Introduction

Agriculture, at all scales of production, is dependent on the natural capital of soils which yield a flow of services upon which humans depend, not only for food, fibre and biomass production, but also for other ecosystem services such as provision of fresh water, regulation of nutrient cycling, flood mitigation, water purification, carbon sequestration and climate regulation (Kibblewhite, Ritz & Swift 2008; Haygarth & Ritz 2009; Dominati, Patterson & Mackay 2010; Robinson et al. 2013). During the 20th century, the rising demand for food globally was met by conversion of natural and semi‐natural habitats into agricultural land, and the intensification of farming methods, including mechanization and use of synthetic fertilizers (Robinson & Sutherland 2002; Haygarth & Ritz 2009). However, intensification of agriculture has depleted the natural capital of soil organic carbon (SOC) and nutrients resulting in serious losses of regulating and supporting ecosystem services (Franzluebbers 2002). These include impaired water and nutrient holding capacity, reduced pollutant immobilization and water filtration, loss of soil aggregates and strength (Watts & Dexter 1997) leading to increased erosion, CO 2 release to the atmosphere and eutrophication of aquatic ecosystems (Robinson & Sutherland 2002; Loveland & Webb 2003; Dominati, Patterson & Mackay 2010; Robinson et al. 2013). Loss of organic matter (OM) content is of particular concern for food security as yields of staple cereal crops typically increase linearly with SOC concentration (Lal 2010).

One of the greatest challenges now facing humanity is to improve the sustainability of agriculture and reduce its environmental impact, whilst also meeting the food demands of the growing global population, which exceeds 7 billion (DEFRA 2010; Godfray et al. 2010). A crucial goal in agricultural sustainability is to reverse the historic losses of SOC from farmland and to increase soil C : N ratios which are important controls on nutrient cycle regulation (Robinson et al. 2013). High C : N‐rich soil amendments are particularly important in reducing the risk of N leaching from soils (Dungait et al. 2012).

Concurrent with the intensification of agriculture has been rapid urbanization; over half of the world's population is now residing in cities and towns (UN 2008). Indeed, urban areas are increasing in areal extent faster than any other land use (Hansen et al. 2005), a trend set to continue as the proportion of people living in cities and towns rises to 70% by 2050 (UN 2008). This land‐use change is further exacerbated by the expansion of urban areas outpacing population growth, particularly in developed regions such as Europe (EEA 2006). These dynamics bring about a number of significant challenges. Of increasing concern is the food security of urban inhabitants as they become physically more detached from primary food production (Howe & Wheeler 1999).

However, an estimated 800 million people currently practise some form of urban food production globally, with much borne out of necessity for subsistence in the developing world (Lee‐Smith 2010). Urban horticulture operates over spatial scales ranging from potted plants, to vegetable plots in gardens, to allotments, community gardens and city farms (Howe & Wheeler 1999). In Europe, allotments are a common feature of urban areas and in areal extent are often the main areas of own‐grown food production. In the UK, there are c. 330 000 allotment plots, and a standard plot is 250 m2, giving a total area nationally likely to be >8000 ha (Crouch & Ward 1997). Allotments represent a unique type of greenspace, designated specifically for food production (van den Berg et al. 2010). Peak allotment provision in the UK occurred during the First and Second World Wars (Crouch & Ward 1997; Martin & Marsden 1999), and during the latter, allotments and gardens provided c. 10% of food consumed in the UK because of the ‘Dig for Victory’ campaign whilst comprising <1% of the area of arable cultivation (Crouch & Ward 1997; Keep 2009).

After a post‐war decline in own‐growing and associated decrease in plot provision, there has been a resurgence in UK allotment demand reflected in increased waiting lists over the past 17 years, with over 90 000 people now waiting for a plot (Campbell & Campbell 2011). The increase in interest in agriculture is not confined to the UK, for example own‐growing in the USA has risen (Viljoen & Bohn 2012) as a result of a recognition of the importance of provision of healthy food, particularly to disadvantaged neighbourhoods in combination with the availability of ‘vacant lots’ within urban areas (Grewel & Grewel 2012). Amongst scientists, policymakers, the media and public, there is increasing awareness of the multiple benefits of ‘own‐growing’ including access to nutritious fresh produce, stress relief, improved psychological well‐being and physical fitness (Martin & Marsden 1999; Leake, Adam‐Bradford & Rigby 2009; van den Berg et al. 2010; Kortright & Wakefield 2011). The UK government £30 million Healthy Towns Initiative launched in 2008 funded projects aimed at increasing participation in own‐growing to promote healthier lifestyles and tackle the problem of sedentary behaviour, low consumption of fresh fruit and vegetables, and obesity. Other motivations for own‐growing include more sustainable living in response to threats from climate change, peak oil and unsustainable food production systems (Hopkins 2008), widespread concerns about chemical residues of pesticides in conventional agriculture, genetically modified crops and ‘food miles’. More recently, the increase in own‐growing has been attributed to rising global food prices (DEFRA 2010).

Soils in urban greenspaces have recently been shown to make an important contribution to provision of ecosystem goods and services especially in holding large stocks of SOC (Pouyat, Yesilonis & Nowak 2006; Churkina, Brown & Keoleian 2010; Edmondson et al. 2011, 2012, 2014). However, we currently know nothing about how soil management for own‐growing in allotments impacts on the main soil quality indicators. Do these soils suffer significant depletion in SOC and nitrogen stocks compared to other urban greenspaces, as might be expected on the basis of the effects of cultivation seen in conventional agriculture? Are higher SOC stocks maintained under perennial woody fruit bushes and trees, where soil may be less disturbed, compared to frequently dug ground used for annual herbaceous crops?

In this paper, we investigate topsoil properties at allotment sites across an entire mid‐sized UK city, including SOC concentration, total nitrogen (TN) concentration, C : N ratio and soil bulk density (BD), and compare them to urban domestic gardens, non‐domestic greenspace and regional agricultural soils. The comparisons with other urban greenspace soils were made to determine whether allotment cultivation significantly impacts urban soil quality within the same city on the same soil types, and these other greenspaces provide a ‘control’ for soil properties that are affected by the urban environment such as air pollutants. The soil properties were selected as they are positively associated with regulating and supporting ecosystem services (Franzluebbers 2002) and can be directly managed for ecosystem service provision (Kibblewhite, Ritz & Swift 2008; Dominati, Patterson & Mackay 2010). SOC is particularly important as it has a direct positive influence on both ecosystem function including water and nutrient holding capacity, and crop growth and C : N ratio is one of the major controls of both N and C cycling in the soil (Powlson et al. 2011; Dungait et al. 2012). BD is a direct measure of soil pore space, which provides an indication of the ability of soil store water and the rate of storm water infiltration (Lal 2007; Dominati, Patterson & Mackay 2010).

Using a questionnaire, we examine plot management practices which may influence soil quality in allotments including the prevalence of on‐site composting; inputs of manure, fertilizer and commercial compost; and the burning or removal of OM for disposal off‐site.

We hypothesize that (i) intensively managed urban allotments will maintain higher soil quality, as indicated by the above parameters when compared to regional agricultural soil, and (ii) cultivation on allotments will negatively affect soil properties in comparison with other types of urban greenspace, to a greater extent in beds used for annual crops than under woody fruit bushes and trees.