## 5 Soil Sensor Arrays  
Tropical rainforest soils receive large inputs of carbon from the atmosphere through photosynthesis. However, organic matter decomposition is also very high as moisture and temperature are optimal for microbial growth and turnover, releasing that carbon back to the atmosphere. The primary limitation is presumed to be nutrients due to high leaching rates and saturated moisture levels that restrict O2 for aerobic microbial respiration. Nevertheless, when carefully examined, the tropical rainforest soils at L Selva have been found to have high root, fungal, and bacterial standing crops [26]. Further, organic carbon may well be an important sink because of the interconnected mycorrhizal plant-fungal-soil pathways [27]. Therefore, establishing the relative significance of the pathways of carbon flow is an important step (Figure 4). A key element of our sensor studies has been the development of a soil monitoring system installed near the base of the eddy covariance towers. The system consists of three instrumented soil profiles, each with sensors for soil CO2 and O2, soil temperature, and soil moisture at four depths (surface, 5-10, 15-20, 50 cm) totaling 12 sets of sensors. Additional surface sensors for the soil temperature and moisture were placed to generate spatial information about the surface soil conditions and select the representative areas for long-term monitoring. The sensor profiles provide data on the soil dynamics at a level of detail not typically present at most flux tower sites. In our measurements to date, we have found that soil respiration is a major component of the ecosystem carbon flux, which is estimated to comprise approx. half of the total ecosystem respiration. As the new eddy covariance towers become operational, the soil flux data will serve as an important check on the carbon flux budget. The CO\({}_{2}\) sensors consisted of a remote Vaisala probe (GMT220 transmitter and GMP221 probe, 0-10% CO\({}_{2}\) range) and the O\({}_{2}\) sensors (Apogee model SO-110). The sensors were installed inside a sealed top of polyvinyl chloride (PVC) tubing and exposed end sealed with a GORE-TEX lining located at the appropriate depth. The temperature and moisture readings were undertaken using Campbell Scientific CS650 sensors (Logan, UT, USA). All the sensors were connected to the same data logger for synchronized data. The readings were recorded at 5-min intervals and could be analyzed independently or averaged using 24-h analyses. A challenging goal for soil ecosystem research has been the quantification of root and microbial dynamics and coupling those measurements with sensor data for the soil temperature, water content, and CO\({}_{2}\) and O\({}_{2}\). Our future intent is to collect quantitative sufficient data to model the CO\({}_{2}\) flux using HYDRUS or other existing models to utilize our soil sensor data (Figure 5). Our initial studies of soil CO\({}_{2}\) fluxes have been made within the footprint of the past eddy covariance estimates [12] to better understand how soil CO\({}_{2}\) behaves in this ecosystem [28; 29]. Figure 4: Soil Ecosystem Observatory at the La Selva. The white box holds a Campbell data logger for recording the sensor data inputs at 5 min intervals. Attached to the data logger are the wiring routed through PVC tubes into the soil with buried sensors to measure the temperature and moisture (Campbell Scientific, Logan, Utah), CO\({}_{2}\) (Vaisala, Helsinki, Finland), and O\({}_{2}\) at four depths, 2, 8, 16, and 50 cm (see details in text [29]). The black box houses the PC for recording the images from the automated minirhizotron (AMR). The cable runs from the computer to the AMR within the PVC tube at a 45\({}^{\circ}\) angle and a meter in length (70 cm depth). The reported soil respiration values were low at La Selva, suggesting that we still have a poor understanding of the processes that regulate soil carbon dynamics in this ecosystem (Figure 4).