industrialization and urbanization. Moreover, the soil environment is interfacial and heterogeneous, and
the basic characteristics of soil contamination are different from those of air and water, such as
concealment and hysteresis, accumulation, and irreversibility [5, 6]. Thus, the effects of heavy metals
contamination on soil are quite alarming and can cause huge disturbances in the ecological balance and
health of living creatures on earth.
Nitrogen has long been recognized as one of the most limiting nutrients for ecosystem productivity [7].
Plant species, both aboveground and belowground, are in intense competition for available N in
agricultural ecosystems [8], and soil potential nitrification rate (PNR) represents great ecological indicator
of ecosystem productivity [9]. Researches has also show that nitrification was inhibited significantly in
soil amended with sludge containing high level of heavy metals, such as Cd2+ and Pb2+ [10], and the
effects of copper ions (Cu2+) on PNR in 17 kinds of typical soils in China was found to be decreased in
the form of ladder with the increase of Cu2+ [11].
Soil microbes are important reflection for soil quality and enzyme activity involved in the
biogeochemical cycling of carbon, nitrogen, phosphorus, sulphur and other nutrients [12].The major
parameters of soil quality are the soil biological properties. Among these special emphasis is made of the
enzyme activities. Soil enzyme activities have been generally accepted as one of the diagnostic indices of
soil fertility quality and can quantify changes as a result of human disturbance [13]. Application of
enzyme activity as a diagnostic index promotes high sensitivity to external effects, simplicity of definition
and low errors [14]. The negative effects of heavy metals on soil enzyme activity have been recognized,
but the mode of action of heavy metals varies from one enzyme to another, and the extent to which
enzymatic activity is inhibited depends both on the concentration and oxidation state of the heavy metal
and on the soil characteristics, such as pH, texture, organic matter content [15].
The combined effects of Pb2+ and Cd2+ on the soil urease activity and nitrification with different
concentrations in the incubation proceeded periods were studied by indoor incubation method in this
research, which aims to evaluate whether there is a synergistic interaction on soil enzyme activities,
nutrient cycling and pollutants by statistic analysis.