| Less rainfall is the important limiting factor of agriculture development in semi-arid regions on the Western Loess Plateau. Traditional agriculture measure led to declined soil quality, serious soil erosion, low field capacity and productivity in the region. Many studies showed that conservation tillage ,such as no-tillage, stubble cover, and stubble standing could increase soil water holding capacity, fertility and infiltration, water use efficiency and productivity, it could also reduce soil erosion.This study was finacially supported by 2006BAD15B06 project, and carried out from 2007 to 2008 year in Semi-arid Loess Plateau of Gansu Dingxi. Three treatments, traditional tillage (T), no-tillage (NT) and no-tillage with straw mulch (NTS) were involved in W→I and I→W rotations experiment. Effects of conservation tillage on soil physical and biological characteristics, crop physiological and biochemical characteristics, photosynthetic characteristics and yield were studied to explore the mechanism of increasing water use efficiency and improving production under appropriated conservation tillage. Main results showed that:1.Conservation tillage could decrease soil bulk density, increase soil porosity and saturated hydraulic conductivity. In W→I and I→W rotations under T, NT and NTS different tillage methods,soil bulk density showed NTS NT>T too. Soil saturated hydraulic conductivity were NTS>NT>T in two rotation, and showed significant difference between treatments. In comparison, conservation tillage improved soil quality better in I→W rotation.2.NTS could improve soil temperature in cool season, and decrease soil temperature in hot season, this could help crops emergence and roots growth. At sowing and seedling stage of wheat and Isatis, soil temperature in different depths showed NTS> NT>T. Soil average temperature of NTS was increased by 2.33℃than that of T at 0-15cm layer in wheat sowing. At wheat seedling stage, soil average temperature of NTS was increased by 3.00℃than T. At Isatis sowing, soil average temperature of NTS was increase by 2℃than T in 0-15cm depth. In hot season,wheat field average temperature was reduced by 2.71℃than T in 0-15cm soil depth,and that of Isatis of NTS was reduced by 1.96℃than T. Wheat field was better at soil temperature adjusting than Isatis field under different tillage methods.3. NTS could increase soil water content of plough layer significantly in W→I and I→W rotations. During wheat and Isatis growing period, average soil moisture of two rotations in 0-30cm,30-200cm and 0-200cm all indicated NTS>T>NT. In 0-30cm depth,during wheat growing period,soil average water moisture of NTS was increased by 6.67% and 6.64% than NT and T respectively in W→I rotation,and In I→W rotation,soil average water content of NTS was increased by 5.81% and 3.36% than NT and T. In growth period of Isatis, Isatis average soil water content of NTS was increased by 3.87% and 3.16% than NT and T in W→I rotation,and In I→W rotation, wheat field average water content of NTS was increased by 5.46% and 4.79% than NT and T. Comparing two rotations, soil water content in W→I was higher during Isatis growing period, but that of I→W was higher during wheat growing period.4. Conservation tillage could significant improve soil enzyme activities and soil microorganisms quantity in W→I rotation. In Isatis field and wheat field, soil urease, alkaline phosphatase and invertase hydrolase activitise showed NTS>NT>T in same soil depth. In 0-30cm soil depth, all soil average hydrolase and oxidoreductase activities were NTS>NT>T, NTS had the significant effect to increase enzyme activities. Compared with W→I, I→W increased soil hydrolase and oxidoreductase activities better. From T, NT to NTS, quantity of soil microorganisms were increased gradually in W→I and I→W rotations at different layer. All soil bacteria, fungi, actinomycetes and total microbes were NTS> NT> T in 0-30cm depth. The number of soil bacteria in wheat field was reduced about 10 times than that of Isatis field in same layer under different tillages, and the total number of microorganisms decreased by 10 times too. W→I rotation was conducive to microbial proliferation under T, NT and NTS different tillage methods.5. Conservation tillage could help Chla and Chlb to synthesis and delay leaves'Chla degradation. NTS increased crops protective enzymes activities and proline content significantly, and induced MDA content and cell membrane permeability,so that protected cell membrane and were conducive to crop growth and development.Isatis and wheat leaves'annual average content of Chla, Chlb and Chl (a+b) all showed NTS>NT>T. Isatis leaves'averge MDA content was T>NT>NTS,averge CAT activity was NTS>T>NT, CAT activity under NTS was increased by 122.24% and 85.01% than NT and T.This showed that Conservation tillage played an important role in reducing reactive oxygen damage by increasing CAT activity on Isatis. Wheat average SOD activity ws NTS>T>NT, average CAT activity was NTS>T>NT.Wheat average CAT activity of wheat leaves under NTS was increased by 91.45% and 103.58% than under NT and T.Averge MDA content was T>NT>NTS. It also showed that CAT and SOD played positive roles to protect crops cells physiological function.Under different tillages,Isatis average proline content showed NTS>NT>T,and average cell membrane permeability showed T>NT>NTS.Wheat leaves average proline content showed NT>NTS>T,and average cell membrane permeability showed T>NT>NTS. It indicated conservation tillages reduced crops cells membrane damage, and it was beneficial to cell structural integrity and physiological function.6. consevation tillage could improve crop Pn and WUEl. Wheat and Isatis diurnal change of Pn were midday depression and bimodal trend under different tillage,and midday depression of photosynthesis were caused by stomatal limitation.The daily and annual means of Pn were NTS>NT>T, and average annual WUEl were NTS>NT>T on two crops. 7. Conservation tillage helped crops to increase yield and water use efficiency. Isatis's dry roots yield, dry leaves yield and WUEy were NTS>NT>T. Compared with T,dry roots yield under NTS was increased by 20.34%,WUEy under NTS was 20.31% higher than T; Isatis dry leaves yield under NTS was improved by 11.26% than T, WUEy was 11.23% higher than T. Wheat production and WUEy showed NTS> NT> T too.wheat production under NTS was improved by 44.20% than T, and wheat WUEy under NTS was increased by 40.39% than that of T.
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