Brennan_Plot

Brennan_Plot

Published as: Brennan, R.B., Healy, M.G., Grant, J., Ibrahim, T.G., Fenton, O. 2012.

Incidental phosphorus and nitrogen loss from grassland plots receiving chemically amended dairy cattle slurry. Science of the Total Environment 441: 132 – 140.

http://dx.doi.org/10.1016/j.scitotenv.2012.09.078

Incidental phosphorus and nitrogen loss from grassland plots receiving chemically

amended dairy cattle slurry

10 

11 

12 

R.B. Brennan

1, 2, 3*

, M.G. Healy

1

, J. Grant

4

, T.G. Ibrahim

2

, O. Fenton

2

1

Civil Engineering, National University of Ireland, Galway, Co. Galway, Rep. of Ireland.

2

Teagasc, Environmental Research Centre, Johnstown Castle, Co Wexford, Rep. of Ireland

3

Dept. Crop, Soil, and Environmental Sciences, Division of Agriculture, University of Arkansas,

13 

14 

Fayetteville, AR, USA.

4

Teagasc, Ashtown, Dublin 15, Rep. of Ireland

15  *Corresponding author: Tel: +1479 575 5720 E-mail address: [email protected]

;

16  [email protected]

17 

18 

Abstract

19  Chemical amendment of dairy cattle slurry has been shown to effectively reduce incidental

20  phosphorus (P) losses in runoff; however, the effects of amendments on incidental nitrogen (N)

21  losses are not as well documented. This study examined P and N losses in runoff during three

22  simulated rainfall events 2, 10 and 28 days after a single application of unamended/chemically

23  amended dairy cattle slurry. Twenty-five hydraulically isolated plots, each measuring 0.9 m by

24  0.4 m and instrumented with runoff collection channels, were randomly assigned the following

25  treatments: (i) grass-only, (ii) slurry-only (the study-control), (iii) slurry amended with industrial

26  grade liquid alum comprising 8% Al

2

O

3

, (iv) slurry amended with industrial grade liquid poly-

27  aluminum chloride (PAC) comprising 10% Al

2

O

3

, and (v) slurry amended with lime. During the

28  first rainfall event, lime was ineffective but alum and PAC effectively reduced dissolved reactive

29  P (DRP) (by 95 and 98%, respectively) and total P (TP) flow-weighted-mean-concentrations (by

30  82 and 93%, respectively) in runoff compared to the study-control. However, flow-weighted-

31  mean-concentrations of ammonium-N (NH

4

-N) in runoff were increased with alum- (81%) and

32  lime-treated (11%) slurry compared to the study-control whereas PAC reduced the NH

4

-N by

33  82%. Amendments were not observed to have a significant effect on NO

3

-N losses during this

34  study. Slurry amendments reduced P losses for the duration of the study, whereas the effect of

35  amendments on N losses was not significant following the first event. Antecedent volumetric

36  water content of the soil or slope of the plots did not appear to affect runoff volume. However,

37  runoff volumes (and consequently loads of P and N) were observed to increase for the

38  chemically amended plots compared to the control and soil-only plots. This work highlights the

39  importance of considering both P and N losses when implementing a specific nutrient mitigation

40  measure.

41 

42 

Keywords: alum; poly-aluminum chloride; lime; runoff; amendments; management

43 

44 

1. Introduction

45  Incidental losses of phosphorus (P) and nitrogen (N) occur when rainfall interacts directly with

46  inorganic and organic fertilizers spread on the land surface (Preedy et al., 2001; Smith et al.,

47  2001a; Withers et al., 2003; Buda et al., 2009). Incidental P and N losses are dependent on

48  factors such as: the amount and type of fertilizer or manure applied (Kleinman and Sharpley,

49  2003), timing of the rainfall event after application of fertilizer or manure (Pote et al., 2001;

50  Smith et al., 2007; Allen and Mallarino, 2008; Hanrahan et al., 2009), the volume of runoff

51  generated, antecedent hydrologic conditions and field position, flow path length (McDowell and

52  Sharpley, 2002), vegetative cover (Zhang et al., 2003) and surface slope (Alaoui et al., 2011).

53  Incidental P losses in runoff following land application of dairy cattle slurry are dominated by

54  particulate P (PP) (Withers and Bailey, 2003) and N losses by ammonium-N (NH

4

-N) (Smith et

55  al., 2001a). While P is generally considered the limiting nutrient in freshwater systems (Correll,

56  1998; Hudnell, 2010; Paerl, 2008; Shindler et al., 2008), N losses also pose a significant risk to

57  water quality (Johnes et al., 2007; Vitousek et al., 2009).

58 

59  Chemical amendment of dairy cattle slurry (Elliot et al., 2005; Torbert et al., 2005; Brennan et

60  al., 2011a, b) and poultry litter (Moore and Edwards, 2007) has been effective at reducing P

61  losses in surface runoff following land application. As a result, manure amendment is a

62  recommended best management practice (BMP) in the USA, and federal support is available to

63  aid its implementation (Sharpley et al., 2006; SERA-17, 2012; USDA-NRCS, 2012). There have

64  been a large number of laboratory-scale studies that have examined the effect of amendments on

65  P solubility in dairy and swine slurry (Dao, 1999; Dao and Daniel, 2002; Dou and Cavigelli,

66  2003; Torbert et al., 2005). Torbert et al. (2005) amended composted dairy manure with ferrous

67  sulphate, gypsum and lime (each at 3:1 metal-to-total phosphorus (TP) ratio) before surface

68 

69  application and immediately prior to a 40-min overland event equivalent to a rainfall intensity of

12.4 cm h

-1

. Ferrous sulphate reduced dissolved reactive phosphorus (DRP) loss by 66.3%, while

70  gypsum and lime amendments increased DRP loss. In a plot study, Smith et al. (2001b) amended

71  swine manure with alum and aluminum chloride (AlCl

3

) at two stoichiometric ratios (0.5:1 and

72  1:1 Al: TP). Dissolved reactive phosphorus reductions for alum and AlCl

3

at the lower ratio were

73  33% and 45%, respectively, and 84% for both amendments at the higher ratio.

74 

75  While the effectiveness of amendments is well established, there is less information on the effect

76  of amendments on N loss to runoff and runoff properties. It is known that land application of

77  dairy cattle slurry on grassland (Nunez et al., 2001) and arable land increases runoff volumes

78  which affects N and P losses (Smith et al., 2007). In addition chemical amendment of dairy cattle

79  slurry affects the texture and rate of drying of slurry following land application (Brennan et al.,

80  unpublished data) which may impact runoff volumes. Approximately 50% of the N in dairy

81  cattle slurry is in an inorganic form (NH

4

-N from urea in the urine component of slurry) and

82  although this is plant available, as much as 80% of it is lost through volatilization in a short time

83  period after slurry application. Chemical amendments have been shown to significantly reduce

84  ammonia (NH

3

) volatilization following land spreading of dairy cattle slurry (Lefcourt and

85  Meisinger, 2001). This is likely to increase the NH

4

-N available for uptake by plants and

86  potentially runoff.

87 

88  Chemical amendments reduce P solubility in poultry, swine and dairy cattle manure. However,

89  slurry N is much more mobile than P and its loss pathways are more complex. Therefore,

90  amendments which change the properties of slurry may influence N transformations following

91  land application, and may result in increased N losses to the atmosphere or in surface runoff.

92  This is sometimes referred to as ‘polluting swapping’ (Stevens and Quinton, 2009).Therefore,

93  any study investigating the efficacy of any potential P mitigation measure, such as those

94  described above, must also consider the ‘pollution swapping’ that may arise from their use. To

95  the authors’ knowledge, this is the first study to examine the impact of chemical amendment of

96  dairy cattle slurry on incidental losses of both N and P in runoff.

97 

98  The specific objectives of this study were to investigate (i) incidental N and P losses from soil-

99  only, slurry-only and amended slurry treatments (ii) the effect of chemical amendment of dairy

100  cattle slurry on runoff volume, volumetric water content, and time to runoff, and (iii) the short-

101  term effect of land application of chemically amended dairy cattle slurry on soil chemical

102  properties.

103 

104 

2. Materials and Methods

105 

106 

2.1. Study site characterization

107 

108  The site work was carried out between 11 th

September 2010 and 18 th

October 2010, on a 0.6-ha

109  isolated plot on a beef farm located at Teagasc, Johnstown Castle, Environmental Research

110  Centre (latitude 52º 17’N, longitude 6º 29’W), in the southeast of Ireland. This area has a cool

111  maritime climate, a mean annual precipitation of 1002 mm (effective rainfall (rainfall -

112  evapotranspiration) from between 400 to 500 mm), and a mean annual temperature of 10°C

113  (Ryan and Fanning, 1996).

114 

115  The location of 25 isolated plots within the 0.6 ha site was determined by: topography/slope, soil

116  texture/drainage assessment, depth to watertable, and soil nutrient analysis. Within the 25 plots

117  (0.9 m by 0.4 m), treatments were randomly assigned in five blocks (Fig 1). The site had

118  undulating topography with a 6.7% slope along the length of the site and an average slope of

119 

120 

3.6% across the site. For textural analysis (pipette method, B.S.1377-2:1990 (BSI, 1990)), 10 cm-deep soil samples (n=3) were taken from a 1-m

2

area at the top, middle and bottom of the 0.6

121  ha plot (Fig 1). Electromagnetic conductivity (characterization to 4 m below ground level (bgl))

122  and resistivity of the 0.6-ha site were used to infer overall textural and drainage characteristics.

123  The top of the plot comprised gravelly clay with pockets of silty/clayey gravel underlain by

124  silt/gravel (20 to 26 mS m

-1

), and was relatively well-drained compared to the lower part of the

125  site, which comprised silt/clay and was poorly drained (>26 mS m

-1

). The median perched

126  watertable depth in three piezometers (top, middle and bottom of slope) was 0.6 m bgl on site.

127  The nutrient status of the soil at these locations (P, potassium (K), and magnesium (Mg)),

128  determined using Morgan’s extractant (Morgan, 1941), are presented in Table 1. Soil pH (n=3)

129  was determined using a pH probe and a 2:1 ratio of deionised water to soil (Table 1). Each plot

130  was installed, isolated and instrumented with a runoff collection channel (Fig 1). A composite

131  soil sample (100 mm) was taken from each plot (before (t

0

) and after the experiment (t

30

)) and

132  soil pH, Morgan’s P, K, Mg and lime requirement (LR) were determined. In addition, composite

133  soil samples (25 mm) were taken from each plot at t

0

and t

30

for water extractable P (WEP)

134  determination.

135 

136 

2.2. Slurry analysis

137 

138  Dairy cattle slurry was collected from the dairy farm at the Teagasc, Environmental Research

139  Centre, Johnstown Castle, in September of 2010. The storage tanks were agitated and slurry

140  samples were transported to the laboratory in 25-L drums. Slurry samples were stored at 4°C

141  prior to land application. Slurry pH was determined using a pH probe (WTW, Germany). The TP

142  of the dairy cattle slurry was determined after Byrne (1979). Total potassium (TK), total nitrogen

143  (TN) and TP were carried out colorimetrically using an automatic flow-through unit (Varian

144  Spectra 400 Atomic Absorption instrument). The WEP of slurry and amended slurry was

145  measured at the time of land application (1:100 dry matter slurry: deionised H

2

O) after Kleinman

146  et al. (2007), and NH

4

-N of slurry and amended slurry was extracted by shaking 50 g of slurry in

147  1 L of 0.1 M hydrochloric acid (HCl) on a peripheral shaker for 1 hr and filtering through No. 2

148  Whatman filter paper at the time of application. The results of the slurry analysis are shown in

149  Table 2. The slurry used in this study was typical of slurry found on farms in Ireland (Fenton et

150  al., 2011). The slurry TN, TP, NH

4

-N and TK were constant across samples. The WEP of slurry

151  was decreased significantly by all alum and PAC amendments. Alum addition reduced the slurry

152  pH from approximately 7.1 to 6.5, while lime addition increased the slurry pH to 8.8.

153 

154 

2.3. Treatments

155 

156  The five treatments examined in this study were (i) grassed soil-only (referred to as soil-only

157  hereafter) (ii) slurry applied to grassed soil (the study-control) (iii) slurry amended with

158  industrial grade liquid alum (Al

2

(SO

4

)

3

.nH

2

O), comprising 8% Al

2

O

3

(iv) slurry amended with

159  industrial grade liquid PAC (Aln(OH)mCl

3 n-m), comprising 10%Al

2

O

3

,and(v) slurry amended

160  with lime (Ca(OH)

2

). The slurry and amendments were mixed by shaking in 2-L containers for

161  30 s immediately prior to land application. In practice, it is likely that amendments would be

162  mixed with the slurry in storage tanks during slurry agitation, which normally occurs within 24 h

163  of land application. Two days before the first rainfall simulation, slurry and amended slurry were

164 

165  applied directly to the surface of the grassed soil. Slurry application rates were equivalent to 33 m

3 slurry ha

-1

(42 kg TP ha

-1

), the rate most commonly used in Ireland (Coulter and Lalor, 2008).

166  Amendments were applied at stoichiometric ratios determined based on results of Brennan et al.

167  (2011b). Alum was applied at a rate of 1:1 (Al: TP); PAC at a rate of 0.85:1 (Al: TP); and lime at

168  a rate of 3.9:1 (Ca:TP). Land application of treatments was staggered over three days and applied

169  in blocks to allow for the first rainfall event (RS1) two days after land application of slurry.

170 

171 

2.4. Rainfall event simulation and plot design

172 

173  Two identical portable multi-drop ‘Amsterdam type’ rainfall simulators, described by Bowyer-

174  Bower and Burt (1989), were used in this study. These rainfall simulators have been used on

175  similar permanent grassland sites and soil types (Kurz et al., 2006; Kramers et al., 2009;

176 

177 

O’Rourke et al., 2010). The rainfall simulators were designed to distribute rainfall over a surface area of 0.5 m

2

and were calibrated to deliver rainfall at an intensity of 11 mm hr

-1

. The rainfall

178 

179  simulator water had average concentrations for the three rainfall simulation events of 0.05 mg

NH

4

-N L

-1

, 4.61 mg nitrate-N (NO

3

-N) L

-1

, 0.002 mg DRP L

-1

and 0.004 mg TP L

-1

.

180 

181 

182 

In order to ensure the absence of edge effects, the rainfall simulators were located directly above study plots – each measuring 0.36 m

2

in area. The plots were isolated using 2.2 m-long, 100 mm-

183  deep rigid plastic sheets, which were pushed 50 mm into the soil to isolate three sides of the plot.

184  The runoff collection channel was placed at the bottom of the slope (Fig 1). Plots were orientated

185  with longest dimension in the direction of the slope (average 3.6%). The runoff collector

186  comprised a polypropylene plastic U-shaped channel piece, which was cut in half and wedged

187  against the soil at a depth of approximately 25 mm below the soil surface (Fig 1). A 400 mm-

188  wide edging tool was used to cut the soil to ensure a good seal between soil and collector. The

189  plots were left uncovered for two weeks prior to first rainfall simulation to allow natural rainfall

190  to wash away soil disturbed by inserting the isolators. Natural rainfall was excluded from the

191  plots between time of slurry application and RS1. Thereafter, plots were exposed to natural

192  rainfall. Natural rainfall, together with the average simulated rainfall applied for each of the

193  rainfall simulations, is shown in Fig 2. The grass on all plots was clipped to a height of 50 mm

194  two days prior to application of treatments to simulate the spreading of slurry following silage

195  cutting, which is common practice in Ireland. The second rainfall event (RS2) was 10 days after

196  the original application (t = 12 d) and the third (RS3) after 28 days (t = 30 d).

197 

198  Soil Moisture deficit (SMD) for the entire landscape position was estimated using the grassland

199  Hybrid model of Schulte et al. (2005). For all events, rainfall simulator amounts (mm) were

200  added to actual daily rainfall data and the SMD for each subsequent day was estimated (based on

201  well, moderately and poorly drained soil). When SMD values returned to values achieved using

202  actual rainfall data, the subsequent simulated rainfall event took place. The volumetric water

203  content of soil in each plot was measured immediately prior to each rainfall simulation event

204  using time domain reflectrometry (Delta-T Devices Ltd., Cambridge, UK), which was calibrated

205  to measure resistivity in the upper 50 mm of the soil in each plot.

206 

207 

2.5 Runoff sample collection and analysis

208 

209  Surface runoff was judged to occur once 50 ml of water was collected from the runoff collection

210  channel and the time from start of rainfall simulation to runoff of 50 ml being the time to runoff

211  (TR). Samples were collected every 5 min for RS1, and every 10 min for RS2 and RS3. Surface

212  runoff was collected for 30 min once runoff commenced until the rainfall simulator was switched

213  off to allow the flow-weighted mean concentration (FWMC) to be calculated (Kurz et al., 2006).

214  For the third rainfall event, water was sprayed gently on the plots using a watering can until

215  surface ponding occurred in order to complete rainfall simulations in daylight hours.

216 

217  Immediately after collection, runoff water samples were filtered through 0.45µm filter paper and

218  a subsample was analyzed colorimetrically for DRP, NO

3

-N, NO

2

-N and NH

4

-N using a nutrient

219  analyzer (Konelab 20, Thermo Clinical Labsystems, Finland). A second filtered subsample was

220  analyzed for total dissolved phosphorus (TDP) using acid persulphate digestion. Unfiltered

221  runoff water samples were analyzed for TP with an acid persulphate digestion. Particulate

222  phosphorus was calculated by subtracting TDP from TP. The DRP was subtracted from the TDP

223  to give the dissolved un-reactive phosphorus (DUP). All samples were tested in accordance with

224  the Standard Methods (APHA, 2005).

225 

226 

2.6 Data analysis

227 

10 

228  Runoff ratio (RR) for each plot and for the duration of each simulated rainfall event was

229  calculated by dividing the amount of water generated in overland flow by the amount of rainfall

230  applied. As the plots were the same size, there was no scale effect (Wainwright and Parsons,

231  2002; Norbiato et al., 2009). Differences in RR between plots result from differences in soil

232  permeability (Norbiato et al., 2009) (runoff ratio increases with a decrease in permeability), slope

233  (Alaoui et al., 2011) (increasing slope will increase RR) and depth of unsaturated zone. A higher

234  RR results from wetter rainfall pre-events and/or rainfall event conditions.

235 

236  The structure of the data set was a blocked one-way classification (treatments) with repeated

237  measures over time (rainfall events (RS1-RS3)). The analysis was conducted using Proc Mixed

238  in SAS software (SAS, 2004) with the inclusion of a covariance model to estimate the correlation

239  between rainfall events. A large number of covariates were recorded, including measurements on

240  the simulators and for each analysis; this set of covariates was screened for any effects that

241  should be included in an analysis of covariance. The interpretation was conducted as a treatment

242  by time factorial. Comparisons between means were made with compensation for multiple

243  testing effects using the Tukey adjustment to p-values. Significant interactions were interpreted

244  using simple effects before making mean comparisons. In order to ensure that variation did not

245  affect the experiment, STP was included as a variable in the statistical analysis. Slurry

246  concentration, which was of much greater significance in terms of P concentrations in runoff

247  following slurry application, was uniform within each block.

248 

249 

3. Results

250 

11 

251 

3.1 Incidental nutrient losses over three rainfall events

252 

253  The FWMC and total loads of DRP and TP for all treatments over the three rainfall simulation

254  events are presented in Fig 3. Slurry application increased the FWMC and total loads of DRP

255  and TP. Alum and PAC were equally effective at reducing FWMCs of DRP and TP compared to

256  the study-control. Lime amendment resulted in increased FWMCs of DRP and TP compared to

257  the study-control, with total loads for the lime treatment approximately 2 times greater than for

258  slurry DRP and TP. When total loads were considered, PAC performed better than alum in

259  reducing total loads of DRP. The effects of amendments on P loss were not significant for RS2

260  and RS3, which is likely a result of available P being leached from the soil.

261 

262  The FWMC and total loads of NH

4

-N and NO

3

-N for all treatments are presented in Fig 4. The

263  addition of alum resulted in an increase in the FWMC of NH

4

-N compared with the study-

264  control, while both lime and PAC treatments decreased the NH

4

-N loss. In contrast, all

265  amendments resulted in an increase in the FWMC of NO

3

-N compared with the study-control.

266  The PAC amendment was the only amendment which decreased total loads of NH

4

-N to below

267  those of the study-control. In contrast, both alum and lime amendments resulted in an increase in

268  NH

4

-N loads compared with the slurry treatment. Nitrite losses were negligible and were

269  equivalent to approximately 1.9% of NO

3

for all samples and, for this reason, were not plotted in

270  Fig 4.

271 

272 

3.2 Runoff characteristics

273 

12 

274  The time from start of rainfall simulation event to commencement of runoff event is shown in

275  Fig 5. Time to runoff was generally longer for RS2 and shorter for RS3 (pre-wetted plots). No

276  clear patterns were observed between treatments and differences were not significant. Total

277  runoff volumes for the study were similar for soil and alum treatments (3990 ml 3930 ml), lower

278  for the slurry treatment (3670 ml) and higher for lime and PAC treatments (4780 ml and 4460

279  ml). The differences observed between treatments were not statistically significant. There was no

280  experimental effect on TR across all treatments when rainfall and rainfall intensity were included

281  as covariates in the model. Both covariates showed a quadratic effect. Although there were no

282  treatment effects observed for volumetric water content (VMC), RR and volume runoff,

283  significant event effects were observed. Antecedent SMD conditions before all rainfall

284  simulations for different drainage classes are presented in Fig 2. Soil moisture deficit was similar

285  for all three rainfall events.

286 

287 

3.3 Soil test P, K, LR and pH

288 

289  Soil test P, WEP, Mg, K, pH and LR results from analysis of plots before (t

0

) and at the end of

290 

291 

292 

293  the experiment (t

30

) are presented in Table 3. Average STP, Mg and K concentrations before the start of the experiment were similar for soil (5.5, 182 and 58 mg L

-1

), slurry (4.5, 173 and 57 mg

L

-1

) and amended plots (from 4.3 to 5.9 mg L

-1

, from 160 to 194 mg L

-1

and from 53 to 59 mg L

-

1

). At the end of the experiment, STP increased by 13% in soil-only plots and by 28 to 34% in

294  slurry, PAC and alum. Lime showed an 8.8% decrease in STP. At the end of the experiment, soil

295  K increased for all treatments. Soil WEP decreased between t

0

and t

30 for soil-only, alum-

13 

296  amended and PAC plots (20, 4 and 37%) and increased for study-control and lime-amended plots

297  (42 and 64%).

298 

299 

4. Discussion

300 

301  Under the European Union (EU) Water Framework Directive (WFD) (EU WFD; 2000/60/EC,

302  OJEC, 2000), the water quality of surface and ground waters should be of ‘good status’ by 2015.

303  Small amounts of P losses may contaminate large quantities of water and, therefore, incidental

304  losses are of concern, in particular, for flashy events during baseflow conditions. Chemical

305  amendment of dairy slurry has been shown to be effective in this regard. Moving from laboratory

306  to field scales allows incidental losses to be simulated using in-situ soil and drainage conditions.

307  The impact of slurry and amended slurry on soil pH, infiltration and runoff volumes,

308  concentrations and loads, are all important when assessing the feasibility of a particular

309  amendment.

310 

311 

4.1 Incidental losses for all rainfall events

312 

313  In order to assess the adverse effects of discharge of incidental losses to a surface waterbody, it is

314  critical to examine both runoff nutrient concentrations and total loads. Statistical analysis showed

315  that differences in runoff volume between treatments were not significant. The addition of lime

316  to soil or slurry, which is applied directly to soil, can change soil hydraulic characteristics such

317  as infiltration, water retention and hydraulic conductivity, and may lead to lower (Roth and

318  Pavan, 1991) or higher (Tarchitzky et al., 1993) runoff volumes. The increase in P loss as a result

14 

319  of lime amendment may be also due to an increase in the pH of the lime-amended slurry. Penn et

320  al. (2011) found that in order for calcium (Ca)-phosphate bonds to remain stable, the pH must

321  remain in a range of 6.5 to 7.5. In the present study, the average pH of the soil on the study site

322  was 6.0 and the pH of the lime-amended slurry was 8.8 at the time of application. Brennan et al.

323  (2011a) showed that the pH of lime-amended dairy cattle slurry increased in the first 24 hr

324  following land application. The slurry pH was too high for Ca-P bonds to be stable during RS1

325  and when the slurry and soil interacted and reached equilibrium, the soil pH was lower than the

326  optimal pH for the formation of Ca-P bonds. This may explain why reductions were not observed

327  during RS2 and RS3. In the Brennan et al. (2011b) study, lime was applied at 10:1 Ca:TP

328  compared to 3.9:1 in the present study, and this is possibly the reason for the difference in

329  performance. In addition, the soil used in the Brennan et al. (2011b) study had a pH of 7.45

330  compared to 5.94 in the present study.

331 

332  The reductions achieved in this study are consistent with the findings in Brennan et al. (2011b)

333  with alum being the most effective amendment at reducing incidental PP and TP losses, while

334  PAC was most effective at reducing DRP losses. Incidental P losses accounted for the majority

335  of P losses from the study-control plots, with approximately 75% of DRP, 72% of DUP, 94% of

336  PP and 83% of TP losses, measured over the three rainfall events, occurring during RS1. While

337  incidental losses were significantly reduced in the alum and PAC-amended plots, the effect of

338  amendments on chronic loss of P from the plots was not clear, as differences in runoff

339  concentrations during RS2 and RS3 were not statistically different to the study-control. Studies

340  have shown that chemical amendments can reduce incidental and chronic P losses (long-term P

341  losses to runoff arising from elevated STP (Buda et al., 2009)) from soils receiving amended

15 

342  poultry litter (Moore and Edwards, 2005 and 2007). Amendments must be an ongoing practice

343  for every manure application to effectively reduce P losses. Ultimately, P application must be

344  balanced with crop P requirements to avoid chronic P loss.

345 

346  In the present study, chemical amendment of dairy cattle slurry had no significant effect on NO

3

-

347  N concentration or load in runoff water. Alum increased the FWMC and load of NH

4

-N

348  compared to the study-control during the first rainfall event, PAC reduced the FWMC and load

349  of NH

4

-N and lime had no effect on the FWMC but increased the load of NH

4

-N due to an

350  increase in runoff volume. Dairy cattle slurry is high in NH

4

-N which explains the high NH

4

-N

351  in runoff during RS1 (Smith et al., 2007). In a gas chamber experiment, Brennan et al.

352  (unpublished data), using the same amendments as Brennan et al. (2011b), found that alum and

353  PAC reduced NH

3

emissions from land applied slurry by up to 93% while lime amendment

354  resulted in a two-fold increase in NH

3

emissions. The increase in NH

4

-N load observed for the

355  alum treatment during RS1 was likely caused by a decrease in NH

3

volatilization, which resulted

356  in more NH

4

-N remaining on the soil surface and being available for uptake by runoff. The

357  difference between alum and PAC treatments indicates that PAC maybe more effective at

358  binding NH

4

-N which has not been volatilized on the soil surface, thereby reducing loss to

359  runoff. The reduction in NH

4

-N concentrations in runoff between RS1 and RS2 across all

360  treatments, including the study-control, was likely due to nitrification occurring in the soil

361 

362 

363 

364  following slurry application and interaction with the soil. Smith et al. (2007) added dairy cattle slurry at a rate 75 m

3

ha

-1

to grassed plots and reported soluble N (NH

4

-N+NO

3

-N) concentrations ranging from 2 mg L

-1

to 14 mg L

-1

, which was comparable to the average

FWMC of soluble N observed in the present study (6.3 mg L

-1

). The results of the present study

16 

365  results suggest that PAC is the most suitable amendment, as there was no increase in N losses

366  compared to the study-control. This study did not examine the effect of amendments on N

367  leaching losses. This work highlights the need to examine the pollution swapping effects of all P

368  mitigation practices.

369 

370 

4.2 Runoff characteristics

371 

372  In the current study, differences in slope of plots were not shown to be significant. All plots had

373  the same landscape position mid-way between a down-gradient river and an up-gradient

374  groundwater divide. Other studies have shown greater differences in slope at different landscape

375  positions. Kleinman et al. (2006) investigated P and N losses in runoff from 1 x

2 m plots under

376  simulated rainfall conditions during wet and dry periods in two landscape positions, foot slope

377  (6%) and mid-slope (30%). Kleinman et al. (2006) showed that antecedent soil moisture at the

378  foot-slope during the spring resulted in quicker runoff generation times and greater volumes of

379  runoff.

380 

381  In a homogeneous soil, runoff ratios should increase with VWC. The fact that this relationship

382  was not always found in the current study for soil-only plots may be due to local heterogeneity.

383  After slurry application, this relationship was more evident, which infers that mixing of soil and

384  slurry leads to greater spatial homogeneity of water distribution and saturation. For amended

385  slurry, the higher variability between VWC and runoff ratio (often a variable relationship)

386  suggests that the amendments had a sealing effect. Within the timeframe of this study, it was not

387  possible to assess the long-term effect of amendments on soil physical characteristics. As time

17 

388  from slurry application increases, soil conditions will return to a more heterogeneous state,

389  whilst amendments may delay this process.

390 

391 

4.3 STP, K, LR and pH

392 

393  In the present study, observed differences in soil nutrient concentrations following chemical

394  amendment were not statistically significant. There were, however, noticeable changes in soil pH

395  for some plots. These changes identify a need to examine the effect of chemical amendments on

396  long-term P dynamics in soil following application of chemically amended dairy cattle slurry.

397  Studies to date involving chemical amendment of dairy slurry have largely focused on reducing

398  P solubility in dairy cattle slurry (Dao and Daniel, 2002; Dou et al, 2003; Brennan et al., 2011a)

399  and mitigating incidental P losses in runoff studies (Smith et al., 2001b; Elliot et al, 2005;

400  Torbert et al., 2005; Brennan et al., 2011b), but little attention has been given to the effect of

401  chemical amendments on short and long-term nutrient availability to plants. In the US, where

402  chemical amendment of poultry litter is a BMP, Moore and Edwards (2005) and Moore and

403  Edwards (2007) reported results from a 20-year study, which began in 1995 and examined the

404  effects of chemical amendment of poultry litter on soil productivity and water quality. They

405  found that long-term land application of alum-amended poultry litter did not acidify soil in the

406  same way as NH

4

-N fertilizers, long-term P losses were reduced, and Al availability was lower

407  from plots receiving alum-treated poultry manure than NH

4

-N fertilizer.

408 

409  With the exception of Kalbasi and Karthikeyan (2004), there has been little research on the effect

410  of land spreading of chemically amended dairy cattle slurry to soil. Kalbasi and Karthikeyan

18 

411  (2004) examined three silt loam soils with different STPs (12, 66 and 94 mg kg

-1

Bray-1 P,

412  respectively) in an incubation experiment conducted over a 24-mo period. Kalbasi and

413  Karthikeyan (2004) found that alum and ferric chloride had no effect on soil pH, while lime

414  increased soil pH slightly. This was consistent with the findings of the present study. These

415  results were also consistent with another study by Brennan et al. (unpublished data). In that

416  study, 5 soils, including soil taken from the same study site as the present study, were amended

417  with chemical amendments and incubated for 9 months. While chemical amendments

418  consistently reduced WEP, the STP and soil pH were not significantly affected by application of

419  amended slurry, with the exception of FeCl

3

-amended slurry in some instances. Due to the

420  relatively short duration of the present study, it was not possible to examine the relationship

421  between the STP of incubated soils and the in-situ STP when subject to a similar treatment.

422 

423 

4.4 Management implications of using chemically amended dairy slurry

424 

425  Ireland has committed to meeting the requirements of the WFD to achieve at least ‘good status’

426  of all surface and groundwater by 2015. While current practices are effective, there will be a

427  time-lag before current changes in farming practices will result in an observable reduction in

428  nutrient losses and a reduction in risk to water quality. The time-lag will be site-specific and

429  while it is likely that in many areas the effects will be shown relatively quickly, there may be a

430  need for some new P mitigation measures. Results show that chemical amendments can

431  significantly reduce P losses and that a once-off application of any of the chemical amendments

432  examined will not result in a significant change in soil physical and chemical properties. It is,

19 

433  however, critical that the long-term effect of repeated applications of chemical amendments to

434  slurry on STP, soil pH, soil WEP, soil microbiology and macro-biology be examined.

435 

436 

5. Conclusions

437 

438  The findings of this study validate findings at laboratory-scale, with amendment of dairy cattle

439  slurry with alum and PAC reducing DRP and TP losses (FWMC and loads) compared to the

440  study-control. Alum was the most effective amendment at reducing PP and TP losses, while PAC

441  was the most effective at reducing DRP losses. This study also showed that chemical amendment

442  of dairy cattle slurry with alum increased NH

4

-N loss (FWMC and loads) to runoff, while PAC

443  reduced NH

4

-N losses. Future work must examine the effects of chemical amendment of dairy

444  cattle slurry on the N cycle and gaseous emissions. In addition, these results indicate that

445  amendments may affect runoff volume for events occurring 48 hr after slurry application.

446  Following from this study, the next step will be to examine the targeted use of chemical

447  amendments at field and catchment-scale. In future, farm nutrient management must focus on

448  examining all farms within a catchment and identifying areas which pose the greatest risk. It is

449  possible that P mitigating methods, such as chemical amendment of dairy cattle slurry, may be

450  used strategically within a catchment to bind P in cow and pig slurries. This work highlights the

451  importance of considering both P and N losses when implementing a specific nutrient mitigation

452  measure.

453 

454 

Acknowledgments

455 

20 

472 

473 

474 

475 

476 

477 

478 

464 

465 

466 

467 

468 

469 

470 

471 

456  The first author gratefully acknowledges the award of a Walsh Fellowship by Teagasc to support

457  this study. The authors are also grateful for assistance provided by Teagasc and NUI Galway

458  staff and colleagues with special mention to Peter Fahy, Theresa Cowman, Carmel O’Connor,

459  Denis Brennan, Linda Finn, Paddy Sills, Pat Donnelly, Stan Lalor, Nyncke Hoekstra and Paddy

460  Hayes. The authors would also like to thank Michael Brennan for his assistance with fieldwork.

461 

462 

463 

21 

479 

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28 

672   

673   

674   

675   

676   

677   

678   

658 

659 

660 

661 

662 

663 

664 

665 

666 

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667   

668   

669   

670   

671   

679   

680   

681   

682   

29 

683 

684 

Table 1 Soil pH, Morgan’s extractable P, K and Mg, sand silt, clay factions, textural class of soil within 0.6 ha plot.

Position Piezometer No.

1 pH Morgan’s P P index

2

K Mg Sand Silt Clay Textural Class

mg %

18 Loam

696 

697 

698 

699 

700 

701 

702 

703 

685 

686 

687 

688 

1

The location of the piezometers is illustrated in Fig 1.

2

P Index 2 expects a likely response to fertilizers whereas a P index of 3 expects a tenuous or unlikely response.

689 

690 

691 

692 

693 

694 

695 

30 

707 

720 

721 

722 

723 

716 

717 

718 

719 

712 

713 

714 

715 

708 

709 

710 

711 

724 

725 

726 

704 

705 

706 

Table 2 Slurry DM, pH, water extractable phosphorus (WEP), total nitrogen (TN), total phosphorus (TP) and total potassium (TK) and average concentrations of NH

4

-N (n=5).

4

+

-N g kg

-1

Slurry (control)

Alum

PAC

Lime

9.1 (0.54) 7.1 (0.62)

9.6 (0.58 6.5 (0.44)

9.42 (0.64) 6.9 (0.47)

9.4 (0.38) 8.8 (0.67)

3.19 (0.37)

0.003(0.001)

0.007 (0.008)

2.48 (0.99)

3960 (741)

4410 (590)

3980 (1280)

5010 (725)

1240 (145)

1260 (190)

1200 (270)

1390 (150)

5170 (870)

5210 (640)

4330 (1290)

5610 (840)

1200 (260)

1160 (270)

1180 (290)

1210 (300)

31 

727 

728 

729 

730 

Table 3 The average slope, soil pH, soil water extractable P (WEP), Morgan’s extractable P, potassium (K), magnesium (Mg) and lime requirement (LR) on the day before the experiment (t

0

) and after the experiment (t

30

) for all of the treatments.

Treatment Slope WEP

0

/WEP

30

P

0

/P

30

K

0

/K

30

Mg

0

/Mg

30

LR

0

/LR

30

% mgkg

-1

mg

Soil 3.05 4.13/6.32

Slurry (control) 2.90 5.82/5.97 8.11/4.78 6.31/8.98 56.8/91.0 173/186 6.00/4.20

737   

738   

739   

740   

741   

742   

743   

731 

732   

733   

734   

735   

736   

744   

745   

746   

747   

748   

32 

749 

750 

751 

Fig 1 Map of study site showing ground elevation, topography, slope, soil conductivity, groundwater flow direction, location of subplots, piezometers and diagram of runoff collection channel and plot isolation.

752 

753 

 

33 

754 

755 

756 

Fig 2 Measured daily rainfall (mm) and simulated soil moisture deficit (SMD) for well, moderate and poorly drained soils. Rainfall applied to plots during RS1-3 is added to measured daily rainfall and used for simulated SMD calculation. X axis is in Julian Days.

757   

758 

34 

759 

760 

Fig 3 Flow-weighted mean concentration and total loads of particulate phosphorus (PP), dissolved un-reactive phosphorus (DUP) and dissolved reactive phosphorus (DRP).

12

10

DRP

DUP

PP

8

6

4

2

0

1 2

Soil

3 1 2

Control

3 1 2

Alum

3 1 2

Lime

3 1 2

PAC

3

20

16

12

8

4

DRP

DUP

PP

0

1 2

Soil

3 1 2

Control

3 1 2

Alum

3 1 2

Lime

3 1 2

PAC

3

761 

 

Treatment

35 

762 

763 

Fig 4 Flow-weighted mean concentration and total loads of nitrate (NO

3

-N) and ammonium

(NH

4

-N).

10

NO3-N

8

NH4-N

6

4

2

0

1 2

Soil

3 1 2

Control

3 1 2

Alum

3 1 2

Lime

3 1 2

PAC

3

764 

765 

20

15

NO3-N

NH4-N

10

5

0

1 2

Soil

3 1 2

Control

3 1 2

Alum

3 1 2

Lime

3 1 2

PAC

3

Treatment

 

36 

766 

767 

768   

Fig 5 Average rainfall intensity, runoff volume, time to runoff and soil volumetric water content for the first (RS1), second (RS2) and third (RS3) rainfall events.

20

15

10

5

0

RS1 RS2 RS3

6

5

4

3

2

1

0

RS1 RS2 RS3

Soil Slurry Alum PAC Lime Mean Soil Slurry Alum PAC Lime Mean

140

120

100

80

60

40

20

0

RS1 RS2 RS3

Soil Slurry Alum PAC Lime Mean

60

50

40

30

20

10

0

RS1 RS2 RS3

Soil Slurry Alum PAC Lime Mean

769 

Mean (average value for all plots)

 

37 

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