Ecosystem Significance of Soil as a Long Term Sink for Anthropogenic Additions of Nitrogen (LT63)
Objective: The overall objective of this study is to
assess the capabilities of soils to retain N in long-term pools that are
relatively unavailable for biological use.
Our two specific objectives are:
- To determine the mechanisms and potential magnitude of N retention in long-term storage pools in soils.
- To determine the ecosystem consequences of increased N retention in grassland soils.
This is a large project encompassing different study areas and laboratory work, however objective two is being addressed with a large field study being conducted at the Central Plains Experimental Range.
Methods for field experiment at CPER addressing objective two:
Site
Description
Study plots are located in the piedmont of north central Colorado
approximately 61km (38 miles) northeast of Fort Collins and 40km (25 miles)
south of Cheyenne, Wyoming (40 degrees 49" N latitude, 104 degrees 46" W longitude). This
site is on the Shortgrass Steppe Long Term Ecological Research site.
The topography of the area consists of gently rolling hills with broad
tops separated by wide ephemeral stream courses.
The average elevation is 1650m (5400 ft).
The experiment was conducted within a fenced enclosure on a level upland.
Previous to the fencing in 1969 the area had been lightly grazed by
cattle for at least 10 years.
Mean monthly temperatures range from -4 to 22 degrees C seasonally
and have a daily average max-min range of 17 degrees C. Annual precipitation at
the CPER averaged 322 mm over the past 51 years and ranged between 107 and 588
mm. Approximately 70% of the mean annual precipitation occurs during the April
to September growing season.
We established 6 carbon treatments on a preexisting water and nitrogen
addition experiment. The intent of this historical experiment was to assess the
influence of excess water and N availability on the structure and function of a
native shortgrass steppe ecosystem (Lauenroth et al. 1978). These historical
treatments consisted of a factorial combination of nitrogen and water
applications replicated in two 1-ha blocks.
Application of these treatments began in 1971 and continued through 1974.
Plots were irrigated sufficient to maintain water potential at a depth of
10 cm between 0 and -0.08 MPa from 1 May to
1 September and resulted in water additions of 460 to 710 mm H2O yr-1
(Lauenroth et al. 1978). N fertilizer (100 - 150 kg NH4NO3
ha-1 yr-1) was added to maintain a difference between the
treatment and control plots of at least 50 kg ha-1 of soil mineral N.
Results from these experiments are discussed in Lauenroth et al. (1978),
Milchunas and Lauenroth (1995), and Vinton and Burke (1998). Water and N
additions ceased in 1974 and with excepting occasional soil and vegetation
sampling have remained undisturbed by further manipulation until 1997. Soil
organic C and N for the top 10 cm are significantly higher in treated plots
relative to control plots (Vinton and Burke 1995)
We overlaid 6 C amendment treatments on both blocks of these historical
water and nitrogen manipulation plots in a slit plot design to assess the
influence of various C substrates on N availability and community composition in
historically N and water enriched ecosystems. We randomly located three transects within each 1 ha historic
treatment, each one of these transects was divided evenly into six 9 m2
plots, for a total of 144 subplots. We
randomly assigned one of the six new treatments, control, sugar, lignin,
sawdust, lignin and sugar, and sawdust and sugar, to each one of the six
subplots in each transect. The
lignin was obtained in purified form from Capital Resin Corporation (Columbus,
Ohio), the aspen sawdust was obtained from Delta Timber Company (Delta,
Colorado), and the sugar was purchased in bulk as table sugar from a variety of
sources. The lignin and sawdust were applied once a year in April at a rate of
777 g m2 and 1061 g m2 respectively.
The sugar was applied in ten even increments throughout the year, making
a yearly application rate of 833 g
m2. The new C treatments
began in 1998 and continue into today.
Data Collected
In Situ Nitrogen Mineralization
We estimated net N mineralization in the field using an uncovered core incubation method. At each core installation time, we took an initial core using aluminum tubes 5 cm inside diameter and 17.5 cm in length. This initial core was placed on ice in a cooler for transport back to the laboratory and subsequent processing and analysis for NH4 and NO3. After taking the initial core, another core close to the initial was extracted, 2 cm of soil at the bottom of this core was scraped out, ands a resin bag composed of mixed cation and anion exchange resin (Polymetric, Colorado Springs, Colorado), 5 cm in diameter and 2 cm thick, was inserted. This core was placed back into the hole it was extracted from, flush with the soil surface, and remained in the field for 28-30 days.
At the end of
the incubation period, the core was removed from the field, and placed on ice
for transport back to the laboratory. Processing
of the soil in both the initial and final cores was the same.
The soil was sieved through a 2mm screen to remove plant parts. 50 mL of
2 M KCL-PMA (phenylmercuric acetate to prevent microbial growth) was added to a
15 gram soil subsample and to the resin bag from the final core.
The soil and resin bag KCL- PMA mixtures were placed on an orbital shaker
for 30 minutes and then filtered through Whatman no. 40 paper. The extract from this filtration was analyzed for inorganic
NH4 and NO3 on a Perstop Autoanalyzer.
We calculated net N mineralization as the difference of the final plus
resin bag N concentrations and N
concentrations in the initial soil core, divided by the number of incubation
days. Net N mineralization was
converted to an area basis by using bulk density.
We installed and processed three
sets of mineralization cores during the growing season (May-Sept) of the years
1998, 1999, and 2000.
We began collecting the density and cover of the vegetation for each plot in 1997, sampling that continues into today.
Literature
Cited
Lauenroth, W.K., Dodd J.L., Sims, P.L. 1978. The Effects of Water- and Nitrogen-Induced Stresses on Plant Community Structure in a Semiarid Grassland. Oecologia 36: 211-222.
Milchunas, D.G. and Lauenroth, W.K. 1995. Inertia in plant community structure: state changes after cessation of nutrient-enrichment stress. Ecological Applications. 5:452-458.
Vinton, M.A. and Burke, I.C. 1995. Interactions between individual plant species and soil nutrient status in shortgrass steppe. Ecology 76: 1116-1133.
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| Two work study students collect nitrogen mineralization cores |
Research associate Petra Lowe finishes spreading |
