Effects of Lysimeter Tension and Sampling Interval on Water Volume Collection and Soil Solution Chemistry.

Lysimeters are used to sample soil solution as an indicator of plant nutrient availability, nutrient leaching, and physical characteristics related to source and age through stable isotope analysis.  Lysimeters are constructed of a porous component and a sample collector. At Coweeta we use falling tension lysimeters constructed of a porous clay cup attached to a PVC pipe.  Tension is put on the PVC pipe and soil water moves through the clay cup and is stored in the PVC pipe until collection. The amount of water collected is controlled by the total lysimeter volume, the difference between lysimeter and soil water tension, and the duration of sample collection, these factors affect not only sample volume but also chemistry and isotopic composition.  We installed an array of falling tension lysimeters representing a full factorial of lysimeter length (40, 70 and 100 cm), tension (-0.03, -0.06, -0.09 MPa), and sample duration (1, 7, 14 days) installed at two soil depths (20 and 60 cm); soil water content within the sample array was monitored continuously.  We sampled soil solution monthly for one year to determine the effects of lysimeter size, tension, and collection duration on volume, NO₃, and isotopic composition (¹⁸O and ²H).  Longer lysimeters, greater tension, and increased sample duration resulted in greater soil solution volume collected; 7 and 14 day sample duration did not differ.  NO₃ concentrations were greater in solutions collected after 1 day at -0.03 MPa in lysimeters at 60 cm soil depth.  Lysimeter length, tension and sample duration did not affect nitrate concentrations in surface soil lysimeters.  Water isotope results suggest that the two lysimeter depths sampled distinct pools of water whose physical characteristics evolve through time and reflect new inputs of precipitation and mixing of existing soil water.