Effect of reclaiming cotton field into paddy and straw application on rice seedling growth

In recent years, panicle deformity of rice (Oryza sativa L.) characterized by blank florets/spikelets and distorted lemma and palea has caused significant yield loss of rice grown in uplands reclaimed into paddy lands in Jianghan Plain, Hubei Province, China. Up to now, it remains unclear why rice panicle deformity is prevalent in reclaimed paddy lands from uplands. However, studies have shown that panicle deformity is largely related with soil characteristics. Using old paddy field as control, a pot soil experiment was carried out to investigate the effects of old cotton fields reclaimed into paddy fields and rice straw application on the growth of rice seedlings. The study also analyzed rice mineral elements uptake, soil Eh, and soil available Fe, Mn, Cu and Zn contents. Soil waterlogged incubation test was adopted to study the effects of straw application on dynamic changes of DTPA-Fe, DTPA-Mn, DTPA-Cu and DTPA-Zn contents in the old paddy fields and paddy fields from cotton field. The results showed that rice seedlings in cotton fields reclaimed into paddy fields growth poorly and had symptoms of chlorosis, with dry weight and chlorophyll content of respectively 30% and 20% those of rice seedlings in old paddy fields. While Fe content was significantly lower, Cu and Zn contents were markedly higher in rice plants in cotton fields than in old paddy fields. Eh was significantly higher in soil of cotton field reclaimed into paddy than in old paddy soils. In the pot experiment, when submerged under water for 10 days, DTPA-Cu and DTPA-Zn contents in upland soils were respectively by 1.4-2.5 times and 1.6-1.8 times of those in old paddy soils while DTPA-Fe content was only 7% of that in old paddy soils. With prolonged inundation, DTPA-Fe content in cotton field soils gradually increased while DTPA-Mn, DTPA-Cu and DTPA-Zn contents initially increased and then decreased. Incubation test results showed that after submergence for 28 days, the differences in soil DTPA-Fe, DTPA-Mn, DTPA-Cu and DTPA-Zn between upland soils and old paddy soils diminished. Straw application of upland soils reclaimed into paddy soils reduced soil Eh and soil DTPA-Cu and DTPA-Zn contents but increased soil DTPA-Fe content. This resulted in a significant increase in chlorophyll content of rice seedlings. In conclusion, it was better to sow rice seeds or transplant rice seedlings after one month of flooding old cotton fields reclaimed into paddy fields. This prevented imbalance in soil trace elements that in turn inhibited rice growth. Straw application partly but not completely eliminated inhibiting effects on rice seedlings growth of reclamation of cotton field into paddy.