International Journal of AgronomyVolume 2012 (2012), Article ID 101074, 10 pagesdoi:10.1155/2012/101074

Research Article

Comparison of Raw Dairy Manure Slurry and Anaerobically Digested Slurry as N Sources for Grass Forage Production

Olivia E. Saunders,¹ Ann-Marie Fortuna,1,2 Joe H. Harrison,³ Elizabeth Whitefield,³ Craig G. Cogger,¹ Ann C. Kennedy,⁴ and Andy I. Bary¹

¹Department of Crop and Soil Sciences, Washington State University, Pullman, WA 99164, USA²Department of Soil Science, North Dakota State University, 1301 12th Avenue North, Fargo, ND 58105, USA³Department of Animal Sciences, Washington State University, Puyallup, WA 98371, USA⁴Land Management and Water Conservation Research Unit, USDA-ARS, Washington State University, Pullman, WA 99164, USA

Received 29 November 2011; Accepted 23 January 2012

Academic Editor: H. Allen Torbert

Copyright © 2012 Olivia E. Saunders et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

We conducted a 3-year field study to determine how raw dairy slurry and anaerobically digested slurry (dairy slurry and food waste) applied via broadcast and subsurface deposition to reed canarygrass (Phalaris arundinacea) affected forage biomass, N uptake, apparent nitrogen recovery (ANR), and soil nitrate concentrations relative to urea. Annual N applications ranged from 600 kg N ha⁻¹ in 2009 to 300 kg N ha⁻¹ in 2011. Forage yield and N uptake were similar across slurry treatments. Soil nitrate concentrations were greatest at the beginning of the fall leaching season, and did not differ among slurry treatments or application methods. Urea-fertilized plots had the highest soil nitrate concentrations but did not consistently have greatest forage biomass. ANR for the slurry treatments ranged from 35 to 70% when calculations were based on ammonium-N concentration, compared with 31 to 65% for urea. Slurry ANR calculated on a total N basis was lower (15 to 40%) due to lower availability of the organic N in the slurries. No consistent differences in soil microbial biomass or other biological indicators were observed. Anaerobically digested slurry supported equal forage production and similar N use efficiency when compared to raw dairy slurry.

1. Introduction

There is a need for a set of best management practices that addresses how to utilize the growing quantity of reactive nitrogen (N) produced by livestock operations. Animal agriculture in the United States has become more specialized with farms consolidating and growing in size [1]. The number of dairy farms has decreased by 94% since 1960, but the number of animals has remained constant [2]. Animal consolidation has created challenges with respect to on-farm N surplus, waste management and nutrient loading in the environment [3, 4]. Annually in the United States, more than 5800 Mg of manure N is produced [5]. One approach to ameliorate negative environmental impacts associated with animal manures is through adoption of anaerobic digestion technologies to treat farm-generated manures and food processing wastes [6–9]. Digestion of wastes can provide a stable and consistent source of nutrients comparable to inorganic fertilizers such as urea.

Anaerobic digestion converts organic carbon into methane used to generate electricity, and it also converts organic N to plant available ammonium (), increasing the ratio of /total N in the effluent [10]. Carbon is removed during both the methane production and fiber removal processes, resulting in a smaller C : N ratio of the effluent [11]. Therefore, digested effluent can serve as low-cost source of readily available nutrients for crop production. Some studies [12] have found increased yield and N availability with application of anaerobically digested material as compared to nondigested material, possibly due to increased N availability and reduced carbon (C) content. Anaerobically digested manure can provide sufficient nutrients to support biomass and crop yields equivalent to synthetic fertilizers and raw manures [13, 14]. The apparent mineral nitrogen recovery of tall fescue (Festuca spp.) receiving raw dairy manure slurry was reported by Bittman et al. [13] as 55 and 51% at early and late applications, respectively, using the drag shoe method (band applied directly to soil, under plant canopy). When surface applied, the ANRM was 37% applied early and 40% when applied late. Similar results were presented by Cherney et al. [15] with orchardgrass (Dactulis glomerata L.) and tall fescue.

Perennial systems that contain living plants year round tend to remove more reactive N than annual systems. Mean reed canarygrass biomass measured in trials in Minnesota was 13 Mg ha⁻¹ under modest N applications (168 kg N ha⁻¹ yr⁻¹) [16]. Bermudagrass (Cynodon spp.) fertilized with 89–444 kg manure N ha⁻¹ yielded a mean of 7.92 Mg dry matter ha⁻¹ over four or five cuttings per year [17]. However, the forage crop recovered only 25% of the N applied over the four years included in the study. Reed canarygrass (Phalaris arundinacea) is an ideal candidate for N removal because of its ability to store any left-over N applied during the growing season in rhizomes overwinter, providing a significant advantage to the forage in early spring when soil-N may be limited [18].

As with any N source, application of manure N in excess of crop uptake can result in leaching [19]. Up to 46% of applied manure N may persist in the soil, increasing the potential for loss of N after multiple applications in a growing season [20]. Some studies indicate that manure N poses less of a risk to leaching than the same amount of N in the form of synthetic fertilizer [21] due to immobilization of N that often occurs as humic materials build up in soil. Other researchers have determined manure increases leaching [22]. Irrespective of the source and properties of the N fertilizer applied during winter months when plants are dormant, leaching can be the main source of N loss [23].

Manure additions can enhance soil fertility and quality through their short and long-term contribution to soil C and N [24–27]. Current research demonstrates that long-term manure applications increase soil organic matter, basal respiration, microbial biomass, and enzymatic activity (measures of soil quality), while mineral fertilizers can decrease pH, enzymatic activity, and microbial biomass C [28]. Organic amendments such as manure also have an effect on microbial community structure in addition to enhancing the activity, C content, and size of soil microbial biomass [29, 30]. A study by Zhong et al. [31] demonstrated total phospholipid fatty acid (PLFA), gram-negative, and actinobacterial PLFA were highest in treatments of organic matter and organic matter + mineral NPK fertilizer. Functional diversity from organic manure and organic manure + mineral NPK fertilizers increased over time far more than with additions of synthetic fertilizers alone. We anticipate that long-term application of raw dairy slurry and digested slurry will enhance soil quality affecting microbial community structure and activity overtime.

The goal of this study was to determine the fate of applied N in anaerobically digested slurry (derived from mixed dairy slurry and food waste), raw dairy slurry, and urea during forage production. Specifically, we compare the biomass, ANR and N uptake of forages to determine which N source(s) has the potential to maintain forage biomass and reduce reactive N. In addition, we evaluated the effectiveness of subsurface deposition versus broadcast application of raw slurry and anaerobically digested slurry to improve forage biomass production, ANR and N uptake of forages, as well as reduce residual reactive N. Our hypotheses were (1) digested slurry would have more available N and generate a greater forage response than raw slurry, (2) subsurface deposition would conserve more N than surface application, resulting in greater forage response, particularly for the raw manure with higher solids content, and (3) application of digested slurry would reduce soil nitrate-N concentrations relative to urea.

2. Materials and Methods

2.1. Site Description

A field-based experiment, located on a commercial dairy in Monroe, Washington, was established in 2009. The field was mapped as 90% Puget silty clay loam (fine-silty, mixed, superactive, nonacid, mesic Fluvaquentic Endoaquepts) and 10% Sultan silt loam soils (fine-silty, isotic, mesic, Aquandic Dystrochrepts) [32] and had a history of manure applications. The site had climatic conditions typical of the Maritime Pacific Northwest with cool wet winters and dry summers. The 2009 growing season had a drier than normal summer, while 2011 had a cool spring and dry summer (Table 1). The 2010 season had the best growing conditions, with a warm spring and more summer rainfall than 2009 or 2011.

Table 1: Average air temperature and total precipitation by month beginning at the start of plot implementation through the 3rd growing season.

Year
Month
Average air temp (°C)
Total precipitation (mm)

2009
April
9.2
61

May
12.8
73

June
16.8
19

July
20.0
6

August
18.0
13

September
15.5
54

October
9.9
100

November
7.6
137

December
1.4
35

2010
January
7.3
89

February
7.0
44

March
7.8
54

April
9.4
55

May
11.2
66

June
14.4
61

July
17.0
7

August
17.1
22

September
15.3
85

October
10.6
85

November
5.7
107

December
5.3
184

2011
January
5.1
120

February
3.4
80

March
7.1
119

April
7.2
79

May
10.6
74

June
14.1
39

July
15.7
18

August
16.4
3

September
15.5
23

October
10.2
71

Data from Washington State University AgWeatherNet, 21-Acres Station.

The experimental design included six treatments in a randomized complete block design with four replicates . Treatments included two dairy manure slurries (raw and anaerobically digested), two slurry application methods (broadcast and subsurface deposition), inorganic N (pelletized urea), and a zero-fertilizer treatment that received 0 kg N ha⁻¹.

The raw dairy slurry was flushed from the barn floor and obtained fresh from a holding tank. Digested slurry was produced in an anaerobic digester with a plug-flow design, operating within mesophilic (23.5°C) conditions, with an approximate 17-day retention time, and storage capacity of ~6,100,000 liters. Liquid slurry from a single dairy consisting of 1,000 lactating cows was codigested with pre-consumer food-waste substrates. Food-waste consisted of no more than 30% of the total digester input and included whey, egg byproduct, processed fish, ruminant blood, biodiesel byproduct, and Daf grease (dissolved air flotation). After digestion, materials were passed through a rotating drum screen solid separator where solids were removed for composting and liquids pumped to a storage lagoon. The digested slurry applied to plots was obtained just after liquids-solids separation and prior to lagoon storage. A 250 mL subsample of each slurry was taken during each application (Table 2), cooled, and analyzed for total-nitrogen, nitrate-N, ammonium-N, total-phosphorus, and total solids [33] (Table 3).

Table 2: Dates of forage harvest and fertilizer (slurry and urea) applications for field study in Monroe, WA for 2009–2011.

        2009
2010        
2011        

Forage Harvest
Fertilizer applicationa
Forage Harvest
Fertilizer applicationa
Forage harvest
Fertilizer applicationa

17-Apr-09b

4-Mar-10

7-May-09c
14-May-09
26-Apr-10
11-May-10
5-May-11
19-May-11

2-Jun-09
8-Jun-09
10-Jun-10
22-Jun-10
10-Jun-11
22-Jun-11

1-Jul-09
20-Jun-09d
7-Jul-10
15-Jul-10
14-Jul-10
4-Aug-11

28-Jul-09
11-Aug-09
12-Aug-10

22-Aug-10
31-Aug-11

31-Aug-09

15-Sep-10
30-Sep-10
20-Sep-10

29-Sep-09

2-Dec-10

18-Oct-11

30-Nov-09

aSoil samples taken 1-day prior to fertilizer application.

bEarly season manure application by grower, prior to plot establishment.

cHarvest prior to plot establishment, yield data from this harvest does not include replicates.

dUnintended slurry application from grower.

Table 3: Annual mean N and P concentrations of raw and anaerobically digested slurries applied to pasture plots.

2009
2010
2011

Raw dairy
Digested
Raw dairy
Digested
Raw dairy
Digested

Slurry
slurry
slurry
slurry
slurry
slurry

Percent Total Solids (%)
2.8
1.9
3.4
2.0
3.4
1.4

Total N, mg kg-1a
1441
1617
1653
2672
1475
2000

NH₄–N, mg kg⁻¹
707
1038
776
1253
760
930

Organic N, mg kg-1b
734
578
877
1419
715
1070

Total P, mg kg⁻¹
350
300
331
292
330
210

aN and P concentrations reported as is.

bOrganic nitrogen (N) = total N − NH₄–N.

A mix of reed canarygrass (Phalaris arundinacea) cv. “Palaton” and white clover (Trifolium repens) was over-seeded into the field at 62 kg ha⁻¹ in May 2006, three years before the start of this experiment. Plots were sprayed with broad leaf herbicides on 18 June 2009, 10 July 2010, and 8 August 2011 with 1.17 L ha⁻¹, 2, 4-Dichlorophenoxy acetic acid, 73 mL ha⁻¹ Carfentrazone-ethyl (Aim), and 410 mL ha⁻¹, dicamba (Banvel) to control the clover.

2.2. Slurry Application Method

Slurries were applied via two application methods, subsurface deposition and surface broadcast application. Subsurface deposition was accomplished with a 4169-liter capacity manure tank fitted to a National Volume Equipment pump (model MEC 4000/PALD) with a 3.05 meter Aerway Sub-Surface Deposition (Model AW1000-2B48-D) and custom Banderator attachment for application of manure through eight PVC pipes attached directly behind the Banderator tines. Tines were set 19 cm apart on the roller and allowed to drop 10 cm below the soil surface creating intermittent slices 12.5 cm in length at the surface. Visual observation of the plots suggested that the tines created slices at random locations throughout the growing season. Surface broadcast of raw and anaerobically digested slurries were accomplished using an Aerway system with the tines raised above the soil surface.

Application rates for the raw and anaerobically digested slurry were projected to be equal in total N and allowed to vary in ammonia-N, for a total yearly application of approximately 600 kg N ha⁻¹ in 2009, 500 kg N ha⁻¹ in 2010, and 300 kg N ha⁻¹ in 2011 (Table 4). We reduced the amount of N applied on urea and slurry treatments each year of the study from 2009 to 2011 based on the fall soil nitrate concentrations. When soil nitrate-N is above 35 mg N kg−1 in the fall, it is recommended that applications be eliminated after August 1st, N application rates be reduced in the subsequent year by 25–40 percent and sidedress N at planting be eliminated [34]. An early season raw dairy slurry application (Table 2, application 1 in 2009) was applied by the grower to all plots prior to establishment of the field plots and is not included in the statistical analyses. An inadvertent application of 143 kg N ha⁻¹ across all plots by the grower in June of 2009 (Table 2, application 4) is included in the analysis. This accounts for the higher annual application rate in 2009. Application rates were lowest in 2011 because wet conditions prevented an early season application (Table 4), and the slurries had lower mean N concentrations. Plots were fertilized no more than five days after grass harvest. There were a total of five manure applications per year during 2009-2010 and four in 2011 (Table 2).

Table 4: Application rate of fertilizer source at each application period, and seasonal total N and P inputs, 2009–2011.

Application
1
2
3
4
5
1
2
3
4
5
Seasonal total

Total N kg ha⁻¹
NH₄⁺–N kg ha⁻¹
Total N kg ha⁻¹
NH₄⁺–N kg ha⁻¹
Total P kg ha⁻¹

2009

Control
111a
0
0
143b
0
51
0
0
83b
0
254
135
0

Urea
111a
112
112
143b
112
51
112
112
83b
112
590
471
0

Raw
111a
121
168
143b
47
51
64
78
83b
23
590
300
130

Digested
111a
176
115
143b
81
51
114
93
83b
47
626
389
121

2010

Control
0
0
0
0
0
0
0
0
0
0
0
0
0

Urea
112
112
112
112
0
112
112
112
112
0
448
448
0

Raw
92
112
113
99
84
53
24
58
53
53
500
240
92

Digested
86
121
63
67
129
55
40
35
40
98
466
268
54

2011

Control
0
0
0
0
0
0
0
0
0
0
0
0
0

Urea
0
112
112
112
0
0
112
112
112
0
336
336
0

Raw
0
52
60
84
63
0
34
24
29
47
258
135
51

Digested
0
33
136
69
70
0
23
52
33
27
308
134
26

aEarly season slurry application by grower, prior to plot establishment.

bApplication 4 in 2009 was an unintended application from the grower to all plots. Urea fertilizer considered equal to in plant availability.

2.3. Field Management and Analysis

The aboveground biomass from grass swaths, 0.6 × 0.6 m, was harvested from the center of each plot every 28–35 d (Table 2) using hand-held hedge clippers. Three subsamples were taken within each of the four plot replicates for each treatment. The three subsamples were divided into grasses, clover, and weeds to adjust the aboveground biomass and ANR measurements. Due to herbicide applications, weeds were minimal all years. White clover biomass was significant in two of the cuttings in 2011. Samples were bagged, and weighed immediately. Forage was then dried at 55°C for 24 hrs, weighed, and ground in a Wiley Mill (Arthur H. Thomas Co., Philadelphia, PA) with a 1 mm screen. Ground samples were analyzed for forage nitrogen content with a Leco FP-528 Nitrogen Analyzer (Leco Corporation, St. Joseph, MI; AOAC, 2001) by Cumberland Valley Analytical Services Inc. (Hagerstown, MD).

Soil samples were collected from each plot and analyzed for Bray-1 P, exchangeable K, and pH at the beginning of the experiment 2009 and again at the end of 2010. Six soil cores per plot were taken to a 30-cm depth using a 2.54 cm diameter soil sampling probe and composited. Additional soil samples were collected for nitrate-N analysis monthly throughout the growing season using the same method, except for biweekly from mid-September through the end of November. Nitrate-N below the 30 cm depth was not measured. Soil chemical properties were analyzed by Soiltest Farm Consultants (Moses Lake, WA) using the methods of Gavlak et al. [33]. Ammonium-N was determined using a salicylate-nitroprusside method and nitrate-N using the cadmium reduction method. Soil samples for gravimetric water content were homogenized by mixing, and a subsample was dried at 38°C for 72 hrs [35].

Whole-soil phospholipid fatty acid (PLFA) procedures generally followed Bligh and Dyer [36] as described by Petersen and Klug [37] and modified by Ibekwe and Kennedy [38]. Fatty acid methyl esters were analyzed on a gas chromatograph (Agilent Technologies GC 6890, Palo Alto, CA) with a fused silica column and equipped with a flame ionizer detector and integrator. ChemStation (Agilent Technologies) operated the sampling, analysis, and integration of the samples. Extraction efficiencies were based on the nonadecanoic acid peak as an internal standard. Peak chromatographic responses were translated into mole responses using the internal standard and responses were recalculated as needed. Microbial groups were calculated based on the procedure of Pritchett et al. [39].

2.4. Slurry Analysis

Slurries were analyzed for total N, ammonium-N, and total P (Table 3). Nitrogen was extracted via the Kjeldahl method [33]. Phosphorus was analyzed using a Thermo IRIS Advantage HX Inductively Coupled Plasma (ICP) Radial Spectrometer (Thermo Instrument Systems, Inc., Waltham, MA) by the Dairy One Forage Analysis Laboratory (Ithaca, NY).

2.5. Statistical Analyses and Calculations

An analysis of variance (ANOVA) was run using SAS PROC MIXED on the aboveground forage biomass, nitrogen content in forage, soil nitrate-N, and soil biological groups for all treatments across the three years [40]. Data were analyzed as a randomized complete block design with each of the six treatments analyzed independently. Crop biomass and crop-nitrogen content from each year were analyzed separately using ANOVA with treatment and sample day as fixed effects. Significance is indicated with a [40].

Forage apparent N recovery (ANR%) was calculated in 2010 and 2011 as a percentage of N (total and inorganic) applied during the season based on the work of Cogger et al. [41] and Bittman et al. [13]:

Estimates of N fixed in white clover were set to 80% of total N in clover biomass based on ¹⁵N studies conducted in a pasture of similar forages and N fertilizer management [42]. Using the above correction, 80% of clover N was subtracted from the forage N uptake values used in the ANR calculations for the two cuttings in 2011 with significant amounts of clover.

3. Results

3.1. Baseline Soil Data

Soil data sampled in May 2009 prior to the start of the field experiment (Table 5) indicated that fertility was not different across the field site. Organic matter, a source of inorganic-N, averaged 5.5 percent (55 g kg⁻¹). Bulk density of soils in the field ranged from 1.14 to 1.30 g cm⁻³, with a mean average density of 1.21 g cm⁻³.

Table 5: Soil pH, Bray-P, and exchangeable K at start of experiment and after two years of slurry applications.

Plot
pH
Bray P
NH₄OAc K

mg kg⁻¹

Baseline, 12 May 2009

Control
6.0
173
591

Urea
6.0
176
608

Raw-subsurface
6.0
160
598

Raw-broadcast
6.0
165
632

Digested-subsurface
6.1
140
616

Digested-broadcast
6.0
168
612

2 December 2010

Control
6.2a
173
379b

Urea
6.0b
176
286c

Raw-subsurface
6.3a
186
479a

Raw-broadcast
6.2a
185
465a

Digested-subsurface
6.2a
173
447ab

Digested-broadcast
6.2a
162
440ab

Letters in a column within a year indicate significant differences at , letters are not included when no significant differences were found. Samples from different dates were analyzed separately using an ANOVA.

3.2. Forage Biomass, N Uptake and ANR

Analysis of variance results for cumulative forage biomass in 2009 to 2011 are presented in Table 6. Total yield was greatest in 2010 (14.1–18.0 Dry Mg ha⁻¹) and lowest in 2011 (9.2–11.1 Dry Mg ha⁻¹). The 2009 data (8.08–9.5 Dry Mg ha⁻¹) did not include the first cutting of the year (7.7 Mg ha⁻¹) because it was harvested before plots and treatments were established. The growing conditions in 2010 were the most favorable of the three seasons. Forage biomass in 2011 was reduced by cool spring temperatures and low summer rainfall (Table 1). Urea had the highest yield in 2009, (Table 6). In 2010, urea and digested broadcast slurry had higher yield than the digested slurry applied subsurface. Slurry type and application method did not affect yield in 2009 or 2011.

Table 6: Annual forage yield and N uptake, 2009 to 2011.

Forage yield
Nitrogen uptake

Treatment
Dry Mg ha⁻¹
N kg ha⁻¹

2009a
2010
2011
2009b
2010
2011

Control
8.0b
14.1c
9.2b
283c
362cd
192c

Urea
9.5a
18.0a
11.1a
389a
655a
296a

Raw- subsurface
8.6b
16.6ab
10.5a
330b
507b
263b

Raw- broadcast
7.9b
17.0ab
10.8a
308bc
531b
254b

Digested-subsurface
8.6b
16.1b
10.9a
332b
501b
239b

Digested-broadcast
8.7b
17.8a
10.9a
338b
550ab
255b

Letters within a column indicate significant differences at .

aValues for forage yield from the first harvest prior to implementation of nitrogen fertilizer treatments and application method were 7.7 Mg ha⁻¹.

bThe N content in forage yield from the first harvest prior to implementation of nitrogen fertilizer treatments and application method was 253 kg N ha⁻¹.

Similar trends occurred when comparing crop N uptake in the forage grasses (Table 6). Urea-treated plots accumulated the most plant N, ranging from 296 to 655 kg N ha⁻¹ removed per year. Uptake of N in forage grasses was greatest in 2010 (Table 6). Slurry type and application method did not have a significant effect on N uptake any year. Nitrogen uptake was lowest in 2011, likely a result of lower N application rates (Table 4) and poorer weather during the spring and summer. Forages in 2011 also contained significant amounts of clover, an N fixer (27% of the dry mass of forage yield at harvest 1 and 34% at harvest 2). Less than 10% of the forage biomass was clover in 2009 and 2010.

In the first full season of the study (2010), the recovery of applied N in the forage (ANR) was higher than in 2011 (Table 7). More favorable weather patterns for growth in 2010 compared with 2011 probably increased ANR in 2010. Urea treatments had an ANR of 65% in 2010 and 31% in 2011. Calculations based on total N applied in slurries were lower, ranging from 29 to 40% in 2010 and 15 to 24% in 2011, and similar between the two types of slurry. ANR calculations based only on the amount of total NH₄⁺–N applied in slurries were 52 to 70% in 2010 and 35 to 53% in 2011, similar to ANR observed for urea.

Table 7: Apparent nitrogen recovery (ANR) in harvested forage as percentage of total and ammonium N applied, 2010 and 2011.

ANR 2010
ANR 2011

Treatment
% of Total N
% of –N
% of Total N
% of –N

Urea
65
65
31
31

Raw-subsurface
29
60
15
35

Raw-broadcast
34
70
24
47

Digested-subsurface
30
52
23
53

Digested-broadcast
40
70
20
46

Urea fertilizer considered equal to in plant availability.

3.3. Soil Nitrate-N

Plots receiving urea had the highest concentration of soil nitrate-N over the three seasons, while there were few differences among the slurry treatments (Table 8). Soil nitrate-N concentrations were highest in all fertilized treatments from July to the start of the fall rainy season, when the potential for leaching increases. Soil nitrate-N levels were greatest in 2009, likely because of the high rates of N applied that year. Lower soil nitrate-N in 2010 reflected the high N uptake during the favorable growing conditions that year. Soil nitrate-N increased again in 2011, particularly in the fall. This was despite a lower N application rate and may reflect the reduced yield and N uptake by the forages during the less favorable growing season in 2011.

Table 8: Soil NO–N (mg kg⁻¹) at 0 to 30 cm depth, 2009–2011.

Soil –N (mg kg⁻¹)

Treatment

Sample Date
Control
Urea
Raw subsurface
Raw broadcast
Digested subsurface
Digested broadcast

2009

12-May
20gh
21gh
18gh
19gh
19gh
20gh

4-Jun
18gh
28fg
24fg
23 g
30fg
24fg

6-Jul
35fe
80b
71bc
76bc
68c
65cd

3-Aug
34f
86ab
80b
76bc
86ab
71bc

9-Sep
20gh
91a
66cd
82ab
72bc
67c

21-Sep
20gh
81b
53de
62cd
78bc
55d

1-Oct
17gh
62cd
52de
50de
56d
45de

19-Oct
14gh
91a
35fe
54de
44e
45de

3-Nov
11h
54de
23g
30fg
29fg
23g

19-Nov
10h
22gh
9.8h
12h
11h
10h

30-Nov
11h
11gh
12h
12h
10h
12h

2010

26-Feb
12fg
11fg
15ef
15ef
13f
14ef

11-May
13f
23d
20de
20de
20de
18ef

16-Jun
6.1g
9.2fg
7.9g
7.0g
7.4g
7.3g

13-Jul
10fg
25cd
18e
18de
16ef
14ef

17-Aug
13fg
61a
23cd
22de
18ef
22de

30-Sep
18de
36b
23cd
28c
22de
19de

12-Oct
12fg
28bc
22de
22de
27cd
24cd

26-Oct
7.2g
12fg
15ef
17ef
21de
16ef

2-Dec
6.9g
8.2g
11fg
9.5fg
10fg
10fg

2011

4-Apr
6.1g
6.2g
7.0g
6.5g
7.0g
8.0g

21-Jun
7.9g
18ef
11fg
11fg
12fe
12fg

4-Aug
8.7g
21ef
15fg
17ef
18ef
18ef

30-Aug
12fg
43cd
22ef
16f
23ef
17ef

16-Sept
17ef
48bc
39cd
48bc
48bc
32d

29-Sept
17ef
46c
36d
42cd
55b
36d

13-Oct
19ef
66a
45c
44c
50bc
44c

4-Nov
8.4g
32d
28de
24e
30de
23ef

Letters within a year indicate significant differences at .

3.4. Microbial Groups

Microbial groups in general did not vary with treatment, but rather varied by year (Table 9). The control and urea treatments varied from the other treatments most consistently for most groups, while no consistent differences were observed among the slurry treatments. By 2011, the control treatment had significantly lower bacteria and anaerobic markers than the other treatments, but similar levels of overall microbial biomass and fungi.

Table 9: Soil microbial analyses from field plots in the spring, 2009–2011.

Biomass
Bacteria
Fungi
Bacteria to fungi
Anaerobe
Mono-unsaturated

g kg⁻¹
Mole percenta
Mole percent
ratio
Mole percent
Mole percent

May 2009

Control
535 ab
0.246
0.098
3.01
0.091
0.338

Urea
433 c
0.246
0.092
3.23
0.092
0.348

Raw-B
538 ab
0.243
0.093
3.18
0.094
0.335

Raw-SSD
454 bc
0.237
0.094
3.07
0.091
0.330

Digested-B
473 bc
0.242
0.092
3.18
0.093
0.324

Digested-SSD
623 a
0.238
0.083
3.45
0.091
0.322

May 2010

Control
610 a
0.243 b
0.071 abc
4.18 ab
0.115 ab
0.328 b

Urea
333 b
0.215 c
0.074 ab
3.48 c
0.101 b
0.322 b

Raw-B
401 a
0.266 ab
0.084 a
4.04 bc
0.116 ab
0.357 ab

Raw-SSD
297 b
0.268 a
0.066 bc
4.91 a
0.127 a
0.414 a

Digested-B
258 b
0.259 ab
0.071 abc
4.65 ab
0.123 a
0.398 a

Digested-SSD
279 b
0.267 ab
0.066 c
4.97 a
0.125 a
0.406 a

April 2011

Control
512
0.221 b
0.087 ab
3.11 d
0.082 c
0.341 b

Urea
447
0.250 a
0.078 c
3.96 a
0.094 ab
0.380 a

Raw-B
489
0.257 a
0.093 ab
3.35 bcd
0.092 b
0.335 b

Raw-SSD
428
0.253 a
0.095 a
3.25 cd
0.100 ab
0.345 b

Digested-B
441
0.258 a
0.085 bc
3.68 ab
0.101 ab
0.357 ab

Digested-SSD
491
0.255 a
0.085bc
3.61 abc
0.102 a
0.359 ab

Letters within a column within a year indicate significant differences at . No letters indicate no significant differences within that column.

aMole percent = (mole substance in a mixture)/(mole mixture) %.

4. Discussion

4.1. Forage Biomass, N Uptake and ANR

Forage biomass, plant N-uptake, and nitrate concentrations during the 2009–2011 growing seasons were affected by seasonal and long-term N management (a history of manure applications) that resulted in high N uptake from the control treatments. Also, favorable growing conditions in 2010 allowed for a more productive field season in this year. For this study, total harvest yield during each season was within the range of other published work where animal manures were applied to forages harvested multiple times over a season [16, 17, 41].

While other studies have shown incorporation of manure to increase yield and crop N content by reducing gaseous losses [13], we did not see an improvement in crop N content from incorporation of slurries in this system. Forages grown in plots with broadcast applied slurries took up the same amount of N or more N than with subsurface deposition, which may have been caused by plant-growth disturbance from the airway banderator when subsurface applying effluent. Additionally, the infiltration rate of the anaerobically digested slurry may have been rapid enough that gaseous losses in the field were not different among subsurface deposition and broadcast applications. From an agronomic perspective, the two slurry types performed equally well as urea over the three growing seasons. Anaerobically digested slurry was suitable for forage production when applied at rates equal to raw dairy slurry. Moller and Stinner [8] also reported no differences in N uptake between digested and undigested slurry. How the system will respond after many years of anaerobically digested slurry application is unclear as the quantity of organic N applied is less than that of raw dairy slurry, supplying less recalcitrant N to the pool of soil organic matter.

4.2. Soil Nitrate-N and Microbial Groups

We found few differences between slurry treatments in seasonal soil concentrations. There was, however, significantly more nitrate-N in urea-treated plots on many dates, even though there was slightly less total N applied to the urea plots in some years. The spike in nitrate concentration in October on soils where urea was applied in place of slurries indicates a greater potential for N leaching from urea compared with the slurries. All treatments declined in concentrations to levels that were not significantly different from control treatments after the fall rains began. Lower soil nitrate-N during the growing season of 2010 compared with 2009 may be due in part to a lower amount of total nitrogen applied. Also, little rainfall during the 2009 growing season may have caused a buildup of soil nitrate in the surface layers. Higher late-season nitrate in 2011 compared with 2010 may have been the result of poorer growing conditions reducing N uptake.

Postharvest soil nitrate-N is a measure of residual plant-available N subject to leaching loss, and an indicator of excess applied N and/or poor yield. The recommended timing of postharvest soil nitrate testing in forage systems that utilize animal manure as a source of fertility in the Maritime Pacific Northwest is prior to October 15 [34]. Nitrate concentrations from soil samples collected from our site in mid-October showed that all treatments except the control exceeded 30 mg NO₃–N kg ha⁻¹ in 2009 and 2011, with NO₃–N levels highest in the urea treatment. Fall nitrate-N levels above 30 mg kg⁻¹ are considered excessive in manured pastures, and reduced rates and adjusted timing of applications are recommended [34].

While soil nitrate concentrations decreased during the fall 2009 months, it is likely that some of this nitrate was not entirely leached from the system, but stored in the canary grass rhizomes over winter as described by Partala et al. [18]. This is evident in the significantly higher yields and nitrogen content of forages during the early season harvest on 26 April 2010.

While the focus of this study is N, dairy manure also contains high levels of P. Runoff from high-P soils can lead to eutrophication in fresh water. Soil P levels were already excessive at the start of this study, because of the history of dairy manure applications at the site, and P tended to increase in the slurry-treated plots during the study (Table 5). The anaerobically digested slurry contained less P than the raw dairy slurry, probably because it had a lower solids content, which would lead to less P accumulation over time.

Microbial groups varied with year more than treatment in these field studies. Urea treatments varied from the other treatments to the greatest extent. The raw and anaerobically digested materials did not alter the soil microbial components as determined by PLFA. Our results may partially be the result of past manure applications.

5. Conclusions

Subsurface deposition did not increase yield or N uptake compared with surface broadcast application, possibly because the slurries were low enough in solids to infiltrate readily into the soil, and because the subsurface injectors could have disrupted plant growth. Anaerobically digested dairy slurry was shown to provide adequate soil fertility and N availability for crop uptake and forage production over the three field seasons. In the short term, anaerobically digested slurry did not significantly increase yield or N uptake compared with similar rates of raw slurry.

This study indicated that soil nitrates measured to a 30 cm depth were fairly consistent across slurry treatments and application methods during each of the field seasons. Soil nitrate-N was lower in 2010 due to favorable growing conditions and lower total applied N relative to 2009. Although urea treatments had the highest apparent N recovery value, the potential for nitrate leaching was also greatest under this management. Anaerobically digested slurry did not increase soil concentrations or alter the microbial composition and provided equal forage production and similar N use efficiency when compared to undigested dairy slurry.

Acknowledgments

Funding for this project was provided by WA Dairy Prod. Commission 113547-001, USDA-NRCS 68-3A 75-5-178 and Hatch Project 0711. The authors would like to thank our student aides: Jeni Maakad, Taylor Graves, Mike Pecharko, Jacob Turner, Clayton Waller, and Allison Wood. They would also like to welcome Vivienne Marie Whitefield to their work group born November 21, 2011.

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