NTF01 Volume and chemistry of throughfall in tallgrass prairie


Amounts and nitrogen content of water passing through the canopy of tallgrass prairie are compared to similar measurements of bulk precipitation. Measurements include nitrate, ammonia, phosphate and organic nitrogen and phosphorus content of bulk precipitation and throughfall. Variables of interest include vegetation type and amounts, time of year, and time since burning.

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To measure the volume and nutrient content of water (throughfall) actually reaching the surface of the prairie soil, in order to calculate net fluxes of nutrients through the prairie vegetation and net nutrient input to the prairie soil.


Location of Sampling Stations: When this study was established in 1982, bulk precipitation and throughfall samples were collected on four watersheds 001D, 001C, N01B, and N04D). All sampling sites were lowland positions (on Tully soils) just above stream banks. Beginning in 1983, throughfall collectors were placed on only two of these watershed (001C and N04D). In 1983 bulk precipitation was limited to three watershed (001C, N01B and N04D). In 1984, bulk precipitation was collected from 001C and N01B through June, with N04D being added in mid-June. An additional site for collection of bulk precipitation at headquarters (HQ) was added in April 1984. Bulk precipitation and throughfall collectors were moved from N04D to 020B in February of 1992 following the bison introduction. Throughfall collection was discontinued after 1995, but bulk precipitation continues to be collected at four sites (001C, N01B, 020B, and HQ).

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 rain gauges 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. Throughfall collectors were removed prior to scheduled fires, and not replaced until the grass was high enough to influence throughfall volumes and chemistry.

Variable Measured: Total sample volume of throughfall or precipitation collected is recorded for each sample, and concentration of NO3 are measured in each sample collected. Since January 1986, NH4, organic-N and -P and ortho-phosphate are analyzed in composite samples created by volume-weighting individual samples into a single monthly composite sample for each collector. Prior to 1986, these variables were measured in individual samples. Throughfall measurements were discontinued after the growing season of 1995, but measurements of bulk precipitation chemistry are ongoing.

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 were not used after 1983.

Since 1984, phenylmercuric acetate (PMA) was 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.

Troughs for throughfall collections are located on gradual slopes with the collecting jugs downhill from the troughs. Prior to 1984, each site had four stainless steel and four PVC troughs. From 1984-1995, each site being used of throughfall collections was instrumented with six troughs, 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. The use of naphthalene crystals was discontinued in 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.

Each site used for bulk precipitation collection has a single bulk precipitation collector (plastic funnels glued into the caps of polyethylene collection jars). Bulk precipitation collectors are used to collect samples for chemical analysis, but not for precipitation volume measurement. Instead, volumes for calculating precipitation inputs are determined using rain gauges located at each sampling site. In addition, a sub-sample of precipitation from the NADP wetfall collector at headquarters (see dataset ANA01) has been routinely analyzed 'in-house' since early spring of 1983. These samples allow us to compare our numbers (and analytical procedures with those provided 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:

In 1982, throughfall was measured on four watersheds (001D, 001C, N01D, and N04D). Each site had one rain gauge, 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 collectors were placed on only two watersheds (001C and N04D), and bulk precipitation was limited to three watersheds (001C, N01B and N04D). In 1984, bulk precipitation was collected from only 001C and N01B through June, with N04D being added in mid-June. An additional site for collection of bulk precipitation at headquarters was added in April 1985. Bulk precipitation and throughfall collectors were moved from N04D to 020B on February 20, 1992, following the introduction of bison into Phase II. For throughfall collections after 1983, each site was instrumented with six stainless steel collectors (PVC collectors were discontinued due to rodent damage). Throughfall measurements were discontinued following the 1995 growing season, although bulk precipitation samples continue to be collected at four sites (001C, N01B, 020B and HQ).

Phenyl mercuric acetate (PMA) as a preservative was added to samples starting in 1983; its use was discontinued as of June 27, 1994. Samples collected in 1982-1983 were analyzed according to the methods outlined in Appendix A of the 1983 LTER Methods Manual. In 1984, methods of chemical analysis were modified. Samples collected in 1984 to present were analyzed according to the methods given in Appendix A of the 1984 LTER Methods Manual.

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