Closing the phosphorus loop in England: The spatio-temporal balance of phosphorus capture from manure versus crop demand for fertiliser

Resources, Conservation and Recycling 55 (2011) 1146–1153

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Closing the phosphorus loop in England: The spatio-temporal balance of phosphorus capture from manure versus crop demand for fertiliser Anna Bateman a , Dan van der Horst b , David Boardman a , Arun Kansal c , Cynthia Carliell-Marquet a,∗ a b c

Department of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom Department of Geography, Earth and Environmental Science, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom Department of Natural Resources, TERI University, New Delhi 110 070, India

a r t i c l e

i n f o

Article history: Received 19 April 2011 Received in revised form 11 July 2011 Accepted 14 July 2011 Keywords: Phosphorus Livestock manure Fertiliser Sustainability

a b s t r a c t Every year 90 million tonnes of housed livestock manures are produced in the UK. This is a valuable reservoir of global phosphorus (P) and a point in the cycle where it is vulnerable to being lost from the terrestrial system. Improved manure management for the effective reuse of phosphorus is vital to simultaneously tackle a major source of water pollution and reduce our dependence on imported fertilisers. This paper quantifies, for the first time, the spatial and temporal challenges of recycling the required amount of manure P from areas of livestock production to areas of crop production in eight regions of England. The analysis shows that England has a P deficit and therefore the capacity to fully utilise the manure P on arable land, but that uneven spatial distribution of livestock poses a significant challenge to closing the P loop in agriculture. Two of the eight regions were shown to have surplus manure P, with the remaining six regions having P deficits, indicating that an annual export of 4.7 thousand tonnes P (2.8 million tonnes manure) must take place from the west to the east of the country each year to balance P supply and demand. Moreover, housed manure production peaks between October and February, requiring an excess of 23.0 thousand tonnes P (15 million tonnes manure) to be stored until it can be used for crop fertilisation from March onwards. The results demonstrate the scale of the challenge in managing manure P in an agricultural system that has separated livestock production from crop production, a pattern that is echoed throughout the developed world. To overcome the spatial and temporal challenges, a logistical system is recommended that will balance the nutrient potential (nitrogen and P content and availability) and pollution potential (eutrophication, greenhouse gas emissions, particulates and nitrous oxide from transport) for cost-effective and environmentally compatible redistribution of manure P from areas of surplus to areas of deficit, when required. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Historically phosphorus cycled within farming systems. Food was produced close to where it was consumed and phosphorus was returned to soil locally by land application of the resulting agricultural and human wastes (Liu et al., 2008). Population growth, technological innovations and globalisation in the 20th century have driven an intensification of farming that has resulted in geographical separation of crop production from livestock production and the distancing of food production from food consumption. Mineral fertilisers, which are produced in a handful of mining areas, have largely replaced animal manure in modern crop production. This has led to the dual problems of dependence on imported phosphate fertilisers, and the concentrated production of bulky livestock

∗ Corresponding author. Tel.: +44 121 414 5140; fax: +44 121 414 3675. E-mail address: [email protected] (C. Carliell-Marquet). 0921-3449/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2011.07.004

manures that are often considered as a waste product. Manures are generally disposed of by application to land within a narrow radius of where they are produced (Sharpley et al., 1994; Defra, 2004), a practice which has resulted in the build up of phosphorus in soils surrounding livestock farms (Haygarth et al., 1998). Where manures are applied as organic fertiliser however, application rates tend to be based on crop N requirements which generally leads to over application of P (Sharpley et al., 1994). The high soil phosphorus levels that arise as a result of this cause increased transfer to surrounding water bodies, in both particulate and soluble forms, where it is lost from the agricultural system (Defra, 2004). The opening of the phosphorus loop in this way has two major consequences: First, elevated phosphorus concentrations in receiving surface waters can lead to eutrophication. Related damage costs directly attributed to agricultural phosphorus losses have been estimated by the Environment Agency (England and Wales) at £19 million (Environment Agency, 2002). Second, the continual need to import fertiliser to replace phosphorus lost from the system is

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leading to concerns for phosphorus security and global food supplies as phosphate rock, from which fertilisers are derived, must be considered a finite resource (Cordell et al., 2009). Smil (2000), Liu et al. (2008) and Cordell et al. (2009) have all identified animal wastes as being a significant global reservoir of reusable P, with animal manures having the potential to supply up to 50% of all phosphorus required for agricultural use in Western Europe and 25% of that required for the United States (Smil, 2000). Cordell et al. (2009) suggests that reuse of animal manure will become increasingly important as a supply side measure to reduce our reliance on mineral phosphate fertilisers over this century. The bulky nature of manure however, its uneven geographical distribution and particularly the geographical separation of livestock farming from crop production are currently seen to be major barriers to achieving this goal (Smil, 2000; Cordell et al., 2009). This paper quantifies, for the first time, the spatial and temporal challenges of recycling the required amount of manure P from areas of livestock production to areas of crop production, in order to exploit its nutrient potential, in eight regions of England. This was approached with the objectives of: (1) quantifying phosphorus inputs in terms of manure production; (2) quantifying phosphorus fertiliser requirements in cropping regions; (3) assessing the extent to which need for and availability coincide, on both spatial and temporal scales; and (4) considering the strategies required for improved phosphorus management. 2. Methods 2.1. System identification It is only the phosphorus in wastes produced from confined (housed) animals that is considered to be recyclable to croplands, with unconfined animal wastes being returned to the pastures on which they graze (Liu et al., 2008). Every year around 90 million tonnes of housed livestock waste is produced in the UK, divided predominantly between Cattle (78%), Swine (13%) and poultry (5%) (ERM, 2006); it is these three major wastes on which the analysis in this study was focussed. 2.2. System characterisation In order to characterise the system, phosphorus was considered simultaneously as a commodity and a pollutant: its presence representing the potential for use on land in place of inorganic fertilisers, but an excess beyond crop needs representing the risk of runoff. Phosphorus inputs and requirements were quantified based on the following principles: • Phosphorus inputs result from livestock waste production. The amount of phosphorus present in the waste is equal to the amount available for, and needing, land application. • Phosphorus requirement is equal to the quantity removed in harvesting the crop which therefore needs to be replaced in order to maintain soil fertility. Average demands represent the capacity for manure utilisation. • In terms of calculating application rates, over a crop rotation manure P can be considered the same as inorganic P fertiliser, as advised by the Agriculture Development and Advisory Service (ADAS, 2001). Phosphorus surpluses and deficits were calculated as the difference between inputs and requirements: a surplus indicates the need to export or store manure, a deficit indicates the need and capacity to import manure.

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2.3. Regional scale in England England is formally sub-divided into the following 8 regions. London is included in the South East. • • • • • • • •

North East North West Yorkshire East Midlands West Midlands Eastern South East South West

2.3.1. Inputs The quantities of housed livestock wastes produced in the UK were identified by waste type (manure/slurry) and by livestock category (cattle/pig/poultry) (AEAT, 2005). UK livestock numbers have declined since 2005: Cattle and poultry both by 6%, and pigs by 8% (Defra, 2010a). The quantities of each waste were scaled by these factors to estimate the current production rate. Further scaling factors were applied to calculate the proportion of these wastes that are produced in England: 55% of UK cattle, 81% of UK pigs, and 76% of UK poultry are kept in England (Defra, 2010a). Both of these scaling methods assume that the quantity and distribution of housed livestock waste production mirrors the quantity and distribution of livestock. Cattle waste in England was calculated as 37.1 million tonnes (11.6 million tonnes as slurry; 25.5 million tonnes as manure), swine waste as 5.5 million tonnes (1.8 million tonnes as slurry; 3.6 million tonnes as manure) and poultry waste as 3.1 million tonnes. Regional livestock populations were identified from the June Survey of Agriculture (Defra, 2010a). The quantity of wastes produced in each region was estimated assuming that distribution of the total waste production mirrors distribution of the livestock. For example, the North East has 5% of the national cattle population and is therefore assumed to produce 5% of the cattle manure and 5% of the cattle slurry. The phosphorus input to each region was calculated based on phosphorus contents of 1.4 kg P/t and 0.5 kg P/m3 for cattle manures and slurry respectively, 2.6 kg P/t and 0.8 kg P/m3 for swine manures and slurry respectively, and 6.1 kg P/t and 10.9 kg P/t for layer and broiler manure respectively (Defra, 2010b). These are average values for each type of waste and were converted to P from figures of P2 O5 . As practiced by Defra in the RB209 Fertiliser Manual, the assumption was made that for slurries units of m3 could be interchanged with tonnes. 2.3.2. Requirements Fertiliser requirements were determined by assessment of the distribution of crop production. The areas of land used in each of the regions for growing 18 major crops and grass were identified from the June Survey of Agriculture (Defra, 2010a). Average phosphorus demands published in the Defra RB209 Fertiliser Manual (Defra, 2010b) for each of the arable crops were used to calculate firstly the phosphorus requirements for each crop in a region, and then the total phosphorus requirements of the region. These requirements are based on maintaining phosphorus levels at soil index 2 at optimum yield rates. The phosphorus demand for grass was taken as the average fertiliser application rate published in the British Survey of Fertiliser Practice (Defra, 2011a). This was to allow for the fact that the majority (91%) of grassland in the UK is grazed (Defra, 2011a) and a large proportion of the nutrient requirements will be provided by dung deposited by grazing livestock. Overall application rate, defined as quantity of nutrient divided by the total extent

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of crop area was used rather than average field rate to allow for the fact that not all grassland receives an application of fertiliser. 2.4. Temporal scale 2.4.1. Inputs The housed period, during which wastes are collectable, does not cover the whole year for all livestock. Cattle are generally kept outside during the summer and wastes are collectable only over the winter. The composition of wastes is fairly constant at all times of the year (Hjorth, 2009). It was assumed that the housed cattle waste is produced just between the months of October to April, and that the swine and poultry wastes are collectable continuously throughout the year. The phosphorus input was calculated on a monthly basis by distributing the previously calculated total annual inputs for England from each livestock between the months of the year according to when each of the wastes are produced. Inputs on a monthly basis were calculated for each region in the same way. 2.4.2. Requirements The rate of application of phosphate fertiliser also follows a pattern within the year. The application rate for each month of the year, as a percentage of total annual application, was identified from the British Survey of Fertiliser Practice (Defra, 2011a). Fertiliser requirements were calculated on a monthly basis for England by dividing the previously determined total annual requirement over the course of the year according to this distribution. Regional monthly requirements were calculated in the same way. Graphs of cumulative inputs and cumulative requirements were plotted to identify when states of deficit and surplus exist, and the scale on which these occur.

Fig. 1. Phosphorus surpluses and deficits on a regional scale. Units are thousand tonnes P; shaded regions have a surplus of phosphorus. The total quantity of phosphorus requiring export is 4.7 thousand tonnes which arises in the North West and South West. Map outline: Defra (2010d). Original data sources: AEAT (2005) and Defra (2010a,b, 2011a).

rates (Johnston and Dawson, 2005), it is unlikely that over the country this is not accounted for at all. Organic farms are an example of where this would be the case.

3. Results and discussion 3.1. Manure production and phosphorus consumption: country scale Based on summing the results of each region, the total phosphorus content of housed livestock wastes in England is 80.7 thousand tonnes P and the phosphorus requirement for crop production is 113.7 thousand tonnes P. On a country scale manure phosphorus is equivalent to 71% of the demand. Arable land in England therefore has the capacity to fully utilise all livestock manures as organic fertiliser at rates in line with crop requirements for P, thus recycling the phosphorus within the agricultural system. Nitrogen requirements would not be met at these application rates and an additional source of N fertiliser is likely to be required. Use of nutrient demands of arable crops to maintain phosphorus levels at soil index 2 may provide an over-estimation of fertiliser requirements in the short term. This is because, due to increased awareness of the risks of phosphorus pollution, fertiliser application rates have been declining over the last few decades in an attempt to use up the residues that accumulated due to overapplication in previous decades (Johnston and Dawson, 2005). As a result of reduced fertiliser application, the phosphorus soil surface balance in England fell by 64% between 2000 and 2009 to 26 thousand tonnes P (Defra, 2010c). In the longer term however, the nutrient demands to maintain soil fertility should offer a good representation of requirements. Use of the average fertiliser application rate to calculate the phosphorus demand for grassland may provide an under-estimate of fertiliser requirements because this is based on the assumption that none of the housed livestock manure is considered to contribute to the fertiliser requirements. Whilst it is commonly reported that the nutrient content of manures is often not accounted for when determining fertiliser application

3.2. Manure production and phosphorus consumption: regional scale The calculated phosphorus inputs and requirements for each region are shown in Table 1. On a farm scale, the phosphorus content of each of the types of waste is subject to local variation due to the nature of livestock feed and the methods employed for waste collection. The regional scale however was considered large enough for the use of average figures to provide a good estimation of phosphorus inputs. Only 40% of all tillage and 38% of grassland actually receives an application of phosphate fertiliser (Defra, 2011a), and requirements are likely subject to regional variations in soil type and management legacies. Use of average crop requirements may therefore not accurately reflect the phosphorus requirement of each region. Based on these calculations, 6 of the 8 regions in England have a deficit of phosphorus (Fig. 1). The arable land in each of these regions has the capacity to fully use the manure produced locally, with the remainder of requirements needing to be imported as either organic or inorganic fertiliser. This accounts for 65% of the total phosphorus content of livestock wastes produced in England. Phosphorus in locally produced manure could theoretically contribute 56, 75, 52, 92, 44 and 47% of fertiliser requirements in the North East, Yorkshire, East Midlands, West Midlands, Eastern and South East regions respectively. Transport of the wastes to achieve sufficient distribution avoiding instances of over application will be required only within the region. 35% of the total phosphorus input in England is contained in livestock wastes produced in the 2 regions identified as being in a state of surplus. Only 70 and 91% of manure can be used within the region of production in the North West and South West respectively, with the remainder of wastes beyond the capacity of local

5213 6758 14,877 18,998 11,312 25,761 13,756 17,041 113,715 2920 9683 11,179 9789 10,410 11,402 6517 18,775 80,675 2 7 12 17 15 24 9 14 100 274 432 3529 1062 598 3059 659 1269 10,883

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arable land. A total of 4.7 thousand tonnes P requires export from these regions which represents 6% of the P contained in the wastes produced in England. Due to the large quantities produced in these regions, transport of wastes within the region may also have to take place over large distances to achieve sufficient distribution. On a regional scale, 1.7 thousand tonnes P requires export from the South West. Assuming this were to be exported as a mix of livestock wastes representative of the ratio of livestock categories in the region, this equates to around 1.2 million tonnes of manures. If cattle and pig slurries were dewatered and exported as 20% dry matter, then the quantity of manure requiring export is around 1.0 million tonnes. The nearest regions which could import this manure are the South East and West Midlands, each at an average distance of approximately 130 miles. The South East has a deficit of 7.2 thousand tonnes P so would be able to import all of the surplus and would therefore likely be able to achieve this with transport distances less than 130 miles. The West Midlands has a deficit of 0.9 thousand tonnes P so would be able to import around half of the surplus, but as this region increased imports towards its theoretical limit the required transport distances to achieve sufficient distribution would increase. In practice the most efficient solution would likely involve exporting to both of these regions. On a regional scale 2.9 thousand tonnes P requires export from the North West. This equates to around 2.0 million tonnes of manures, or 1.8 million tonnes after dewatering the slurries. The nearest regions which could import this manure are the North East and Yorkshire, each at an average distance of approximately 50 miles. Yorkshire has a deficit of 3.7 thousand tonnes P so would be able to import the entire surplus; the North East has the capacity for an additional 2.3 thousand tonnes P. Again, the most efficient solution would likely involve export to both of these regions. Patterns of farming practices that have been established in England have separated manure production from the land where the nutrients are required to the extent that large phosphorus deficits exist in the east of the country and large surpluses exist in the west. In order to exploit the nutrient potential of manures within agriculture and minimise the risk of water pollution, a net export of phosphorus from west to east is necessary.

546 2100 3353 4912 4138 6748 2495 4032 28,324

c

Data source: Defra (2010a). Original data sources: AEAT (2005) and Defra (2010a,b). Original data sources: Defra (2010a,b, 2011a). a

North East North West Yorkshire East Midlands West Midlands Eastern South East South West England

5 17 10 9 14 4 8 32 100

2100 7150 4298 3815 5673 1596 3363 13,473 41,468

3 4 32 10 5 28 6 12 100

3.3. Manure production and phosphorus consumption: temporal scale

b

Total P Inputb (tonnes) Livestocka (%) Livestocka (%)

Poultry Pigs

Livestocka (%)

P Inputb (tonnes) Cattle

Table 1 Phosphorus inputs and requirements for each region and England.

P Inputb (tonnes)

P Inputb (tonnes)

Phosphorus

Requirementsc (tonnes)

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The calculated phosphorus inputs and requirements for each month of the year are shown in Tables 2 and 3 respectively. It is likely that there is an under-estimate of summer inputs and overestimate of winter inputs. This is because, whilst these figures are based on dividing the annual total between the months assuming zero cattle waste is collectable during the summer, dairy cattle in fact spend a number of hours of the day inside for milking during which times the wastes produced are available for collection. Nearly 60% of the annual phosphorus fertiliser requirement for England occurs in March, April and May. The 21.8 thousand tonnes of phosphorus available in manures produced in these months is less than a third of that required for application. Between November and January arable land has the capacity for application of 6.8 thousand tonnes of phosphorus, however more than 4 times this quantity is produced in wastes during these months. Fig. 2 shows the calculated cumulative temporal production of housed manure in England and the corresponding cumulative fertiliser requirements at each month of the year. It can be seen that at the end of the growing season (August), there is an overall deficit of phosphorus. Between October and March, however, there is a large surplus of livestock wastes. The accumulated excess phosphorus in England increases to a maximum of 23.0 thousand tonnes in February before this can start to be utilised for crop fertilisation in spring. Storage facilities for sufficient manure to accommodate for

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Table 2 Phosphorus inputs to each region and England on a monthly basis. Units are tonnes P. Original data sources: AEAT (2005) and Defra (2010a,b).

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

North East

North West

Yorkshire

East Midlands

West Midlands

Eastern

South East

South West

England

377 365 377 377 340 377 365 70 67 70 70 67

1261 1220 1261 1261 1139 1261 1220 215 208 215 215 208

1213 1174 1213 1213 1095 1213 1174 584 566 584 584 566

1065 1031 1065 1065 962 1065 1031 507 491 507 507 491

1232 1192 1232 1232 1113 1232 1192 402 389 402 402 389

1066 1032 1066 1066 963 1066 1032 833 806 833 833 806

760 735 760 760 686 760 735 268 259 268 268 259

2420 2342 2420 2420 2186 2420 2342 450 436 450 450 436

9394 9091 9394 9394 8484 9394 9091 3330 3222 3330 3330 3222

Table 3 Phosphorus requirements for each region and England on a monthly basis. Units are tonnes P. Original data sources: Defra (2010a,b, 2011a).

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

North East

North West

Yorkshire

East Midlands

West Midlands

Eastern

South East

South West

365 104 52 156 365 1408 1043 626 156 104 313 521

473 135 68 203 473 1825 1352 811 203 135 405 676

1041 298 149 446 1041 4017 2975 1785 446 298 893 1488

1330 380 190 570 1330 5130 3800 2280 570 380 1140 1190

792 226 113 339 792 3054 2262 1357 339 226 679 1131

1803 515 258 773 1803 6955 5152 3091 773 515 1546 2576

963 275 138 413 963 3714 2751 1651 413 275 825 1376

1193 341 170 511 1193 4601 3408 2045 511 341 1022 1704

this quantity of phosphorus will therefore be necessary in order that applications can be made at the appropriate time. At a scaled average of 1.5 kg P/t that considers the relative amounts of each waste produced between October and February, without dewatering, this equates to 15 million tonnes of manure. The North East, Yorkshire, East Midlands and South East have an overall deficit of phosphorus by the end of the growing year (Fig. 3) but between October and March/April, there is a surplus in these regions. In each case the accumulated excess phosphorus reaches a maximum in January/February at values of 820, 2930, 1760 and 1230 tonnes P respectively. The Eastern region has a small surplus during the winter months, reaching 880 tonnes in January, but this is quickly outweighed by fertiliser demand as soon as requirements start to increase in February. In the North East and Yorkshire storage requirements will be necessary for 26–28% of the total wastes produced if over-application in the winter is to be avoided. This is 18–19% of the total wastes in the East Midlands and South East, and

Thousand tonnes P

120 100 80 60 40

England 7960 2274 1137 3411 7960 30,703 22,743 13,646 3411 2274 6823 11,372

7% in the Eastern region. These results show that even in the east of England, where there are large deficits of phosphorus overall, storage facilities to accommodate manures for up to 6 months in the year are likely to be required in order that phosphorus can be applied at appropriate rates in line with varying crop requirements throughout the year. The West Midlands has a surplus through the majority of months; it is not until month 10 that the capacity for phosphorus consumption matches the input. The accumulated excess reaches a maximum of 3740 tonnes P, equivalent to 36% of the total production. The North West and South West remain in a state of surplus throughout the whole year. The accumulated excess phosphorus reaches a maximum in February of 4790 tonnes in the North West and 8380 tonnes in the South West, representing 50 and 45% of the total local input respectively. As a state of deficit is not reached in these regions, export will be required throughout the year. Exporting manures on a regional scale from the west to the east is a key strategy that will be necessary to ensure sustainable applications of phosphorus. The export of surplus manure however cannot take place until another region is in a position to accept further inputs of phosphorus. Export from the South West to the South East cannot take place between October and March, and to the West Midlands not until near the end of the year in August. Export from the North West to Yorkshire cannot take place until April, but to the North East slightly earlier–from March. Storage of large quantities of manures over the winter in the west of the country will therefore be necessary until this can be exported in the spring.

20

Fertiliser Use

Jul

Apr

Oct

Jan

3.4. Nitrate vulnerable zones 0

Waste Production

Fig. 2. Cumulative input and requirement over the course of a year in England. The difference between the graphs represents the accumulated excess phosphorus at a given time. Original data sources: AEAT (2005) and Defra (2010a,b, 2011a).

Under the EC Nitrates Directive, farms in areas of land that have been designated as Nitrate Vulnerable Zones (NVZs) are already required to adhere to regulations regarding the storage of livestock wastes during ‘closed periods’ in which the spreading of organic manures with a high readily available N content is prohibited. These regulations stipulate that by January 2012, five months’ storage

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Fig. 3. Cumulative input and requirement over the course of a year for regions in England. Units are thousand tonnes P. Original data sources: AEAT (2005) and Defra (2010a,b, 2011a).

capacity must be provided for cattle slurries (1st October–1st March) and 6 months’ storage capacity for pig slurries and poultry manures (1st October–1st April). Although not dictated by the P content of the wastes, these regulations will result in the storage of a certain quantity of phosphorus. No minimum storage requirements have been set for solid wastes such as farmyard manures (Defra, 2009). The phosphorus content of slurries and poultry manure produced in England between the months stated above is 19.0 thousand tonnes P. 62% of England has been designated as an NVZ, so, based on an assumption that 62% of the wastes that are produced in the country are produced in an NVZ, the phosphorus contained in the wastes requiring storage under NVZ regulations is 11.8 thousand tonnes P. The storage capacity required under NVZ regulations therefore covers approximately half of the 23.0 thousand tonnes P that was calculated as requiring storage, so on a country scale this will need to be doubled in order to provide for improved phosphorus management. Continuing the assumption that 62% of the wastes produced in each region are produced in an NVZ, a quarter of the phosphorus requiring storage in the North West and South West is covered under NVZ regulations. This rises to 40–50% in the North East, Yorkshire and West Midlands, and 82% in the South East. In the East Midlands and Eastern region, where there is a high ratio of poultry to other livestock, the phosphorus storage requirements are actually exceeded by NVZ storage requirements. The proportion of land that has been designated an NVZ however is not equal in each region. The map provided by Defra (Defra, 2011b) shows that the NVZs are focused primarily in the central and eastern areas of the country, and exclude large areas of the south west and the north. This means that a larger proportion of the phosphorus storage requirements than calculated above will actually be met by NVZ regulations in central and eastern regions, and a smaller proportion in the south west and north. In the North West and South West therefore, the large phosphorus storage requirements will be covered only to a very small extent by NVZ regulations. 3.5. The role of manure in the agricultural phosphorus cycle Growing awareness of resource depletion is focussing attention on the way phosphorus is currently used in agriculture. Intensive, high-yield agriculture is dependent on addition of fertilisers (Tilman et al., 2002) and global food production, which has already doubled in the past 50 years, may need to increase by up to 70%

over year 2000 levels by 2050 to meet demands from population growth (MacDonald et al., 2011). Developing countries, with the most exploitable ‘yield gap’, could have significant yield increases with use of appropriate technologies (Tilman et al., 2002), of which increased access to mineral fertilisers is central. This additional crop and livestock production however must be achieved without an increase in the negative environmental impacts associated with agriculture, particularly nutrient pollution, which is one of the most significant negative impacts of agriculture across the globe. Management of the agricultural phosphorus cycle is therefore key to achieving the desired agricultural productivity whilst also safeguarding water quality and slowing down consumption of phosphate rock resources. Central to managing the agricultural phosphorus cycle is the effective recycling of manure P from areas of surplus (high livestock densities) to areas of deficit, which has even been proposed on a global scale (MacDonald et al., 2011). This makes theoretical sense when one considers that some countries, for example the Netherlands, have an overall manure P surplus due to intensive livestock and dairy industries (Smil, 2000), whereas large parts of Eastern Europe for example, have P deficient soils (MacDonald et al., 2011). Furthermore, many scenarios for the future forecast an increase in global livestock production due to increasing dietary shifts towards a higher proportion of meat (Tilman et al., 2002; Cordell et al., 2009; Van Vuuren et al., 2010) and these same authors identify recycling of this increasing manure P stockpile as playing a significant part in meeting future P fertiliser demand for agriculture. All this leads to one conclusion, that P in livestock manure is increasingly the linchpin of the agricultural P cycle and by separating livestock production from crop production, the potential to close this loop has been severely compromised. Meeting global food production demands however relies on intensive, high yield agriculture, which has evolved to mean intensive animal feedlots separated geographically from large areas of monoculture crop production because they require different physical environments to be successful. This is the case for the eight regions of England studied in this paper, with livestock production established primarily in the hillier topography of the west and large-scale crop production taking place in the flatter regions in the east of the country. The patterns of P surpluses in the areas of livestock production and P deficits in areas of crop production, reported in this paper, are echoed throughout the developed world (Smil, 2000; MacDonald et al., 2011) for similar reasons. So, although it is well understood that effective nutrient management in agricul-

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ture requires recycling of manure P to croplands, this is unlikely to be achieved by a full-scale shift towards the more traditional mixed livestock-cropping systems of the past. What then, is the solution for England and what lessons could be transferred to global P management? Transport and storage of excess livestock wastes (either as raw manures and slurries, or as digestate if plans to increase the uptake of anaerobic digestion in the UK are successful) will play a key role in improving the management of phosphorus in English agriculture. To transport and store wastes efficiently, the first step is dewatering. Mechanical dewatering, using centrifuges or gravity belt presses, can achieve a solids fraction of 20–30% solids (WRAP, 2009; Defra, 2010b) that contains the majority of the phosphorus with increased concentration per mass of dry solids. As well as reducing the mass of material requiring storage or transport, this has benefits in terms of exploiting the full nutrient potential of the manure: The phosphorus and nitrogen can only be applied to land in the ratio in which it occurs in the manure product. In whole manure the N:P ratio is generally 2–6:1 which is phosphorus rich compared to crop requirements of 7–11:1 (Sharpley et al., 1994). In this case, stopping application once the crop phosphorus demand has been met will mean that the nitrogen potential of the manure will not be fully utilised. Nitrogen will end up being exported or stored along with the excess phosphorus and mineral fertiliser will then need to be imported in order to complete the nitrogen application. The majority of phosphorus in manures is in particulate form and separation using a belt separator has been shown to remove 79% of the phosphorus into the solids fraction leaving the separated liquid with an N:P ratio of 24:1 (Hjorth et al., 2008). This process therefore essentially produces a solids phosphorus fertiliser and a liquid nitrogen fertiliser which can be managed independently according to crop requirements, thus increasing the opportunity to maximise utilisation of both the N and P nutrient potential of the manure. Excess phosphorus may then be stored or exported as the less bulky solids fraction. With a water content of 70–80%, the solids fraction is however still a bulky material. Storage and transport will therefore still incur high costs, both financial and environmental, which from a farmer’s point of view may quickly outweigh the value of the nutrients. This implies that if livestock wastes are to be directly and efficiently reused in the agricultural P cycle a logistical system will be required to determine the most cost-effective routes for redistribution of manure P from areas of surplus to areas of deficit, at the times of the year required. Such a system is currently the subject of further investigation by these authors. The long term goal to close the P loop in agriculture demands a management system with a high degree of flexibility, so that P can be delivered when and where required without entailing excessive environmental or economic costs. This required flexibility might be difficult to achieve with manures alone because even after dewatering, its bulky nature means that the supply route could be vulnerable to interruptions. For example, in 2001 in the UK a foot and mouth epidemic led to the destruction of 1.2 million animals (Tilman et al., 2002) and prevented digestate from sewage sludge being spread to farmland. This resulted in vast quantities of digestate having to be stored in excess of 6 months, which exceeded the calculated storage requirements of many UK water companies. It seems that, long term, further treatment of the manure to extract P in a mineral form must be considered if we are to truly close the P loop in agriculture, both in the UK and globally. In the Netherlands, where there is a large surplus of animal manure due to the intensive livestock industry, full-scale crystallisation processes are already capable of precipitating P as struvite (magnesium ammonium phosphate) from veal calf manure and the process has proven successful in laboratory and full-scale tests on swine wastes (Burns et al., 2001; Nelson et al., 2003). Struvite is a dry, mineral form of P that can be stored and transported as mineral P fertiliser,

effectively decoupling the P content of the manure from the bulky, water and organic containing mass. In the UK two plants using the Ostara struvite crystallisation process are planned for treating digestate liquors at wastewater treatment works owned by Thames Water and Severn Trent Water. Precipitation of struvite could, potentially, be used to extract P from manures and agricultural digestate in the UK but it is unlikely to be viable at a single farm scale, particularly for dairy wastes which may require the addition of expensive chelating agents to release phosphorus from calcium compounds which form due to the high calcium content of cattle feed (Zhao et al., 2010). This suggests that wastes would need to be transported to regional processing facilities and therefore presents questions about the optimum siting and sizing of plants to maximise phosphorus recovery whilst minimising the economic and environmental impacts. Another potential technological option which has considerable potential for manure P management is supercritical water oxidation (SCWO), a process which results in a slurry of inorganic ash in a pure water phase entirely free from organic contaminants. This technology has already been used at full scale to treat digested sludge from sewage treatment (Stendahl and Jäfverström, 2003), with 90% of the P originally present in the digestate being effectively extracted from the resulting ash using a caustic solution. The P is recovered in the form of sodium orthophosphate, which can then be used to form mineral P fertiliser products. Moreover, the concentrated P solution is free from metal contaminants, which is a considerable advantage over mined phosphate rock. This technology is of increasing interest to the UK water industry and research is being undertaken to establish this technology for energy and mineral resource recovery. Again, as for struvite recovery from wastes, this technology is unlikely to be viable at the single farm scale, at least in the coming decade and implies the need to consider larger processing sites either for raw manure or resulting digestate. Overall, potential strategies for improved livestock waste management need to be weighed up considering a number of factors and it is unlikely that there will be a ‘one size fits all’ solution; the primary goal is to return as much P from areas of surplus to areas of deficit, when it is needed, and this is going to require a flexible management system. Such a management system will balance the nutrient potential (N and P content and availability) and pollution potential (eutrophication, GHG emissions, particulates and nitrous oxide from transport) in order to achieve the most environmentally compatible strategy. With Defra targets to encourage uptake of anaerobic digestion in UK agriculture in the coming decade, the potential for energy recovery will also need to be factored into the equation.

4. Conclusions Livestock manure is a valuable reservoir of phosphorus and a point in the P cycle where it is vulnerable to being lost. Improved manure management for the effective reuse of phosphorus offers an opportunity to simultaneously tackle a major source of water pollution and reduce our dependence on imported manufactured fertilisers, and is a measure that will be necessary to maintain agricultural productivity sustainably at rates required for future generations. At current livestock production rates, feeding and housing practices, housed livestock manure can theoretically provide 71% of the phosphorus requirements for crop production in England and could replace significant quantities of imported fertiliser. Attitudes must therefore shift from disposing of the wastes to developing strategies for achieving the most efficient use of the resources it contains. Whilst maximising the return of phosphorus from livestock manure to crop production is an important consideration for

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improving agricultural sustainability, the analysis presented in this paper demonstrates that the spatial and temporal separation of housed livestock waste production and crop fertiliser demand pose a challenge for the efficient redistribution and reuse of P in English agriculture. In many cases the storage and transport that is required will present costs to the farmer that outweigh the value of the nutrients; transport of manure also presents its own pollution potential so in many situations will not be an acceptable solution. Treatment processes to concentrate the phosphorus stream will reduce the storage capacity requirements, increase the distance over which it can be economically transported and increase the opportunity to maximise use of both the N and P content of the manure. The economic and wider environmental impacts of each option however would need to be weighed up against those of the phosphorus to determine the extent of the benefits that these may offer. Closing the phosphorus loop in agriculture therefore poses a multi-dimensional challenge. Both technological and strategic management innovations are required to minimise the economic and environmental costs of capturing, extracting, storing, transporting and applying P from livestock waste to crop production. To overcome the spatial and temporal challenges of manure P management that have been highlighted in this paper, a logistical system is being developed by these authors to balance the nutrient potential and pollution potential for cost-effective and environmentally compatible redistribution of manure P from areas of surplus to areas of deficit, when required. Acknowledgements This research has been funded through a College of Engineering and Physical Sciences (EPS) (University of Birmingham) research partnership fund for joint research with TERI University. References ADAS. Making better use of livestock manure on arable land. Notts, UK: Agriculture Development and Advisory Service; 2001. AEAT. Assessment of methane management and recovery options for livestock manures and slurries. AEAT Report ENV/R/2104, Oxfordshire, UK; 2005. Burns RT, Moody LB, Walker FR, Raman DR. Laboratory and in-situ reductions of soluble phosphorus in swine waste slurries. Environ Technol 2001;22: 1273–8. Cordell D, Drangert J, White S. The story of phosphorus: global food security and food for thought. Global Environ Chang 2009;19:292–305.

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