| Drought is one of the most serious natural disaster for agriculture.Seasonal drought occur frequently even in wet and rainy subtropical red soil region.The occurring mechanism of seasonal drought and its regulation were in-depth studies and practices.However,a single research was more about the response of soil or crop to seasonal drought.And systematic study was lack about the response of soil-crop system and the countermeasure to seasonal drought.As a basis on the temporal and spatial characteristics of seasonal drought in subtropical red soil region was fully known in this study firstly.The ways were explored to slow the seasonal drought from time and space by by setting different crop sowing time and tillage measures.The relationship was analyzed between the seasonal drought and soil moisture,the crop growth and its root distribution and soil structure change by setting different fertilization and soil and water conservation measures(e.g.straw mulching et al.).The occurring mechanism of seasonal drought and its effect on the soil-crop system were explored from soil,crop,fertilization and field management and main conclusions were obtained as following:1.The seasonal drought of subtropical red soil region had distinctive temporal and spatial characteristics.The monthly evaporation was more than monthly rainfall from July to October every year in this region.The evaporation was 2.1 times and more 265.8 mm than rainfall in July and August in which the drought was most likely to occur.The vigorous water requirement period of crop was in farmland during July and August every year.And the most roots distribution was at 0-30 cm soil layer in farmland.Thus,the seasonal drought occurred easily at mainly 0-30 cm soil layer of the farmland more than woodland,grassland and bare land.It is disadvantageous to crop growth with more than 12 days of continuous drought.It would run a higher risk of the yield reduction at the earlier period of summer maize than at the later period as meeting drought.Improving the sowing time and crop root depth will be effective measures to relieve a crisis of seasonal drought in this region.2.It would be beneficial for crop growth to avoid the crop vigorous water requirement period meeting with the seasonal drought.We seeded maize from the middle of May and then sowed five times with one time per 15 days in serial two year.Compared to the general date of maize sowed at the middle of June,the maize yields were no significant change with moving up the sowing time,but the maize grain yield significantly cuts of 23.3%-52.6%% with deferring the sowing time.And the dry weight of stem and leaf and roots mass density were no significant difference for five sowing times.The maize roots of the different sowing times distributed more than 90% at the 0-30 cm soil layer and had no layering.The crop may avoid later drought by moving up sowing time,but sowing time change had no improvement in the root distribution and drought had no obvious effect of avoiding drought.Unfortunately,crop yield reduced by deferring the sowing time.Disappointing to mitigate the seasonal drought through adjusting sowing time.3.Compared with conventional tillage(CT),deep-tillage(DT),no-tillage(NT)and compaction-tillage(PT)increased 0-40 cm soil field capacity of 3%-5% and soil available water content of 22%-50% and above all the main influence was at 0-20 cm soil layer.Compared to CT,DT increased the maize grain yield of 25.8%,but NT and PT decreased the yields of 16.7% and 21.6%.DT increased total root mass density by 21%,but NT and PT decreased them of 21% and 50.5% in 15 cm x 40 cm soil body.DT decreased the maize roots proportion about of 10%,but NT and PT increased the proportions of 10.5% and 6.2% at 0-10 cm surface soil layer.DT reduced the maize roots deep proportion by 43%,but NT and PT increased them of 10.5% and 54.5% at 30-40 cm soil layer.The seasonal drought occurred frequently at 0-30 cm soil layer and the lower soil water is enough.The function of NT and PT was limited to ease the seasonal drought through only improving the surface soil available water capacity.DT promoted root deep growth and then absorption the deep moisture.So in view of subtropical red soil seasonal drought with the distinctive temporal and spatial characteristics,DT was superior to other tillage measures to alleviate the drought.4.Compared with no fertilization with long-term no-tillage(CK),chemical fertilizer(NPK),organic fertilizer(chicken manure,OM)and straw mulching coordinated with chemical fertilizer(NPK+S)with long-term no-tillage above all significantly improved both soil organic matter content and soil N,P,K contents;slightly increase(less than 7%)on 0-40 cm soil layer field capacity,soil available water content and water absorption rate,especially on 20 cm soil layer.NPK and OM lowered surface of unsaturated hydraulic conductivity of 50.7% and 67.7% respectively,while NPK+S increased the conductivity of 128.0%.NPK,OM and NPK+S lowered the surface soil water loss rate of 37.6%,45.1% and 46.9% at first evaporation stage,while improved the maize leaf area of 37.6%,45.1% and 46.9%,root mass density of 47.7%,133.0% and 103.4%.The maize root dry weight,root depth and the distribution proportion of deep roots from high to low was NPK+S,OM,NPK,CK and 90% of which were distributed on 20 cm upper soil layer.Compared with CK,three kinds of fertilization raised the maize yield and water use efficiency 3-4 times,but during the event of continuous drought,the fertilization amplified CWSI 1.1-5.2 times.Although long-term fertilization with no tillage improved the root growth,high root proportion distributed at 0-20 cm soil layer,besides the obviously increased leaf area,it led soil water loss fast,crop CWSI high,even aggravated surface soil drought and crop water stress.5.It is occur frequently rainfall erosion early and seasonal drought later over the course of a year in the subtropical red soil region of China.The objective of this study was five soil and water conservation measures(B,contour Bahia-grass strip;SM,straw mulching;PAM,polyacrylamide surface application;SPAM,straw mulching and polyacrylamide surface application;and BPAM,contour Bahia-grass strip and polyacrylamide surface application)and only crop as CK were conducted in order to ascertain the response of rainfall-induced topsoil structure changes to seasonal drought later.This results showed that the all discrepancies of soil water storage at 0-30 cm soil layer were less than 5% between CK and B,PAM and BPAM.The discrepancies were 7.4% and 8.2% between CK and SM and SPAM.It was different with five soil and water conservation measures that CK significantly(p<0.05)increased the soil crust coverage significantly(p<0.05)decreased the WSA0.25 of 0-20 cm soil layer and 0-30 mm soil porosity and mean pore size after raining.The WSA0.25 were 63%-88% and soil porosity were 79%-97% in CK to compared with that in five conservation measures.The index of soil drought intensity(I)and degree(D)was established using soil water loss rate and soil drought severity.The correlation analysis showed that I was significantly(p<0.05)positively correlated with crust size,crust coverage and was significantly(p<0.05)negatively correlated with WSA0.25.D was significantly(p<0.05)positively correlated with crust coverage and was significantly(p<0.05)negatively correlated with WSA0.25,0-15 mm soil porosity.Among of soil structure factors,the soil porosity had the biggest impact on I and D.The average value of I for 20-day drought duration were 1.2-2.5 times in CK than that in five soil and water conservation measures.The final value of D after 20-day drought duration were 1.1-1.4 times in CK than that in these conservation measures.The Change of topsoil structure induced by water-erosion had little impact on soil water storage,but significantly increased the rate of soil water loss and drought degree.The rate of surface soil water loss quickened by water-erosion is one of the reasons for seasonal drought occurring.6.Straw mulching was widely used as a soil and water conservation measure.Three levels of straw mulching and N fertilization were conducted to investigate the response of the soil-crop system to soil hydro-thermal regime.Straw mulching slightly increased 0-20 cm soil moisture less than 5%.The effect of mulching on decreasing soil temperature was greater than that on increasing the temperature.Straw mulching most reduced 0-5 cm soil temperature of 3.3-6? and most elevated the temperature of 0.7-2.5?.N fertilization increased the maize yield,adding straw mulching the yield was more.The highest mulching level significantly(p<0.01)reduced maize grain yield with no N fertilization.The maize grain yield reduced 21% with mulching of 10000 kg/hm2 and N of 0 kg/hm2 than with no mulching and no N fertilization.Straw mulching decreased total soil NO3--N content and increased total NH4+-N content.Inversely,N fertilization increased total soil NO3--N content and decreased total NH4+-N content.Straw mulching and N fertilization increased maize total nitrogen uptake more than 16.9% and 23.6%.Nitrogen productivity raised with the increase of N fertilization with mulching.Straw mulching reduced nitrogen productivity under low N fertilizer level and improved the productivity under high N level.The effect of straw mulching was restricted by N level.Straw mulching and N fertilization made up for deficiencies each other and not only improved the field hydro-thermal regime but also raised crop yield and water use efficiency.7.On the basis of explaining the temporal and spatial characteristics of seasonal drought in subtropical red soil region,the thesis discusses the cause and countermeasures of seasonal drought in different sowing time,tillage,fertilization and soil and water conservation measures.The results showed that the root distribution and surface soil water loss of subtropical red soil were two causes of seasonal drought.In order to promote root deep growth and reduce surface soil water loss,we recommend tillage more deeply,sowing and fertilizing early as far as possible.The best tillage depth should be over 30 cm.What's more,the middle of June later can be avoided as sowing time,at the same time with the organic fertilizer combining with chemical fertilizer as basal fertilizer applied,it would be better to keep mulching straw over the entire year. |
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