Urban lawn
During the 2011 season,
NEE was dominated by net uptake of CO
2 as more than 70% of daily
NEE sums indicated net uptake. Brief periods of net emission occurred in response to specific environmental conditions and management. For example, a distinct period of net emissions in mid-May 2011 coincided with a rapid increase in soil temperature (daily averages increased 8.0°C within 7 days) and the start of irrigation. The influence of soil temperature on soil respiration is well established and remains a subject of research (Fang and Moncrieff
2001; Risk et al.
2002; Hibbard et al.
2005; Davidson et al.
2006; Graf et al.
2008; Karhu et al.
2014). But soil respiration in most terrestrial ecosystems is also influenced by soil moisture (Risch and Frank
2007; Balogh et al.
2011; Lellei-Kovács et al.
2011). The observed pulses of soil CO
2 efflux following the re-wetting of (dry) soils (also known as the “
Birch effect” – see discussion below) have been reported in other studies (e.g., Lee et al.
2004; Jarvis et al.
2007; Chowdhury et al.
2011; Kim et al.
2012). The immediate response of
NEE to precipitation/irrigation events, however, was difficult to estimate since data during these events was often incomplete due to the interference of water on the open-path IRGA instrument. Fertilization may also have led to short periods of net emissions or clearly reduced uptake in both seasons as fertilizer application can considerably enhance soil respiration (Fierer et al.
2003; Verburg et al.
2004). For most of the remaining period in 2011, net uptake dominated but was not uniform in strength.
NEE noticeably slowed in August 2011 despite both, high irrigation input and LAI. We observed small, positive daily sums of
NEE (net emission) during some of the warmest days. High air temperatures and high VPD likely stressed vegetation, reducing photosynthetic activity and weakening uptake (Mathur et al.
2014).
NEE in 2012 appeared to be impacted by above average heat, especially drought conditions, and lack of sufficient irrigation. Above-average temperatures and earlier vegetation development (as indicated by LAI) in spring of 2012 led to increases in soil respiration and photosynthetic fluxes, resulting in greater diurnal amplitudes of
NEE compared to 2011. Irrigation was a major influence on
NEE through feedbacks via soil moisture and LAI. These factors primarily determined the seasonal course of
NEE until early September, together with record high air temperatures and high VPD (Fig.
11). The effect of irrigation limits became apparent in late May and more so in late July/August. When irrigation was stopped for periods of a few days or more, soil moisture and LAI declined, photosynthetic flux was reduced and net losses of carbon followed. The resilience of the lawn preserved seasonal net uptake of CO
2 as LAI repeatedly recovered once irrigation resumed, resulting in negative
NEE, indicating uptake.
The effect of lawn-mowing on NEE could not be clearly identified as mowing varied temporally and spatially on the lawn. Reduced LAI may have contributed to decreased net uptake of CO2, but data that coincided with or followed lawn-mowing, also showed other, potentially influencing factors (e.g., high temperature, low soil moisture, low photosynthetically active radiation (PAR), increased photosynthesis by previously shaded vegetation), confounding interpretation.
To evaluate
NEE of CO
2 at the urban lawn, data drawn for comparison also included urban studies that contained a significant portion of lawns (e.g., suburban neighborhoods, park landscapes), since studies investigating
NEE of lawns are still relatively rare. Bergeron and Strachan (
2011) studied
NEE at a suburban site in Montreal, Canada, where in spring (April-May) and fall (September-November)
NEE was generally close to zero and in summer (Jun-Aug) showed an average midday uptake of −7 μmol CO
2 m
−2 s
−1. In comparison, diurnal averages observed at this study’s urban lawn were usually stronger, reaching up to −10.6 (spring 2012), −11.8 (fall 2011), and − 13.3 μmol CO
2 m
−2 s
−1 (summer 2011). For other suburban sites, Kordowski and Kuttler (
2010) reported average summer
NEE maxima of up to −10 μmol CO
2 m
−2 s
−1, while Buckley et al. (
2014) working in Syracuse, New York, found a midday average CO
2 flux of −11 μmol CO
2 m
−2 s
−1 during the summer months. Average
NEE during June–August in suburban Baltimore, Maryland, ranged between −14 and + 10 μmol CO
2 m
−2 s
−1 and diurnal amplitudes showed a sensitivity to PAR and soil temperature (Crawford et al.
2011).
NEE measured at suburban/residential sites could also be influenced by anthropogenic emissions (e.g., traffic) but, in general, these results compare well to this study’s data.
Studies that aim to quantify annual net carbon uptake of turfgrass are often based on the analysis of changes in soil carbon stocks as the carbon is mostly stored in the soil and not in shoots or roots (Guertal
2012). Qian and Follett (
2002) investigated turfgrass sites at golf courses of different ages (mainly in Colorado) and found average net uptake rates of −90 to −100 g C m
−2 a
−1 during the first 30 years following establishment while Qian et al. (
2010) reported a narrower range of −34 to −78 g C m
−2 a
−1. Milesi et al. (
2005) estimated that turfgrass in the United States could uptake between −36 to −100 g C m
−2 a
−1. Carbon uptake for ornamental lawns in Irvine, California, was −140 g C m
−2 a
−1 (Townsend-Small and Czimczik
2010). These values compare well to the estimated annual uptake for the urban lawn in this study in 2011 (−131(±24) g C m
−2 a
−1) but also illustrated how severely summer drought conditions impacted carbon uptake potential in 2012 (−18(±22) g C m
−2 a
−1).
Tallgrass prairie
Like the urban lawn, the tallgrass prairie showed substantial
NEE of CO
2 variability day-to-day, through the course of a season, and between years. Similar variability has been observed in various other (multi-year) studies in the Great Plains, including changes in ecosystem function from carbon sink to source (Frank and Dugas
2001; Sims and Bradford
2001; Polley et al.
2008; Parton et al.
2012). These differences in (annual) carbon budgets are often attributed to climatic variability which can directly and indirectly impact
NEE, for example, by influencing aboveground-net primary productivity in grasslands (Knapp and Smith
2001; Flanagan et al.
2002; Xu and Baldocchi
2004).
NEE at the prairie site during the 2011 season followed a seasonal cycle strongly influenced by soil moisture and temperature conditions, solar radiation, VPD and feedbacks via LAI. Diurnal cycles of
NEE in spring were weak but were evidence that rising soil temperatures stimulated soil respiration while early vegetative growth initiated uptake of CO
2, which strengthened further into the season. However, similar to the lawn, periods of net emission were also observed, e.g., in May (Fig.
9), following a rapid increase in soil temperature and significant precipitation but also low DLI. The individual impact of temperature and moisture on soil respiration is difficult to estimate as both parameters have been shown to influence soil CO
2 efflux in prairie ecosystems (Mielnick and Dugas
2000; Frank and Dugas
2001; Chimner and Welker
2005). Net CO
2 uptake dominated until late July/early August with rising LAI and significant precipitation input, but slowed abruptly thereafter, likely due to stress conditions (high temperatures and VPD, depleted soil moisture) impacting photosynthetic activity. Cooling temperatures, declining VPD, and moderate precipitation in early September led to a partial recovery of net uptake, but senescence of vegetation in early October ended net uptake.
Anomalous climate conditions in spring and summer of 2012 impacted
NEE at the prairie site. Amount and timing of precipitation appeared as important parameters affecting strength and direction of CO
2 flux (uptake/emission) and annual carbon balance, a finding consistent with other studies (e.g., Frank and Dugas
2001; Sims and Bradford
2001; Huxman et al.
2004; Harper et al.
2005; St. Clair et al.
2009). Similar to the lawn, above-average springtime temperatures led to an earlier and more rapid development of vegetation. CO
2 uptake sharply increased in mid-May, enhanced by nearly regular precipitation and a further increase in LAI. However, vegetation vitality was impacted towards the end of the month and into June by changing environmental conditions. Lowering soil moisture with air temperature and VPD increasing towards annual maxima led to drastically declining net uptake. Uptake temporarily resumed as precipitation in early July replenished soil moisture, but as air temperatures and VPD remained high and soil moisture decreased, daily uptake sums gradually decreased. By the end of August, drought conditions resulted in net emissions and the prairie became a net source of CO
2 until the end of October.
NEE of CO
2 of the prairie displayed a distinct sensitivity in summer to heavy precipitation events after periods of low or no precipitation during both years. Net uptake ceased after precipitation pulses, followed by a peak of net release of CO
2 (up to 10 g C m
−2 within one week) before uptake usually resumed. Notable events occurred around the same time in both years, i.e., early July and early-mid September; although during the drought year 2012 resultant net emissions appeared stronger. The CO
2 emission spike in a semiarid ecosystem following a precipitation pulse after a dry period is known as the “
Birch effect” (Birch
1958), a subject of various studies (e.g., Huxman et al.
2004; Parton et al.
2012) including those in shortgrass (Munson et al.
2010) and tallgrass prairies (Liu et al.
2002). Interest in these precipitation-induced carbon losses is due to the fact that these events can lead to large CO
2 effluxes, representing a considerable portion of annual respiration (Ma et al.
2012).
CO
2 emission pulses may be the result of multiple processes (Ma et al.
2012). Initially, the infiltration of rain into the soil causes the physical displacement of CO
2-rich air from soil pores. The CO2 is also generated by the stimulation of microbial activity by a sudden increase in soil moisture. Microbial activity may be further enhanced by increased carbon and nutrient availability through photo-degradation of dead biomass. This latter process has been shown to cause direct CO
2 emissions from litter (Rutledge et al.
2010). Parameters such as timing and magnitude of precipitation pulses also seem to exert an influence on respiration response (Harper et al.
2005; Munson et al.
2010; Ma et al.
2012).
Comparing cumulative carbon uptake to other studies illustrates that the span in uptake observed between years at the prairie site is not atypical. For example, Frank and Dugas (
2001), measuring
NEE over 4 years at a mixed prairie site, found that cumulative net uptake ranged between −50 to −130 g C m
−2 (April–October; average: −95 g C m
−2). They also noted that, similar to this study’s prairie site, seasonal variability in
NEE was clearly related to LAI/biomass responding to moisture and temperature stress and that maximum CO
2 flux occurred at the time of maximum LAI. Sims and Bradford (
2001) reported an average
annual uptake of −70 g C m
−2 a
−1 for a prairie site, comparable to this study’s prairie in 2011 (−61(±10) g C m
−2 a
−1) and emphasized the importance of the timing of precipitation. Similar observations regarding the change of grasslands from carbon sink to source in response to precipitation patterns have been made by Meyers (
2001), Ma et al. (
2007) and Xu and Baldocchi (
2004). Polley et al. (
2008) found that interannual variability of net uptake was clearly reflected in average daily
NEE during the growing season which varied by more than a factor of 3 between years, comparable to the ratio found for the prairie site. Daily
NEE sums for this study’s prairie site during the growing season displayed a range (May–July 2011: −2.6 to +2.1 g C m
−2 d
−1; July–September 2012: −1.9 to +2.8 g C m
−2 d
−1) similar to that found by Suyker and Verma (
2001) (July–August: −1.8 to +2.2 g C m
−2 d
−1). For the same tallgrass site (Suyker et al.
2003), the
annual uptake was larger (−274 g C m
−2 a
−1) in comparison to this study’s xeric tallgrass prairie but severe drought conditions also reduced annual
NEE by more than 80%, similar to what the prairie site experienced in 2012.