Variable flux closed-dynamic chamber for measuring soil ammonia emissions

Inspired by previous designs available in the literature, a variable flux closed-dynamic chamber was constructed to measure soil ammonia emissions from the soil after applying different N fertilizers on a field experiment. The chamber includes a polyvinyl chloride (PVC) ring of 237 mm i.d. and 100 mm long, which is inserted into the soil, then a polyamide lid close the ring on the top at the time of the measurement. The lid has an O-ring greased with Vaseline to improve sealing. The height of the lid over the soil inside the chamber is of 40 mm, the soil area covered by the chamber is of 441.2 cm2. The inner volume of the chamber is 1.765 L. A vacuum pump sucks the air through the chamber at a maximum flow rate of 12.5 L . min(-1). Before entering the chamber the air passes through a saturated solution of boric acid to remove the ammonia, and through silica gel, to remove the water vapour. Once pumped from the chamber, the air passes through a 150 ml of 1 M boric acid solution to recover the ammonia. As the air that enters in the chamber is ammonia- free, the ammonia recovered from the chamber is the one emitted by the soil. It is well known that ammonia emission from the soil increases due to wind velocity up to 0.135 m.s(-1). For this reason, the chamber is equipped with a flow meter and a hand operated flow regulator to adjust air flow through the chamber. In this way it is possible to obtain an air velocity inside the chamber similar to the wind velocity at the time of the measurements. At each measurement the air flows through the chamber during 3 min. The measurement starts by switching on the vacuum pump manually and finishes when a timer stops the pump automatically. At the lab, the samples are analysed for N content by colorimetry with a continuous flow auto analyser. The chamber has proven to be reliable in field conditions. Due to its volume, the system does not generate high wind speeds inside the chamber. This can be improved by reducing the height of the chamber above the ground. It has been determined that inadequate sealing between the PVC ring and the soil that can be fixed by increasing the insertion depth of the ring in the soil.