T6713 Sensor – Discussion

The CO2 sensor could detect differences in CO2 accumulation over time and under different treatments. Some of these differences could be explained due to changes in the external environment on the day of sampling. However, due to time constraints and the focus of this thesis being to design a prototype for a wireless sensor system, environmental factors such as relative humidity, soil temperature and moisture etc, were not documented. Nevertheless, on the day of measuring CO2 under the no-glucose treatment from the chamber, rain and hailstones fell between 4-6pm. This may help to explain why we see a greater range of values in the no-glucose treatment in comparison to the glucose treatment. Sakabe et al. (2015) confirmed increases in CO2 emissions from soils after Asian monsoon rainfall. Deng et al. (2017) found similar results in a tropical forest in China. Their findings showed an 85% increase in soil CO2 fluxes after rains. This was also accompanied by an increase in both fungal and bacterial phospholipid fatty acids (58 and 51% respectively). Laporte et al. (2002) also found a correlation between soil moisture and soil surface CO2 efflux in a grassland ecosystem in Northern Ontario. It is therefore important to consider the abiotic environment when taking GHG measurements. The lowest CO2 levels in the chamber were recorded at 4pm under the glucose treatment. As the glucose solution was only applied once and at 2pm it is possible that its influence on soil CO2 flux may not have been as potent 2 hours later. Unlike sampling on the no-glucose treatment day, the glucose treatment day had no outbursts of precipitation and as a result we don’t see as extravagant CO2 levels accumulating in the chamber.

Despite the sensor being able to detect differences in CO2 over time and across treatments, values recorded are still hugely over estimated in comparison to standard atmospheric concentrations (~400ppm, (IPCC, 2014)). This may be the result of a fault in the sensors automatic background calibration (ABC) or the calibration being slightly offset during soldering as a result of high temperatures.

It is possible that a more expensive CO2 sensor would have produced better quality results. One of the challenges in this dissertation was choosing which sensor to pick from an endless list. For example, the CO100-Meter measures CO2 concentrations as great as 9999ppm and records air temperature and humidity (Extech Instruments, 2014) as well as the K-30 CO2 sensor which measures CO2 concentrations up to 10,000ppm (CO2 Meter, 2017). Similar technical challenges arose when constructing the chamber as materials listed in Parkin and Ventura (2010) were not always easily supplied to Ireland. In this instance, alternative materials had to be sourced and in the majority of cases, there were delays with ordering and delivering parts.

Had additional time been available, it would have been interesting to investigate the sensitivity of the sensor to CO2 at even lower concentrations of NaHCO3 and at different depths in the soil, not just the surface. Tang et al. (2003) used chambers and small solid-state sensors buried at different depths (2cm, 8cm and 16cm) to investigate soil CO2 efflux and Pingintha et al. (2009) used GMP343 CO2 sensors at different depths (0.02m and 0.12m) to also examine CO2 efflux. Likewise, Sakurai et al. (2015) measured CO2 concentrations using pairs of nondispersive infrared (NDIR) CO2 sensors at different depths (5, 10, 20, and 50cm), vertically in the soil. In this study, a sensor which was either mobile (i.e. didn’t require being physically connected to an external device or programme) or had WiFi would have facilitated the task of monitoring GHG emissions at different soil depths. For example, the S05-CO2 outdoor IP66 Wireless CO2 sensor which has a wireless transmission range of 500m and is compatible with WiFi (Nietzsche Enterprise Co., Ltd, 2015). Likewise, Lambebo and Haghani (2008) used WSNs to monitor temperature and GHGs which had a WiFi receiver node which could communicate with other parts of the network system. Studies concerned with exploring and testing environmental technology are important in developing a better understanding of the dynamics of CO2 emissions in soils.

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