Root and leaf senescence of maize subject to spatial differentiation of soil water and CO₂ in sandy fields with plastic film mulching

High intensive irrigation is commonly found in newly reclaimed oasis sandy land in arid area, which easily causes plant senescence of spring maize in farmland with plastic film mulching. The previous studies have found that land cover significantly affected soil moisture and soil aeration, but the data of spatial differentiation of soil moisture and soil aeration under mulching condition was lacking. In order to explore whether soil moisture and aeration causes plant senescence, an experiment was conducted to investigate the effects of plastic film mulching on spatial differentiation of soil water and partial pressure of soil CO₂ (pCO₂) in root zone, root growth, green leaf duration, leaf photosynthetic physiology, grain yield and grain quality of spring maize in a sandy farmland in the Hexi Corridor (100°12′E, 39°20′N, 1370 m above sea level). Two treatments plastic film mulching (PFM) and no mulching (NM) were set up in this experiment. In the current study, oven drying method was used to determine soil water content. Soil gas was collected by gas well method, which was composed of collecting pipe, transmission pipe and a sampler. The CO₂ concentration of gas samples was analysed by a gas chromatograph (Agilent 7890A, Agilent, Palo Alto, USA). Plant roots were sampled using a steel drilling, washed into a nylon mesh bag, scanned with root a scanner (EPSON Perfection V700) and then obtained root length density (RLD) using WinRHIZOPro software. Soil moisture showed no significant difference in horizontal direction in NM treatment, whereas, the average soil moisture of mulched soil (narrow row and wide row at 5 cm distance from plants) was 28.1% (P<0.05), 15.2% (P<0.05) and 21.7% (P<0.05) higher than that under non-mulched soil (wide row at 25 cm distance from plants) before, 6-days after, and 9-days after irrigation. In PFM treatment, soil pCO₂ in mulched zone was 40% higher than that in the in non-mulched zone. Compared with NM, the excessively high soil pCO₂ under mulched zone under PFM treatment decreased the maize root activity by 19.7%, but the root distribution and activity in the non-mulch zone increased by 22.7% and 9.6%, respectively; the leaf photosynthetic rate and transpiration rate in PFM were increased by 20.0% and 8.5% at the jointing stage, respectively, but the corresponding value at grain filling stage were −40.0% and −18.0% respectively. In addition, compared with the NM, the senescence-start time and senescence-maximum time of green-leaves were 1.7 d and 7.1 d earlier in PFM, respectively, while the average and maximum green-leaves senescence rates were increased by 6.7% and 21.7% in PFM, respectively. The above effects of mulching did not significantly affect the yield of corn grain, but reduced the starch content and protein content of grain by 20.1% and 22.1%, respectively. The above results show that the excessively high soil pCO₂ after maize flowering in the newly reclaimed oasis sandy land in the arid area may be an important reason of plant senescence and grain quality degradation, and it is recommended to take timely film-uncovering, controlled irrigation (e.g., deficit irrigation, root alternating irrigation) or aerated irrigation to improve soil aeration.