PURPOSE:
To measure:
1) the volume and nutrient content of water (throughfall) actually reaching the surface of the prairie soil,
2) the chemical composition of bulk precipitation inputs, in order to calculate net fluxes of nutrients through the prairie vegetation and net nutrient input to the prairie soil.
LOCATION OF SAMPLING STATIONS:
Sampling sites are located on Tully soils just above stream banks on watersheds 001C and N04D. Each site has one bulk precipitation collector and (at least since 1984) six throughfall collectors. Additional sites for collection of bulk precipitation only are on watershed N01B and at headquarters.
FREQUENCY OF SAMPLING:
Samples are collected as soon as possible after each rain event (or when there has been an accumulation of at least 4 mm in the on-sight raingauges after a number of small precipitation events) during the period May 1 to October 31. During the winter, collections are less frequent, depending upon the freeze-thaw patterns.
VARIABLES MEASURED:
Total volume of throughfall or precipitation collected and concentrations of N03- for individual samples. Since January 1986, NH4+, organic -N and -P, and ortho-phosphate are analyzed in composite samples created by volume-weighing the individual samples into a single monthly estimate for each collector.
METHODS:
Throughfall collectors are either 5 x 100 cm V-notch stainless steel troughs or 5.6 x 100 cm split PVC pipe (as of 1988, all troughs in use are stainless steel). All troughs have a drainage tube at one end which is fitted with Tygon tubing to transfer canopy leachates into a 4 L collecting jug. Prior to 1984, nylon mesh filters (0.05 mm mesh) prevented most particulates from entering the collecting jugs. However, the mesh filters tended to become clogged and have not been used since 1984.
Since 1984, phenylmercuric acetate (PMA) has been used to inhibit microbial activity in collecting jugs. A stock solution of 1 mg/g PMA was made and 0.5 mL is added to each bulk precipitation collector prior to placement in the field. One mL of PMA solution is added to each throughfall collector. Because of the toxicity of this preservative, all collections are returned to the laboratory for volume measurements. No PMA-treated samples are disposed of on the prairie.
Further details explaining field collection are located in Bushnell Rm 218 in the "Copy of Field Notes" file.
Troughs for throughfall collections are located on gradual slopes with the collecting jugs downhill from the troughs. Previously, each site had four stainless steel and four PVC troughs. Currently, the six troughs at each site are all stainless steel. Considerable physical damage to troughs and tubing resulted from rodent activities. These creatures even occasionally defecated and urinated in the troughs despite naphthalene crystals being placed at each trough. Naphthalene crystals have not been used since 1984.
Prior to 1984, the volume of throughfall obtained from each collector was measured in the field using a graduated cylinder, and then the samples were returned to the laboratory for chemical analyses. It was sometimes necessary to composite samples across watersheds in order to obtain enough material for chemical analysis. Since initiating use of PMA as a preservative in 1984, all collecting jugs are returned to lab for volume determinations. Laboratory procedures for NO3-, NH4+, PO4-, and organic -N and -P are described in Appendix A.
Volumes for calculating precipitation inputs are determined using raingauges located at each sampling site. Samples of bulk precipitation for nutrient analyses are obtained using bulk precipitation collectors (plastic funnels with tubing to drain into collection jugs). In additions, a sub-sample of precipitation inputs from the NADP wetfall collector at headquarters has been obtained routinely since early spring of 1983. These samples allow us to compare our numbers (and analytical procedures) with those obtained by the NADP laboratory, and also allow us to compare nitrogen inputs in wetfall versus those in bulk precipitation. The difference in concentrations of nitrogen in these two types of samples allows us to estimate nitrogen inputs from dryfall.
SUMMARY OF ALL CHANGES UP TO 1993:
In 1982, throughfall was measured on four watersheds (001D, 001C, N01D, and N04D). Each site had one raingauge, one bulk precipitation collector, and eight throughfall collectors (four collectors made of PVC and four collectors made of stainless steel). Fifty micron mesh screens were used to keep particulate material out of collection jugs. No preservatives were used in 1982. Beginning in 1983, throughfall measurements were restricted to two watersheds (001C and N04D). Each site used six stainless steel collectors (PVC collectors were discontinued due to rodent damage). PMA preservative was added to samples starting in 1983. In 1984 methods of chemical analysis were modified. Samples collected in 1982-1983 were analyzed according to the methods outlined in Appendix A of the 1983 LTER Methods Manual. Samples collected in 1984 to present were analyzed according to the methods given in Appendix A of the 1984 LTER Methods Manual.