Can organic carbon and water supplementation sustain soil moisture–carbon balance under long-term plastic mulched semiarid farmland?
Introduction
Plastic mulching significantly increases crop yield worldwide ([18], [13], [24]), especially in semiarid areas ([9], [25]; Wang et al., 2015; Wang and Shangguan, 2015), where cover 40% of the global land surface (Huang et al., 2016) and are the main regions suffering starvation and poverty (FAO, 2019). The soil thermal–hydraulic effect of plastic mulching is clear ([20], [13], [9], [24], [36], [39]), and is regarded as a promising way to increase crop production and resolve poverty in semiarid areas. However, there has been increasing attention on soil quality and sustainable agricultural production ([16], [18], [43], [22], [38], [4], [12]), especially on soil organic carbon (SOC) and annual water balances, which are the key factors affecting the sustainable production of plastic mulching technology.
The effect of plastic mulching on SOC concentration is not only a key issue for technological development, but also a scientific issue of great contention. There are contradictory statements on the effect of plastic mulching on SOC concentration. Some researchers reported that plastic mulching can decrease soil organic matter concentration ([16], [18], [43], [11], [37], [7], [4], [12]), mainly because plastic mulching increases soil water content (SWC) and temperature, resulting in decreased dissolved organic carbon and increased microbial biomass carbon and soil respiration ([11], [4]), thus being a disadvantage to soil carbon accumulation. However, other studies demonstrated that plastic mulching can significantly increase SOC, sustainably enhance crop productivity and maintain soil quality ([20], [22], [19]), especially for an aridity index of 0.15–0.4, thus SOC more likely benefits from plastic mulching. Zhu et al. (2017) reported that integrated arbuscular mycorrhizal fungi and a plastic film mulching farming system promoted both SOC and economic profitability and was an environment-friendly and promising management to ensure food security and ecosystem sustainability. Additionally, Fu et al. (2017) reported that plastic mulching did not change SOC concentration in a wheat field in a sub-humid rain-fed area after 7 years. Consequently, the effects of plastic mulching on SOC was sharply different, this mainly because change in SOC is a long-term accumulative effect, and differs according to the crop and the area. The short-term (i.e. less than 10 years) and single-crop field investigations is not sufficient to estimate this key issue.
The soil water balance (SWB) is another key factor in determining the plastic mulching in sustainable agricultural production, which is also doubted widely. Some work verified that plastic mulching increased SWC, accompanied by significantly increased evapotranspiration (ET) ([40], [39], [26], [36]). Conversely, Ding et al. (2018) reported that soil water was depleted by 125.6–193.0 mm, down to 180 cm depths, in winter wheat on the northwest Loess Plateau. Wu et al. (2018) also found that soil water storage (SWS) in the 100–200 cm profile of a plastic-mulched maize field decreased by 28 mm at harvest stage compared with a no-mulching field. Our previous study also showed that after 6 years of maize continuous cropping on a semiarid rain-fed field, SWS with plastic mulching decreased by 163.59 mm, comprising 5.21 mm in the 0–140 and 158.37 mm in the 140–300 cm layers, compared with that at sowing stage in the first experimental year (Wang et al., 2018). However, wheat continuous cropping increased SWS in the deep soil profile (Hou et al., 2014). Otherwise, the effect of plastic mulching on SWB differs according to local rainfall, crops and even the measured soil depth of 0–200 or 0–300 cm soil profiles. For example, plastic-mulched maize depleted deep soil water in a semiarid area (Wang et al., 2018) but increased it in a sub-humid area (Ren et al., 2017), with annual rainfall of 410 and 550 mm, respectively.
In sum, studies on the plastic mulching in sustainable agricultural production are well documented, and both the SOC and SWB have conflicting results, for three main reasons. (1) Previous studies concerned one kind of crop on the same site, conducted over the short term or the measured depths of SWC (0–200 cm) were insufficient to obtain accurate information on the effect of plastic mulching on crop production sustainability. (2) The experimental treatment designs usually only showed the difference between with and without plastic mulching treatments, and did not address the two key factors – water supplementation and organic carbon addition – although these are very important to determine a helpful method to sustain this balance. (3) The experimental analysis investigated the SOC and not crop carbon sequestration, and thus could not accurately estimate the effect of plastic mulching on carbon accumulation of the soil–crop system. For the above reasons and based on our previous work, we hypothesized that plastic mulching causes soil water depletion and decreases soil SOC, and designed a field experiment, spring maize (Zea mays L. Xianyu 335) and wheat (Triticum aestivum L. Longchun 27) were selected as tested crops. The field experiment was conducted in a typical semiarid area (the aridity index is 0.28) from 2010, and the SWC down to 300 cm and SOC down to 100 cm in the soil profile were measured. The objectives were (1) to determine the SOC and water balance over the long term for a deep soil profile in a typical semiarid area (annual rainfall around 400 mm, in a critical region for agriculture); (2) if PM was disadvantage to the balance of soil water and organic carbon, could the water supplementation and organic carbon addition sustain this balance? and (3) assess the production and ecological effect of plastic mulching by analyzing the total carbon and water budgets of the soil–crop system.
Section snippets
Site description and experimental treatments
The study site was located at the National Agricultural Experimental Anding Station for Soil Quality (Anding District, Gansu Province, 104°36′E, 35°35′N), with 415 mm mean annual rainfall and nearly 68% occurring during June–September (relative variability of rainfall is 24%), and 1500 mm mean annual evaporation, in a typical semiarid rain-fed area in China. The altitude is approximately 1970 m, mean annual temperature is 6.2 °C and average annual sunshine hours are 2500 h. The soil is a light
Rainfall
Rainfall was determined using meteorological data of the National Agricultural Experimental Anding Station for Soil Quality. On average, around 73% of the rainfall occurred in the growth period and 27% in the fallow period for maize, and half and half for wheat (Fig. 2). Serious drought occurred in 2011 and 2016 for maize, with annual rainfall of 78.3% and 65.4% of average, and rainfall in the maize growth period of 87.3% and 65.2% of average, respectively. In 2013, 2018 and 2019, rainfall
Discussion
Plastic mulching has increased crop production significantly worldwide ([13], [33], [24]). This benefit basically depends on inhibiting evaporation and increasing soil temperature ([40], [9], [6], [26]), resulting in a relatively suitable micro-environment for crop development. The well-developed crop population improves soil water utilization, which may cause soil water depletion and destroy the soil water annual balance. Some research has shown that continuous cropping of maize and wheat
Conclusions
After 10 years monocropping, PM decreased soil organic carbon content but well sustained the soil water budget in wheat field, as compared with CK. PMI and PMO increased grain yield and water use efficiency both for wheat and maize, and well sustain soil water budget. However, PMO accelerated soil organic carbon loss for both maize and wheat, resulting in decrements in the soil organic carbon budget. PM significantly increased the carbon accumulation in the soil–crop system, and the
Declaration of Competing Interest
Zhang Xucheng and Hou huizhi have received research grants from the National Natural Science Foundation of China. Zhang Xucheng has received research grants from the Gansu Academy of Agricultural Sciences. The authors declare that they have no conflict of interest.
Acknowledgments
None.
Compliance with ethical standards
None.
Funding
This work was supported by the National Natural Science Foundation of China (No. 31960398 and 31560355) and the Agricultural Innovative Special Plan of Gansu Academy of Agricultural Sciences (2020GAAS32).
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