Tab. 11.1: Names and characteristics of the established treatments regarding nitrogen (N) and carbon (C)
content of the residues, C/N ratio of the residues and application rate per microcosm.Table GGG: Names and characteristics of the established treatments regarding
nitrogen (N) and carbon (C) content of the residues, C/N ratio of the residues and application rate per cosm
N C C/N CN of applied
residues
application
rate residues name % %
application rate g (microcosm)-1
control control - 0
low low CN 4.9 39.8 8.2 3.6
medium low low mass 3.0 39.7 13.2 1.2
medium medium medium mass & CN 3.0 39.7 13.2 3.6
medium high high mass 3.0 39.7 13.2 10.7
11.2 Material and Methods
N2O and CO2samples were taken from soil columns over a period of 24 days. The soil columns were part of a microcosm system which allowed continuous flushing of the system with constant gas flow rates. Temperature was kept at a steady 20°C with the help of a climate control unit. Because it is known that both rewetting or dried soil as well as aggregate disruption (which could have happened during sieving) can lead to short time N2O peaks (Letey et al., 1980; Gregorich et al., 1989; Ruser et al., 2006; Bergstermann et al., 2011), a preconditioning phase preceded the measurement.
After 3 weeks, constant flux rates were measured and the following treatments were realized (see Tab. 11.1): (i) a control treatment without addition of plant residues;
(ii) three treatments with input of ground cauliflower residues with low, medium and high C/N ratios and (iii) three treatments with low, medium and high addition of residue input with medium C/N ratio.
The treatment with the medium addition of residues was identical with the medium C/N ratio treatment, re- sulting in 6 different treatments. 4 replicates were prepared for each of the treatments.
Soil was taken from the upper horizon (0 - 25 cm) of the field described in Chapter 4 - 10, air dried and sieved through a 5 mm sieve. Plexiglas cylinders (height 30 cm, inner diameter 14.4 cm) were packed up to a height of 25 cm with soil compacted to a bulk density of 1.3 g cm−2 (corresponding to 5 kg of dry soil for each column). The microcosms were sealed airtight by a removable top lid and a bottom lid with a Cellulose Acetat membrane filter (pore size 0.2µm, Whatman, UK). An opening with an airtight screw-on cap was placed at the top of each cylinder to permit irrigation. The microcosm system was continuously flushed with atmospheric air. Several samples of ambient air were taken from the system inlet and from the inlet of each treatment during each measurement. The concentrations of these samples were considered for the calculation of flux rates. For each treatment, the gas flux could be separately regulated with a needle valve to keep the gas concentration above the soil column in an optimal range concerning the GC sensitivity. Exhaust air left the system through an outflow tube connected to a wash bottle which avoided entrance of ambient air into the system from this side. At each sampling, flow rates were measured at the exhaust using a high precision digital flow meter (Thermo Scientific, Langenselbold, Germany).
For the gas sampling, 22.4 ml vials with crimp-cap septa (Häberle, Lonsee-Ettlenschieß, Germany) were con- nected to the exhaust air of the microcosm system and to the washing bottles by means of tubes, needle valves and capillary tubes. This enabled the flushing of vials with exhaust air. N2O and CO2 in the gas samples was then analyzed using a gas chromatograph equipped with a63Ni electron capture detector (ECD) (5890 series II, Hewlett Packard) and autosampler (HS 40, Perkin Elmer).
Fresh cauliflower leaves were dried, ground and C/N ratios were determined using an elemental analyzer (vario MAX CN, Elementar Analysensysteme, Hanau). The amount "medium input" corresponds to a fresh weight of 6.67 Mg fresh weight (75 kg N ha−1 and 800 kg C ha−1 dry matter) which was calculated to be the mean input of cauliflower on the vegetable field from which the soil was taken. The mean C/N ratio of the cauliflower residues used for the study was 13.2. Due to different fertilization levels in this field study, cauliflower residues with lower (8.2) and higher (17.1) C/N ratios were also available. Amounts, C/N ratios and abbreviations of the treatments are shown in Table 11.1. Residues were carefully mixed into the upper 5 cm of the soil columns. The upper layers of the control soil columns were treated in the same manner but without addition of residues.
Water-filled pore space (WFPS) was adapted to a value of 75 % with 10−2M CaCl
2solution at the beginning of the study and kept at this level by periodically weighing and readjusting the WFPS value gravimetrically. Therefore, the lid was opened and a precise irrigation was carried out with the help of a jet-nozzle which fitted into the opening at the top of the cylinders. After 14 days, WFPS was increased to 85 % and after another 2 days to 90 %. The study was completed after a total of 24 days. Soil moisture was analyzed gravimetrically after drying the soil at 105°C for 24 h. Samples from all microcosms were taken at the beginning, when increasing the WFPS and at the end of the study. The water-filled pore space (WFPS) was calculated as described by Ruser et al. (1998).
For the quantification of mineral N contents, 20 g of fresh soil were extracted with 40 ml of a 0.5 M K2SO4 solution. At the beginning of the study, only concentrations of soil nitrate were measured using a quick- check (RQ easy Nitrate test, Merck, Darmstadt). At the end of the study, the microcosms were opened and samples were taken of the upper layer (0 - 5 ) and the lower layer (5 - 25 ). Concentrations of nitrate (NO3−)
11 Incubation study: C-to-N-ratio and amount of residue addition
and ammonium (NH4+) in the extracts were determined using a flow injection analyzer (3 QuAAtro, SEAL Analytical, UK). Furthermore, concentrations of total dissolved organic carbon (DOC) were measured according to method introduced by Ruser et al. (2008) with a fully automated "multi N/C 2100S" analyzer (Analytic Jena, Jena, Germany).