Interaction Effects Of Film Mulching,nitrogen Rate And Plant Density On Grain Yield,Water And Nitrogen Use Efficiency Of Spring Maize Under Dryland System

Plastic film mulching applicaiton has been proved as an effective measure to improve crop yield,water and nitrogen use efficiency in semiarid and arid regions,particularly in dryland agroecosystem.However,irrational nitrogen（N）application and low planting density limit the further improvement of crop yield,water and N use efficiency in this mulching system.With the aim of establishing and optimizing the high yield and high resource use efficience cultivation system of dryland maize production,two field experiments were conducted at the Changwu Research Station of Agriculture and Ecology on the Loess Plateau of China（35.12°N,107.40°E and 1220 m elevation）.Experiment 1:Two mulching models[film mulching（FM）and non-mulching（NM）],four N application rates(0,170,200 and 230kg N hm^-2 in 2013,respectively;0,170,225 and 280 kg N hm^-2 in 2014,respectively)and three plant densities(50000,65000 and 80000 plants hm^-2)were adopted during 2013 and2014 maize growing seasons.We investigated the interaction effects of mulching,N application rates and plant density on maize growth,yield,soil moisture and nitrate-N content.The interaction effects on water and N use efficiency and dry matter and N accumulation dynamics of spring maize were also addressed.Experiment 2:Two mulching models[film mulching（FM）and non-mulching（NM）]and two N application rates(0 and 250 kg N hm^-2)were adopted during 2015 and 2016 maize growing seasons.We investigated the architectural and anatomical responses of maize roots to mulching practices and N rates.Furthermore,the relationships between architechtural and anatomical traits of root and crop water,nutrient absorption and utilization were revealed.The main results were showed as follows:（1）FM markedly accelerated the progresses of maize growth development.The emerging time and silking stage of FM treatments were shortened by 3—4 days and 4—15days,respectively,compared with NM.FM could promote accumulation of dry matter and increase crop yields through increasing chlorophyll content（SPAD）and leaf area index（LAI）.The field experiment showed that a treatment combination of 200—225 kg N hm^-2 and 80000plants hm^-2 for FM yielded 96%of the potential maximum(average 14.2 t hm^-2),while a treatment combination of 170 kg N hm^-2 and 50000 plants hm^-2 for NM yielded 95%of the potential maximum(average 8.7 t hm^-2).（2）Flim mulching and N application significantly affected the architechtural and anatomical traits of maize root.Crown and brace root angles to the soil line decrease in response to film mulching and nitrogen fertilization.Crown roots under plastic film mulching showed a significantly decreased distance to branching,reduced lateral root length,and overall increased root diameter.Similarly,nitrogen application significantly decreased the distance to branching,yet induced more compact and denser crown roots,and increased the root diameter.Brace roots exhibited an increased distance to branching,greater lateral root length and density,as well as a larger root diameter in response to plastic film mulching and nitrogen fertilization.Additionally,the accumulated number of nodal roots increased greatly under plastic film mulching and nitrogen treatments.At the anatomical level,nitrogen application reduced the proportion of the root cortical aerenchyma area.In contrast,aerenchyma area,cortex cell size,and late metaxylem vessel diameter were increased as a result of plastic film mulching.The architectural and anatomical traits would facilitate efficient use of available water and nutrients.（3）FM significantly increased water use efficiency.FM markedly increased soil surface water content in the early stage,increased deep soil water consumption during the middle and late stages.This effect was more obvious when maize suffered from severe drought stress during the pre-silking periods in 2014,implied that FM could improved soil water supply capacity,but also satisfied the moisture demand of plant during the dry season.Water consumption of spring maize appeared interannual difference.No significant difference in water consumption was observed between FM and NM in 2013,while FM significantly increased it during the pre-and post-silking periods and the whole growth stages by 9%,16%and 12%,respectively,compared with NM in 2014.N application significantly increased water consumption compared with no fertilizer treatments,especially during the post-silking period.The results showed that a treatment combination of 200—225 kg N hm^-2 and 80000plants hm^-2 for FM could achieve 97%of the potential maximum WUE(average 34.0 kg hm^-2mm^-1),while a treatment combination of 170 kg N hm^-2 and 50000 plants hm^-2 for NM achieved 94%of the potential maximum WUE(average 22.1 kg hm^-2 mm^-1).（4）Suitable N supply and plant density significantly improved N use efficiency under FM or NM system.Compared with NM,FM significantly increased above-ground N uptake by 32%—55%,decreased residual soil NO₃^–-N content in the 0—100 cm layer and NO₃^–-N losses by 28%—100%and 53%—69%,respectively.N recovery efficiency（RE）,agronomic efficiency（AE）and partial-factor productivity（PFP）were significantly increased by44%—45%,24%—65%and 54%—62%,respectively.Residual soil NO₃^–-N content in the0-100 cm layer and losses of NO₃^–-N increased with increasing N application rates,while N use efficiency showed an opposite trend.A combination of 280 kg N hm^-2 and 50000 plants hm^-2 resulted in soil NO₃^–-N leached to 120 cm depth under FM system;while more NO₃^–-N would leach to 140 cm depth even at N rate of 170 kg N hm^-2 under NM system due to its coincidence with the most rainfall during the middle and late stages.Taken above-ground N uptake,N use efficiency and leaching of NO₃^–-N into account,the optimum N fertilizer rate and plant density were 200—225 kg N hm^-2 and 80000 plants hm^-22 for FM and 170 kg N hm^-2and 50000 plants hm^-22 for NM,respectively.（5）FM significantly increased grain yield and grain N accumulation through increasing dry matter and N translocation,promoting dry matter and N accumulation during the post-silking periods.Under the FM system,the dry matter,N assimilation and its contribution to grain did not increased in treatment with N fertilizer at rates of 200—225 kg N hm^-2 and plant density of 80000 plants hm^-2 during the post-silking periods,but the improved N translocation during the pre-silking periods significantly increased grain yield and grain N accumulation eventually.However,under the NM system,N fertilizer more than 170 kg N hm^-2 or plant density above 50000 plants hm^-22 decreased dry matter and N accumulation during the post-silking periods and its contribution to grain,which failed to increase grain yield and grain N accumulation.In conclusion,―N fertilizer input gap‖and―plant density gap‖might exist between FM and NM systems for dryland spring maize.Appropriate N supply and suitable plant density could not only increase dry matter and N accumulation and translocation,promote root architectural and anatomical traits,but significantly improved grain yield,water and N use efficiency.Consequently,the whole film mulching double-furrow technology combined with N fertilizer rate of 225 kg N hm^-2 and plant density of 80000 plants hm^-22 was the better cultivation practice in dryland farming systems.