Dataset: Effects of flooding and nutrients on Spartina patens and Sagittaria lancifolia soil shear strength: a greenhouse study

This dataset is from a greenhouse study on the effects of flooding and nutrient-enrichment on two marsh plant species common to Louisiana. The greenhouse study tested the effects of flooding (5, 45, and 90% time flooded) and nutrients (control and 2 mgN/L + 0.1 mgP/L) on growth and strength of Sporobolus pumilus and Sagittaria lancifolia. Sporobolus at higher elevation had higher aboveground biomass and higher shear strength than at low elevation. Measurements include above-and belowground biomass and soil shear stength.

Methods
To test the hypothesis that Sporobolus and Sagittaria differ in their response to flooding and nutrient-enrichment, we conducted a controlled greenhouse experiment. Plant material was collected from a Sporobolus dominated marsh in Sabine National Wildlife Refuge, Calcasieu, Louisiana in the Chenier Plain region of Louisiana where large robust stands of Sporobolus occur.  Sagittaria plants were collected from an intermediate marsh in Barataria Bay. At each site 48 PVC plant plugs of each species (15.5 cm diameter and 30 cm deep) were collected and transported to the Louisiana State University greenhouses. The plugs consisted of 27 clay Sporobolus,19 organic Sporobolus and 48 Sagittaria. Sporobolus samples were separated later on at greenhouse by either clay or organic. Core locations were chosen haphazardly to get a random sample of the area. Aboveground biomass in each plug were clipped to the soil surface prior to collection.  

Ninety-six plants were placed in one of eight tanks each outfitted with a tidal control system (Aquabiotech Inc.)(Figure 13). Each tank held 519 liters and was attached to a water reserve carboy that holds 1540 liters of water. Water levels were controlled by pumps that raised and lowered the water in the tanks over a period of 12 hours (i.e., diurnal tide). Water was pumped at a rate of 760 mL/minute for each tide. A split-plot design was used for two levels of nutrients treatment and three elevations.

Three elevation treatments (low, medium, and high) were established for each species within each tank (Figure 12). Six Sporobolus and six Sagittaria plugs were placed in each tank with two of each species at each elevation treatment. The surface of low elevation plugs were 4.6 cm above low water and 33 cm below high water. Middle elevation plugs were 19.6 cm above low water and 18 cm below high water. High elevation plugs were 34.6 cm above low water and 3 cm below high water. The percentage time that the surface was flooded was approximately 90, 45, and 5% for low, middle, and high elevations, respectively. The location of elevation and species treatments were randomized within each tank. These differences in elevation will create different percent time flooded for each group with the low elevation group having the highest percent time flooded and the high elevation group having the lowest percent time flooded. 

The nutrient treatment (Ambient or Nutrient) was a whole plot factor applied to each tank.  Four tanks were filled with tap water (Ambient) and four tanks had a single application of fertilizer treatment of 2mg/L of N and 0.1mg/L of P (Nutrient). The volume of water in each tank was 519 L. To achieve this ratio, 6.49 g of fertilizer with a ratio of 16-0-0 were added and 0.1153 g of fertilizer with a ratio of 0-45-0 were added to each tank.

 Average canopy height and stem density by species were collected for each of the 96 plugs 3 times over the 293-day study period. At the end of the study, shear vane measurements were taken using a Humboldt H-4227 Vane. This process involves the insertion of a cross-shaped vane into the material, followed by the measurement of the necessary torque to shear the material by rotating the rod. Three shear vane measurements were collected in each plug at depths of 0, 15 and 30 cm. 

Above- and belowground biomass was harvested at the end of the study. Aboveground biomass, was clipped to soil level, separated into live and dead stems by species, and dried at 60°C to a constant weight in dry pre-weighed paper bags. For belowground biomass, soil plugs were extracted from the PVC and cut into three depth segments of 0-10, 10-20, and 20-30 cm. These segments were bagged and refrigerated until processed. A 4.5 x 4.0 cm core tube was used to collect a soil sample from each depth segment for determination of ash weight (loss on ignition; LOI). The core was taken approximately 5 cm from the edge of each depth segment. The soil samples were dried to a constant weight and placed in a muffler furnace at 550°C for 8 hours. LOI was calculated by  . Belowground biomass was measured for each depth segment by rinsing the soil through a sieve (0.71mm) and separating the live and dead roots. The biomass was then dried at 60°C and weighed.