ESC 511 Lab 5 DECOMPOSITION
Brian Strahm and Jeff Hatten
11/6/2002
Soil respiration, or CO
2
evolution, is often used to determine rates of decomposition. Two different methods are commonly used in assessing CO
2
evolution.
The static soda lime trap is a laboratory method, involving mass gain of given weight of soda lime as a result of CO
2
absorption, which has been respired from a designated mass of soil. The other method commonly used is a sodium hydroxide (NaOH) trap, which is used in situ to determine a respiration rate for a given area of a particular site.
These respiration rates can be directly related to decomposition, and thus, mass loss of a given substrate. When examined over time, a measure such as mass loss creates a negative exponential curve with an exponential decay rate of k with units yr -1 . We will attempt to compare the two methods of assessing soil respiration and discuss them in terms of microbial community size and rates of decomposition.
The purpose of this lab is to give us the opportunity to determine CO
2
evolution of soils sampled from a deciduous forest, coniferous forest and garden plot by using a static soda lime method. This data will then be related to the respiration data obtained earlier using a sodium hydroxide (NaOH) trap method. In addition to comparing the two methods of measuring respiration with one another, the data will be discussed in the context of the sized of the respective microbial populations. Finally, we will work through a problem set regarding mass loss of conifer needles observed over a five year period.
2
Field samples from the upper 10 centimeters were sieved to separate the <2mm soil fraction. A 150g sample of this field moist, sieved sample was placed in a glass jar with airtight lid. An additional 150g sample was weighed and oven dried at 100

C overnight to calculate moisture content. A 25mL aliquot of distilled water was added to the soil sample in the glass jar. After oven drying at 100

C, 10 grams of soda lime was added to a 150mL beaker. The 150mL beaker with soda lime was then placed atop the soil in the glass jar and the lid sealed. After repeating for each of the three soil samples,

a fourth jar was prepared to serve as a control. The control had neither soil, nor distilled water, to ensure the physical stability of the beaker containing the soda lime. After 48 hours, the soda lime was again oven dried at 100

C overnight and the weight was recorded. The difference between this oven dried weight and the original weight of the soda lime sample represents mass gained as a result of CO
2
absorption.
Calculation of CO
2
evolution as a result of these measurements was determined by subtracting the mass gain of the control from the mass gain of the sample. This value was then divided by the weight of the oven dried soil sample and multiplied by the universally accepted correction factor of 1.48.
2
Table 1: Summary of Respiration Methods and Plate Count
Method
Soda Lime Trap
NaOH CO
2
Trap
(g of CO
Unit
2
(g of CO
2
/g soil/day)
/m
2
/day)
Garden
3.79
2.89
Conifer Deciduous
2.44
8.14
5.15
2.83
Plate Count
(Billions of Fungi,
Yeast and Bacteria/g dry soil)
31.73
15.69
17.03
Figure 1: CO
2
Evolution v Plate Counts
9
8
7
6
5
4
3
2
1
0
15 y = -0.0731x + 6.3631
R
2
= 0.0477
y = -0.0805x + 5.3524
R
2
= 0.2949
Soda Lime Trap (g CO2/g dry soil/day)
NaOH Trap (g CO2/m2/day)
Linear (Soda Lime Trap (g CO2/g dry soil/day))
Linear (NaOH Trap (g CO2/m2/day))
17 19 21 23 25 27 29
Plate Counts (Billions of Fungi, Yeast, and Bacteria/g dry soil)
31
2
33

Figure 1 shows the relationship between the two methods used to determine respiration and the plate counts obtained from the three soils. Both methods have a negative relationship with the plate counts as determined by a linear regression model.
However the strength of the relationship is in question due to the low R 2 values and the lack of replicates. The strength of the relationship is strongest between the NaOH Trap method (R 2 =0.29) and the plate counts, than the soda lime trap and the plate counts
(R 2 =.05). This may be due to the age of the sample used in the soda lime trap method experiment. The sample was stored at room temperature for approximately 4weeks until it was used. In the NaOH trap the soil remained at age 0 since it was done in-situ. While the plate counts were done on a soil sample soon after collecting, closest to age 0. This difference in age may have an effect on the activity of some organisms that would have gone into a suspended state from the drying of the soil during storage.
The negative relationship is perplexing as well due to the fact that we would expect that the more organisms there are the higher the respiration will be. Roots may be the main heterotrophic organisms in the soils, in this case the results of the NaOH trap would be more or less reflecting the abundance of roots in an area and not the abundance of fungi, yeast, or bacteria. To shed further light on this conundrum it would help to know the relationship between microorganisms and roots. It seems to me however that that relationship would be positive and not negative as would be needed to explain the disparity we are seeing.
9.00
Figure 2: CO2 Evolution Soda Lime Trap v NaOH Trap Methods
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
2.50
3.00
3.50
y = -1.5819x + 10.522
R
2
= 0.4912
4.00
4.50
5.00
NaOH Trap Method (g CO2/m2/day)
5.50
2
2
The negative relationship between the two methods suggests that there may be a negative relationship between root respiration and microorganism respiration (Figure
2). This could come about because roots at the conifer site (highest NaOH trap method

respiration) are doing a majority of the respiration. There are more than likely better explanations for the difference such as a change in the amount of soil sampled (100g for the soda lime method and (area x bulk density x depth) of soil for the NaOH trap method).
Figure 3 exhibits the percent mass loss of conifer needles over time. As expected, the data represents a negative exponential curve for the function of mass loss, after being retrieved and weighed over the course of five years.
Figur e 3: M as s Re m aining Ove r Tim e
100
80
60
40
20
0
0 1 2 3
Tim e (ye ars )
4 5 a. The k-values decrease with time due to the increasing sampling time and the decreasing rate of decomposition through time. The best k-values to use would depend on what question you are asking. If asking a question about the rate early on in the trial then a k-value from year 1 or 2 may be appropriate. If asking a question about the overall decomposition rate then a k-value towards the later stages of decompostion may be appropriate.
Table 2: Decomposition problem
2
3
4
Time (yr)
0
1
5
Mass (g)
10
5
2.5
1.5
1
0.5 k
0.693
0.693
0.632
0.576
0.599

b. Time to 50% decay. Looking at the k-value column on Table 2 I would want to use a k-value near the time that there is 50% of the mass left, as compared to the mass at time
0. So:
Time to 50% decay = 0.693/(k-value at year 1 = 0.693) = 1.0year c. Time to 95% decay.
Time to 95% decay = 2.996/(k-value at year 5 = 0.599) = 5.0years d. Time to 99% decay.
Time to 99% decay = 4.605/0.599 = 7.7years