Modeling Carbon Sequestration in Home Lawns

Soil organic carbon (SOC) sequestration and the impact of carbon (C) cycling in urban soils are themes of increasing interest. A model was developed to investigate the potential of C sequestration in home lawns. The model contrasted gross C sequestered versus the hidden C costs (HCC) associated with typical lawn maintenance practices. The potential of SOC sequestration for U.S. home lawns was determined from SOC sequestration rates of turfgrass and grasslands. Net SOC sequestration in lawn soils was estimated using a simple mass balance model derived from typical homeowner lawn maintenance practices. The average SOC sequestration rate for U.S. lawns was 46.0 to 127.1 g C/m/year. Additional C sequestration can result from biomass gains attributable to fertilizer and irrigation management. Hidden C costs are the amount of energy expended by typical lawn management practices in grams of carbon equivalents (CE)/m/ year and include practices including mowing, irrigating, fertilizing, and using pesticides. The net SOC sequestration rate was assessed by subtracting the HCC from gross SOC sequestration rate. Lawn maintenance practices ranged from low to high management. Low management with minimal input (MI) included mowing only, a net SOC sequestration rate of 25.4 to 114.2 g C/m/year. The rate of SOC sequestration for doit-yourself (DIY) management by homeowners was 80.6 to 183.0 g C/m/year. High management, based on university and industry-standard best management recommendation practices (BMPs), had a net SOC sequestration rate of 51.7 to 204.3 g C/m/year. Lawns can be a net sink for atmospheric CO2 under all three evaluated levels of management practices with a national technical potential ranging from 25.4 to 204.3 g C/m/year. Research on abrupt climate change and the C cycle have become major thematic foci since the 1990s. SOC sequestration is one of the strategies proposed to stabilize atmospheric carbon dioxide (CO2) (Lal, 2004a; Smith et al., 2007b). The interest in urban soils is derived from the fact that 75% of the U.S. population lives in urban areas where individuals can potentially affect C sequestration in their home landscape (United States Census Bureau, 2010). Lawn grasses are the predominant plants in the urban landscape that are managed by the homeowner (Beard, 1973). A simple C footprint benchmark of home lawns can be developed from three components: the capacity of urban soils to store C, the capability of grass plants to fix and sequester C, and the C footprint of lawn maintenance practices. Several studies have evaluated C sequestration potential of agricultural and urban soils as one of several options to stabilize atmospheric CO2 abundance (Blanco-Canqui and Lal, 2004; Bruce et al., 1999; Lal, 2004a, 2008; Leified, 2006; Pataki et al., 2006; Pickett et al., 2008; Pouyat et al., 2002, 2006; Smith et al., 1993). SOC is comprised of the historic accumulation of humus in the soil. Long-term storage of SOC occurs when humus reaches a point of stability and gains exceed losses (Whitehead and Tinsley, 2006). Variations in the SOC pool occur in different ecosystems because of differences in the rate of soil organic matter decay through microbial decomposition, temperaturefluctuations,andprecipitation amounts and frequencies (Pouyat et al., 2002). The SOC pool is important for soil structure maintenance and other ecosystem services (Lal, 2004a, 2009). It improves numerous soil properties and processes including soil tilth, aggregation, plant-available water and nutrient capacities, reduction in susceptibility to erosion, and filtering of pollutants (Blanco-Canqui and Lal, 2004). Soil organic carbon is depleted through soil cultivation and land use conversion (Lal, 2004a; Post and Kwon, 2000) and can be enhanced through those soil conservation and restoration practices, which add biomass C and influence the rate of its decomposition (Lal, 2004a). Common conservation and restoration practices include no-till (NT) agriculture, perennial plant cover, fertilization, irrigation, and organic amendments (Lal, 2004a; Post and Kwon, 2000; Post et al., 2004). Moreover, lawn grasses are a perennial plant cover and have the potential for longterm SOC sequestration (Pataki et al., 2006; Pouyat et al., 2006). Urban lawns are potential C sinks and their prevalence in urban landscapes suggests that they can store a significant amount of C (Pataki et al., 2006; Pouyat et al., 2002, 2006). Urbanized land covers 40.6 million hectares (Mha) in the United States (United Nations, 2004a). The National Census Bureau estimates that 75% to 80% of North American population lives in urban areas (United Nations 2004b). Urban land use is 3.5% to 4.9% of the U.S. land area (National Association of Realtors, 2001; Nowak et al., 2001). As urbanization increases, the percentage of land converted into turfgrass is also increasing (Bandaranayake et al., 2003; Lorenz and Lal, 2009a; Milesi et al., 2005; Qian and Follett, 2002). Approximately 41% of the U.S. urban area is under residential land use (Nowak et al., 1996, 2001). Turfgrasses cover 16 to 20 Mha in the United States, which includes residential, commercial, and institutional lawns; parks; golf courses; and athletic fields (Grounds Maintenance, 1996; Milesi et al., 2005). There are 80 million U.S. single-family detached homes with 6.4 Mha under lawns (Augustin, 2007; National Association of Realtors, 2001; National Gardening Association, 2004). The size of home lawns varies regionally (north to south and east to west) as well as locally (rural versus suburban). Home lot size differs from that of home lawn size (National Association of Realtors, 2001). Home lot size includes the house and land owned by the homeowners, where home lawn size includes the area covered by turfgrass. In this study, the average size of household lawns in the United States is 0.08 ha (Augustin, 2007; National Association of Realtors, 2001; Vinlove and Torla, 1995). The estimated SOC pool in the U.S. urban soils is 77.0 ± 2.0 Mg ha (Pouyat et al., 2006). A compilation of research showing conversion of crop land into perennial grasses sequestered an average of 0.3 Mg C/ha/year (Post and Kwon, 2000), and the rate can be as high as 1.1 Mg C/ha/year with fertilizer and irrigation management (Contant et al., 2001; Gebhart et al., 1994; Qian and Follett, 2002). Qian and Follett (2002) modeled SOC sequestration with historic soil testing data from golf courses and reported that soils under golf course sequester SOC at a rate of 1.0 Mg C/ ha/year. Land under the Conservation Reserve Program also sequesters SOC at a similar rate (Qian and Follett, 2002). Ohio farmland converted to golf courses sequesters SOC at an initial rate of 2.5 to 3.6 Mg C/ha/year as a result Received for publication 11 Nov. 2010. Accepted for publication 4 Mar. 2011. To whom reprint requests should be addressed; e-mail [email protected]. 808 HORTSCIENCE VOL. 46(5) MAY 2011 of permanent groundcover and increased management inputs of fertilizer and irrigation (Selhorst, 2007). Although home lawns have potential to sequester C, information on SOC dynamics in urban lawns is limited (Pouyat et al., 2006), yet the technical potential for urban lawns to sequester SOC is high as a result of perennial turfgrass cover and improved management. Lawns provide a perennial groundcover and the soils beneath established grasses are relatively undisturbed. Therefore, the lawn ecosystem has been compared with perennial grasslands and NT agricultural systems (Falk, 1976, 1980; Follett et al., 2009; Qian and Follett, 2002). Lawns have the capacity to produce biomass at a rate similar to those of managed crops such as corn (Zea mays), wheat (Triticum aestuvum), and prairie grasses (Falk, 1976, 1980; Qian and Follett, 2002). A complete turfgrass C cycle accounting for turfgrass maintenance practices of mowing, irrigating, fertilizing, and applying pesticides must be completed to determine net C sequestration rates (Bandaranayake et al., 2003; Pickett et al., 2008; Pouyat et al., 2006). In general, fertilizer and irrigation practices can increase the rate of SOC sequestration (Campbell and Zenter, 1993; Glendining and Powlson, 1991; Gregorich et al., 1996; Lal, 2003; Paustian et al., 1997). Thus, use of fertilizers and irrigation as lawn maintenance practices could increase plant biomass and enhance the SOC pool. An increase in input of plant biomass also increases the rate of humification (Duiker and Lal, 2000; Puget et al., 2005). A wide range of techniques exist for estimating the technical potential of SOC sequestration (Bruce et al., 1999; Rickman et al., 2001; Smith et al., 1993, 2008). Changes in the SOC pool can be measured directly over time (Bruce et al., 1999). Direct measurements are an efficient technique for a small scale (plot scale) but can be complicated by spatial and temporal differences in soils for large regional scales (Bruce et al., 1999; Qian et al., 2003; Smith et al., 1993). Mathematical modeling of SOC is well developed and is widely used to study SOC dynamics under a range of environmental conditions at regional scales (Bruce et al., 1999; Lal, 2004a; Post et al., 2004; Qian et al., 2003; Smith et al., 1993, 2008). Modeling has been used extensively to estimate changes in the SOC pool resulting from management practices (Blanco-Canqui and Lal, 2004; Bruce et al., 1999; Lal, 2004a, 2004b). Although atmospheric enrichment of CO2 is cited as a principal driver of climate change, N2O and CH4 are other green house gases (GHGs) of concern. The global warming potential (GWP) of N2O and CH4 can be expressed in terms of CO2-C equivalents by knowing their radiative forcing and residence time. Radiative forcing is the difference in the amount of radiation energy entering and exiting the earth’s atmosphere. On a 100-year time scale, one unit of N2O has the same GWP as 310 units of CO2 and one unit of CH4 has the same GWP as 21 units of CO2 (Intergovernmental Panel on Climate Change, 2001). Soil emissions of CO2, methane (CH4), and nitrous oxide (N2O) are highly impacted by soil properties and climate (Kaye et al., 2005; Khan et al., 2007; Maggiotto et al., 2000; Smith et al., 2007a). Carbon dioxide represents over 98% of the soil GHG flux and is accounted for by NPP estimates in the basic model (Phillips et al., 2009). Emissions of CH4 are formed from anaerobic fermentation of organic matter under conditions typical of flooded rice paddies but not of typical home lawn ecosystem conditions. Normal well-drained soils tend to act as a sink for CH4 (Janssen et al., 2009; Phillips et al., 2009). Soil emissions of N2O are less than 1% of the GHG soil flux and result from soil microbial activity (Kaye et al., 2005; Phillips et al., 2009). Soil N2O emissions are increased under saturated soil conditions (Eichner, 1990; Smith et al., 2007a). Average soil N2O flux is comprised of 65% to 77% background emissions and 23% to 35% fertilizer-induced emissions (Snyder et al., 2007). Significant potential exists for the mitigation of these GHG fluxes from soils by management practices according to the Intergovernmental Panel on Climate Change (Smith et al., 2007b). Determining N losses from turfgrass and soil ecosystems is a useful strategy for developing an appropriate fertilization program that promotes healthy turfgrass as well as addressing the environmental concerns associated with N losses. Losses of N from turfgrass occur through denitrification, leaching, volatilization, runoff, and in some cases by erosion (Baird et al., 2000; Foth and Ellis, 1997; Petrovic, 1990; Tinsdale et al., 1985). Groffman et al. (2009) reported few differences in N2O fluxes above four urban grassland and eight forested ecosystems. The flux of N2O from intensively fertilized grasslands did not exceed that from forest ecosystems, indicating that N cycling in urban lands is a complex process. The data by Groffman and colleagues also suggests that N retention may be significant in these ecosystems. Denitrification losses are most likely low for many turfgrass/soil conditions (Carrow et al., 2001). Some conditions such as soils compacted with poor drainage and algae covered surfaces may be conducive to denitrification. Kaye et al. (2004) studied the fluxes of CH4 and N2O from urban soils and compared these with those from non-urban ecosystems. The ecosystems studied consisted of urban lawn, native shortgrass steppe, dryland wheat fallow, and flood-irrigated corn. The urban lawn fluxes of CH4 and N2O were comparable to those from irrigated corn (Zea mays) but were more than those from wheat (Triticum aestivum) fallow or native grasslands. Limited information is available for field comparisons of soil–atmosphere exchange on N2O and CH4 fluxes from turfgrass/soil ecosystems. Although this model did not account for CH4 and N2O, future modeling scenarios should consider inclusion of soil GHG when dictated by specific climate, soil conditions, or management practices known to greatly influence GHG fluxes (Groffman et al., 2009; Lorenz and Lal, 2009b; Neeta et al., 2008; Raciti et al., 2008). The objective of this research was to investigate a simple mass balance model that compares the rate of SOC sequestration under a range of management scenarios for singlefamily home lawns practiced in diverse ecoregions of the United States. This article specifically explains methods to estimate the net pool of SOC sequestration under MI, medium input determined as DIY homes based on average current practices, and high input determined as homes using BMPs. Net SOC sequestration rates of each category were determined by subtracting the HCC from gross SOC sequestration. Materials and Methods Soil organic carbon sequestration rates for U.S. home lawns were modeled using data available from published literature. All data for SOC sequestration rates were compiled for the 0to 15-cm soil layer. The net SOC sequestration rate was the amount of gross C accumulated minus the HCC of lawn maintenance practices expressed as C equivalents. Home lawns are cared with a number of agronomic and maintenance inputs with the majority including mowing, use of fertilizers and pesticides, and irrigation. Forty million home lawns use a MI system (mowing only), 30 million lawns are maintained by the homeowner, and 10 million use a lawn care service or apply fertilizer multiple times a year (Augustin, 2007). Do-It-Yourself lawn practices focus on average current lawn maintenance practices to calculate average net SOC sequestration rate in U.S. home lawns. Estimates of lawn maintenance practices for MI and BMPs are calculated to benchmark C sequestration of lowto highrange lawn maintenance regimes. These ranges also provide an estimate of lawn management practices under a wide range of regional environmental conditions. The parameters, data, and assumptions used in the model are summarized in Table 1. This equation is expressed in units of g/m/year (Eq. [1]). Net C sequestration rate = Gross SOC sequestration rate -HCC [1] Soil organic carbon sequestration. The net rates of SOC sequestration were compiled from published literature on NPP and SOC dynamics (Tables 2 and 3). Data on net primary productivity (NPP) were used to estimate the average rate of SOC sequestration after the humification of plant material (Smith et al., 1993). The only data sets selected for use in this study consisted of gross primary productivity minus the respiration using both the belowground (root) and aboveground (shoot) growth rate for U.S. grasslands and turfgrasses (Table 2). The grassland sites ranged widely in geography and climate across the United States. The NPP data used in this model included direct measurements of dry plant biomass over 12 different sites. The average range of NPP was 5.89 to 12.71 Mg dry matter/ha/year. Each year, 10% of the biomass added to the soil may be humified (Duiker and Lal, 2000; HORTSCIENCE VOL. 46(5) MAY 2011 809 TURF MANAGEMENT