A novel robotic chamber system allowing to accurately and precisely determining spatio-temporal CO2 flux dynamics of heterogeneous croplands
Graphical abstract
Introduction
The precise and accurate assessment of terrestrial sources and sinks for anthropogenic carbon dioxide (CO2) is crucial to understand the global carbon (C) cycle, support the development of climate policies, and project future climate change (Friedlingstein et al., 2019). However, current estimates for croplands are still subject to considerable uncertainties from field to global scale (Carlson et al., 2016; Luo et al., 2016; McDermid et al., 2017; Rosenzweig et al., 2020). Mbow et al. (2019) for example, estimated global cropland emission with an uncertainty of almost 34%. This is mainly due to insufficient knowledge regarding the effect of management practices in relation to specific soil conditions on net CO2 exchange (NEE) and the resulting CO2 source and sink function (Chenu et al., 2019).
In croplands, soil is often characterized by small-scale spatial patterns, for instance as a result of soil erosion caused inter alia by ploughing that leads to lateral redistribution of soil C (Berhe et al., 2018; Doetterl et al., 2016). This induces extreme differences in soil C, which alters soil fertility, biological activity and crop growth even within a distance of a few meters only. In addition to that, these small-scale spatial patterns might change rapidly due to altered management practices. Both together cause strong small-scale spatial differences in the C-dynamics and CO2 exchange of such croplands (Dialynas et al., 2016; Nadeu et al., 2015). However, precise information is hardly available so far, since current measurement techniques are strongly limited when it comes to evaluate the influence of management practices on the CO2 exchange of heterogeneous croplands.
While the widely used eddy covariance (EC) technique provides precise high temporal resolution CO2 flux data, the spatial resolution is relatively coarse (Baldocchi, 2014). As a result, neither multifactorial treatment comparisons on field trials (Baldocchi, 2014) nor the detection of small-scale spatial differences are possible using EC (Paustian et al., 2019), since both would require spatially distinct measurements in two dimensions (2D). Compared to that, the manual chamber (MC) measurements have the advantage of being spatially distinct, which enables them to detect small-scale spatial differences and allow its use within field experiments (Hoffmann et al., 2017; Koskinen et al., 2013; Pavelka et al. 2018). Anyhow, these measurements are labor intensive and thus characterized by either a low temporal resolution, wherefore they are usually in need of extensive gap filling, commonly conducted using e.g., empirical modeling approaches (Moffat et al., 2007), or limited to shorter term studies (< 1 year; e.g., Srivastava et al., 2018). As a result, short-term changes in CO2 dynamics caused by e.g., weather conditions and crop development might be misrepresented in annual or multiyear CO2 exchange measurements using MC. Accordingly, it has been shown that measurements using MC are less reliable than EC measurements (Lucas-Moffat et al., 2018). In principle, automatic chambers (AC) can combine the advantages of EC (temporal resolution) and MC (spatial resolution) measurements (Baldocchi, 2014; Pumpanen et al., 2004). However, AC placed permanently above measurement plots, are substantially influencing the microclimate and crop development underneath (Hoffmann et al., 2017), which might bias CO2 flux estimates. In addition to that, they are restricted to the total number of measurement plots (Morin et al., 2017), which limits the amount of treatments to be compared and directly affects the number of repetitions being possible.
To overcome these disadvantages, we developed a novel robotic chamber system for conducting CO2 exchange measurements in a high temporal and spatial resolution. The developed robotic chamber system is mainly inspired by former developments described by Kiese et al. (2018) and Pütz et al. (2016), who used opaque chambers capable to move in two dimensions (2D) connected to a small gantry crane to monitor GHG emissions at grassland sites. This represents a further development of the 1D approaches presented by Keane et al. (2018, 2019), who used a single chamber moving along a line. We hypothesize that the presented novel robotic chamber system will enable us to record not only short-term temporal dynamics but also the impact of management practices and small-scale spatial differences in soil condition on measured NEE and derived flux components, ecosystem respiration (Reco) and gross primary production (GPP). To test this hypothesis, measurements were carried out from the beginning of July to the beginning of September 2019 on heterogeneous cropland, on which the establishment of a block trial manipulation experiment, dedicated to new solutions for permanent additional C and N storage is planned. The novel robotic system should help us to find out whether the spatial heterogeneity of C-dynamics is low within the areas of the individual soils, which is a prerequisite for obtaining trustworthy results in the planned field experiment.
Section snippets
Study area
The study area “CarboZALF-D” is located within the Uckermark region, an intensively agricultural used hummocky ground moraine landscape in the northeast of Germany (53° 23´ N, 13° 47´ E; ~50-60 m a.s.l; Fig. 1a). The soil types of the selected undulating field (110 × 16 m, height difference between center and edges 1m) at a summit of the study area represent the full gradient of erosion stages present in this type of landscape and usually occur in a very close proximity (<30m; Sommer et al.,
Data reliability
Using the described robotic chamber system a total of 14,099 measurements were conducted (34% during nighttime (Reco); 66% during daytime (NEE)) during the study period. Out of these measurements, 11 % did not meet linear regression assumptions when a linear regression was applied to the entire measurement length of 7 min, with a small difference between chamber 1 (8 %) and chamber 2 (13 %). Hence, measurements of chamber 2 seem to be slightly less reliable and more prone to potential chamber
Data availability statement
The data that support the findings of this study are openly available through the ZALF open-research-data repository at https://www.doi.org/10.4228/ZALF.DK.143
Declaration of Competing Interest
None.
Acknowledgements
This study was supported by the German Federal Ministry of Food and Agriculture (FNR Grant: 22404117). The authors are very grateful to the Pfannenstiel ProProject GmbH for the excellent collaboration in designing as well as constructing the gantry crane system and ATTEC Automation GmbH for programming the system control.
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