Soil organic matter dynamics in the tabonuco forest, a plantation and a secondary forest in Guzman

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Data set ID: 



In this project we try to find out the relationship between the primary production and the soil organic carbon fractions.

Date Range: 
1996-07-01 00:00:00 to 1997-06-30 00:00:00

Publication Date: 

2011-06-07 00:00:00



Additional Project roles: 

Name: Eda Melendez-Colom Role: Data Manager


DATA SET METHODS: This study was conducted in the Luquillo Experimental Forest (LEF) in the subtropical life zone of northeastern Puerto Rico. The forest is dominated by tabonuco (Dacryodes excelsa). The site receives 3500 mm of rainfall each year with a monthly mean temperature of 22.1 °C (Brown et al. 1983). Soils of the area are a complex of upland Ultisols and Oxisols (Soil survey 1995). The general topography is mountainous with deeply dissected drainage at elevations between 340 m and 460 m above sea level.

The experimental treatments were initiated in November of 1989 two months shortly after hurricane Hugo passed over the LEF (Zimmerman et al. 1995). The hurricane defoliated the forest and generated plant debris that was equal to the total amount of annual litterfall in the forest (Lodge et al. 1991). The experimental design had four blocks, and each block had three treatments: one time removal of hurricane-generated debris, fertilization, and control.

Fertilization was conducted by a quarterly addition of nutrients to the plots at an annual rate of 300 kg-N/ha, 100 kg-P/ha, 100 kg-K/ha, 8 kg-B/ha, 15.4 kg-Cu/ha, 2.2 kg-Fe/ha, 25 kg-Mn/ha, 26 kg-Zn/ha, and 19 kg-Mg/ha. The plot for each treatment was 20 m 20 m in size with an interior 10 m x 10 m area for measurement. All blocks were located within the tabonuco forest zone, although there were differences both within and among blocks in tree species composition, tree density and stem basal area (Parrotta and Lodge, 1991).

Five baskets ( 0.50 m2 in surface area, 0.65 m above ground) were placed in each plot of the three treatments (debris removal, fertilization and control) in each block. The litterfall was collected every two weeks from July 14, 1996 to June 30, 1997. The collected materials were separated into leaves, wood and miscellaneous materials and then oven dried at 50 0C for one week before they were weighed. Forest floor litter was collected every two months from July 1996 to May 31, 1997 from a random area of 0.5 m x 0.5 m in each plot, divided into leaf, wood and miscellaneous materials and weighed after being oven-dried for one week (50 0C).

Litter samples were ground with a Wiley mill to pass through a 0.85 mm mesh sieve. Soil organic carbon was determined with the Walkley-Black method (Nelson and Sonmers 1982). Nitrogen was analyzed with a Kjedahl digestion method (Bremner and Mulvaney 1982). Extractions for soil exchangeable K, P, Mn and Fe was done using a modified Olsen s method (Hunter, 1982). Calcium, Mg, Na and Al were extracted with 1 N KCl. All extractions were analyzed by a Beckman plasma emission spectrometer.

Soil respiration was measured using NaOH in field chambers (Carter, 1993). Six cylinder chambers (102.5 cm2 in opening area and 20 cm in height) were placed in each plot. Three of them included ground plant litter and three had no litter. A plastic bottle with an opening area of 10.75 cm2 containing 15 ml of 1 M NaOH was placed inside each chamber for 24 hr and then brought to the laboratory for titration using 1 M HCl to determine the amount of CO2 which was absorbed in the NaOH solution during the incubation. Microbial biomass was measured using a fumigation-incubation procedure (Jenkinson and Powlson 1975) in August 1996 and March 1997, representing a wet and dry season, respectively. A soil sample from seven cores (3.75 cm in diameter) from each plot was collected to a depth of 10 cm and weighed for soil bulk density determination. Approximately 15 to 20 g was oven-dried at 1050 C for three days for determining soil water content. Two sets of soil samples were prepared: one as a control and the other for fumigation treatment. Each sample contained 30 g of soil that was placed into a 100 ml beaker. For the fumigation treatment, beakers were placed into a clean glass vacuum desiccator that was lined with moistened filter paper. A beaker containing 40 ml of alcohol-free chloroform was placed into the desiccator which was then evacuated for 2-3 min. During the evacuation period, the chloroform was boiled vigorously until the atmosphere in the desiccator was near saturation. Samples were left in the desiccator for 18-24 hr. The fumigated samples were vaccumed for 3 times to extract the remaining chloroform in soil. Each sample was inoculated with 1.0 g well mixed inoculum and then placed into wide month glass jars of 2-L volume. A beaker containing 20 ml 1 M NaOH was placed into each jar. The jars were closed using lids with rubber septa and incubated for 10 days at 25 0C. The quantity of NaOH remaining in each beaker was titrated with 1 M HCL with phenolphthalein as an indicator. The same soil sample was incubated with another 20 ml 1 M NaOH for another 10 days and the remaining NaOH was titrated with HCl at the end of the incubation.

Nitrogen and carbon in the light fraction was determined using a density isolation method (Sollins et al., 1984). Air- dried soil was passed through a 2 mm mesh sieve and then 1 g of the soil was suspended in 20 ml of soidium iodide (NaI) solution adjusted gravimetrically to 1.85 g/ml density. The suspension was sonicated for 15 min, vacuumed (70 kPa) for 10 min, and then left to for 24 hrs at room temperature to separate light and heavy fractions. The light fraction at the surface of the high- density liquid was aspirated and trapped onto a filter paper (GF/A), rinsed with deionized water, and then analyzed for C and N with a Perkin-Elmer CHN analyzer.

Soil respiration rate was calculated using the following formula (Carter 1993): Milligrams of C or CO2 = (B - V) NE, where B = volume (milliliters) of acid needed to titrate the NaOH in the beakers from the control sample, V = volume (milliliters) of acid needed to titrate the NaOH in the beakers from the CO2 enriched samples, N = normality of the acid, and E = equivalent weight. To express the data in terms of carbon, E = 6; to express it as CO2, E = 22.
Microbial biomass was calculated using the formula: B = Vf -Vc; G = B/100*S*0.0004955 g C/cm3; M = G*H*D/W where B = CO2 flush (as % by volume), Vf = CO2 flush of 10 day fumigated soil, Vc = CO2 flush of 10 day control soil, G = Grams of CO2-C flush, S = jar volume, M = soil biomass, H = soil bulk density, D = soil depth, and W = dry weight of soils. Statistical analysis of the data was conducted using Statistix software. I used the SAS to calculate the decay constant k (SAS 1985). Differences among the treatments were tested using Sheffe s multiple range test. The significance level was set at alpha= 0.05.



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