Effects of Different Plastic Mulching Methods on Soil Water , Temperature and Nitrate Accumulation in a Dryland Winter Wheat Field

This study has investigated the effects of two different plastic mulching methods on soil water, temperature, and nitrate (NO3-N) accumulation in a dryland winter wheat field after one-year experiment. The drought-resistant wheat (Triticum aestivum) variety Chang-8744 was grown by (i) furrow planting with ridge mulching, (ii) bunch planting with flat mulching, and (iii) conventional flat planting without mulching (or control). Results showed that dryland winter wheat effectively utilized soil water down to 2 m depth, mainly in the first 140 cm. Plastic mulching increased the evapotranspiration during wheat growing season and mostly r flat plastic mulching, by ~18% over the value recorded in the control plots. Soil temperature of the 20-40 cm-layer was higher than the one recorded at 5-10 cm depth during seedling-overwintering stages. Ridge plastic mulching and flat plastic mulching increased soil temperatures at 5 cm, 10 cm, and 40 cm depths during seedling–overwintering stages with reference to the control (no mulching), then lowered them at the same depths during reviving–ripening stages. Residual NO3-N was always detected in the soil after harvesting irrespective of the mulching method. It was mainly concentrated in the first -60 cm accounting for ~50% of soil NO3-N accumulation within the 2-m profile. The highest soil NO3-N accumulation occurred under flat plastic mulching, which represented ~107% of the mean value of the remaining treatments. Finally, flat plastic mulching showed the greatest effects on soil water, temperature, and NO3-N accumulation in dryland wheat field.


Introduction
Dryland winter wheat is one of the major staple crops in China's Shanxi Province, which is a typical rainfed agricultural area.Winter wheat production in this area is constrained by the uneven distribution of precipitation (~500 mm yearly) and its inconsistency with the water requirement of wheat.Therefore, effectively harvesting rainwater, preserving soil moisture, and coupling water with fertilizers is critical for stable production of rainfed winter wheat.
In recent years, mulching techniques have been widely promoted and applied as an important measure to increase soil temperature, preserve soil moisture to ensure optimum crop yields in the rainfed agricultural areas of northern China (Li et al., 2001;Gao et al., 2005;Debashis et al., 2008;Chen et al., 2010).A previous study found that the water consumption (i.e., evapotranspiration) of plastic mulching was significantly higher than that of no mulching during the growing season in Lanzhou, Gansu (Yang et al., 2015).Plastic mulching showed a remarkable effect on harvesting rainfall and effectively increased soil water content in the surface layer in Wuchuan, Inner Mongolia (Wang et al., 2013).Additionally, plastic mulching markedly enhanced crop utilization of soil water in the deep layer in Yangling, Shaanxi (Wang et al., 2004).
Many studies have indicated that plastic mulching can affect the variation in soil temperature.Plastic mulching resulted in an obvious temperature rise in the Jungar Banner of Inner Mongolia; the largest temperature difference was found at 10 cm and 20 cm depths, with a temperature increase of ~2.5 °C (Li et al., 2010).Compared with the control (no mulching), plastic mulching also showed a common effect of increasing soil temperature in Yuzhong, Gansu (Cheng et al., 2016).With regard to soil nitrate (NO 3 -N) accumulation, the highest NO 3 -N concentration was found at the 0-10 cm depth under plastic mulching treatment in Dingxi, Gansu (Zhang et al., 2011).Soil NO 3 -N concentration was influenced by plastic mulching within a range of 0-80 cm in the loess highland area of northwest China (Wang et al., 2015).There exist regional differences in the effects of plastic mulching on soil water, temperature, and NO 3 -N accumulation in dryland wheat fields due to different environmental conditions.In the present study, field experiments were conducted during 2014-2015 in a rainfed agricultural area of southern Shanxi, China that aimed at assessing variations in soil water, temperature, and NO 3 -N concentration under two different plastic mulching methods.The results would provide reference data for the high yield and stable production of dryland winter wheat in southern Shanxi, China.

Experimental Site
The experiment was carried out during 2014-2015 in a dry highland wheat area of Dongliang Village, Liujiayuan Town, Hongtong County, Linfen City, Shanxi Province, China.This area has a warm temperate continental monsoon climate with an average annual sunshine of 2419 h.The effective accumulated temperature was 3326.9 °C and the average annual temperature 12 °C.The frost-free period lasted 180-210 days, with the average annual rainfall of ~500 mm.The experiemental site's soil was calcareous cinnamon with a medium loam texture.The chemical characteristics of the topsoil before planting are shown in Table 1.

Experimental Design
Three treatments were applied in the experiment including (i) control i.e. conventional flat planting without plastic mulching, (ii) furrow planting with ridge plastic mulching, and (iii) bunch planting with flat plastic mulching, with four replications.The experiment was arranged in a randomized complete block design.The elementary plot area was 470 m 2 .All fertilizers were applied once as basal fertilizers before sowing, and no irrigation was applied during the growing season of wheat.The drought-resistant wheat (Triticum aestivum) variety Chang-8744 was sown at a rate of 150 kg/hm 2 during September 30-October 1, 2014 and harvested on June 7, 2015 for estimation of the whole plot yield (Gao et al., 2016).
For the control, conventional flat planting was conducted with shallow rotary tillage 13 cm deep before sowing; after breaking the clods and flattening of the soil, seeds were sown with a row spacing of 20 cm.For the ridge mulching, ridges were covered with plastic film and seeds were sown in 30 cm-width furrows at the side of the plastic film; seeds were sown in two rows spaced 20 cm apart, with a ridge width of 35 cm.For the flat mulching, the whole ground was covered with flat plastic film fixed with an overlying layer of soil approximately 0.5-1 cm thick; seeds were sown to 3-5 cm depth, in rows spaced 15-16 cm apart and bunches spaced 12 cm apart; the plastic film was 120 cm wide each lane, with 7-8 rows per lane.

Soil Physicochemical Analysis
Soil samples were collected down to 2 m depth at 20 cm intervals before sowing (September 23) and after harvesting (June 7) during the growing season of 2014-2015.Three parallel samples were taken for each treatment.For soil NO 3 -N analysis, 5 g of fresh soil sample was weighed and transferred into 50 mL of 0.01 mol/L CaCl 2 and oscillated for 30 min.The NO 3 -N concentration in the extracts was measured using an AA3 continuous flow analyzer (Brown rupee, German).Additionally, soil water content was measured by the oven drying method (105 °C, 12 h).
Soil water content = (Quality of aluminum box and soil sample before drying -Quality of aluminum box and soil sample after drying)/(Quality of aluminum box and soil sample after drying -Quality of dry empty aluminum box) × 100 %.
Evapotranspiration (mm) = Soil water storage in the 2-m profile before sowing -Soil water storage in the 2-m profile at harvesting + Effective precipitation during the growing season of wheat. jas.ccsenet.

Data A
The experi

Effects
Soil Figure

Table 1 .
Chemical characteristics of the topsoil before planting