(Institute of Desert Research, Chinese Academy of Sciences and State Forestry Administration, Lanzhou 730000, P. R. China)
Abstract The aims of rice cultivation on sandy land are summarized as: (1) to prevent water and nutrient from leakage through the porous course-sandy soil and salt from up immigrating to the ground surface through the capillary action by laying a thin plastic film at the depth of 20-25cm, in order transform the bare sands to fertile paddy field in a few days; (2) to increase fertilizer use efficiency by using course aeolian sands as cultivation matrix which possess good properties such as better porosity, higher water and nutrient availability and smaller heat capacity; (3) to alleviate roots suffering from obstacles of reduction to soil nutrient bank availability and emission of toxin (such as H2S) caused by deep waterlogging by restricting the plastic film above the possible reduction layer of sandy soil; (4) to supply water and fertilizers restrictively according to the changes in water and nutrient requirements of rice in different development stages so as to increase the proportion of non-structural substance and its accumulation rate at the same time to decrease the long-term stock of the soil nutrient bank, and finally to gain a high production of grain (about 9,000 kg/ha); (5) to perform minimum tillage system for saving labours and energy; (6) to put the transformation of sand in the process of its utilization and to promote the former with the later, abandoning the outmoded notion that sand had been considered as available soil for cultivating rice.
The essential techniques of rice cultivation on sand land include: (1) land preparation, canal management and dune stabilization; (2) construction and maintenance of film mulching laid under sandy paddy plots; (3) rice seedling raising with sand mixed with mud in small green house; (4) density and canopy structure controlling; (5) irrigation scheme and fertilization formula for different development stages; (6) the prevention and control of rice diseases and elimination of pests and weeds; (7) selection of rice varieties for sandy land; (8) construction of small eco-farm on sandy land .
Desertification process in China is accelerated greatly by such negative human activities as over-reclamation of cropland, undue collection of fuel wood, over-grazing etc. Desertification caused by sand encroachment at the marginal areas of farming-grazing regions of semi-arid zone and those areas surrounding oasis is rapidly spreading. Those areas with most fragile ecosystem, the fragility are the imbalance between the limited resource environment capacity and irrational land use as well as heavy pressures from human being and livestock. The marginal area of farming-grazing region of the semi-arid zone in the north of China used to be rainfed rangeland. As population was increased, frequent human disturbance has brought great threats to the fragile semi-arid environment, and has resulted in desertification. The productive land was degraded, or even destroyed by desertificication, leading to much intense irrational land use, and as consequence, the sand enchroachment and dune movement have occurred. The peoples living environment in the desertified area is severely impacted. This vicious cycle of irrational land useresource and environment degradationpovertymore abuse and degradation of resource is getting worse than ever before. It is impossible to rehabilitate naturally these degraded lands. There appear two kinds of material differentiation in the process of desertification. One is that the surface roughness resulting from the wind erosion increases the permeability of sand dune. The other is that more ground water is accumulated. There is a wrong idea that desertification is considered to be inevitably accompanied by aridization. In fact, in some regions, due to the reduced water consumption by plant transpiration following the decreased vegetation caused desertification, greater permeability of sand dunes and more water are concentrated in the inter-dune lowlying areas. Thats why some water ponds, even lakes can be seen in the depressions of megadune. Frequent damages caused by waterlogging in summer or autumn season can also be witnessed.
Based on the knowledge of desertification mechanism and water characteristics in desertified land region, it can be concluded that, the two material differentiation processes mentioned above have to be considered while rehabilitating desertification. The water resources accumulated in the interdune depressions and the coarse sand bed should be fully utilized while restoring ecosystem and improving the degraded land. The key issue and main solution in sand land reclamation are to making rational utilization of the limited water resource in efficient way, to readjust the land use orientation, to alleviate the pressure on steppe and rangeland and rainfed farmland, to gain maximum income, to improve peoples living standard while implementating both integrated local areas development programme and action plans to combat desertification at local level.
In light of temperate desert characteristics, a series of techniques for cultivating rice on sandy soil has been studied. May efforts have been done for rehabilitating the severely degraded environment at the marginal areas of farming-grazing in the semi-arid zone, for ascertaining the highly sustainable ecological model at the marginal areas of farming-grazing regions and for carrying out the rehabilitation of the whole fragile zone. These techniques do not only solve the issues of water and fertilizer-losing inherited by aeolian sand soil, but also do exploit them as renewable resources. Estimated ecological and economic benefits have been achieved in the regions where the adaptation and extension service of the technique is well done.
1 Requirement to
Save Water
1.1
The thin plastic film prevents water from down-leaking and salt from
up-concentrating
The shifting and semi-shifting aeolian sand soil, with more than 95% sand particle over 0.1 mm in diametre and 38.76% porosity, has low structural capacity, poor water and nutrient holding properties. Such severe water-leaking, is as high as 8,640 mm a day, will result in high cost and frequent irrigation to the cultivation of some common dryland crops due to the lack of water-holding horizon. It is also impossible to cultivate rice with such condition. Introduction of plastic film will prevent the irrigation water from leaking and form a water-holding horizon. Salinization is another problem in the desertified area. The ground water in desertification affected land areas in the semi-arid area does usually have rich salt content, which is concentrated on the surface by capillary attraction force and rapid evaporation under high temperature. The existence of water-holding layer will prevent the groundwater from up-moving, alleviating the threat of salinization on rice field on sandy land.
1.2 Make
full use of existing conditions
Large increase of crop production can be realized by saving water and fertilizer, by making the best use of loose structure of coarse sands, active role of aerobic microorganism, rapid increase of daytime temperature, rapid decrease of nighttime temperature, small heat-capacity, and rapid transformation of fertilizer. The growth of root system could be facilitated, the effective material accumulation could be increased, the duration of growth and maturation will be shortened, the consumption of water and fertilizer at different stages will be controlled.
1.3 Increase
of oxygen
Due to long-term domination of water layer, the rice field on sandy land lacks oxygen, which was resulted in by reduction condition in long term waterlogging. Such reduced materials as H2S and organic acid, low valence iron and manganese are produced. These reduced matters restrain the respiration, absorption of phosphate, potassium, even poison the root system. The reducing environment generally results in lack of O2 caused reduction and the harm to root system from H2S. Proper regular drainage, sunshine sterilization of the top soil, proper application of fertilizer and water can improve the ratio and accumulation of preheating chemical compound, reduce the volume of slow-release nutrition.
1.4
Increase of soil fertility
The mono-purpose to control wind erosion and to combat desertification will be replaced by the multi-purpose practices of returning the rice residue to the field to improve the fertility of sandy soil. Fibrous rice roots will cohere sandy soil and prevent it from wind erosion. The amount of rice residue left on the field in a year is nearly 3590,8 kg per ha According to the decomposition measurement, the rice residue can be decomposed completely in one year for supplying the sandy soil with N.P.K and organic material and increasing the fertility.
1.5
Multi-purposes for land use
A beneficial farming-grazing compound cycle can be established by combining water-saving rice cultivation with herds farming. Man-made fodder farms and grazing land can be created in the low-lying areas by using the drained water from the rice field sandy land. Water from the rice field is abundant in nutrient, which is favourable for the growth of fodder. The animal manures can be applied as natural fertilizer for the sandy rice field for improving the rice productivity.
Cultivation of rice on film-underlain sandy land was firstly succeeded in Caiman County (42°55′N, 120°42′E) of Horn Sandy Land, Northeast China, then in Shapotou of Zhongwei County in the Tengger Desert (37°27′N, 104°57′E), and Linze County in the Badan Jilin Desert (39°14′N, 100°05′E).
Horqin Sandy Land is located in the boundary area of Inner Mongolia, Liaoning and Jilin Provinces. It is characterized by temperate continental semi-arid monsoon climate. The annual mean precipitation is 362 mm; the annual mean evaporation is 1,935 mm. The annual mean temperature is 6.5℃ and the accumulated temperature 10℃ is 3,000℃. The frost-free period is 3,000℃ days. The ground water is abundant and ground water table is 0.5-5 m deep. Its natural landscape is semi-arid steppe, predominated by the marginal area of farming-grazing. The surface is covered by deep and loose sandy soil composition. Destruction of vegetation caused by over-reclamation has turned the aboriginal steppe into degraded rangeland with frequent blown sand encroachment.
Shapotou is located at the southeastern edge of Tengger Desert. It is the transitional zone of steppe and desert. The annual mean precipitation is 186.2 mm; the mean annual evaporation is 3,000 mm. The mean annual temperature is 9.6C and the accumulated temperature 10C is 3,010C. The frost-free period is 179 days. The natural landscape is mobile transverse dunes with sparse vegetation and coverage is about 1%. The ground water is deep and almost impossible available. However, the Yellow River water is available for irrigation at the four seasons.
Linze Oasis is located in the middle part of Hexi Corridor, Qilian Mountain in the south, Badan Jilin Desert in the north, is characterized by typical continental climate. The mean annual precipitation is 117.1mm. The mean annual evaporation is 2,337.6 mm. The mean annual temperature is 7.6℃ and the mean annual accumulated temperature 10℃ is 3085℃. The frost-free period is 160 days. The ground water is abundant in a depth of 1-3m. The waters from Heihe River and Lihe River originating from Qilian Mountain are available for cropping farming forestry plantation.
3 Method
3.1 Seedling raising of rice on sandy bed
Raising and transplanting of rice seedlings in the north of China in dry, windy and dusty spring are of great significance in making full use of solar energy, in saving water and increasing production. The first step in rice seedling raising is to prepare seedling bed, which is selected near a yard with rich sunlight, easy drainage, and lower ground water table. The size of rice nursury bed can be determined by the size of planned rice field, in general 1m2 rice seedling bed could provide enough seedlings for 100 m2 of rice field. Thin plastic film, of 0.08mm thick, is mulched on the flattened under bed. A plastic shed is constructed to shelter the nursary bed. The time of raising seedling is selected when the mean temperature of a period of ten days is over 10C. such preparation could prevent the seeds from drying in the sunlight. Seed selection, soaking and pre-germination should be done before raising in the nursery. The estimated amount of seeds is 200-250 g/m2. The rice seeds are covered by 0.5cm thick sand, the seedling bed should be kept with moisture, pH value should be adjusted periodically to the range of 4.5-5.5, mixed fertilizer of N, P, K should be added every 5-7 days, the temperature in the shed should be controlled in a range of 20-30 C and ventilation by opening the plastic film is needed when the temperature is over 30C. Seedlings are ready to be transplanted when they have more than five leaves.
3.2 Establishment of leak-proof rice field and water conservancy
facilities
The canal system has to be installed when the selection of mobile or semi-mobile dune field was flattened. The irrigation water for the rice cultivation on sandy land is usually pumped from wells, but water available from reservoir and lake could be pumped through canal system. Both the irrigation and drainage canals should be installed and the bottoms of canals have to be mulched with plastic films or concrete for avoiding the leaking. Banks between field plots are made after the canal system is completed to enable the size of every plot is around 1 mu (0.33 ha) and the relative relief within plots should be less than 4 cm. The prepared plot beds without trims weeds root or other unwanted plants have to be mulched with 0.05-0.08 mm leaking-proof plastic film and then buried with 25cm sands cover. Banks between plots should also be covered by film to prevent both downward and sideward water leaking. Suitable measures to control shifting sands are needed to control the encroachment of sands surround the rice field.
3.3 Field management
The seedlings are ready to be transplanted when the ten-day mean temperature reaches 15C. Base fertilizer is added while irrigating the transplanting. 5-7 individuals of seedlings should be transplanted in a bunch spaced as big as 20 cm15 cm. Fertilizer should be applied at different stages according to the requirements in early promotion, middle regulation, and final conservation. At the beginning stage, on the basis of sufficient base fertilizer, further need to be arranged before tilling to increase the N concentration, to facilitate earlier tilling and more ears. No fertilizer is needed during the full tilling and elongation, to control ineffective tilling and improve the growth of root system so that the metabolism predominated by N is replaced by that dominated by both N and C and more carbohydrate, needed by heading is accumulated. Proper adding of ear and grain fertilizer around heading period is necessary to improve the photosynthesis, ear-bearing grains and weight per thousand grains.
The shallow moisture irrigation strategy is adopted on rice field on sandy land. Shallow water of 3-5cm is kept between recovery and tilling to create favourable environment for temperature increase, root growth and tilling. Irrigation should be stopped for sunning topsoil to control ineffective tilling, to strengthen the resistance to stress in the ineffective tilling stage. Sufficient water, generally 5-7 cm deep, is ensured during the elongation and heading period, which is the key stage to meiosis and the final production. The filling stage is with alternated shallow water, moistening and sunning to ensure earlier maturation.
Care should also be paid to controls of pest and waterlogging. The key to pest control is to select proper pesticide. The main types of pests are leaf miner, rice plant skipper and rice midge.
Keeping proper density, ventilation environment and avoiding redundant application are important, as they are the key factors to cause later maturation, waterlogging and lower production.
Ploughing is not needed every year because rice field on sandy land does usually have few weeds and good porosity. Therefore minimum-tillage is encouraged.
4 Results and
Discussion
The rice seedlings on sand land have vigorous growth, but effective tilling rate is low. Compared with clay soil, sandy soil has higher temperature in spring due to its relatively low heat capacity. For example, in a depth of 5 cm, the temperature of clay varies from 16.1℃ to 25.4℃ while that of sand ranges between 17℃ to 27.7℃ at the same time. The accelerated photosynthesis by high daytime temperature enables the nutrition accumulation. On the other hand, the rapid temperature increase but short sunlight time in the spring season will result in low tilling rate. So, in order to enable more effective ears, the number of seedlings in a bunch should be properly increased.
To know the advantage of sandy land rice, comparative study was conducted between the rice cultivated on sandy land and that on common land using the same rice variety. Compared with the common rice field, the sandy rice field does not only save water and fertilizer, but also does have higher productivity. In the north of China, the water leakage is generally 360-1440 mm, accounting for 43-63% of the total consumption, the water leakage of sandy land is 864-8640 mm/d. However, little water leakage is possible on the rice field on sandy land mulched with plastic film and this technique does save a lot of water. Shapotou is an active example (Table 1). The irrigation water for the common field is 1400 mm, accumulative fertilizer applied 900 kg/ha, while the irrigation water for the rice field on sandy land is 1200 mm and cumulative fertilizer applied is 850 kg/ha
Sandy, due to its low water and fertilizer-holding capacity and the moisture content below the withering point under dry condition, can hardly be simply used as transplantation bed material. The mulching of plastic film under sand layer could prevent water leakage on one hand and improve the degraded environment on the other hand, which is hard for the common mesophyte, but favourable for rice. But even for rice cultivation, the waterlogging will cause some problem of unstable distribution of nutrition. To make the best of the less nutrition absorption and lower nutrition-holding capacity of the sand, in the light of nutrition needs, fertilizer have to be applied at different stages so that it can be absorbed soon. The nutrition problem for the sandy land rice can be essentially solved by mixed use of common fertilizer with the slow releasing granulate fertilizer.
The seedling raising on sand and mud mixture in plastic shed does not only enable adequate seedling supply, but also elongate the growth time. The limited growth time decided by the short frost-free period in the north of China is unfavourable for crop growth. The green house afforded by plastic shed makes it possible to raise rice seedling with low temperature. Seedlings should be transplanted under suitable temperature condition.
Water-saving rice cultivation technique needs high investment input and as a return it will produce high output. However, rice cultivation on sandy land will consume large amount of water. Extension of sandy land rice must thoroughly consider the regional water resource sustainability. Particularly in the area where underground water is available for irrigation, over-exploitation of ground water is not permitted.
Table 1. Comparison of
sandy rice field and common rice field
|
|
Type |
|
|
Sandy |
Common |
|
|
Basic seed-ing (seedling/m2) |
400 |
180-240 |
|
Muture ears (ear/m2) |
800 |
400-500 |
|
Ear length (cm) |
16-20 |
15-17 |
|
Ear grains (Grains /ear) |
170-210 |
160-180 |
|
Blighted grains (%) |
5-11 |
10-15 |
|
Weight per thousand grain(g) |
24-27 |
23-25 |
|
Harvest index (%) |
46-56 |
43-55 |
|
Irrigate water (mm) |
1200 |
1400 |
|
Fertilizer Used (kg/ha) |
850 |
900 |
|
Yield (kg/ha) |
1950 |
1200-1950 |