr. 5) -2 4 L urine m + compaction (Tr. 12) -2 4 L urine m + dung (Tr. 14)
1/06/2001
Urine application
1/08/2001
Urine application
01/09/2001
Urine application
01/10/2001
Urine application
25
20
15
10
5
Temperature, °C Figure 1. Water-filled pore space, soil temperature, timing of urine applications and N2O fluxes for three treatments during the measurement period.
20 pattern is consistent for all treatments. Major peaks in emission are not reached before approximately 10 days after urine application, and in both years the highest peaks are for the August urine applications. As can be expected from rainfall patterns in the Netherlands, WFPS is highest around October in both years, and below detection in June–July 2001. Total N2O emissions for several treatments, averaged for the eight application dates, are shown in Figure 2. The highest average emissions were found for urine in combination with dung (treatment 14) or with compaction (treatment 12). However, this did not result in an overall significant effect of dung on the emission (P ¼ 0.054; Table 2). Only for August 2000 could a significant increase be detected (P ¼ 0.006; Table 2). The overall effect of compaction was highly significant (P ¼ 0.002, leading to an increase for both low and high N applications (Table 3). The amount of urine-N with similar volumes of urine was significant for two application dates (P ¼ 0.019 for May 8th, 2001; P < 0.001 for October 2nd, 2001), but this did not result in an overall significant effect (P ¼ 0.586; Table 4).Figure 3 denotes the N2O emissions, relative to the N application rate for October 2001, which did show significant differences. No significant effect of different urine volumes with constant amounts of urine-N could be detected (P ¼ 0.441;
Table 5). Although average emissions for cattle urine (0.81%) were lower than either artificial urine (1.67%) and urea (1.89%), these results were not significant (P ¼ 0.307; results not shown). Figure 4 depicts the effect of a high urine volume addition, compaction and dung on the WFPS. Averaged over all 8 application dates, WFPS returned to background levels after 8 days for the large urine volume addition. Although the initial WFPS increase was slightly lower for both compaction and dung addition, WFPS remained about 4% higher than background levels for about 18 days. The overall effect of WFPS on N2O emissions, regardless of treatment, season or time after urine application, is shown in Figure 5. Although problems with the detection limit of the TDR system used in 2001 resulted in few measurements below 30% WFPS, measurements were available in the range 12–100%. Emissions peaked at a WFPS of 60–70%.
Discussion and conclusions Seasonal variation in N2O fluxes There were large seasonal differences in N2O emissions, with respect to both overall average
2.0
N2O emission, g N2O-N m-2
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 Background (Tr. 1)
Water (Tr. 2)
Dung (Tr. 13)
Urine (Tr. 5)
Urine + Dung (Tr. 14)
Urine + Compaction (Tr. 12)
Figure 2. Average emissions over all application periods for different treatments. All urine applications are 37.32 g N m)2 in 4 L urine m)2. Error bars denote standard error (n ¼ 3).
* = 0.05; ** = 0.01; *** = 0.001; NS = not significant; 1P = 0.054.
Source of variation, whole experiment Dung
NS ** Source of variation per application date Dung
NS
NS
NS
NS
NS
NS
NS1
2.21 0.98 ± 0.27 3.26 ± 0.54 2.64 ± 2.26 0.67 ± 0.21
5.15 ± 1.33
3.63 ± 1.15
)0.02 ± 0.07
1.91 ± 0.31
2.82 ± 0.88 0.61 ± 0.42 2.07 ± 0.47 3.89 ± 0.95 0.07 ± 0.10 5.24 ± 1.87 4.40 ± 4.54 1.13 ± 0.09
5.68 ± 2.75
1.60 ± 0.71 1.34 ± 0.25 1.75 ± 0.48 2.62 ± 0.36 )0.11 ± 0.07 0.88 ± 1.34 0.20 ± 0.05
No dung (treatment 5) Dung added (treatment 14) Average
4.62 ± 1.03
2.02 ± 0.69
7/8/2001 8/8/2000
5/9/2000
3/10/2000
8/5/2001
12/6/2001
4/9/2001
2/10/2001
avg. Application date Treatment
Table 2. The effect of dung application on the N2O emission factor (in%) from urine patches. Emissions are corrected for dung-derived N2O. Indicated ranges denote standard errors (n = 3 for individual treatments)
21 emissions and effects of treatments (Tables 2–4). However, these differences are difficult to interpret as the data from the 2 years differed considerably. Although for both years peak emissions were measured in August, with decreasing peaks in September and October (Figure 1), this probably reflects patterns in background emissions due to fertilization more than emissions from urine. When corrected for background emissions, average emissions in the period August–October 2000 actually increased for almost all treatments, whereas during the same period in 2001 they decreased (Tables 2–4). Overall, emissions were highest in October 2000 (average 6.93% for all treatments in the compaction test – Table 3) and lowest in June 2001 (average )0.03%). Yet, in October 2001, a similar addition under similar soil moisture conditions as one year earlier and only slightly higher temperatures resulted in average emissions of 1.00% (Table 3). The literature does not provide a consistent seasonal pattern either. Allen et al. (1996) reported a shift from a slight consumption of N2O during summer (4–17 mg N2O–N kg)1 urine) to a relatively high production during winter (148–197 mg N2O–N kg)1 urine). As no urine-N composition was reported, no emission factors could be calculated from their data. However, Allen et al. (1996) reported a general emissions of 0% in summer and 0.8–2.3% in winter, depending on soil type and urine composition. Anger et al. (2003) reported a decrease from 1.4–4.2% in May to 0.3–0.9% in September. Obviously, seasonal patterns will vary strongly with soil type and meteorological conditions, and will drive parameters such as plant N uptake, plant growth, root turnover and WFPS/ anaerobicity, which influence N2O emissions more directly, as is shown for WFPS in Figure 5. An average background emission of 0.3 g N2O–N m)2 (3 kg N2O–N ha)1) for the eight application dates may appear high. However, this was mainly due to fertilization with CAN on July 26, 2000 and August 7, 2001. Total emissions from fertilized ungrazed grasslands in the Netherlands have been reported to be within this order of magnitude (e.g. Velthof et al., 1996). For several treatments, negative emissions were reported for the second application date (Tables 3, 4 and 5). These did not originate from measured negative fluxes by the gas monitor. Rather, as far as
NS NS
0.52 1.15 0.20 0.64 0.63
± ± ± ± ±
NS NS
0.34 0.62 ± 0.30 5.41 ± 0.05 0.88 ± 0.18 )0.58 ± 0.15 1.58 ±
5/9/2000
8/5/2001
** NS
NS NS
NS NS
0.14 0.40 0.07 0.01 0.10
12/6/2001
10.08 3.92 ± 1.83 )0.04 ± 0.20 )0.16 ± 6.08 10.79 ± 2.89 1.79 ± 0.43 0.25 ± 1.34 4.62 ± 1.03 2.02 ± 0.69 )0.11 ± 1.58 8.39 ± 1.17 1.94 ± 0.66 )0.11 ± 2.64 6.93 ± 1.16 1.34 ± 0.34 )0.03 ±
3/10/2000
** *
1.40 4.01 2.62 7.04 3.77
± ± ± ± ±
0.46 1.40 0.36 0.36 0.72
7/8/2001
* = 0.05; ** = 0.01; *** = 0.001; NS = not significant; 1low N = 18.64 g urine-N m)2; high N = 37.32 g urine N m)2.
Source of variation, whole experiment Compaction Amount of N
Compaction Amount of N
Source of variation per measurement period
Treatment1 Low N, no compaction (treatment 3) Low N, compaction (treatment 5) High N, no compaction (treatment 11) High N, compaction (treatment 12) Average
8/8/2000
Application date
NS NS
0.47 2.16 1.75 1.77 1.54
± ± ± ± ±
0.38 0.76 0.48 0.36 0.29
4/9/2001
NS *
0.61 0.97 1.34 1.08 1.00
± ± ± ± ±
0.11 0.27 0.25 0.11 0.12
2/10/2001
** NS
0.92 3.32 1.67 2.52 2.00
avg.
± ± ± ±
0.46 1.23 0.53 1.18
Table 3. The effect of soil compaction on N2O emission factors (in%) for two different N applications. Interactions between factors were not significant and are not listed. Indicated ranges are standard errors (n = 3 for individual treatments)
22
*** NS NS NS * NS Source of variation, whole experiment Urine N
NS Source of variation per measurement period Urine N NS
* = 0.05; ** = 0.01; *** = 0.001; NS = not significant.
10.08 5.28 1.34 2.55 2.51
3.92 5.89 4.62 2.59 4.25
± ± ± ± ±
1.83 0.78 1.03 0.16 0.60
)0.04 ± 0.20 1.03 ± 0.61 2.02 ± 0.69 3.64 ± 0.35 1.66 ± 0.46
)0.16 )0.13 )0.11 )0.04 )0.11
± ± ± ± ±
0.14 0.10 0.07 0.04 0.04
1.40 5.71 2.62 4.01 3.44
± ± ± ± ±
0.46 3.45 0.36 1.47 0.94
0.47 0.91 1.75 1.02 1.04
± ± ± ± ±
0.38 0.11 0.48 0.12 0.19
0.61 0.89 1.34 2.94 1.45
± ± ± ± ±
0.11 0.13 0.25 0.18 0.28
NS
0.92 2.12 1.67 1.96 1.67
± ± ± ±
1.10 0.81 0.36 0.43
they exceed measurement errors, they reflected background emissions that were higher than treatment emissions, mainly within one block. As these higher background emissions were consistent over many measurement dates and could not be traced back to specific causes or inputs, we chose to retain them in our data set. Effects of urine concentration and -volume
± ± ± ± ± 0.62 2.09 0.88 1.12 1.18 0.34 0.24 0.05 0.14 0.10 ± ± ± ± ± 0.52 0.60 0.20 0.37 0.42 Amount of urine)N, g N m)2 18.64 (treatment 3) 27.96 (treatment 4) 37.32 (treatment 5) 74.60 (treatment 6) Average
8/8/2000
5/9/2000
3/10/2000
8/5/2001
12/6/2001
7/8/2001
4/9/2001
2/10/2001
avg. Application date
Table 4. The effect of different amounts of urine))N in similar volumes of urine (4 L m)2) on the N2O emission factor (in%). Interactions between factors were not significant and are not listed. Indicated ranges are standard errors (n = 3 for individual treatments)
23
The urine-N concentrations reported in the literature vary widely. In a review, Oenema et al. (1997) reported ranges between 1 and 20 g N L)1. We feel therefore that the range of our urine concentrations (4.66–18.65 g N L)1) is appropriate to cover most field situations. Similarly, urine volumes vary considerably, with 4 L m)2 as an average values, and 2–8 L m)2 as a possible range (Oenema et al., 1997). The relative composition of nitrogenous compounds varies even more widely with ration in cows (Doak, 1952; Oenema et al., 1997; Van Vuuren and Smits, 1997); however, we think that the chosen composition was representative for current livestock systems. The amount of urine-N (Table 4) and the urine volume (Table 5) did not have an overall significant effect on the N2O emissions. For urine volume, these results deviated markedly from those of Van Groenigen et al. (2005), who found a clear effect which they related to optimal WFPS values. However, they presented an incubation study with partially closed jars in which higher volumes of urine resulted in prolonged periods of higher WFPS due to inhibition of evaporation, and the absence of leaching or uptake by plants. In our current field study, the effects of even large amounts of urine volume on WFPS did disappear within 8 days after application (Figure 4), which was generally before the major N2O peaks were detected (Figure 1). We therefore conclude that the observed effect of urine volume by Van Groenigen et al. (in press) was largely attributable to constraints of incubation studies, rather than to effects that can regularly be observed in practice. Differences in N2O emissions between different amounts of N in equal urine volumes are more difficult to interpret. Although there was no overall effect, there were significant effects for two separate application dates (Table 4). Application dates 8 May and 2 October 2001 both showed a considerable increase in emissions for
24 N2O emission, mg N 2O-N · g -1 urine - N · d-1
3.5
2.8
18.64 g N . m (Tr. 3) -2 27.96 g N . m (Tr. 4) -2 . 37.32 g N m (Tr. 5) -2 74.60 g N . m (Tr. 6) -2
2.1
1.4
0.7
0 01/10/2001
16/10/2001
31/10/2001
15/11/2001
30/11/2001
Date (urine application on 1/10/2001)
Figure 3. Nitrous oxide emissions for different amounts of urine-N in similar volumes of urine (4 L m)2) after application data 1/ 10/2001. Emissions are expressed per unit of applied urine-N. Error bars denote the standard error (n ¼ 3).
larger amounts of urine-N. For 2 October 2001, fluxes from the highest N application (treatment 6) were notably delayed, peaking at 21 October compared to 15 October or earlier for the other treatments (Figure 3), and decreased only towards the end of the measuring period. Conceptually, a distinction might be made between N2O emissions from nitrogenous urine compounds and emissions from soil N induced by increased WFPS after urine application. In our study, this difference may be addressed by comparing treatment 1, where nothing was applied, with treatment 2, which received 4 L water per m2 (Table 1). Figure 2 shows that there were no significant differences between N2O emissions from these two treatments. When treatment 2, rather than treatment 1 was used to correct the treatments for background emissions, no major changes in emission patterns could be detected (results not depicted). Therefore, we conclude that the effect of urine water on soil organic N emissions is negligible in our study. However, this may be different for the pH effect of urea hydrolysis. Effects of dung patches and soil compaction Soil compaction had a highly significant effect on the N2O emissions, raising it on average with a factor 2.2 from 1.30 to 2.92% (Table 3). This is in
between results from Yamulki and Jarvis (2002) and Hansen et al. (1993), who reported increase emissions with factors 3.5 and 1.4 after compaction, respectively. For dung patches, one individual application date (8 August 2000) yielded a strongly significant effect (P ¼ 0.006; Table 2). Although the overall effect (P ¼ 0.054) failed our level of significance (a ¼ 0.05) by a small margin, we think that the average increase in emissions from 1.60 to 2.82% warrants serious discussion. Both effects were considerably smaller than the five- and eightfold increase reported for compaction and dung, respectively, by Van Groenigen et al. (2005). Also, the strong delay of up to 35 days in major peaks reported in the same study for both effects was absent in the present study. Rather, general patterns were similar for dung, compaction and urine-only treatments, with peaks mostly varying in height (Figure 1). In our view, Figure 4 shows most clearly the mechanism through which dung and soil compaction affected N2O emissions. Both resulted in a prolonged period of higher WFPS (Ball et al., 1999; Douglas and Crawford, 1993), which averaged a 4.0% increase for 13 days after urine application in absolute terms (e.g. from 50% to 54% WFPS). As Figure 5 shows, such a 4.0% increase may lead to considerable changes in N2O emissions. On the first application date (8 August 2000), when dung patches made a significant
)0.02 )0.11 )0.11 )0.08 )0.39 ± 0.10 2.02 ± 0.69 0.51 ± 0.36 0.71 ± 0.42 3.36 4.62 2.80 3.59 )0.02 ± 1.48 0.90 ± 2.32 3.47 ± 3.61 1.45 ± 1.35
0.06 0.07 0.07 0.04
NS
4.63 2.62 0.84 2.70
± ± ± ±
1.42 0.36 0.29 0.70
NS
2.20 1.75 0.66 1.54
± ± ± ±
0.30 0.48 0.24 0.29
NS
1.43 1.34 0.79 1.19
± ± ± ±
0.10 0.25 0.07 0.13
NS
1.51 ± 0.40 1.67 ± 0.36 1.15 ± 0.33 1.44
difference, this pattern was especially strong, with dung patches resulting in an WFPS increase of 2– 3% consistently for 17 days (results not depicted). Whether the effect of dung patches would be different if urine would be applied prior to the dung instead of afterwards remains to be determined. In our study, soil compaction and dung patches were artificially created on well-established swards. This would result in relatively quick uptake of mineral N by the plants. However, camping areas in the field tend to combine dung, urine and soil compaction with the partial absence of vegetation. Future research efforts may therefore focus on more prolonged residence time of mineral N in the soil solution due to absence of N uptake. Given the results presented above, the effects of soil compaction and dung patches were crucial in adequately monitoring total N2O emissions from urine patches in pastures. For this purpose, modeling efforts of the spatio-temporal concurrence of urine patches, dung patches and soil compaction are crucial. Although modeling efforts have been published on the distribution of urine patches across a field (Lantinga et al., 1987; Petersen et al., 1956) or the chances of urine patches overlapping (Hack-ten Broeke et al., 1996), we are not aware of any modeling efforts on the spatio-temporal overlap of urineand dung patches, or of hoof threading.
NS
± ± ± ±
0.02 1.03 0.33 0.41
NS
NS
± ± ± ±
12/6/2001 8/5/2001 3/10/2000
Source of variation, whole experiment Urine volume
NS Urine volume
Source of variation per measurement period
* = 0.05; ** = 0.01; *** = 0.001; NS = not significant.
0.27 0.05 0.09 0.14
NS
Conclusions
± ± ± ± 0.85 0.20 0.22 0.42 2 (treatment 8) 4 (treatment 5) 8 (treatment 7) Average
8/8/2000
5/9/2000
7/8/2001
4/9/2001
2/10/2001
avg. Application date Urine volume L m)2
Table 5. The effect of different volumes of urine containing similar amounts of urine–N (4 L m)2) on the N2O emission factor (in%). Interactions between factors were not significant and are not listed. Indicated ranges are standard errors (n = 3 for individual treatments)
25
In conclusion, the average emission factor for urine-only treatments in our study was 1.55%. Most significant effects on N2O emissions in both the field study and the previous incubation study could be linked to changes in WFPS. Both soil compaction and dung patches led to a longer period of high WFPS after urine application. For compaction, this led to an increase of a factor 2.3 under field conditions and 5 in laboratory studies. Seasonal changes could also be linked to WFPS. The effect of urine volume that was detected in the laboratory studies did not remain in the field. Similarly, no significant effect of N compound in the urine could be detected. Therefore, mitigation strategies for N2O emission should in our view focus on avoiding combinations of moisture, compaction, dung and urine in
26
Excess water-filled pore space, %
12.5
8 L urine m-2 (Tr. 7) Compaction + 4 L urine m-2 (Tr. 12) Dung + 4 L urine m-2 (Tr.14)
10.0
7.5
5.0
2.5
0.0 0
5
10
-2.5
15
20
25
Days after urine application
Figure 4. Changes in water-filled pore space (relative to background–treatment 1) due to urine application, compaction and manure, irrespective of season. The error bars denote the standard error.
483
10
N2O flux, mg N2O-N m-2 d-1
9 8 7 312
6
513
5 4
205
3
113
2 1
144 7
10
40
2
0 15
25
35
45
55
65
75
85
95
100
Water-filled pore space, % Figure 5. The relation between water-filled pore space and N2O flux for the whole experiment, irrespective of season or timing of urine application. The error bars denote the standard error, the numbers above the error bars are the number of measurements per WFPS range.
the field and the corresponding increase in WFPS. Obviously, reducing the grazing period would be most effective to achieve this aim. Measures to avoid so-called ‘camping areas’ in the field are especially relevant. This might for example be possible by rotating locations of water supply or creating more shading areas throughout the pasture. Finally, estimates for N2O emissions such as those made using IPCC
methodology should include the effects of compaction and dung.
Acknowledgements This research was funded by NOVEM within the Dutch Research Program on reduction of nonCO2 greenhouse gases, section agriculture (con-
27 tract 374299/0011) and by the DWK research program 415 on ‘Gaseous emissions from animal husbandry’ from the Dutch Ministry of Agriculture, Nature management and Fisheries. We thank Willy de Groot, Eduard Hummelink and Han te Beest for their assistance in the field.
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