During the past 50 years, the amount of agricultural
fertilizer used in Northern China increased from
about 7.5 kg ha-1
in the 1950s to approximately
348 kg ha-1
in the 1990s. Given that little is known about
the effects of nitrogen fertilization on soil labile carbon
fraction in Northern China, this paper evaluated such
effects in terms of microbial biomass and dissolved organic
carbon in the Sanjiang Plain located in Northeast China.
Soils with different cultivation time and undisturbed marsh
with Deyeuxia angustifolia
were selected to study the
effects of nitrogen fertilization on the soil labile organic
fractions microbial C (biomass C, microbial quotient, and
basal respiration) and to estimate the contributions of
nitrogen input on the dynamics of soil labile carbon.
Continuous nitrogen application decreased total organic
and dissolved organic carbon concentrations significantly,
leading to the lack of carbon source for microbes. Therefore,
continuous nitrogen fertilizer application induced
negative effects on measured soil microbiological properties.
However, a moderate nitrogen application rate
(60 kg N ha-1
) stimulated soil microbial activity in the
short term (about 2 months), whereas a high nitrogen
application rate (150 kg N ha-1
) inhibited measured soil
microbiological properties in the same period.