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DOI: 10.1002/jpln.201200214
J. Plant Nutr. Soil Sci. 2012, 175, 840–845
Short Communication
Replacing inorganic fertilizer with anaerobic digestate may maintain agricultural productivity at less environmental cost John J. Walsh1, Davey L. Jones1, Gareth Edwards-Jones1, and A. Prysor Williams1* 1
School of Environment, Natural Resources & Geography, College of Natural Sciences, Bangor University, Gwynedd, LL57 2UW, United Kingdom
Abstract We applied digestate generated from the anaerobic digestion of slurry, undigested slurry, or inorganic N (ammonium nitrate) or NPK compound fertilizer to pots of grass and a grass–clover mix grown in two soils. Crop yields were equal or enhanced with digestate, and analysis of soil water showed that there was less potential for loss of nutrients via leaching. Replacing inorganic fertilizer with digestate may therefore maintain grassland productivity but with less impact on the environment. Key words: agriculture / biogas / eutrophication / livestock / organic farming / water pollution
Accepted July 21, 2012
1 Introduction Inorganic fertilizers are becoming increasingly expensive due to the energy-intensive nature of their production, and their use is responsible for a significant proportion of the greenhouse-gas (GHG) emissions and water-pollution incidences from agriculture. Spreading organic wastes (e.g., animal slurries) can reduce dependency on inorganic fertilizers; however, this too can lead to nitrate (NO3 ) and phosphate (PO34 ) pollution of groundwater (Strebel et al., 1989; Fraters et al., 1998) and the storage and application of slurry also emits GHGs (Banks et al., 2007). There is therefore both a need to reduce the amount of inorganic fertilizers utilized and to improve the management of organic wastes to reduce the environmental impact of agriculture. Anaerobic digestion (AD) generates two products: methane which can subsequently be used as a source of renewable energy, and digestate, which can be separated into a dry and liquid fraction suitable for land application. Other benefits of AD include reducing both the odor (Smet et al., 1999) and pathogen load (Sahlstrom, 2003) of wastes, improving weedseed kill (Engeli et al., 1993), and reducing both the chemical and biological oxygen demand of wastes (Anonymous, 2003; Clemens et al., 2006). Anaerobic digestion is particularly appealing to livestock systems as the wastes generated (manure and slurry) are suitable for digestion and hence can provide an additional source of income and reduce costs (Demirer and Chen, 2005). In agricultural systems, N is the most frequent limiting factor for crop growth, especially on organic farms where inorganic fertilizer cannot be applied (Berry et al., 2002). During the AD process, the ammonium (NH 4 ) content of wastes increases;
digestate therefore has a higher content of plant-available N than undigested wastes (Gutser et al., 2005). Anaerobic digestion could therefore help farmers maximize nutrient returns from slurry and reduce their reliance on inorganic fertilizer. There is, however, a paucity of information on the agronomic effects of applying digestate as a replacement for undigested slurry and inorganic fertilizer. Although Pain and Hepherd (1985) and Tafdrup (1995) are extensively cited, the evidence they provide on the positive benefits of digestate application on crop yield is highly subjective and lacks scientific rigour. A recent study did, however, show that application of digestate, rather than manure gave significantly greater yield of hay (Bougnom et al., 2012), although there was no comparison with inorganic fertilizer. Some ambiguity also exists as to whether the application of digestate, relative to undigested wastes, leads to greater or less potential for leaching of nutrients (Möller et al., 2008; Sänger et al., 2010, 2011; Goberna et al., 2011). Crucially, a number of such studies have been performed on fallow soil (e.g., Goberna et al., 2011; Sänger et al., 2010, 2011), where no crops can uptake the readily available forms of N (Gutser et al., 2005). Furthermore, no studies have compared leaching from pasture systems to which digestate has been applied relative to inorganic fertilizer. The objectives of this study were to compare how the application of liquid digestate generated from digested cattle slurry, undigested (raw) cattle slurry, and two types of widely used inorganic fertilizer (N and NPK) to different grass leys affected crop yield and the potential for leaching of nutrients in soils from an organic and conventional farm, both of which are productive farms operating on a pasture-based system for livestock.
* Correspondence: Dr. A. P. Williams; e-mail:
[email protected]
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Plant Nutr. Soil Sci. 2012, 175, 840–845
2 Materials and methods 2.1 Soil and fertilizer collection and characterization Samples from a sandy clay loam–textured Eutric Cambisol were collected from a pasture-based system at a conventional (nonorganic) farm (CS) (53°14′05″ N, 4°00′50″ W) and an organic farm (OS) (53°08′16″ N, 2°90′48″ W). The soil samples were collected to a depth of 10 cm, passed through an 8 mm sieve and analyzed in the laboratory within 24 h of collection. Undigested slurry (US) was collected from an organic dairy farm, and liquid digestate (LD) was collected from an AD unit on the same farm. The AD unit is a 1000 m3 mesophilic (38°C) gas-mixing system, continually stirred digester with a retention time of 25 d, and fed with cow slurry only. The digestate was separated mechanically after leaving the digester, and only the liquid fraction was collected. Samples were analyzed within 24 h of collection. Soil and organic fertilizers were analyzed according to Walsh et al. (2012).
Replacing inorganic fertilizer with anaerobic digestate 841 all samples were weighed fresh, dried for 48 h at 85°C, and then reweighed.
2.4 Nutrient levels in soil solution Sterile vacuum tubes were attached for 24 h to the Rhizon® samplers at weekly intervals throughout the experiment, 1 h after a watering event. Volumes of soil solution collected were subsequently measured, and concentrations of NO3 , 3 NH 4 , and PO4 determined. Nutrient sampling was stopped 10 weeks post the first fertilizer application as concentrations in soil solution thereafter were below detection levels (< 0.1 mg L–1). These data were pooled so that mean concentrations of nutrients in soil solution could be determined.
2.5 Statistical analysis Statistical analysis was performed using SPSS v.18. Soil and organic fertilizers were analyzed via a homoscedastic twotailed T-test; crop yield and soil-solution data were analyzed via ANOVA. Post-hoc tests were carried out on all ANOVAs using Tukey HSD test at the level p < 0.05.
2.2 Experimental design A fully randomized pot trial experiment (n = 100) was set up in a greenhouse under controlled conditions (mean temperature [23 ± 3]°C). Pots (150 mm ∅; 1.2 L volume) were filled with 1.3 kg of either OS or CS (dry bulk density, OS: 0.87 g cm–1 and CS: 0.85 g cm–1) and fitted with Rhizon® suction samplers (Rhizosphere Research Products, Wageningen, The Netherlands) at ≈ 100 mm depth for collection of soil solution. Soil moisture was maintained at 70% field capacity throughout the experiment by watering up to a known weight for each pot. Half the pots for each soil type were seeded with perennial ryegrass only (Lolium perenne L.), and the other half with a mixture of perennial ryegrass and white clover (Trifolium repens L.) at a rate equivalent to 40 kg of grass ha–1 and 12 kg of clover ha–1 (Emorsgate Seeds, Norfolk, UK). The final setup therefore consisted of four subgroups, namely (1) OS, grass only; (2) OS, grass–clover mix; (3) CS, grass only; and (4) CS, grass–clover mix. These were subsequently split into subgroups of five treatments (n = 5 of each), to which the following were applied: US, LD, commercial straight ammonium nitrate (34.5% N) fertilizer (N) (GrowHow, Cheshire, UK), a commercial compound (21.8.11 NPK) fertilizer blend (NPK) (Yara, Lincolnshire, UK), or no amendment controls (C); for both soil types. The application rate for each was normalized for N, based on total dissolved N values for US and LD and total N content for the inorganic fertilizers. Nutrients were surface-applied in two stages, with an equivalent of 100 kg N ha–1 applied four weeks following seeding, and the equivalent of 50 kg N ha–1 applied thereafter, one week after the first harvest (see below). Pots were regularly re-randomized during the trial.
2.3 Yield analysis In total, three herbage harvests were taken at weeks 5, 11, and 16 after the first application of fertilizer. After harvesting, 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3 Results 3.1 Soil and fertilizer analysis The physicochemical properties of the soils and fertilizers are presented in Tables 1 and 2. There were some differences between soils such as in levels of nutrients (e.g., P), likely to reflect differences in past management (e.g., nutrient-input regime) between conventional and organic farming systems. There were also differences in some properties of both organic fertilizers. For instance, NH 4 levels were considerTable 1: Physico-chemical properties of both soils used in the study. Values represent means ± SEM (n = 3) and are expressed in terms of dry weight. Organically managed soil 5.3 ± 0.04
pH EC /
lS cm–1
51.0 ± 3.7
Water content / % 19.5 ± 0.11 Total C / mg g–1 Total N / mg
g–1
20.4 ± 0.4 2.0 ± 0.02 10 ± 0.02
C:N DOC / mg g–1
0.35 ± 0.02
NO3 / lg g–1
12 ± 2.12
–1 NH 4 / lg g
P / lg g–1 g–1
Conventionally Statistical managed soil significance 5.5 ± 0.02
p < 0.05
44 ± 5.5
p > 0.05
21.8 ± 0.27
p < 0.05
29.2 ± 0.5
p < 0.05
3.1 ± 0.07
p < 0.05
9 ± 0.13
p > 0.05
0.11 ± 0.01
p < 0.05
20 ± 1.21
p > 0.05
10 ± 3.41
9 ± 1.34
p > 0.05
16 ± 2.2
90 ± 5.41
p < 0.05
14 ± 4.5
30 ± 4.12
p < 0.05
Ca / lg g–1
24 ± 2.2
36 ± 3.31
p < 0.05
Na / lg g–1
40 ± 3.1
65 ± 6.03
p < 0.05
K / lg
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J. Plant Nutr. Soil Sci. 2012, 175, 840–845
Table 2: Physico-chemical properties of the undigested slurry (US) and liquid digestate (LD) used in the study. Values represent means ± SEM (n = 3) and are expressed in terms of dry weight. Undigested slurry
Liquid digestate
7.6 ± 0.12
pH
3.2 Crop yield
p < 0.05
All treatments gave greater yields than the unamended control (p < 0.05), apart for grass–clover grown in the conventionally managed soil (Fig. 1A–1D). In grass and grass–clover grown in the organically managed soil (Fig. 1A and 1B), greatest yields were recovered where LD or NPK had been applied; likewise for grass grown in conventional soil (Fig. 1C), although there was no difference between N and NPK in the latter (p > 0.05). However, in conventional soil, the grass–clover yield (Fig. 1D) showed very different results with no difference (p > 0.05) between control and the inorganic fertilizers; but greater yields (p < 0.05) from pots to which LD or US had been applied.
9.0 ± 0.14
12.2 ± 0.10
p < 0.05
94.8 ± 0.37
p < 0.05
394 ± 8
Total C / mg g–1
Statistical significance
85.7 ± 0.26
EC / mS cm–1 Water content / %
8.6 ± 0.01
274 ± 6
p < 0.05
Total N / mg g–1
22 ± 0.4
22 ± 0.8
p > 0.05
C:N
18.1 ± 0.26
13.1 ± 0.07
p < 0.05
DOC / mg g–1
35 ± 0.2
30 ± 1.0
p < 0.05
DON / mg g–1
12 ± 0.14
27 ± 1.3
p < 0.05
NO3 / mg
0.31 ± 0.15
g–1
–1 NH 4 / mg g
P / mg
0.51 ± 0.04
6.5 ± 0.25
20.4 ± 0.53
11 ± 0.8
g–1
Ca / mg g–1
1.0 ± 0.28
p > 0.05 p < 0.05 p < 0.05
9 ± 0.01
17 ± 0.08
p < 0.05
14 ± 0.13
20 ± 0.10
p > 0.05
K / mg g–1
3.6 ± 0.01
Na / mg g–1
ably higher in the separated liquid digestate, but levels of P were lower.
7.3 ± 0.21
3.3 Nutrient levels in soil solution When the data were analyzed in a 3-way ANOVA, no differences in mean NO3 concentrations in soil solution were evident in both soil types, however, there was a difference in seed type (p < 0.05), with NO3 levels greatest where clover was also present (Fig. 2A and 2B). No difference in NO3 levels in soil solution emerged between C pots and those applied US and LD; all of which were lower than inorganic fer-
p < 0.05
10
Crop yield / g DW pot -1
A. Organic grass
First harvest Second harvest Third harvest Total harvest
8
B. Organic grass-clover
6 c
d
c
d
4
c
b b
b
2 a
a 0
D. Conventional grass-clover b b
Crop yield / g DW pot -1
C. Conventional grass 8
6
a
d cd c
a
a
4 b 2 a 0 C
US
LD
N
NPK
C
US
Treatment
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
LD
Treatment
N
NPK
Figure 1: Yield of (A) organic-farm grass, (B) organic-farm grass–clover, (C) conventionalfarm grass, and (D) conventional-farm grass–clover after the application of different fertilizer types: no-fertilizer control (C), undigested slurry (US), liquid digestate (LD), mineral N (N), and mineral NPK (NPK). Values represent the mean ± SEM (n = 5). Lowercase letters within graphs denote differences (p < 0.05) between treatments within the same subgroup.
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A. Organic
Grass Gras-clover
NO3- / mg N L-1
70
Replacing inorganic fertilizer with anaerobic digestate 843 B. Conventional
b
c
60 50
b
b
b 40
b
b
30 20
ab
10 aa
a a
a a
a a
0 6
a
aa
a
D. Conventional
C. Organic b
NH4+ / mg N L-1
5 4 b 3
ab
ab b
b
2
ab
1
a
a a a
a a
a
C
US
LD
a a
aa
a a
C
US
LD
0 N
NPK
Treatment
Treatment
tilizer (p < 0.05). There were no differences between the inorganic fertilizers (p > 0.05), but levels were greater in comparison to the control and organic treatments (p < 0.05; Fig. 2A and B). Likewise, there were no differences in NH 4 concentrations in soil solution collected from C, US, and LD; and no difference between the inorganic fertilizers. However, levels within controls and those applied organic fertilizers were lower than those applied inorganic fertilizers (p < 0.05; Fig. 2C and D). No significant differences were seen in concentrations of phosphate in soil solution between treatments (p > 0.05; data not shown).
4 Discussion Grasslands dominate livestock systems, and the agricultural industry is increasingly seeking to maximize returns achievable through better utilization of grass with lesser inputs. This study found that grasses applied LD gave similar or better yield than those receiving either N or NPK inorganic fertilizers. This pattern was also evident when comparing LD and US in all but one case. To our knowledge, this is the first study to show such differences with grass leys. It is acknowledged that yield response may differ with crop types; however, the results concur with previous studies with other crop types that implied that AD of organic fertilizers enhances nutrient availability to plants and subsequent yield (Dahlberg et al., 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
N
NPK
Figure 2: Mean concentrations of NO3 and NH 4 in soil solution of an organically (A and C) or conventionally (B and D) managed soil after the application of different fertilizer types: no-fertilizer control (C), undigested slurry (US), liquid digestate (LD), mineral N (N), and mineral NPK (NPK). Values represent the mean ± SEM (n = 5). Lowercase letters within each graph denote significant differences (p < 0.05) between treatments within the same subgroup.
1988; Liedl et al., 2006; de Boer et al., 2008; Bougnom et al., 2012). Further, the application of inorganic N is known to suppress clover growth (Nesheim et al., 1990); and although clover yield was not directly measured, visual observation suggests that there was greater clover in pots with LD than inorganic N. In this study, all fertilizers were surface-applied as 95% of slurry in the UK is applied in this manner (Defra, 2010). However, the thicker texture of US meant that a notable proportion remained on the surface of the soil and hence may be subject to loss of ammonia (NH3) (and therefore N) through volatilization (Sommer and Hutchings, 2001); whereas the LD was readily absorbed into the soil. While this rapid infiltration may make the N within LD more plant-available, the high soluble C and N load of LD could also lead to significant N2O emissions especially if applied in moderately wet soils (Juarez-Rodriguez et al., 2012). A high loss of N2O would negatively offset any benefits seen from reduced syntheticfertilizer use or marginal gains in crop yield. The greater yield from leys applied LD compared to N in all subgroups may also partly be due to LD incorporating other nutrients key for plant growth (e.g., P and K), as LD gave yields akin to NPK in three of the four treatments. This broader nutrient base may be particularly important in soils with yield-limiting factors or deficiencies. Land-application of all the digestate, rather than solely the liquid fraction, may bring further agronomic benefit as the solid (dry fiber) fraction www.plant-soil.com
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is likely to contain higher levels of other important plant nutrients, such as P (Holm-Nielsen et al., 2009), in addition to increasing soil organic matter levels. The loss of N from soil is a major concern due to the economic and environmental cost on water quality and the atmosphere (Stark and Richards, 2008). Schroder et al. (2010) implied that the degree of N leaching from grassland was unaffected by whether the source of N was inorganic fertilizer or cattle slurry, and that the dominant factor is the balance of supply and crop demand. The presence of nitrogenous compounds in soil solution collected at 10 cm depth does not necessarily equate to the leaching of N from the system as the rooting depth of different crops, including grass, can be considerably greater, hence nutrients could be taken up by the plant. Nevertheless, using typical application rates and methods, our results indicate that application of inorganic fertilizer, rather than LD, could lead to greater leaching of N from grassland, especially during periods of high rainfall or when soil nutrient and moisture levels exceed plant/crop requirements. It is acknowledged that a number of other factors may also govern the loss of nutrients through leaching, including climatic patterns, and especially the application method, timing and load of fertilizer application. The results from this trial could be developed and expanded to test such relationships at field-scale.
5 Conclusions This greenhouse-based study was performed with one widespread agricultural soil type, two extensively used fertilizers, and with two grass–ley compositions that are widely used for livestock grazing. Although the findings should not be extrapolated to all soil types and management systems, the results add further evidence as to the potential value of AD and subsequent utilization of digestate. This study indicates that replacing inorganic fertilizers with liquid digestate can maintain or improve yields from grassland systems and concurrently reduce the potential for losses of nutrients to the environment. This may ultimately reduce agricultural dependence on inorganic fertilizer and the energy and economic costs associated with their use. Therefore, AD should not only be considered a source of renewable energy and waste management system, but also a pollution-abatement technology.
Acknowledgments This work was funded by the European Union’s Knowledge Economy Skills Scholarship programme, Fre-Energy Ltd., Wrexham, UK and Calon Wen, Carmarthenshire, UK. We thank Llinos Hughes, Mark Hughes, Rob Brook, Paula Roberts, and Ceri Gwyther for assistance.
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