Rates of soil respiration at a reference temperature of 10°C (Rs10) ranged from 0.25 to 5.49 μmol m−2 s−1 () and increased with increasing mean annual soil temperature, C content of the upper soil layers, peak leaf area index and annual gross primary productivity across sites (). Maximum Rs exceeded 14 μmol m−2 s s−1 for a number of Central European sites, reaching values of up to 15.9 μmol m−2 s−1 on warm summer days (, ). Across sites maximum Rs rates were not correlated with associated soil temperatures, but generally increased with increasing Rs10 of a site (). The temperature sensitivity of Rs (Q10) based on the pooled data sets and calculated for a common temperature range (at 5 cm depth) from 10 to 15°C ranged from 2.0 to 4.9 and was not related to mean annual soil temperature (r2 = 0.01; not shown).
Figure 1 Soil respiration at a reference temperature (10°C) in relation to A mean annual soil temperature (r2 = 0.69, P < 0.001), B soil C content in the uppermost 20–25 cm of the soil (r2 = 0.50, P < 0.05), C peak leaf area index (more ...)
Figure 2 Maximum rates of soil respiration in relation to A the associated soil temperature and B soil respiration at a reference temperature (10°C) (r2 = 0.87, P < 0.001). Meadows (▲), pastures (●), unmanaged Northern grasslands (more ...)
Soil moisture constrained Rs at higher soil temperatures at most sites. This is reflected by the logistic model, whose sigmoid shape levels Rs at higher Ts when limitations due to low soil moisture are frequent, and the Lloyd and Taylor model combined with a Gompertz-function, which modifies the temperature response of Rs in relation to soil moisture. Reductions of Rs by low soil moisture were most pronounced at the Spanish site Alinya, which—with the exception of the northernmost site Värriö—was characterized by the lowest amounts of annual precipitation (). Two time series illustrate the course of Rs at Alinya during two summer periods immediately before and after short rainfall events. In July 2003 Rs was low and its diurnal variation was minor until after a rain event when soil moisture increased from less than 10 to more than 20 vol%. After a short time lag following the rain pulse soil CO2 efflux doubled and then followed a distinct diurnal pattern (). As soil moisture dropped below 10 vol% during the subsequent days, Rs was again reduced to values occurring before the rain, with a decreasing response to Ts fluctuations. In June 2004, soil moisture never decreased to less than 20 vol%, and an increase in soil moisture after rain did not alter Rs rates and their response to Ts ().
Figure 3 Soil respiration (), soil temperature (dashed line) and soil moisture (solid line) before and after rainfall during summer periods in A 2003 and B 2004 at Alinya. Bars indicate the timing and the amount rainfall. Soil respiration was measured (more ...)
The temperature response of Rs was not only influenced by thresholds of soil moisture, but varied under non-limiting water supply in the course of the day. This was particularly obvious on clear days, when a hysteresis effect occurred, which became more apparent when relating soil CO2 efflux to temperature at increasing soil depth (). When related to a given Ts at 1 cm soil depth Rs was higher in the late afternoon as compared to the morning hours (), whereas in relation to a given Ts at deeper soil layers Rs was highest during morning hours and lowest at night (). On a cloudy day at a given Ts at 1 cm evening and early night-time values of Rs were slightly higher than during the rest of the day (), whereas no clear temperature response and temperature-independent pattern of Rs were observed in relation to temperatures at deeper soil layers ().
Figure 4 Diurnal changes of soil respiration (Rs) in response to temperature at 1, 3 and 10 cm soil depth on a clear (18 June 2004; A–C) and a subsequent cloudy day (19 June 2004; D–F) at Alinya. Consecutive hourly means have been connected by (more ...)
At a daily to weekly timescale land use affected Rs and its response to Ts. At Monte Bondone Rs of clipped plots was reduced by approximately 10% relative to adjacent unclipped plots, and recovered after about 2 weeks (data not shown). At Amplero clipping resulted in a reduction of Rs by more than 50% at two periods during August, and by almost 30% later in the season, whereas it caused an increase in soil temperature during all periods (). Additional grazing caused a further significant reduction of Rs only in late August (). At Carlow a silage cut early in the season followed by subsequent extensive grazing resulted in distinct seasonal fluctuations of LAI, which explained much of the variation of observed versus predicted Rs values, as based on the seasonal relationship between Ts and Rs ().
Effects of clipping and grazing on A soil CO2 efflux and B soil temperature at Amplero during the summer of 2004 (for each treatment n = 9).
Figure 6 Residuals of observed minus predicted values of soil respiration at Carlow in relation to seasonal values of leaf area index (LAI) in 2003 (r2 = 0.70, P < 0.001, when excluding the data point indicated with an open symbol from the regression). (more ...)
Annual soil CO2 efflux from the grasslands ranged from 58 to 1988 g C m−2 y−1 (), the average being 1108 g C m−2 y−1. The highest values were found for meadows in the Alps (Oensingen, Stubai Valley, Monte Bondone), the lowest occurred at the northernmost site in Finland (Värriö). The uncertainty of the annual estimates was 9–25% for most sites, but as high as 40 to almost 90% for three sites in different parts of the European transect (). Across sites annual Rs was closely related to mean annual Ts (). Without the two Mediterranean sites (Amplero, Alinya), whose annual totals were distinctly below the regression line, r2 of the regression increased from 0.81 to 0.92 (P < 0.001). Across sites annual Rs increased with increasing soil C content in the upper soil layers (r2 = 0.65; not shown) and peak leaf area index (, without values for Auchencorth Moss r2 = 0.77), and was very well correlated with gross primary productivity (), but exhibited no relationship with peak aboveground biomass (r2 = 0.02; not shown).
Figure 7 Annual soil respiration in relation to A mean annual soil temperature (r2 = 0.81, P < 0.001), B peak leaf area index (r2 = 0.74, P < 0.001) and C annual gross primary productivity (r2 = 0.94, P < 0.001). Meadows (▲), pastures (more ...)
Land use of the grasslands studied was generally related to mean annual soil temperature (), which decreased from meadows (mean annual Ts across sites 9.5 ± 0.6°C) to pastures (8.3 ± 0.7°C), and the natural grasslands (5.1 ± 1.8°C). Thus it is not possible to separate effects of land use on annual Rs from those related to temperature. Average annual Rs pooled for meadows, pastures and natural grasslands was 1520 ± 151, 865 ± 155 and 293 ± 236 g C m−2 y−1, respectively.