关键词: 半干旱区, 养分释放, 干湿交替, 凋落物分解, 科尔沁沙地

Abstract: Low temperature disaster is one of the major disasters of winter wheat in North China, which could occur at its every growth stage. In this paper, low temperature disaster of winter wheat was identified based on daily minimum air temperature and injury threshold at different growth stages of winter wheat in North China in last 50 years, then the spatialtemporal change characteristics of the disaster were analyzed. The results showed that, in last 50 years, both the severity and frequency of the low temperature disaster of winter wheat in North China decreased along with decadalscale variation. The severity index of the disaster decreased from more than 0.6 in the 1960s to less than 0.4 in the 2000s, and the frequency of the disaster reduced from more than 50 days in the 1960s to less than 40 days in the 2000s. As to spatialtemporal change pattern of the disaster in North China, both the severity and frequency in the eastern region decreased faster than in the western region, and more rapidly in recent 20 years than in the former 30 years. At the different growth stages of winter wheat, the change tendency of the disaster frequency was obviously different. At the overwintering stage and turninggreen stage, the frequency of low temperature disaster was reduced rapidly at decadal scale, but it occurred unchangeably at high levels at the stages of tillering, jointing and head sprouting. The mean of daily minimum air temperatures when winter wheat suffered the low temperature disaster rose about 1-2 ℃ during the last 50 years in North China, but there was no clear trend of decrease in the extreme value of the disaster occurring in the same growth stage.

Key words: semi-arid region, nutrient release, drying-rewetting, litter decomposition, Horqin Sandy Land