Location of Sampling Stations: HQC
Frequency of Sampling: Three times a year for microbial biomass and inorganic-N: pre-burn (April), post-burn (June), and fall (October).
Variable Measured: Microbial biomass C and N; Inorganic-N; Soil water content.
Methods: Determination of soil microbial biomass carbon and nitrogen:
Microbial biomass C and N are determined by the fumigation-incubation method (Jenkinson and Powlson, 1976). Soil (25 g) is added to two 125 mL Erlenmeyer flasks. When the gravimetric soil water content is less than 0.26 kg kg-1, enough water is added to bring the soil water content to this level. Both samples are pre-incubated at 25oC for five days. At the end of the preincubation period, one of the samples is fumigated with chloroform. Samples are placed in a vacuum desiccator that has a wet paper towel in the bottom and a beaker with approximately 50 mL of ethanol-free chloroform and nonvolatile granules for distillation. Vacuum is applied three times for approximately 30 seconds to allow the chloroform to boil. Immediately after the last application of vacuum, the desiccator is tightly closed for 20-24 hours to allow for diffusion of the chloroform into the soil. After 20-24 hours, the beaker with chloroform and the wet paper towel are removed, and the desiccator is evacuated eight times for three minutes each time. Fumigated and unfumigated samples are placed into 940 mL mason jars that have water at the bottom to maintain a highly humidified environment. Jars are tightly closed, and the samples are incubated for 10 days at 25oC. At the end of the incubation period, the CO2-C concentration in the headspace of the mason jars is measured using a Shimadzu GC-8A gas chromatograph (Shimadzu Scientific Instruments Inc., Columbia, MD) equipped with a 2 m Porapak Q column and operated at 70oC with an He carrier gas flow rate of 14 mL minute-1. After measuring CO2-C, 100 mL of 1M KCL are added to the erlenmeyer flasks and the flasks are shaken for one hour in an orbital shaker at 300 rmp. The suspension is transferred to 250mL centrifuge bottles and centrifuged at 16,000g for ten minutes. After centrifugation, the supernatant is filtered through a nylon mesh (10cm) and analyzed for NH4-N and NO3-N. Nitrate-N + nitrite-N are determined by the Griess-Ilosvay technique (Keeney and Nelson, 1982), and ammonium-N by the salicylate-hypochlorite method (Crooke and Simpson, 1971), both implemented on an Alphem Autoanalyzer (Alpkem Corp., Clackamas, OR).
We express microbial biomass C and N as carbon (Cf) and nitrogen (Nf) flush, the difference in CO2-C evolved and N mineralized between fumigated and unfumigated samples, to avoid the confusion of using different conversion factors (kc and kn). When comparing to other data, we calculate microbial biomass C (MBC) and N (MBN) as suggested by Voroney and Paul (1984): MBC=Cf/0.41; and MBN=Nf/kn; where: kn = -0.014 (Cf/Nf) + 0.39, Cf = CO2-C evolved from the fumigated treatment; units=mg C kg-1 dry soil, Nf=NH4-N + NO3-N mineralized in the fumigated treatment; units = mg N kg dry soil. Determination of soil inorganic N: + Inorganic-N (NH4-N + NO3-N) is determined at the same sampling dates as microbial biomass C and N. Soil (20 g) is extracted with 100 mL 1M KCl by shaking for one hour in an orbital shaker at 300 rpm. The suspension is transferred to 250ML centrifuge bottles and centrifuged at 16,000 g for 10 minutes. After centrifugation, the supernatant is filtered through a nylon mesh (10um) and analyzed for NH4-N and NO3-N. Nitrate-N + nitrite-N are determined by the Griess-Ilosvay technique (Keeney and Nelson, 1982), and ammonium-N by the salicylate-hypochlorite method (Crooke and Simpson, 1971), both implemented on an Alpkem Autoanalyzer (Alpkem Corp., Clackamas, OR). Inorganic N (NH4-N + NO3-N) is expressed as mg N kg dry soil.
Determination of soil water content: Soil water content is determined gravimetrically. Soil samples, approximately 10g, are weighed into preweighed moisture tins and dried for at least 24 hours at 105oC. The samples are then weighed to determine the weight lost. Water content is expressed as g g-1 dry soil.
For additional metadata information see: http://lter.konza.ksu.edu/sites/default/files/DC.pdf
For additional methods information see: http://lter.konza.ksu.edu/sites/default/files/MM.pdf
