| Water is the main limiting factor for dryland farming.The Loess Plateau is a typical rainfed area,where the precipitation is scarce in space and unevenly distributed at annual or inter-annual scales.To remediate water problem,water conservation measures are widely used.In this research,we focused on the soil surface mulching measures.First of all,the potential yield under field experiment data published in previous researches were researched to acquire a general idea of mulching effect in this region.Based on this information,the Changwu tableland,in the south of the Loess Plateau,was selected as the study area to carry out a series of field experiments to explore how does mulching affect soil reservoir and crop yield.Meanwhile,noticing the issue that straw mulch may lead to a yield reduction in spring corn in this area,we searched out reason(s)and proposed an improved scheme.Finally,the influences of plastic film mulch on soil water use from the scope of precipitation use efficiency(PUE)chain were analyzed to clarify the mechanism of plastic film mulching(PM)on field-scale water use improvement.The main findings of this research were listed as follows.(1)Based on the concept of the French & Schultz's boundary function and the quantile regression method,an improved method to establish the yield-evapotranspiration(Y/ET)boundary function was developed.Taking spring maize cropping experiments carried out on the Loess Plateau of China as a case study,we collected 887 records from 65 papers published between 1987 and 2014.Based on these data,a Y/ET boundary function of dryland spring maize was formulated as y = 60.5 ×(x-50),with the yield maximized to 15,954 kg ha-1 when ET exceeded 314 mm.Without film mulching,the Y/ET boundary function was formulated as y = 47.5 ×(x-62.3),with the yield reaching to 12,840 kg ha-1 when ET exceeded 325 mm.We further compared the maize production under three mulching operations: plastic film mulching,straw mulching,and no mulching,among which,plastic film mulching tended to be the most effective measure in increasing the yield of spring maize and thus water use efficiency,regardless ET.(2)Under the PM treatment,ET of spring maize tended to be higher during the growing season while lower in the fallow period,compared with the other operations(without significance).In that case,PM is the most sustainable treatment with respect to soil water balance.Soil water change of dryland spring maize could reach as deep as 300 cm.A 7-year experiment and the supplemental experiment confirmed that straw mulching at the seedling stage may lead to yield reduction,and this effect can be mitigated by delaying the straw application to the three-leaf stage.(3)Summer fallow mulching made full use of the limited precipitation and increased wheat yield significantly in the dryland region on the Loess Plateau.Ridge-furrow frame combined with plastic film and straw mulch,which also acted as micro-catchment patterns,outperformed the other treatments in improving soil water and wheat production.Results showed that all mulching operations for the summer fallow period could significantly increase soil water storage: T3(ridge-filmed-byplastic + furrow-mulched-by-straw)increased soil water storage by 84 mm(in 2013)and 67 mm(in 2014)during the summer fallow period,which ranged as the highest among the other operations;T6(mulched by straws from wheat and corn)also had a good performance,with soil water storage increased by 59 and 47 mm,followed by T2(ridge-filmed-by-plastic)increased by 56 and 42 mm.Precipitation storage efficiency increased by 9.7%-21.5% and 6.4%-21.3% under different mulch treatments in 2013 and 2014,respectively.Mulch treatments increased wheat yields by 361-732 kg ha-1 in 2013-2014,among which yields were increased by 705,638 and 732 kg ha-1(p<0.05)for the treatment T3,T5(no mention before)and T6,respectively.The yield of inter wheat increased linearly with soil water storage at sowing stage.Higher yield required higher water input,which further led to a uniformity in soil water storage(at a low level)during the harvesting stage.(4)Without the influence of crop growth,both soil temperature and soil water storage was increased linearly as the increase of plastic film mulching degree.When the mulching degree increased from 0% to 100%,PSE increased from 11% to 56%.While with the influence of crop growth,soil temperature mainly increased in the early stage of spring maize.After July,the warming effect of PM decreased as the shade effect of maize leaves.(5)Plastic film mulching increased all parts of dryland water use efficiency chain and further improved precipitation use efficiency,field water use efficiency and crop yield.In the seven consecutive years of field experiment,compared with no mulching treatment(CK),spring maize yield under the PM operation increased by 803-3,102 kg ha-1,PUE also increased significantly.In the mid-late stage of spring maize,the T/ET ratio of PM was 73.3% in 2013 and 82.0% in 2015,17.4% and 18.1% higher than that of CK respectively.With the increase in PM degree,both soil temperature(soil layer: 15 cm)and precipitation storage efficiency increased linearly(p<0.05).Higher soil temperature during sowing and seedling stage significantly enhanced the maize's sprout rate and crop growth,so as to crop biomass and LAI.Water transpiration process was also intensified,which was one of the main reasons to trigger higher T/ET ratios under the PM treatment. |
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