A field experiment was carried out on cultivated loessial soil in semiarid areas with 415 mm precipitation and spring wheat
( Triticum aestivum L.) used as an indicating crop to study the effects of the plastic mulching periods on the soil nitrogen availability.The experiment had three factors and they were water storage in soil profile before sowing,
plastic film mulching period and nitrogen fertilization.The water storage in soil profile before sowing had two levels under each of which there were no mulching,30 day mulching,60 day mulching and full growing period mulching after sowing.Under each mulching period there were two rates of nitrogen fertilization and the rates were 0 kg/hm 2 and 75 kg/hm 2.So the experiment had a complete design with 16 treatments,and all the treatments were replicated three times. The results of the experiments showed that plastic film mulching significantly increased water contents of surface soil (the average water content of 0～20 cm soil with plastic film mulching was 3 2% higher than that without mulching during the period of crop growth and the difference between the two reached a significant level),but it made the water contents of subsoil soil decrease,so the water storage of 0～200 cm soil with plastic film mulching was basically equal to that without mulching. The plastic film mulching could increase soil temperature,and the effect of plastic film mulching on the soil temperature at the depth of 5 cm was related with crop growth.Because there was low crop coverage during the early stage of crop growth (the 1st～30th day after sowing),plastic film mulching could significantly increase the soil temperature;The crop coverage increased during the middle stage of crop growth (the 30th～90th day after sowing) and thus temperature increasing effect of plastic film mulching became weak;During the latter stage of crop growth (the 90th～120th day after sowing),crop leaves drooped and crop coverage declined,and thus the temperature increasing effect of plastic film mulching became strong.So the temperature increasing effect of plastic film mulching showed a U shape pattern with crop growth. Plastic film mulching inevitably resulted in accelerating the decomposition of soil organic matter.The value of soil respiration (the amount of CO 2 releasing from soil) directly measured in situ can reflect the rate of soil organic matter decomposition.According to the experiment results,plastic film mulching could significantly increase the amount of CO 2 releasing from soil,the average released CO 2 amount with plastic film mulching was 251 5 μg/cm 3 higher than that without mulching and this difference reached a significant level of p <0 01.The plastic film mulching with nitrogen fertilization could intensify the decomposition of soil organic matter due to the priming effect of nitrogen fertilizers. Nitrogen fertilization and plastic film mulching significantly affected the
accumulation of nitrate nitrogen in soil profile ( p <0 01).No matter in which treatment,the amount of accumulated nitrate nitrogen declined as wheat growth advanced.The difference of accumulated nitrate nitrogen between nitrogen fertilization and no nitrogen fertilization correlated with the stages of wheat growth and the film mulching period.On the 30th day and 60th day after sowing,the amount of accumulated nitrate nitrogen with the plastic film mulching was significantly higher than that without mulching,but the amount of accumulated nitrate in the plots for 30 day mulching was lower than that mulched for all the growing period on the 60th day after sowing; On the 90th day after sowing and at the time of wheat harvesting,the amounts of accumulated nitrate nitrogen in the plots for 30 day mulching and 60 day mulching were not only lowest,but also did not differ much earth other.After wheat harvesting,the amounts of accumulated nitrate nitrogen with 30 day mulching and 60 day mulching without nitrogen fert