Paddy Rice-Upland Crop Rotation System in Selected Southern Provinces
Through interviews with experts on potato cultivation and detailed reviews of available literature on paddy-dry rotations, the southern region contained approximately 16 million ha of fallow rice fields in winter. On one hand, the large number of fallow fields was because of rural labor shortages and high labor costs. On the other hand, because these paddy rice fields had a relatively poor soil water holding capacity, when fields were drained in preparation for dryland crops in the winter, due to low levels of organic matter and low temperatures, the soil may become hard with a poor condition unsuitable for planting. This unsuitability for multi-cropping is especially prevalent in the southwestern mixed cropping area (Zone IV), where mountainous terrain and small rain-fed field sizes inhibit water management (i.e., irrigation and drainage) possible in other regions. Simultaneously, water resources are scarce in these provinces, irrigation is inadequate, and irrigation facilities are insufficient that is mainly a problem during the dry-cropping period (Wu and Jiang 2014). Winter is also a dry season in the south of China, low precipitation can lead to reduced potato yields, thus affecting farmers' income. At the same time, cost of water-saving irrigation facilities is very high because most winter potato is grown on small and scattered plots. These factors are limiting the development of rice and potato crop rotation system in winter crop areas (Wu and Jiang 2014). The current measures proposed by the Chinese government to transform low- and medium-yielding fields can only be achieved when the government invests in appropriate infrastructure, including water-saving irrigation systems.
The survey also found that the proportion of rice-potato rotations to the total proportion of potato acreage in each province differed significantly between the southern winter crop zone and the southwestern mixed crop zone. The proportion of winter potatoes planted in the southern winter crop area ranged from 88 to 100%, while the proportion of winter potatoes planted in the southwestern mixed cropping area ranged from 4 to 40% (Table 4). The main reasons leading to this difference were the geography and climate variance between the winter crop area and the southwestern mixed crop area. The winter crop areas like Guangdong Province were mainly composed of hills, the annual rainfall being 1000–3000 mm, and the frost-free duration being more than 300 days. These elements provide favorable conditions for planting potatoes in winter. The southwestern mixed crop area mainly consisted of mountains, so the temperature and rainfall were significantly different in low and high altitudes.
These conditions have a significant influence on crop planting in the southwestern mix crop area. For instance, Sichuan winter potatoes accounted for about 4%. The low proportion was due to Sichuan Province being the country's largest winter rapeseed cultivation area and many rice fields being used for rapeseed cultivation (MOA 2020). Yunnan's proportion was high due to the economic benefits of winter potatoes being very high. For instance, in Dehong, Mangshi, Honghe, Jianshui and Kaiyuan in southern Yunnan, many farmers planted winter potatoes using the so-called "1-1-3-9 method". That is planting 1 mu (45 t/ha−1), for 100 days, with a yield of 3000 kg and an approximate revenue of CNY 9000 (USD 138.9)) (DARAYP 2018). Especially in Dehong, potato yields gradually increased to an average of 47 t/ha−1 (Li et al. 2019). Due to the better prices for potatoes than other winter crops such as tobacco, the farmer has more motivation to plant potatoes. Consequently, increases in winter potatoes in Yunnan Province have been swift, becoming the largest winter potato planting area in China (DARAYP 2018).
Conservation Agriculture Practices in the Process of Rice-Potato Rotation
As shown in Table 5, in the rice-potato rotation systems in the three provinces investigated, the leading conservation agricultural practice in potato planting included integrated late blight management, mechanization, plastic film mulching and soil testing and formula fertilization technology. The late blight integrated management is the principal measure taken in potato planting in southern China. It was found that farmers' adoption rate in Sichuan Province was 50%, lower than Yunnan Province and Guangdong Province. The overriding reason was that farmers in Sichuan Province planted potato just for their own household consumption rather than to sell, so they did not spend much on integrated late blight management. The high adoption rate of potato as a winter crop in Guangdong and Yunnan was due to the 'farmers' reliance on agricultural revenues earned from the winter season. Intentions to maximize agricultural production obliged farmers to spend more on farm management and inputs. Plastic film mulching was the primary cultivation measure adopted for winter potato planting (Shan and Han 2015; Mo et al. 2017).
Table 5 also shows that the adoption rates of farmers in Yunnan and Sichuan were almost half of Guangdong. The reason for this difference was that regional geography (Zone IV) limited the use of the mechanization necessary for plastic-mulching. Likewise, the southwestern mix crop area's geography had the same impact on the adoption of mechanization. This impact was due to mountainous agro-ecology, leading to small disparate fields. Soil testing and formula fertilization was a quick and accurate method to determine the relative acidity of the soil (pH) and the level of several essential nutrients (phosphorus, potassium, calcium, magnesium, sodium, sulfur, manganese, copper, and zinc) as a guide to farmer fertilization (Duan 2018; Bradley and Deanna. 2019). The Chinese Ministry of Agriculture has promoted the technology in recent years (Zhu et al. 2016), resulting in the increased adoption rate of technology by farmers. The practices of water and fertilizer integration and drip irrigation were barely present in the three provinces. It was understood that the irrigation facilities' cost was high and cumbersome for field-based production as evidenced by the expert's opinions. Also, recently high winter and spring rainfall in the southern regions resulted in farmers not adopting the practice. The zero-tillage technique was not practiced in Yunnan and Guangdong because paddy fields were not suitable for direct planting. Such practices resulted in tubers rotting before emergence.
Benefit Analysis of the Rice-Potato Rotation System
Economic Benefit Analysis of the Rice-Potato Rotation
The economic benefits of rice-potato rotation mainly referred to the increase in the rotation's production and profitability, closely related to farmers' incomes. This survey found that in contrast to the rice-fallow system, the rice-potato rotation system increased profitability by 266% (Table 6). Compared to the rice-fallow system, rice yields and thus outputs are lower in the rice-potato system. Despite the apparent loss in production, this phenomenon appeared to be due to the duration that a rice variety's crop takes to reach harvest. For instance, in the rice-potato rotation system, farmers plant the early maturing rice to ensure the required growth period for the succeeding potato crop, but in the rice-fallow system, a farmer can plant middle or late-maturing rice varieties resulting in greater rice yield. However, some early maturing rice varieties that fit into rice-potato systems may only result in a small yield reduction compared to medium or late maturing varieties (Hong et al., 2020). Transplanting rice, as practiced in the Indo-Gangetic Plains, could be another farming practice to maintain yields (Gatto et al., 2020). In this case, production increases compared to a scenario where the period after rice is left fallow.
Similarly, findings for intensified systems with potato have been documented by Qiu et al. (2015) who found that rice-potato rotations had higher economic benefits than the rice- rapeseeds rotation when analyzing the economic benefits. Unlike rice-rapeseed rotations, in rice-potato crop rotation systems, potato cultivation does not prevent earlier rice planting.
Li et al. (2016) also pointed out that under the "potato-rice-rice" model in Guangdong Province, fertilization can be reduced by 80% compared to the control. After harvesting potatoes, early and late double-cropping rice are planted, allowing for a significant reduction in fertilization applications. Similar economic benefits were also observed in other parts of Asia where rice-potato systems are prominent (Gatto et al. 2020).
Ecological Benefit Analysis of Rice-Potato Rotation From Reviewed Literature
Intensification of agriculture results in higher input-use and, by default, negatively affects the environment, such as soil and water quality. However, input-use efficiency can be increased in rice-potato systems as both crops jointly require fewer inputs than both crops individually. The ecological benefits of rice and dry rotation refer to the impact on soil structure (Cui 2018), soil fertility (Liu and Jiang 2019), soil quality (Sun et al. 2019), the entire ecological environment and agricultural ecosystem after a particular farming system model is implemented or promoted (Wang et al. 2002; Liu and Jiang 2019). It is further implicated in the larger issue of sustainable development. Field experiments showed that paddy rice and dry rotation can improve the physical and chemical properties of the soil (Wang et al. 2003), accelerate the renewal of organic matter, increase the permeability of the soil, and facilitate the effectiveness of soil nutrients. Yu (2010) analyzed the soil nutrients in the rice-potato rotation system. Yu’s results showed that in this type of rotation system, after the potato is harvested, the quantity of organic matter, total nitrogen, total phosphorus, alkali hydrolyzed nitrogen, available phosphorus, and available potassium increased by 3.39 g/kg−1, 0.159 g/kg−1, 0.029 g/kg−1, 15.2 mg/kg−1, 17.2 mg/kg−1, 42.6 mg/kg−1 compared with rice-fallow. Wang et al. (2009) showed that the residues of the roots of different winter crops such as winter potatoes provide a relatively large amount of organic carbon source for the growth of microorganisms during rice growth and improved the activity of soil microorganisms. Zhang et al. (2009) also showed that different winter crops such as potato, ryegrass, milk vetch and rapeseed all increased the soil microbial activity, carbon and nitrogen content of paddy fields.
Wang et al. (2002) showed that the total weed coverage of the late rice field in the second year of the paddy and dry rotation was 80% less than that of the continuous late-rice field. Other studies also showed that aquatic and xerophytic weeds of the paddy-dry rotation, grass weeds such as oat grass in wheat fields and dodder seeds in the dry rotation with legumes had been effectively controlled. South China Agricultural University (2014) developed a method of planting potatoes in winter fallow rice fields that can effectively control the damage of Ampullaria gigas to early rice in the coming year. Xie et al. (2010) also indicated some soil and seed-borne diseases of potatoes in a rice-potato rotation, such as bacterial wilt, ring rot and scab disease (Wu et al. 2020), can be eliminated or significantly reduced after one or two rounds of flooding the paddy field. In summary, the rice-potato rotation can have positive ecological benefits within an agricultural ecosystem. Not only is the input-use efficiency increased, rotation contributes to improving soil health by increasing the utilization rate of the soil, thus reducing soil-borne diseases.
Constraints of the Rice-Potato Rotation System in Southern China From Reviewed Literature
In the rice-potato rotation system, several general factors affected rice-potato cultivation. First, despite the limited availability of arable land in China, urbanization continues to advance. A large amount of agricultural land has already been occupied. However, the total area of arable land in China was only 130 million ha, and the per capita arable land area was only 0.09 ha per person (Jiao 2018), the stability of agricultural production has been seriously affected. Among all the arable land in China, there are no restrictions on the necessary conditions for paddy and dry rotation. However, only 39% of the arable land with irrigation facilities was of relatively good quality. The remaining 60% of the arable land was affected by various restrictive factors. This non-arable land quality was relatively low, so these lands cannot be used for rice and dry crop rotation (Wu and Jiang 2014).
Another inhibitory factor was limited access to water resources. The major problem facing rice-potato rotation was water scarcity. 90% of water resources in Asia were used for agricultural production, and rice irrigation water accounted for 50% of the total agricultural water consumption (Bouman 2001). In China, nearly 70% of water resources were used for agricultural production, and rice irrigation accounted for 65% of total agricultural water consumption (Si et al. 2000). Winter was typically a dry season in the south of China, where rainfall was low, and rainfall cannot meet potato growth and development needs. Hence, irrigation was necessary to obtain high yields for potato tubers with a moisture content of about 75% to 80%. Therefore, the water crisis was a critical factor restricting adoption of rice and dry crops' rotation (Wu and Jiang 2014).
Third, high-yielding and early maturing varieties that are also resistant to biotic and abiotic stressors were not readily available. In 2015, between 15 and 20% of the area was planted to early/medium-maturing varieties (Gatto et al. 2018). Breeding for resistance/tolerance and earliness are key traits to make potatoes fit in rice-potato systems, to not affect spring sowing, and provide economic viability.
Fourth, the supplies of virus-free seed potatoes was one of the main restricting factors of the high yield of potatoes in the winter cropping areas. Most southern provinces in China have high temperatures and heavy rainfall in the summer. Virus vectors and sources of potato viruses are numerous and complex, which leads to rapid degradation of potato varieties and ultimately affects the yield and quality of commercial potatoes. Therefore, virus-free seed potatoes are generally transported from the north of China. However, recently, the high-altitude areas of the mixed cropping area of the southwest have also begun to produce virus-free seed potatoes, which means that seed potatoes grown in the winter cropping area may also be derived from the mixed cropping area.
Add to these factors the small land sizes and limited access to in- and output markets (Scott and Suarez 2012). While potato production may be more profitable compared with other crops, such as rapeseed, the mentioned factors, combined or individually, pose serious constraints to adoption of sustainably intensified rice-potato systems.