| Perennial vetch(Vicia unijuga A.Br.)is an important forage legume in the grassland agricultural system on the Qinghai-Tibetan Plateau.However,the forage biomass and seed yield are often low and variable,constrained by the cold and low and erratic seasonal precipitation environments.Soil-environmental(soil water and heat condition)modifications are critical for improving the productivity of V.unijuga.Soil surface mulching(i.e.,plastic sheet and crop straw),which modifies soil temperature and moisture conditions,has been widely adopted to efficiently improve crop productivity.A 3-year field experiment was conducted at Xiahe County,Gansu province of the Qinghai-Tibet Plateau(102°29?E,35°11?N,2962 m above sea level)during 2014-2016.The responses of V.unijuga growth and yield to no mulching(CK),plastic film mulching(FM),plastic film mulching during vegetative stage(RM)and threshed barley straw mulching(SM)was investigated in regards to soil temperature,moisture and nutrient conditions,plants establishment,growth and physiological traits,forage biomass,seed yield,yield components as well as seed quality.The aim of the study was to explore a suitable growth condition and provide a theoretical basis and scientific support for V.unijuga large-scale cultivation and sustainable development on the Qinghai-Tibetan Plateau.The main results were as follows:(1)Mulching effects on soil temperature,water content and nutrient of Vicia unijugaFor 0-1 year V.unijuga plants,averaged across the three years,a greater(P<0.05)0-10 cm soil temperature(ST)was measured in FM treatment than in CK,but ST was lower(P<0.05)for SM treatment than CK.Moreover,a greater(P<0.05)0-20 cm soil water content(SWC)was measured in all mulching treatments than in CK.The increase of SWC was in the order of,FM>SM>CK.For 4-6 year V.unijuga plants,averaged across the three years,compared to the CK,the FM treatment significantly(P<0.05)increased the ST during the growing season.The RM treatment had a greater(P<0.05)ST than CK during vegetative stage,but RM treatment(P>0.05)decreased the ST compared to the CK during reproductive stage.Compared to the CK,the SM treatment significantly(P<0.05)decreased the ST during the growing season.Moreover,the SWC was greater(P<0.05)in the FM,SM and RM treatments than in the CK.The increase SWC was in the order of,FM>SM>RM>CK.Mulching treatment greatly affected soil nutrients in 0-20 cm soil layer.Averaged across the three years,SM treatment significantly increased soil organic carbon(SOC)in the topsoil(0-20 cm)at turning green and maturity stages,and variations of SOC between turning green and maturity stages(?SOC),respectively,than CK,FM and RM treatments.Compared with CK,FM treatment decreased(P>0.05)SOC content at turning green and maturity stages,and the?SOC.Compared with CK,FM and RM treatments significantly(P<0.05)increased available nitrogen(AN),available phosphorus(AP)and available potassium(AK)contents,and FM treatment had greater(P<0.05)AN and AP contents than RM treatment.Moreover,SM treatment had greater(P<0.05)AK content than the CK,and FM and RM treatments.(2)Mulching effects on seedling development,accumulation of root reserves and potential for perenniality of Vicia unijugaV.unijuga seeds germinated under a wide range of temperatures ranging from 5to 25°C and water potential ranging from 0 to-1.0 MPa.The adverse effect of low water potential on germination was weaker at optimum temperatures(20°C),resulting in improving germination percentage and rate.For 0-1 year V.unijuga plants,averaged across the three years,compared to the CK,plants in FM treatment had greater(P<0.05)seedling emergence percentage,shoot height,shoot dry matter,root length,root dry matter,starch content,and plants surviving overwinter percentage,increased by 9.6%,58.8%,278.9%,42.6%,158.5%,104.6%and 13.8%,respectively.However,compared to the CK,plants in SM treatment had less(P<0.05)shoot dry matter,starch content,and plants surviving overwinter,decreased by 34.6%,16.5%and 12.1%,respectively.Results from the multiple linear regression indicated that the content of starch in roots was the strongest predictor(R~2=0.322,P<0.05)of overwinter survival.(3)Mulching effects on physiological traits and forage biomass of 4-6 year Vicia unijugaCompared to the CK plots,V.unijuga plants in the FM and RM plots advanced all phenological periods.The entire growing season was 1.3 d longer in the CK plots than that in the FM plots and was 2 d shorter than that in the RM plots.Compared to the CK plots,V.unijuga plants in the SM plots delayed all phonological periods,and the entire growing season was in the SM plots 1.7 d longer than that in the CK plots.Across three years,the change of photosynthetic rate was in the order of,FM>RM>CK>SM.Mulching efficiently stimulated perennial vetch growth,as indicated by a greater(P<0.05)plant height,leaf area index,aboveground and belowground dry matter,and the increased growth traits were in the order of,FM>RM>SM>CK.For example,across three years,aboveground dry matter was1596.9 kg/ha,1452.3 kg/ha,1002.0 kg/ha,and 662.6 kg/ha,for FM,RM,and SM treatments and CK at initial flowering stage(IFS),respectively.Multiple regression analysis revealed that 51.5%and 61.0%of the variation(P<0.05)in photosynthetic rate at the IFS and maturity stage(MS),respectively,was explained by ST and SWC.The relative contribution of the ST accounted for more than the SWC to the variations in photosynthetic rate at the IFS and MS.Moreover,multiple regression analysis found that 75.8%and 68.2%of the variation(P<0.05)in aboveground dry matter(ADM)at the IFS and MS,respectively,was explained by the ST and SWC.At IFS,the relative importance analysis of the SWC accounted for more than the ST to the variations in ADM,where the reverse was true for ADM at MS.(4)Mulching effects on seed yield and quality of 4-6 year Vicia unijugaAveraged across the three years,compared with the CK,FM and RM treatments significantly(P<0.05)increased the seed yield(SY)and water productivity(WP),and FM treatment had more(P<0.05)SY and WP than the RM treatment.The FM treatment significantly(P<0.05)increased SY and WP by 186.4%and 242.6%,respectively,compared with the CK.Examination of the R~2 of the regression in SY showed that 59.1%of the variation(P<0.05)was contributed by the ST and SWC,and the relative importance analysis of the ST accounted for more than the SWC to the variations in SY.Averaged across the three years,compared with the CK,the FM treatment significantly(P<0.05)increased inflorescence/m~2(IS),florets/inflorescence,pods/inflorescence,seeds/pod and 1000-seed weight,by 79.2%,17.0%,29.7%,12.1%,and 5.2%,respectively.The path coefficient analysis confirmed that IS was the most important seed-yield component,having a direct effect on SY of 0.633(P<0.05).The greatest relative importance analysis of the IS in the contribution was reaching 42.0%(P<0.05)to explaining the SY.Averaged across the three years,FM treatment was the greatest compared to other mulching treatments and CK to improve seed quality.Compared with the CK,FM treatment significantly(P<0.05)increased seed soluble sugar,starch,non-structure carbohydrates,nitrogen and amio acid contents,germination percent(GP)and shoot dry matter(SDM),by 6.3%,17.8%,15.8%,9.5%,6.9%,6.1%,and61.7%,respectively.Multiple regression analysis showed that GP and SDM was 35.0%and 76.7%,respectively,of the variation(P<0.05)was contributed by the ST and SWC.The relative contribution highlights a surprisingly dominant role of the ST compared to the SWC to the variation in the GP and SDM. |
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