Effects of mulching on soil CO2 fluxes, hay yield and nutritional yield in a forage maize field in Northwest China

In arid areas of China, water shortage and heavy carbon emissions have been threatening agricultural sustainability and which has become a vital issue. A field experiment was conducted to explore how different mulching affects soil moisture and temperature, CO2 fluxes, forage-maize hay yield and nutritional value during 2 consecutive years: 2014 and 2015. The field experiment showed that mulching materials had distinct effects on soil moisture and temperature and CO2 fluxes. The soil temperature and CO2 fluxes were in order of common plastic film mulching (PFM) > bio-degradable mulch mulching (BMM) > no mulching (CK) > straw mulching (SM), while the soil moisture was in order of PFM > BMM > SM > CK over these two years. Compared with CK, hay yield respectively increased by 23.25%, 22.51% and 5.27% for PFM, BMM and SM, WUE increased by 35.60%, 32.34% and 10.88%, and the total nutrient yields increased by 17.75%, 21.35% and 6.95%, respectively. To sum up, in combination with ecology and environmental protection, bio-degradable mulch could replace common plastic film and bio-degradable mulch should be popular in future. As bio-degradable mulch is green non-pollution, it is conducive to the sustainable development of agricultural ecosystem.


Materials and Methods
Site description. The field experiment was carried out in 2014 and 2015 at the Wuwei experimental station of Gansu Agricultural University in an arid oasis region (37°96′N, 102°64′E). The station, located in the eastern part of the Hexi corridor of northwestern China, is in the temperate arid zone in the hinterland of the Eurasia Continent. Long term (30 years)average solar radiation is 6000 MJ m −2 , annual sunshine duration is >2945 h, annual mean temperature is 7.2 °C with accumulated temperature above 0 °C >3513 °C and above 10 °C >2985 °C 9,10 . Mean annual precipitation is rarely greater than 156 mm, occurring mainly in the summer (Fig. 1), however, annual potential evaporation is greater than 2400 mm. The soil at the experimental site was classified as an Aridisol (FAO/ UNESCO, 1988), and some of the properties are presented in Table 1. At the start of the experiment, the total nitrogen (N), Olsen P, and organic matter of the top (0-20 cm) soil was 0.78 g kg −1 , 1.14 g kg −1 and 14.3 g kg −1 , respectively. Experimental design and field management. The experiment was conducted with a randomized, complete block design. There were 4 treatments (common plastic film mulching (PFM), bio-degradable mulch mulching (BMM), straw mulching (SM) and no mulching (CK)) with 3 replicates constituting a total of 12 plots. Plot size was 8*5 m 2 . Forage-maize cultivar (Xianyu 335) was manually sown. The thickness of the common plastic film and the biodegradable mulching film was 0.008 mm. The common plastic film was manufactured by Shijiazhuang Yongsheng Plastic Plant Co Ltd, China, and bio-degradable mulch film was manufactured by BASF Co Ltd, Germany. The biodegradable mulch film was composed of starch and other bio-materials. Straw mulch used the wheat straw.
The forage-maize was seeded on April 16, 2014 and April 20, 2015, plant spacing was 30 cm and row spacing 40 cm, and the density was 82,500 plants ha −1 . Before sowing, the plots were divided and the land was ploughed once and harrowed. According to local fertilizer application, a base fertilizer containing 150 kg P ha −1 and 40 kg K ha −1 was spread evenly over planting belts and then ploughed into top soil before sowing. In the whole growth period of maize, the nitrogen fertilizer was pure N 430 kg ha −1 (common urea, containing 46.4% of pure nitrogen). N fertilizer was used in 30%, 21%, 42% and 7% of the total fertilizer during feeding maize sowing, elongation   Table 1. Some soil properties across 0-120 cm soil profile at Wuwei experimental station. a Soil texture is determined by using the soil particle percentage. b Soil particle fraction based on the USDA textural soil classification system.
where θ is the soil water content; W 1 is the weight of the aluminum box and wet soil, W 2 is the weight of dry soil and aluminum box, and W 3 is the weight of aluminum box. Evapotranspiration was determined using the equation as follows 19 : Where Pc is the effective precipitation (mm), determined by the USDA soil conservation services method; I is the irrigation quota (mm); U is the upward capillary flow from the root zone(mm); R is the runoff (mm); D w is the downward drainage out the root zone (mm); and ΔS is the change of soil water stored in the 0-120 cm layer (mm). The upward and downward flows were measured previously at a nearby field, and these two items have been found to be negligible in this semiarid area 20,21 . Runoff was also negligible due to small rains, and irrigation was controlled by raised ridges between plots. Therefore, the reduced equation is as follows: Water use efficiency (WUE, kg ha −1 mm −1 ) was calculated as the ratio of hay yield (kg ha −1 ) to ET.
Soil CO 2 fluxes. Soil CO 2 fluxes was measured using a CFX-2 system(Soil Forage-maize hay yield. Forage-maize in each plot was hand-harvested at maturity, and the forage-maize was air dried and weighed for hay yield in film patterns.
Forage-maize hay nutritional value. The crude protein was determined by semi-micro kjeldahl method 22 , crude fat was determined by residual method 22 , the crude fibers were determined by Cooking method of elimination 22 . While the nutritional yield was calculated as the product of nutrient content and hay yield.
Statistical analysis. Data were analyzed using the Mixed model of Statistical Analysis Software (SPSS software, 16.0, SPSS Institute Inc., USA), with the treatment as the fixed effect and replicate as random effect. The mean separation procedure was Duncan's multiple-range test. Due to significant treatment by year interactions for most of the variables evaluated in the study, the treatment effect was assessed for each year separately. All significances were declared at the probability level of 0.05, unless otherwise stated.

Results
Soil temperature. Topsoil temperatures increased with increasing air temperature at seedling stage, and decreased with rainfall and decreasing air temperature in early autumn. Topsoil (in 0-25 cm) average temperatures for various treatments during 2014 and 2015 maize growth period were shown in Fig. 2. In the early forage-maize growth (April to June), the topsoil temperature was significant between difference mulching treatments. While in the later maize growth (July to September), the topsoil temperature had no obvious difference between different mulching treatments. During the same maize growth period, the topsoil temperatures of PFM and BMM were significant higher than CK, while the topsoil temperature of SM was slight lower than CK. www.nature.com/scientificreports www.nature.com/scientificreports/ Soil water storage. Soil water storage is an important index to measure soil water balance. Normal variations in rainfall, evapotranspiration, root depth and mulching led to obvious differences in soil water storage (0-120 cm) during maize growing seasons for treatments in 2014 and 2015 (Fig. 3). From April to June in 2014 and 2015, the soil water storage was highest in forage-maize growth period. Except forage-maize sowing period, the soil water storage was in order of PFM > BMM > SM > CK. Compared with the CK, The average soil water storage increased by 8.20, 6.80 and 4.49% for PFM, BMM and SM in 2014, respectively, but increased by 9.59, 7.23 and 4.94% in 2015.
Soil CO 2 fluxes. The diurnal variation soil CO 2 fluxes for various treatments during 2014 and 2015 was presented unimodal curve, but the time of peak for unimodal curve was different in Fig. 4. The diurnal variation soil CO 2 fluxes of unimodal curve peak for PFM, BMM and CK was afternoon 2 'clock, respectively, while SM was afternoon 4 'clock. Compared with the CK, The average diurnal variation soil CO 2 fluxes increased by 8.94%, 1.44% and −1.92 for PFM, BMM and SM, respectively, While increased by 13.63%, 6.82% and −4.14% in 2015.
Soil CO 2 fluxes for various treatments during 2014 and 2015 maize growing seasons was shown in Table 2. During different maize growth periods, soil CO 2 fluxes was in order of blossom > large-belling > filling > maturity over two years. The average soil CO 2 fluxes in maize growing seasons were 4.06 Forage-maize hay yield and WUE. According to Table 3, the hay yield was in order of PFM > BMM > SM > CK in two years. The hay yield of PFM and BMM was significantly higher than SM and CK, but no significant differences were found between PFM and BMM, SM and CK over 2 years. The hay yield increased by 22.87%, 21.16% and4.09% for PFM, BMM and SM, respectively, in 2014, but increased by 23.63%, 23.85% and 6.45% in 2015 compared with the CK.
Based on hay yield, the WUE was in order of PFM > BMM > SM > CK in two years. The WUE of PFM and BMM was significantly higher than SM and CK, but no significant differences were found between PFM and   Forage-maize hay nutrient contents and nutritional yields. The crude protein and crude fiber content of SM was significantly higher than PFM, but no significant differences were found between others treatments in two years. While the crude fat content of treatments was no significant difference( Table 4). The crude protein content decreased by 5.26%, 2.42% and −3.    www.nature.com/scientificreports www.nature.com/scientificreports/ crude protein, crude fiber, and crude fat yields of PFM and BMM was significantly higher than SM and CK, but no significant differences were found between PFM and BMM, SM and CK in 2 years. In terms of total nutrient yields, PFM and BMM were significantly higher than SM, while SM was significantly higher than CK in 2014 and 2015. The total nutrient yields increased by 18.98%, 19.62% and7.19% for PFM, BMM and SM, respectively, in 2014, while increased by 16.43%, 23.08% and 6.69% in 2015 compared with the CK.

Discussion
Water was the main factor that restricted agricultural and animal husbandry production in the west arid regions of China 23 . Mulching improved the plant growing environment by reducing soil moisture evaporation and regulating soil temperature. Li et al. 24 observed that the average temperature in 5-25 cm tilth soil under common plastic film and biodegradable film mulching were 2.51-3.77 °C and 1.30-2.19 °C, respectively, which is higher than that of the uncovered ground. We found that the use of various mulching had distinct effects on soil temperature and soil water storage. With crops growing, full plant canopy was established during the middle and later growth stages which led to small differences of soil temperature among treatments. The average topsoil temperatures increased by 1.5 and 0.8 °C for PFM and BMM, respectively, and decreased by 0.3 °C for SM over two years. Meanwhile, maize was growth slowly and root distributed shallow in early time, with the rapid vegetative growth and reproductive growth of maize, soil water storage of treatments gradually decreased. Compared with the CK, the average soil water storage increased by 8.89, 7.01 and 4.49% for PFM, BMM and SM over two years, respectively. A field experiment using oats as an indicator crop showed that mulching materials had distinct effects on topsoil temperature 25 . Soil was the largest carbon sink in terrestrial ecosystems, while soil CO 2 fluxes was impacted by soil water and heat. We found that mulching film increased soil fluxes, straw mulching decreased soil fluxes. The reason that mulching film increased topsoil temperature but straw mulching decreased topsoil temperature. The soil CO 2 fluxes showed a seasonal variation and fluctuated with the soil and the atmospheric temperature for upland measured in Zigui, Three Gorges region 26 .
Several investigators have reported that mulching can improve crop yields and water fertilizer rate 27,28 . Zhang et al. observed that fields with recourse to common mulch and biodegradable film mulch showed an average increase in yield 19.23% and 17.82%, respectively, average WUE were 21.49% and 20.25% over two years 29 . We found that hay yield increased by 23.25%, 22.51% and 5.27% for PFM, BMM and SM, respectively, over two years compared with the CK. While WUE increased by 35.60%, 32.34% and 10.88%, respectively, over two years. Investigators have reported that film mulching was significantly higher than straw mulching for yield which was probably because film mulching reduced soil evaporation on ridges and augmented infiltration of rainwater and irrigation into soil 30 . The results of research showed that nutrient contents was in order of SM > CK > BMM > PFM, which was similar to the results of Pan et al. and Li et al. studied on maize and tobacco leaves 31,32 . Possibly because straw mulching could decompose and fertilize soil, which was beneficial for maize growth and organic matter transformation. The crude protein, crude fiber, and crude fat yields of BMM was highest in two years. Compared with CK, the total nutritional yields increased by 17.75%, 21.35% and 6.95% for PFM, BMM and SM, respectively. In agricultural and animal husbandry, we should consider not only the nutrient content of herbage but also calculate the nutrient yield.
Mulching was an important measure to increase agricultural yield in this area. However, common plastic mulch was not easy to degrade and resulting in environmental pollution. Therefore, biodegradable plastic mulch should be promoted in future agricultural production. As biodegradable plastic mulch was green and environmental friendly, it was conducive to the sustainable development of regional agriculture.

Conclusions
The results of research showed that soil temperature and soil CO 2 fluxes were in order of PFM > BMM > CK > SM, while the soil water storage were in order of PFM > BMM > SM > CK over two years. Hay yield increased by 23.25%, 22.51% and 5.27% for PFM, BMM and SM, respectively, compared with the CK over two years, while WUE increased by 35.60%, 32.34% and 10.88%, respectively. The total nutrient yields increased by 17.75%, 21.35% and 6.95%, respectively. To sum up, in combination with ecology and environmental protection, bio-degradable mulch could replace common plastic film and bio-degradable mulch should be popular in the future. As bio-degradable mulch was green and free-pollution, it was conducive to the sustainable development of agricultural ecosystem. However, this paper only focused on the changes of soil water, temperature, soil respiration, maize hay yield and nutrient yield after biodegradable plastic mulching. Next, we will study the degradation rate of biodegradable plastic film and the changes of soil physico-chemical properties after mulching.