Effects Of Straw Return On Soil Fertility And Crop Yields In Paddy-upland Rotation System

In order to study the decomposition characteristics and nutrient release patterns of rapeseed and wheat straws, wheat (Triticum aestivum L.cv.) and rapeseed (Brassica napus) straws were all wrapped in net nylon bags and put on the soil surface or embedded into the soil under different rice cultivations and straw return models in2007and2008growth seasons, respectively, to study the decomposition characteristics and nutrient release patterns of rapeseed and wheat straws. A site-specific experiment (2008-2010) was conducted to study the effect of straw mulching on soil physical and chemical properties and crop yields in paddy-upland rotation system. Effects of straws decomposition on soil biological properties and nutrient status were studied under different water regimes. The major results as follows:The decomposition characteristics and nutrient release patterns of rapeseed and wheat straws were studied under different rice cultivations and straw mulching models in2007and2008growth seasons, respectively. Wheat and rapeseed straws were wrapped in net nylon bags and put on the surface or embedded into the soil under conventional cultivation and water-saving cultivation models. The results showed that the decomposition rate of straw was faster within the first30days and then slowed down until the end of the experiment. Under water-saving cultivation model,90days after straws were embedded into soil, the decomposition rate was high up to61.06%. The cumulative decomposition rates of wheat and rapeseed straw were48.88%-59.95%and50.88%-61.06%, respectively. Under conventional cultivation model, the straws that were mulched decomposed faster than the straws that were embedded into the soil did. With straw mulching, there was no obvious difference on decomposition rates between the two cultivation models. When straws were embedded into the soil, the straw under water-saving cultivation model decomposed faster than the straw under the conventional cultivation model. The sequence of nutrients release rates were K>P>N≈C. C release rates of wheat straw and rapeseed straw were up to48.29%-63.79%and50.29%-66.55%, and N were up to48.35%-52.83%and46.48%-57.67%, and P were up to54.83%-67.49%and56.44%-75.64%, and K were up to91.98%-95.99%and92.31%-96.24%after90days decomposition, respectively. The effect of cultivation model and incorporation method on N, P and C release patterns had almost the same trend with the decomposition of straw. K release rate was more than90%after30days decomposition. It indicated that on the basis of straw mulching, the water-saving cultivation model can promote the straw to decompose and to release the nutrients. At the same time, it also can prevent the loss of nutrients from the soil, prevent pollution and increase water use efficiency.Soil microbial quantity increased rapidly in the early growth stage after wheat straw was returned to the field, but decreased significantly in the middle growth stage and reduced slowly in the late growth stage. The activities of the urease,alkaline phosphatase and sucrase in the soil had the same rules with the microbial quantity after straw mulching. But the rules of cellulase activity were different; it increased rapidly in the early growth stage, improved slowly in the middle growth stage and decreased rapidly in the late growth stage. The numbers of the microbial and the activity of enzyme under water-saving cultivation model were significantly higher than that under the conventional cultivation model. The effects of different rates of straw returned to the soil on microbial numbers and enzyme activities were that increased the amount of the wheat straw properly could improve microbial numbers and enzyme activities, but the excessive straw would have a negative effect on the number of bacteria and actinomycetes. The results also indicated that soil nutrient contents could be improved notablely by straw returned to the field. After straw returned, the content of the organic carbon, total nitrogen, alkali-hydro nitrogen and available phosphorus increased throughout the whole trial period. The available potassium content reached the highest after30days of straw mulching, and then decreased gradually. The nutrient contents under water-saving cultivation model were significantly higher than that under the conventional cultivation model. Increasing the amount of straw had a significantly positive effect on soil nutrient contents.A site-specific experiment (2008-2010) was conducted to study the effects of straw mulching on soil physical and chemical properties and crop yields in paddy-upland rotation system. The results showed that compared with no straw mulching treatment, the soil bulk density was reduced at0-5cm soil layer and soil water content was increased at0-5cm and5-15cm soil layer with straw mulching treatment. Straw mulching increased soil organic matter, alkali N, available P and available K contents at0-25cm soil layer. Influences of straw mulching on soil nutrient conditions at0-5cm soil layer were higher than that of5-15cm and15-25cm soil layer. The nutrient contents increase sequence at0-5cm soil layer after5seasons of straw mulching were:available K (7.64%-15.33%)>available P (7.52%-10.03%)>alkali. N (7.30%-8.74%)> organic matter (6.08%-7.53%). At0-5cm,5-15cm and15-25cm soil layers, compared to no straw mulching treatment, the contents of soil NH4+-N and NO3--N increased significantly with straw mulching treatment. With the enhancement of the time of mulching and the rate of straw, the soil NH4+-N and NO3--N contents increased. After the harvest of the fifth season rice, the soil NH4+-N and N03--N contents of straw mulching treatment were higher than that of no straw mulching treatment by18.01%-38.43%and11.69%-48.16%, respectively. Straw mulching could reduce the contents of NH4+-N and NO3--N, and also could reduce the soil NO3--N leaching during rice earlier growth stage, which can promote the crop reproductive growth in the later growth stages and then increase crop yield. On the other hand, NO3--N had a trend of leaching into deeper soil with the straw mulching treatment. The results also indicated that straw mulching increased crop yield and the effect of straw mulching on wheat and rapeseed was higher than that of rice. Increasing the year of straw mulching and the amount of straw had a significantly positive effect on crop yield. The key components of the yield were effective panicles of wheat and rice and pod number and seed per pod of rapeseed.