Soil ecological effects of plastic film mulching in semiarid agro-ecological system

Field experiments have been carried out on Loess Plateau in middle part with 415 mm average precipitation and in south part with 632 mm precipitation per year.In these experiments,spring wheat (Triticum aestivum) and winter wheat (Triticum aestivum) have been used as indicating crops to study the effects of plastic film mulching (for spring wheat included no mulching,mulching for 30 days and 60 days after sowing,and mulching for the whole growth period,and for winter wheat included no mulching,mulching for 75 days and for 150 days after sowing,and mulching for the whole growth period) and nitrogen fertilizer application (for spring wheat including no nitrogen application and nitrogen application 75 kg N/hm2,and for winter wheat including no nitrogen application and nitrogen application 225 kg/hm2) on soil moisture,temperature,nitrogen availability,the flux of CO2 and N2O in soil.The results showed that,in 2 years experiment on spring wheat,plastic film mulching almost didnt affect the soil water storage amount in 2 m soil layer,but could increase the soil water content in 0-20 cm layer significantly;and the effect of plastic film mulching on soil temperature in 5 cm layer changes as the "U" pattern,that is,the effect was significant at early and latter growth stage,and was insignificant at middle growth stage;plastic film mulching decreased the soil microbial biomass nitrogen,but increased the soil respiration and the mineralization of soil organic nitrogen,so it could significantly affect the accumulation of residual NO-3-N in 0-100 cm layers after harvest:the averages of the soil storage water amount before sowing of 2 levels of nitrogen application showed that the accumulation of NO-3-N respectively increased by -23.0,-10.1 and 49.7 kg/hm2 in treatments of mulching for 30 days,60 days and the whole growing period (126 d) than CK of no mulching in 1999;Under the same condition,in 2000,its respectively increased by 4.6,-5.0 and 8.2 kg/hm2,so it was obvious that plastic film mulching for the whole growing period increased the amount of residual NO-3-N in soil profile after harvest.The effect of different mulching course on the mineralization nitrogen in the soil profile after harvest were different,especially on the accumulation of NO-3-N,the main reasons of resulted in this phenomenon was related to the uptake of NO-3-N by crops and the mineralization of organic nitrogen in soil:to mulching for 30 days and 60 days,these treatments improved the growing of wheat and the uptake of wheat for NO-3-N,but due to the 2 treatments had no mulching between 30-90 days (125 d) and 60-90 days (125 d),the products of organic nitrogen mineralization was reduced,especial the amount of NO-3-N in soil profile are significantly reduced;but to film mulching for the whole growing period,the soil moisture and temperature condition were better in 60-90 days (125 d) in tillage layer,and it was benefit to the mineralization of organic nitrogen,and at this stage,the uptake ability and requirement amount of wheat on the accumulation amount of NO-3-N became weak,so the amount of residual NO-3-N in soil profile was accumulated.The determining results showed that the film mulching can significantly increase the flux of N2O,and also is related to the growth course of wheat.The results of experiment on winter wheat shows that,either mulching or no mulching,the flux of N2O in field is the highest at the flowering stage and is the lowest at the ripening stage;film mulching could increase the flux of N2O,the average of the whole growth period showed that,under the cultivated condition,the flux of N2O was increased by 4.7 μg/(m2·h) after mulching,while under the fallow condition,it was increased by 10.4 μg/(m2·h).Obviously,the longterm mulching for the whole growing period has the negative effect on stabilizing or maintaining the soil organic matter,such as the excessively mineralizing of the soil organic matter.The soil