Decadal variability of soil CO<sub>2</sub>, NO, N<sub>2</sub>O, and CH<sub>4</sub> fluxes at the Höglwald Forest, Germany

Abstract

Abstract. Besides agricultural soils, temperate forest soils have been identified as significant sources of or sinks for important atmospheric trace gases (N 2 O, NO, CH 4 , and CO 2 ). Although the number of studies for this ecosystem type increased more than tenfold during the last decade, studies covering an entire year and spanning more than 1-2 years remained scarce. This study reports the results of continuous measurements of soil-atmosphere C-and N-gas exchange with high temporal resolution carried out since 1994 at the Höglwald Forest spruce site, an experimental field station in Southern Germany. Annual soil N 2 O, NO and CO 2 emissions and CH 4 uptake (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) varied in a range of 0.2-3.0 kg N 2 O-N ha −1 yr −1 , 6.4-11.4 kg NO-N ha −1 yr −1 , 7.0-9.2 t CO 2 -C ha −1 yr −1 , and 0.9-3.5 kg CH 4 -C ha −1 yr −1 , respectively. The observed high fluxes of N-trace gases are most likely a consequence of high rates of atmospheric nitrogen deposition (>20 kg N ha −1 yr −1 ) of NH 3 and NO x to our site. For N 2 O, cumulative annual emissions were ≥0.8 kg N 2 O-N ha −1 yr −1 in years with freeze-thaw events (5 out 14 of years). This shows that long-term, multi-year measurements are needed to obtain reliable estimates of N 2 O fluxes for a given ecosystem. Cumulative values of soil respiratory CO 2 fluxes tended to be highest in years with prolonged freezing periods, i.e. years with below average annual mean soil temperatures and high N 2 O emissions (e.g. the years 1996 and 2006).Furthermore, based on our unique database on trace gas fluxes we analyzed if soil temperature, soil moisture measurements can be used to approximate trace gas fluxes at daily, weekly, monthly, or annual scale. Our analysis shows that simple-to-measure environmental drivers such as soil temperature or soil moisture are suitable to approximate fluxes of NO and CO 2 at weekly and monthly resolution reasonably well (accounting for up to 59 % of the variance). However, for CH 4 we so far failed to find meaningful correlations, and also for N 2 O the predictive power is rather low. This is most likely due to the complexity of involved processes and counteracting effects of soil moisture and temperature, specifically with regard to N 2 O production and consumption by denitrification and microbial community dynamics. At monthly scale, including information on gross primary production (CO 2 , NO), and N deposition (N 2 O), increased significantly the explanatory power of the obtained empirical regressions (CO 2 : r 2 = 0.8; NO: r 2 = 0.67; N 2 O, all data: r 2 = 0.5; N 2 O, with exclusion of freeze-thaw periods: r 2 = 0.65).

Soil Greenhouse Gas Fluxes, Environmental Controls, and the Partitioning of N2O Sources in UK Natural and Seminatural Land Use Types

“…Soil temperature explained most of the variability (47%) of soil respiration across land use types, with an additional 13% explained by soil moisture and bulk density, which confirms the findings of several previous studies on the controls of soil CO 2 respiration in terrestrial ecosystems (Carter et al, 2012;Gritsch et al, 2015;Imer et al, 2013;Luo et al, 2012;Schaufler et al, 2010;Skiba et al, 2013). The temperature dependence of soil respiration can be explained by the stimulation of biological activity (plant roots and microbial communities) with increasing temperature, and this was evident within all the investigated land use types as indicated by the MLR analysis (Table 2).…”

“…The multiple linear regression analysis indicated significant and relatively complex environmental controls of nitrous oxide emission from natural terrestrial ecosystems (Table 2) including nitrate and organic carbon availability, soil pH, and temperature with overall lower predictive power (43%) compared to the models predicting methane and carbon dioxide effluxes from the same land use types. Weak relationships between N 2 O fluxes and environmental variables measured in the field are commonly found in the literature (Carter et al, 2012;Eickenscheidt et al, 2014;Luo et al, 2012;Skiba et al, 2013). This has been attributed to the highly dynamic nature of nitrous oxide production and consumption processes and their enormous spatiotemporal variability (Baggs, 2011;Butterbach-Bahl et al, 2013), which is difficult to predict particularly at weekly or monthly timescales (Luo et al, 2012).…”

“…Soil reactive N (labile organic bound N and inorganic N compounds) availability has been advocated as the major driver of N 2 O emissions from soils (Butterbach-Bahl et al, 2013), while soil moisture and temperature provide additional controls by regulating the availability of oxygen and microbial enzymatic activity, respectively (Butterbach-Bahl et al, 2013;Schaufler et al, 2010). The interaction of these environmental controls, particularly due to the opposing conditions (aerobic versus anaerobic) for N 2 O production, and the contribution of other distal controlling factors such as soil organic carbon quality, soil pH, and texture (Saggar et al, 2013;Sgouridis & Ullah, 2014) commonly lead to weak relationships between N 2 O fluxes and environmental variables measured in the field (Carter et al, 2012;Luo et al, 2012;Skiba et al, 2013), further hampering our ability to predict N 2 O fluxes at the landscape scale and in response to changing environmental conditions. Despite the wealth of studies that have investigated GHG fluxes and their controlling factors, the majority have focused on one or two land use types and only a few have considered all major GHGs across a range of natural and seminatural land use types in the laboratory (Schaufler et al, 2010), in meta-analysis (Mander et al, 2010), and under field conditions (Czóbel et al, 2010).…”

Cold season soil NO fluxes from a temperate forest: drivers and contribution to annual budgets

“…The Höglwald (HGW) research site (48°30′N 11°11′ E, 540 m a.s.l) is a temperate mature spruce forest in an agricultural area with high atmospheric nitrogen (N) deposition (20-30 kg N ha −1 yr −1 ) (Butterbach-Bahl et al 1997, Luo et al 2012, 2013. The climate is suboceanic with an annual bulk precipitation rate of 932 mm (including snow water equivalent) and an annual mean air temperature of 8.6°C (observation period of 1994Luo et al 2012). The soil is a Typic Hapludalf (Soil Taxonomy 2014) (WRB (2015): Dystric Cambisol) with an acidic pH (CaCl 2 ) of 2.9-3.2 in the organic layer and 3.6-4.0 in the uppermost mineral soil layer (Kreutzer 1995).…”

“…Using daily NO fluxes (Luo et al 2012), annual NO budgets were calculated from 1 July to 30 June of the following year to cover the corresponding cold season. Cold season NO budgets consisted of the entire cold season period, i.e.…”