Bobby A. Stewart

Long-term experiments for sustainable nutrient management in China. A review

China is facing one of the largest challenges of this century to continue to increase annual cereal production to about 600 Mt by 2030 to ensure food security with shrinking cropland and limited resources, while maintaining or improving soil fertility, and protecting the environment. Rich experiences in integrated and efficient utilization of different strategies of crop rotation, intercropping, and all possible nutrient resources accumulated by Chinese farmers in traditional farming systems have been gradually abandoned and nutrient management shifted to over-reliance on synthetic fertilizers. China is now the world’s largest producer, consumer and importer of chemical fertilizers. Overapplication of nitrogen (N) is common in intensive agricultural regions, and current N-uptake efficiency was reported to be only 28.3, 28.2 and 26.1% for rice, wheat and maize, respectively, and less than 20% in intensive agricultural regions and for fruit trees or vegetable crops. In addition to surface and groundwater pollution and greenhouse gas emissions, over-application of N fertilizers has caused significant soil acidification in major Chinese croplands, decreasing soil pH by 0.13 to 2.20. High yield as a top priority, small-scale farming, lack of temporal synchronization of nutrient supply and crop demand, lack of effective extension systems, and hand application of fertilizers by farmers are possible reasons leading to the over-application problems. There is little doubt that current nutrient management practices are not sustainable and more efficient management systems need to be developed. A review of long-term experiments conducted around the world indicated that chemical fertilizer alone is not enough to improve or maintain soil fertility at high levels and the soil acidification problem caused by overapplication of synthetic N fertilizers can be reduced if more fertilizer N is applied as NO3− relative to ammonium- or urea-based N fertilizers. Organic fertilizers can improve soil fertility and quality, but long-term application at high rates can also lead to more nitrate leaching, and accumulation of P, if not managed well. Well-managed combination of chemical and organic fertilizers can overcome the disadvantages of applying single source of fertilizers and sustainably achieve higher crop yields, improve soil fertility, alleviate soil acidification problems, and increase nutrient-use efficiency compared with only using chemical fertilizers. Crop yield can be increased through temporal diversity using crop rotation strategies compared with continuous cropping and legume-based cropping systems can reduce carbon and nitrogen losses. Crop yield responses to N fertilization can vary significantly from year to year due to variation in weather conditions and indigenous N supply, thus the commonly adopted prescriptive approach to N management needs to be replaced by a responsive in-season management approach based on diagnosis of crop growth, N status and demand. A crop sensor-based in-season site-specific N management strategy was able to increase Nuptake efficiency by 368% over farmers’ practices in the North China Plain. Combination of these well-tested nutrient management principles and practices with modern crop management technologies is needed to develop sustainable nutrient management systems in China that can precisely match field-to-field and year-to-year variability in nutrient supply and crop demand for both single crops and crop rotations to not only improve nutrient-use efficiency but also increase crop yield and protect the environment. In addition, innovative and effective extension and service-providing systems to assist farmers in adopting and applying new management systems and technologies are also crucially important for China to meet the grand challenge of food security, nutrient-use efficiency and sustainable development.

Chapter 3 Nitrogen in Dryland Soils of China and Its Management

Abstract As an essential nutritional element, nitrogen (N) plays extremely important roles in agriculture. It is needed by plants in large amounts while almost all soils worldwide are deficient in this element, resulting in a great gap between its demand and its supply from soil. Application of N fertilizers has strongly helped promote agricultural development for supplying food to man and fodder to animals, and enabled the enormous and unprecedented expansion of the global human population. More than 55% of the increase of crop production in developing countries is from the use of chemical fertilizers with N fertilizers being dominant. China has consumed about 30% of the N fertilizer in the world and this is one of the reasons for the Chinese success to feed 21.8% of the worlds population with only 6.8% of the world arable land. However, nitrogen is also an important pollutant. Inputs of large amounts of N fertilizer have brought about many malpractices: low N use efficiency and low N fertilizer recovery have led to low economic returns, and the processes of N behavior such as ammonia volatilization, wet and dry deposition of the volatilized ammonium N to lands and waters, eutrophication in rivers, lakes and sea mouths, nitrate N formation by nitrification and nitrate N leaching, nitrate-N accumulation in water and plants (especially in vegetables), and transformation of nitrate N into N2O, NO, and N2 by denitrification. These processes have exerted a great impact on environmental pollution, ecosystem deterioration, and biodiversity as well as on human health. This situation not only affects agricultural outcome at the present, but will have a major impact on agricultural development in the future. Drylands are a large part of agricultural production areas in China, and the arable lands constitute a large proportion of the total cultivated lands. With the growth of population and the decrease of land and water resources, the drylands become more and more important for the Chinese agricultural development and sustainability. Due to poor plant growth induced by shortage of water supplies, serious wind and water erosion, and low input of fertilizes by farmers, the arable dryland soils in China are low in organic matter (OM) and thus in total N that varies from a minimum of 0.001% to a maximum of 0.178% depending on soil type and environmental conditions. Thus, N deficiency is a major nutritional constraint for crop production. In addition to wind and water erosion that causes organic N loss, nitrogen loss from dryland soils and fertilizers added to soil is mainly in mineral forms. Ammonia volatilization, and nitrate N accumulation in soil profile in and beyond root zones are the major pathway for N loss while denitrification loss is negligible. Despite N deficiency in most of drylands, excessive addition of N fertilizer has occurred in some places and resulted in great attention to the rational management of soil N. For improvement of nitrogen fertilizer recovery (NFR) and nitrogen use efficiency (NUE) while providing adequate N for crop production, some achievements have been made and various measures have been adopted. Application of N fertilizer with organic fertilizer (OF) can significantly increase the NUE, and at the same time significantly increase crop yield and water use efficiency (WUE). Since N in OF is slowly mineralized, P is too high for plant requirements. Thus, OF should be applied together with N fertilizers, but separately from P fertilizer. In most dryland areas, P deficiency has limited crop production and N fertilizer efficiency, and in some lands even prevented crops from responding to N fertilizer. Combining the use of N fertilizer with P fertilizer can increase NFR and NUE as well as WUE. If N fertilizer is mixed with acid P fertilizer, N loss by volatilization can be reduced. Deep application can place N fertilizer in the layer where more water is available, nutrients are deficient, and more roots are present for efficiently using N from the fertilizer applied and the moisture from the soil in addition to reduction of N loss by volatilization, and thus can increase crop yield, and fertilizer use efficiency. Deep application can be conducted with deep plowing so that N fertilizer can be placed in a suitable layer for plant use. In areas without supplemental irrigation, early application of N fertilizer to wheat and other autumn-sown crops should be encouraged while for maize with full irrigation, N fertilizer should be divided into four portions with one portion applied at sowing, one portion at elongation, and two portions before heading. A crop often obtains up 45%–70% of its total N from the soil. Therefore, N fertilizer applications should be made according to the soil nitrogen supplying capacity (SNSC). Several biological and chemical procedures have been used for evaluating the SNSC, yet none of the methods has proven suitable for agricultural practice. A large number of field results demonstrated that the cumulative amount of nitrate N from the 0 to 1 m soil layer was significantly correlated to crop uptake N with a correlation coefficient of 0.908, giving a very satisfactory index of soil availability. Due to high nitrate N accumulated in soil profile, the potentially mineralizable N, estimated by either incubation or chemical reagent extraction, did not show a good correlation. In contrast, in soils with low amounts of nitrate N accumulated in soil profile, some methods for determining mineralizable N did exert a positive role in reflecting the SNSC, having certain potentials for use. Crop responses to N forms depend on soil pH and plant species. In dryland soils, nitrate N is the major form existing in soil and due to high pH buffering capacity, it is also the major form taken up by plants, and the major crops, wheat and maize, have responded better to nitrate N than to ammonium N. Since ammonium-based N fertilizer, particularly urea, is the major form produced by industrial processes, enhancing nitrate nutrition may be difficult for some countries or some regions. For solving this problem, rapid nitrification of ammonium N may be a solution. This can be achieved by pretreating the soil with a small quantity of NH 4 + -salt before a large amount of urea is applied. For sustainable agriculture and eliminating N fertilizer pollution of the environment, different strategies have been proposed. Roughly, two ways are noted: agricultural and industrial. The former is improvement of crop growing conditions for efficient use of the N fertilizer whereas the latter improvement of N fertilizer characteristics. Of these, agricultural strategies are fundamental and basic. Applying adequate rates of N fertilizer, rotating legumes in cropping sequences, using organic materials in combination with chemical fertilizers, and improving crop health for better use of nutrients are some important aspects for consideration in the future.

Differences of Some Leguminous and Nonleguminous Crops in Utilization of Soil Phosphorus and Responses to Phosphate Fertilizers

As a vital component of a number of macromolecules and an integral part of energy metabolism and major biological processes in photosynthesis, respiration, and membrane transportation, as well as playing a genetic role through ribonucleic acid and energy transfers via adenosine triphosphate, phosphorous is indispensable for all life forms and cannot be substituted by any other element. Being the life-limiting element in natural ecosystems, regular inputs of P fertilizer to replenish the P removed from the soil by crops are one of the characteristics of modern agriculture. The demand for P resources will outstrip supply in the coming decades because the global commercial phosphate reserves may be depleted in another 60–130 years. In addition, rock phosphate (RP) reserves are under the control of a few countries. The P recovery rate is very low and the surpluses of P in soil have produced variable responses of crops to P fertilizers and environmental pollution. Requirements for direct application of RP and improvement of P fertilizer efficiency have led to adoption of specific plant species. Since leguminous crops in general respond better to P fertilizer than cereals, some scientists have proposed the application of P to leguminous crops as the first priority. Many hypotheses have been proposed to explain the different responses to P fertilizer between the two types of crops, but most of them have not been substantiated. A series of experiments have been conducted by us on different aspects for more than 40 years, and this chapter reviews the current investigation status and reports our viewpoints based on results obtained mainly from wheat [Triticum aestivum (L.) em. Thell], pea (Pisum sativum L.), maize (Zea mays L.), and soybean [Glycine max (L.) Merr.] One reason for the different response to P fertilizer supposes that legumes require more P than nonlegumes. A long-term experiment in a maize–maize–soybean rotation sequence in which maize and soybean were grown in the same season with almost the same duration of growing period showed that the total P uptake by soybean from unit area was similar to that of maize and in some cases uptake by maize was higher than that by soybean. Results of pot and field experiments conducted by us showed that P uptake amount by leguminous crops was not higher than that by cereal crops, and wheat had a higher capacity to use soil P than do pea and vetch. Without application of N fertilizer, P amounts taken up by legumes were equal to or slightly higher than those of nonlegumes, while cereal crops with N application took up much more P than legumes in most cases either with or without application of P fertilizer....

Vegetation, phosphorus, and dust gradients downwind from a cattle feedyard

Abstract A native shortgrass pasture downwind from a 25,000-head beef cattle feedyard near Bushland, Tex. degraded after the feedyard was stocked in 1970. Objectives were to determine pre-1970 vegetation, quantify current vegetation, and describe changes in vegetation, soil P and dust deposition with distance from the feedyard. Pre-1970 vegetation was documented with published measurements. In 2000, plant cover was quantified using 600 quadrats. Soil P, conserved in the local soil, was measured in soil samples from 119 locations. Dust was collected at 12 locations. From 1966–1972, cover was 18.8% blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Griffiths] and 7.4% buffalograss [Buchloe dactyloides (Nutt.) Engelm.]; the 2 species comprised 95% of vegetation cover. In 2000, perennial grass (75–99% blue grama) cover averaged 3.7% at < 150 m from the feedyard, and increased to 28% at > 525 m from the feedyard. Conversely, annual grass (67% Hordeum pusillum Nutt.) and annual forb [72% Kochia scoparia (L.) Schrad.] covers were 49% and 35% nearest the feedyard and decreased to 9% and 1%, respectively, at > 525 m. Over a similar distance, soil P decreased from 75 to 17 mg kg−1. Dust deposition rate decreased with distance from the feedyard. Manure dust contribution to total dust ranged from negligible to 89%. It was estimated that 20–30 kg N ha−1 year−1 were deposited over 30 years to areas nearest the feedyard. Changes in vegetation and soil P were greatest at < 500 m from the feedyard. Vegetation and soil P were near values expected for shortgrass prairie at > 500 m downwind from the feedyard. The pattern of vegetation, soil fertility, and dust deposition gradients strongly suggested that the feedyard was the primary cause of the observed changes, although a direct causal link could not be established, and other factors, such as grazing, could have contributed to the observed changes.

Soil Health Concerns Facing Dryland Agroecosystems

About 40% of the world’s total land area is considered drylands and is of increasing importance to meeting the food and fiber needs of a growing and more prosperous world population. Soils in dryland regions have been seriously degraded by erosion, excessive tillage, compaction, and other processes. Loss of soil organic carbon (SOC) has perhaps been the greatest concern because this affects chemical, physical, and biological properties simultaneously that seriously affect soil health. The restoration of SOC in dryland agroecosystems is particularly challenging because the low and highly variable amounts of precipitation limit biomass production required for carbon sequestration. Dryland regions also generally have high temperatures that hasten the decomposition of organic matter. Cropping systems that include the principles of conservation agriculture appear the best approach to maintain or enhance soil health in these regions. These principles are minimum soil disturbance, permanent soil cover with crops or plant residues, and diversification of crop species.

Investigations into the Beneficial Uses of Ash from the Combustion of Manure from Beef Cattle Feedlots

The potential for beneficial uses or co-products from the combustion of beef cattle manure were investigated. Phosphate concentrations indicate some potential for use as an agronomic soil amendment, but the phosphate is not freely released. Greenhouse studies suggest that neither good nor harm occurs. Ashes from the combustion of coal and lignite have been used effectively as an amendment to Portland cement concrete and a variety of construction applications, indicating potential for similar applications for the manure ash. The strength and chemical properties suggest only limited possibilities for the ash in this arena. The ash can be amended with Portland cement to yield a product suitable for road base or flowable fills, but appears to serve primarily as a fine aggregate lacking the plasticity normally associated with the Class C ashes produced from the combustion of lignite or sub-bituminous coals. Chemical analyses of the coal and manure ashes indicate subtle differences in the composition that may account for the difference in plasticity. Tests with bottom ash suggest that all ash fractions, when sintered and crushed, may be suitable for use on icy roads as a replacement for sand and salt. It is particularly suited to this application because of its essentially inert chemistry.