Chapter 7 Nutrient and Water Management Effects on Crop Production, and Nutrient and Water Use Efficiency in Dryland Areas of China

Abstract

Located in the northern territory of China, the vast semiarid and subhumid regions referred to as dryland areas are stressed by two major constraints for crop production: shortage of water supply and deficiency of nutrients in soil. Low precipitation and its uneven distribution have resulted in soil water, surface water and groundwater deficit, and made crops being under water stress in most cases. As a direct result, except for a few places that can conduct irrigation, most regions remain rainfed agriculture. In addition to shortage of water supply, serious wind and water erosion derived from sparse vegetation coverage, windy climate and frequent rainstorms plus human activities have led to serious soil degradation and nutrient stress. Deficiency of N can be found everywhere and that of P occurs at least in one third of the arable lands, this leading to low productivity. However, the limited water resources have not been fully used and the nutrient use efficiency by crops is very low, both having a certain potential for use and a large room for improvement. Management of water and nutrients are extremely important not only for crop production, but for environmental concern in these areas. Water and nutrients have great interactions that may gain either positive or negative effects on crop production, depending on crop growth stages, amounts, combinations and balance. In the dryland areas, the effect of nutrients and that of water are often limited to each other. Remarkable variations in precipitation from year to year significantly influence soil water and nutrient status, and so do the nutrient input effect. Nutrient input may obtain a good harvest in one year while a poor harvest in another. Considering the precipitation changes and taking effective measures to regulate nutrient supply, crops may not suffer from water limitation in a dry year and from nutrient deficiency in a wet year, and in this way we cannot lose the opportunity to obtain good harvest in both dry and wet year. Nutrient input is the key for crop production. Roots are essential for taking up water and nutrients to support crop growth, and the significance of roots becomes even more important on drylands, since the topsoil is often dry and nutrients are often unavailable, and plants need to extend their roots into deep layer to obtain available nutrients in the moist soil. It has been found that in most cases, crop yield is highly correlated with crop root mass almost in a linear shape. Addition of organic fertilizers can enhance soil organic matter, raise soil water storage capacity, reduce soil bulk density, and therefore create good conditions for root penetration into deep layer. Both organic and chemical fertilizer can provide nutrients for forming strong root system and for roots having a higher capacity to absorb nutrients and water, improve root activities such as raising the root synthetic ability of amino acids by rational N fertilization. Different nutrients have different functions on root growth and its distribution. Nutrient input is also essential for improvement of plant physiological activities. Regulating plant water status and osmotic pressure, increasing the activity of nitrate reductase in plant leaves and raising photosynthesis and transpiration intensity whereas decreasing evaporation constitute some important aspects. All these benefit plants in optimization of the use efficiency of water and nutrients. Experimental results show that the osmotic regulation effect is higher with fertilization. The increase of N-supply level reduces disorder of N metabolism in plants deficient in water and increases plant resistance to drought. Under water stress, rational N supply could make wheat leaves to have high activity of nitrate reductase, high levels of proteins, and better water status. Bleeding sap amount increase per plant by N fertilization provides evidence that water intake by plants is increased. Addition of K can make leaf stomata quickly closed under dry and hot wind conditions. With normal water supply, transpiration rate is increased by fertilization while reduced in a water deficit case. Due to vigorous growth, rapid leaf emergency, large leaf area and high coverage rate of plants on the ground with fertilization, soil surface evaporation is reduced and more water is used by transpiration. It has been found that by rational N fertilization, the ratio of water lost by transpiration to that by evapotranspiration was increased from 0.32 to 0.65, and water loss by evaporation was decreased by 1/3, the water use efficiency (WUE) for both grain and dry matter production being increased. Addition of nutrients, particularly K, can increase chlorophyll, protect the photosynthetic organs from dryness and make the photosynthetic organs fully played their role, and therefore increase the photosynthesis that is regarded as the main cause for crop yield reduction under dry conditions. All these have made the dryland crop production increased. Wise input of fertilizer and manure may do more to prevent soil erosion than some of the more obvious mechanical means of control, since the growing of bumper crops by fertilization not only gives a maximum ground cover but supplies sufficient organic matter to aid in the maintenance of all important soil constituents; and the increase of soil permeability to water under such conditions is certainly a factor of major importance. Effective water management can increase nutrient availability, transformation of nutrients in soil or from fertilizers. Mineralization of organic N is proportional to soil water, and the net mineralized nitrate-N is increased with the increase of water content in an adequate range under suitable temperature. A very closely linear relationship has been found between water content and mineralized N. Due mainly to good aeration induced by deficit of water on drylands, ammonium-N both from soil and fertilizers can be quickly nitrified into nitrate-N. Thus, a large amount of nitrate-N often accumulates in soil profile that has been used as a good index for reflecting soil N-supplying capacity. Adequate water content can promote nitrification of ammonium N while the process is inhibited when moisture content is too high or too low. Water influences mineral nutrient movement from soil to roots and then from roots to aboveground parts of plants. The difference of nitrate N concentrations at different distance points of soil from a plant being greatly declined by adequate irrigation is a typical example showing that some nutrients could be transferred as solute to plant roots with water movement. Adequate soil water content can significantly transfer a large portion of N to aboveground part, and increase N contents in seeds. All in all, water promotes total nutrient uptake by plants and nutrient use efficiency, and affects nutrient composition of plants. It has been reported that N recovery was increased about 20% at any N rate by an adequate supply of water. Water deficit, on the other hand, not only causes water stress to plants, inhibits plant root growth, reduces roots-absorbing area and capacity, increases the viscosity of sap in hadromestome, and thereby decreases nutrient transfer, but also reduces the availability of soil nutrients, nutrient movement in soil, and nutrient uptake and efficiency. Plant growth and crop yield are thus reduced. However, the reduction rate of plant growth is more serious than nutrient uptake, leading to a relative increase in nutrient concentration. Too much supply of water may cause nitrate N leaching and decrease N recovery. Since water supply and nutrient efficiency are closely related, balanced application of nutrients, and determination of their types, ratios, amounts, timing and methods should be based not only on the nutrient-supplying capacity, but also on water status of soil. Rational combinative supply of water and nutrients can increase efficiency of both and produce good interaction. When available water supply is less than a certain range, crops may have little response to fertilizers at any rate, and with sufficient supply of water, nutrient efficiency is increased. An intense interaction exists between available water and fertilizer, and one being changed will likewise lead to the change of the other. The interaction of water and fertilizer is time dependant, and application of water and fertilizer at different stages of plant growth may produce different interaction effects. Oversupply of either or both may delay crop maturation by encouraging excessive vegetative growth, while deficit of water may result in high nutrient concentration in soil, making it difficult for crops to take up and use both water and nutrients, and in a worst case, plants may die resulting in “haying off” effect. The different results obtained for the optimal time of application of water and fertilizer may relate to soil water and nutrient supply at different time. For promotion of water and nutrient fully playing their role and realization of maximum yield, high quality and high efficiency, while protection of the environment from fertilizer ill impact, one important thing is to fully understand and utilize their positive interaction, and attention should be paid not only to input of water and nutrients, but to their rational combination. That is, for addition of water, one should consider nutrient supply, and for addition of nutrients, one should consider water coordination, so that limited nutrient and water can produce optimum effect. Short supply of fresh water and fertilizer pollution has promoted investigations into the interaction effects of water and nutrients on crop yield and nutrient efficiency and WUE, and some achievements have been made. However, there still exist a large number of issues that need further studies in the future. Delineating drylands into different regions and determining the priority issue in each region, determination of most efficient time or growth stage for input of nutrients and water to different crops, and interaction mechanism of water and nutrients are some important aspects.