Fjölrit RALA - 15.06.2004, Side 111

C and N mineralization rates of cultivated Icelandic Andosols
Rannveig Guicharnaud and Hólmgeir Björnsson
Agricultural Research Institute, Iceland
Organic matter tumover plays a significant role in the functioning of ecosystems as a source
of plant nutrients and as an energy source for decomposition. Organically complexed
aluminum, non-crystalline hydroxide, allophane and imogolite have been reported to have a
protective effect against biodegradation resulting in slower mineralization rates in andic soils
(Boudot, 1992). Most studies of mineralization rates of soils with andic properties have
primarily focused on soils formed under warmer chmates. The present study aims to explain
C and N minerahzation in andic soils formed in sub- arctic climate giving this experiment
soils a unique position in the Andosol taxonomy.
Soils with andic properties were sampled (0-20 cm depth) at three sites in Iceland, Korpa
(South West Iceland), Eyjafjörður (North Iceland) and Skagafjörður (North Iceland). At each
site three fields with different cultivation history were sampled, a) un-ploughed, b)
continuously ploughed for a short period of time, c) continuously ploughed for a longer
period of time. However, results can not be used for direct comparison of the effect of
cultivation history because of differences between the fields of the beginning the experiment.
C and N minerahzation was measured by incubation for 21 weeks at 15°C. For mineralized
C, CO2 was measured by titration and for mineralized N, NO3" and NH4+ was measured by the
Kjeldahl-method (Bremmer & Mulvaney, 1982). Incubation samples for mineralized C and N
were collected and measured regularly. For studying the minerizable N pool, the function N
= No*(l - exp(-kt)) was fitted to measured N mineralization pattems where N was the
accumulated mineralized N (mg/kgSOii) at each time t (days) and k was the mineralization rate
constant (days1).
Measured nitrate concentrations were highest initially during the incubation period due to
easily decomposable organic N fractions (Korpa, Eyjafjörður and Skagafjörður). Measured
NH4+ concentrations were under detection limits. C mineralization rates, relative to N
mineralization, were in general higher.
The percentage of readily mineralizable organic N in the pool of total organic N is small in
Andosols compared to that of non-andic soils. Saito (1990) compared the N mineralization
potential (No) to the total organic N (M ) between Andosols and non-andic soils from North
Eastem Japan which had experienced various soil management and cropping. When the
Icelandic results were compared to Saito’s results from Japan (table 1) Icelandic rates
constants k were greater than those obtained from Japanese andic-soils and comparable with
rates constants obtained from Pálmason et al., (1996) (table 1).
Andosols from Japan had a greater mineralization potential (No) than Icelandic soils.
Furthermore the percentage of mineralizable N (No/ M values) were higher in Japanese andic
soils. Andic soils from both countries had smaller percentage of mineralized N (No/ M
values) than non-andic soils. This supports the theory that organic N and C in Andosol is
resistant to microbial decomposition which is most likely due to A1 bound in insoluble
complexes, non-crystalline A1 hydroxides, allophanes and imogolite (Boudot, 1992). In
addition micro-structures are commonly observed in both surface and subsurface horizons of
Andosols and have been reported to be unfavourable for enzymatic reactions with organic N
compounds (Saito, 1990).
88