The Effects And Mechanism Of Straw Retaining On Winter Oilseed Rape Growth And Soil Fertility

Seasonal drought, low temperature in winter and low soil fertility are main limiting factors for winter oilseed rape production in the middle and lower basin of Yangtze River. Therefore, identifying management practices that modificate the soil micro-enviroment, meanwhile enhance soil fertility is desirable for winter oilseed rape in this region. On the other hand, intensive rice production in the region contributes to enormous amounts of rice straw every year. Because of having not found an alternative method to use the straw, formers just burn it, which causes the waste of resources and environmental pollutio. Considering the advantages of straw retaining in improving soil moisture, soil temperature as well as soil fertility, rice straw retaining in winter oilseed rape season may be a potential practice not only to improve the productivity of winter oilseed rape, but also to fully utilize rice straw. In this study, pot experiment, short-term field experiment and long-term field experiment were conducted to (1) evaluate the effects of rice straw retaining on soil water and soil temperature;(2) explore the influences of rice straw retaining on winter oilseed rape growth, yield and nitrogen use;(3) analyze the changes of soil fertility and crop productivity under the condition long-term straw retaining. Results from this study may provide some information that can be used to (1) determine whether rice straw retaining has the potential of improving the productivity of winter oilseed rape;(2) preliminarily judge whether winter oilseed rape requires different management practices under straw retaining. The main results were as follows.1. Straw retaining had significant effects on soil moisture at0-30cm depth. Both straw incorporation and straw mulching resulted in higher soil water content at all growth stages of winter oilseed rape. At seedling stage of winter oilseed rape, straw mulching significantly reduced diurnal fluctuations of soil temperature, but straw incorporation had no significant influence on soil temperature. The results of pot and field experiments showed that both straw incorporation and mulching reduced the emergence percentage of winter oilseed rape, however, the relative mangnitude of the decrease was much bigger when straw mulching was applied than when the straw incorporaction was applied. Straw incorporaction improved the dry matter accumulation of winter oilseed rape throughout the whole growth periods. While straw mulching also increased the dry matter accumulation of winter oilseed rape at all growth stages, nevertheless, the relative magnitude of the increase in dry matter accumulation of winter oilseed rape between the mulched and un-mulched treatments reduced after seedling stage. Straw mulching resulted in more topdressing N loss through ammonia volatilization and a significantly lower soil inorganic N content after seedling stage, which might be the main factor that was responsible for decreasing positive effects of straw mulching on oilseed rape growth after seedling stage. Straw incorporation significantly increased rapeseed yield, but the effect of straw mulching on winter oilseed rape yield varied depend on seasons. In drought season (2010-2011), the mulched treatment had significantly more seed than the un-mulched treatment, however, in relatively normal season (2011-2012), straw mulching merely resulted in an insignificant increase in rapeseed yield.2. Different straw-mulching rates treatments (0,3750,7500and15000kg/hm2) were arranged in a field experiment to preliminarily estimate proper straw-mulching rates for winter oilseed rape. The results showed that compared with no straw mulching, mulching could increase significantly the rapeseed yield by16.0%on average. While between the three mulching rates treatments, no significant difference in rapeseed yield was observed. According to this result, meanwhile give consideration of the fundamental of easy operation, the mulching rates of7500kg/hm was recommended in actual farming, this rate roughly corresponded to the amounts of pervious rice straw. The effect of different sowing rates on emergence and yield of winter oilseed rape was studied under the condition of straw mulching. The results showed that the increase in sowing rates had no significant influence on emergence percentage of winter oilseed rape, but it significantly increased the plant density and yield of winter oilseed rape, indicating that increasing the seeding rates was an effective practice to improve rapeseed yield when straw mulching was applied in winter oilseed rape production.3. In the double rice-winter oilseed rape cropping system, continuous straw mulching increased the crop yield, moreover, the yield response was more significant in winter oilseed rape season than in rice season. Similar to yield, nutrient uptake of early-season rice, late-season rice and winter oilseed rape were also improved by continuous straw mulching. Straw mulching resulted in significantly lower internal efficiency of potassium (K) both in rice season and winter oilseed rape season, but it had no significant influence on internal efficiency of nitrogen (N) and phosphorus (P). The results of system-level nutrient balance showed that significant positive N, P and K balances had occurred without straw mulching, when straw mulching was applied, N and K surplus of cropping system were furthermore intensified. At the initial stage of long-term field experiment, there was no significant difference in soil total N content between mulched and un-mulched treatments, but with prolongation of experimental years, a gradually significant increase effect of straw mulching on soil total N content was observed. Straw mulching had no sigficant influence on soil available P content, but it had significantly increased soil exchangeable K content since the beginning of experiment. Straw mulching also resulted in an increase in soil total organic C content, moreover, under the condition of no-till, this increase was more significant.