A. Dobermann

Agroecosystems, nitrogen-use efficiency, and nitrogen management.

Abstract The global challenge of meeting increased food demand and protecting environmental quality will be won or lost in cropping systems that produce maize, rice, and wheat. Achieving synchrony between N supply and crop demand without excess or deficiency is the key to optimizing trade-offs amongst yield, profit, and environmental protection in both large-scale systems in developed countries and small-scale systems in developing countries. Setting the research agenda and developing effective policies to meet this challenge requires quantitative understanding of current levels of N-use efficiency and losses in these systems, the biophysical controls on these factors, and the economic returns from adoption of improved management practices. Although advances in basic biology, ecology, and biogeochemistry can provide answers, the magnitude of the scientific challenge should not be underestimated because it becomes increasingly difficult to control the fate of N in cropping systems that must sustain yield increases on the worlds limited supply of productive farm land.

Opportunities for increased nitrogen-use efficiency from improved resource management in irrigated rice systems

Abstract Research and extension work to improve nitrogen (N) management of irrigated rice has received considerable investment because yield levels presently achieved by Asian farmers depend on large amounts of N fertilizer. Most work has focused on placement, form, and timing of applied N to reduce losses from volatilization and denitrification. In contrast, less emphasis has been given to development of methods to adjust N rates in relation to the amount of N supplied by indigenous soil resources. As a result, N fertilizer recommendations are typically made for districts or regions with the implicit assumption that soil N supply is relatively uniform within these domains. Recent studies, however, document tremendous variation in soil N supply among lowland rice fields with similar soil types or in the same field over time. Despite these differences, rice farmers do not adjust applied N rates to account for the wide range in soil N supply, and the resulting imbalance contributes to low N-use efficiency. A model for calculating N-use efficiency is proposed that explicitly accounts for contributions from both indigenous and applied N to plant uptake and yield. We argue that increased N-use efficiency will depend on field-specific N management tactics that are responsive to soil N supply and plant N status. N fertilizer losses are thus considered a symptom of incongruence between N supply and crop demand rather than a driving force of N efficiency. Recent knowledge of process controls on N cycling, microbial populations, and soil organic matter (SOM) formation and decomposition in flooded soils are discussed in relation to N-use efficiency. We conclude that the intrinsic capacity of wetland rice systems to conserve N and the rapid N uptake potential of the rice plant provide opportunities for significant increases in N efficiency by improved management and monitoring of indigenous N resources, straw residues, plant N status, and N fertilizer.

Rice yields in tropical/subtropical Asia exhibit large but opposing sensitivities to minimum and maximum temperatures

Data from farmer-managed fields have not been used previously to disentangle the impacts of daily minimum and maximum temperatures and solar radiation on rice yields in tropical/subtropical Asia. We used a multiple regression model to analyze data from 227 intensively managed irrigated rice farms in six important rice-producing countries. The farm-level detail, observed over multiple growing seasons, enabled us to construct farm-specific weather variables, control for unobserved factors that either were unique to each farm but did not vary over time or were common to all farms at a given site but varied by season and year, and obtain more precise estimates by including farm- and site-specific economic variables. Temperature and radiation had statistically significant impacts during both the vegetative and ripening phases of the rice plant. Higher minimum temperature reduced yield, whereas higher maximum temperature raised it; radiation impact varied by growth phase. Combined, these effects imply that yield at most sites would have grown more rapidly during the high-yielding season but less rapidly during the low-yielding season if observed temperature and radiation trends at the end of the 20th century had not occurred, with temperature trends being more influential. Looking ahead, they imply a net negative impact on yield from moderate warming in coming decades. Beyond that, the impact would likely become more negative, because prior research indicates that the impact of maximum temperature becomes negative at higher levels. Diurnal temperature variation must be considered when investigating the impacts of climate change on irrigated rice in Asia.

Management of phosphorus, potassium, and sulfur in intensive, irrigated lowland rice

Abstract Management of soil phosphorus (P), potassium (K) and sulfur (S) resources in intensive, irrigated rice systems has received less attention than increasing cropping intensity and yields with new cultivars, irrigation, and fertilizer N. Crop requirements, input-output balance, and soil supplying capacity of P, K and S in irrigated lowland rice are reviewed. Based on projected rice production requirements, we estimate that the total annual nutrient demand for irrigated rice will be about 9 to 13 × 106 t N, 9 to 15 × 106 t K, 1.2 to 2.4 × 106 t P and 0.9 to 1.5 × 106 t S in 2025, amounts that represent an increase of 65 to 70% above 1990 requirements. At present, negative K balances are widespread and K deficiency has become a constraint to increasing yields, even on heavy-textured lowland soils with high inherent fertility. Because opportunities are limited for breeding cultivars that acquire more P, K or S from soil or have higher internal nutrient-use efficiencies, long-term management strategies must focus on maintaining adequate nutrient balances in the topsoil layer. Interactions among nutrients have a large influence on physiological and agronomic efficiency that result from nutrient applications. Strategies that only aim at increasing P or K application rates without considering the indigenous supply from soil reserves are inefficient; they may not sustain yield increases to meet rice demand. Little improvement in fertilizer use efficiency can be expected from the present system of providing blanket recommendations for a given production domain. Instead, site-specific nutrient-management approaches will be needed to accommodate the tremendous variability in indigenous nutrient supply found in the irrigated lowlands of Asia.

Plant nutrient management for enhanced productivity in intensive grain production systems of the United States and Asia

Are present nutrient management recommendations for the worlds major cereal cropping systems adequate to sustain the productivity gains required to meet food demand while also assuring acceptable standards of environmental quality? To address this question, the current nutrient management approaches and their scientific basis in large-scale, mechanized maize (Zea mays L.)-based cropping systems of the USA and more labor-intensive, small-scale irrigated rice (Oryza sativa L.) production systems in Asia were evaluated. The principal challenges in both systems are similar: (1) there is no compelling evidence for significant increases in the genetic yield potential in both systems during the past 30 years, (2) farm yields are presently about 40–65% of the attainable yield potential, and (3) nutrient management mostly relies on approaches that do not account for the dynamic nature of crop response to the environment. Because average farm yield levels of 70–80% of the attainable yield potential are necessary to meet expected food demand in the next 30 years, research must seek to develop nutrient management approaches that optimize profit, preserve soil quality, and protect natural resources in systems that consistently produce at these high yield levels. Achieving these goals will require novel strategies for more precise plant nutrient management tailored to the technologies, dynamics and spatial scales relevant to each system. Significant advances in soil chemistry, crop physiology, plant nutrition, molecular biology, and information technology must be combined in this effort. Future field-oriented plant nutrition research must be of a more strategic, interdisciplinary, and quantitative nature. Systems approaches at micro- to meso-scales are required for gaining a more quantitative understanding of crop response to nutrients based on interactions among the essential crop nutrient requirements and on response to dynamic environmental conditions.

Improving nitrogen fertilization in rice by site-specific N management. A review

Excessive nitrogen (N) application to rice (Oryza sativa L.) crop in China causes environmental pollution, increases the cost of rice farming, reduces grain yield and contributes to global warming. Scientists from the International Rice Research Institute have collaborated with partners in China to improve rice N fertilization through site-specific N management (SSNM) in China since 1997. Field experiments and demonstration trials were conducted initially in Zhejiang province and gradually expanded to Guangdong, Hunan, Jiangsu, Hubei and Heilongjiang provinces. On average, SSNM reduced N fertilizer by 32% and increased grain yield by 5% compared with farmers’ N practices. The yield increase was associated with the reduction in insect and disease damage and improved lodging resistance of rice crop under the optimal N inputs. The main reason for poor fertilizer N use efficiency of rice crop in China is that most rice farmers apply too much N fertilizer, especially at the early vegetative stage. We observed about 50% higher indigenous N supply capacity in irrigated rice fields in China than in other major rice-growing countries. Furthermore, yield response of rice crop to N fertilizer application is low in China, around 1.5 t ha− on average. However, these factors were not considered by rice researchers and extension technicians in determining the N fertilizer rate for recommendation to rice farmers in China. After a decade of research on SSNM in China and other Asian rice-growing countries, we believe SSNM is a matured technology for improving both fertilizer N use efficiency and grain yield of rice crop. Our challenges are to further simplify the procedure of SSNM and to convince policy-makers of the effectiveness of this technology in order to facilitate a wider adoption of SSNM among rice farmers in China.

Soil organic matter and the indigenous nitrogen supply of intensive irrigated rice systems in the tropics

Soil organic matter (SOM) has been proposed as an index of N supply in paddy soils although field validations are few. We evaluated the relationship between the indigenous N supply (Ni) of the soil-floodwater system and soil organic carbon (SOC) or total N (Nt) in surface soil of long-term fertility experiments (LTFEs) at 11 sites, in 42 farmers fiels with similar soil type, and in the same field in ten consecutive rice (Oryza sativa L.) crops. The Ni was estimated by crop N uptake from plots without applied N (No plots) under otherwise favorable growth conditions. There was a tight linear correlation between yields and N uptake in No plots and tremendous variation in both parameters among LTFE sites, farmers fields, and in the same field over time. Correlation between Ni and SOC or Nt explained little of this variation. Factors likely to contribute to the poor correlation were: (1) inputs of N from sources other than N mineralization of SOM in surface soil, (2) degree of congruence between soil N supply and crop demand, which is sensitive to soil drying, length of fallow, crop rotation, and residue management, and (3) differences in SOM quality related to intensive cropping in submerged soil. Better understanding of the processes governing the Ni of tropical lowland rice systems would contribute to the development of crop management practices that optimize utilization of indigenous N resources.

Fertilizer inputs, nutrient balance, and soil nutrient-supplying power in intensive, irrigated rice systems. I. Potassium uptake and K balance

Research in many countries indicates a negative K balance in intensive, irrigated rice systems but comparative studies across different environments are few. Using a uniform sampling methodology, we measured K uptake, K use efficiency, and K balance in six different fertilizer treatments of long-term fertility experiments with rice at 11 sites in five Asian countries. Depending on the absolute yield level, K uptake requirements of rice ranged from 17 to 30 kg K per ton of grain. For yields greater than 8 t ha-1, total K uptake exceeded 200 kg ha-1. The K balance at most experimental sites was negative, with an average net removal of 34–63 kg K season-1. There was significant depletion of soil K reserves at many sites. Based on these data, we estimated that the amount of K cycled annually from the soil into rice plants is 7–10 million t in irrigated rice systems of Asia. About 1 million t of this total amount is removed with the harvested grain. Present recommendations for K addition in most intensive irrigated rice domains are insufficient to replace K removal. However, response to K can only be expected on soils with deficient supply capacity and where other nutrients, particularly N and P, are not limiting. Efficient K management for rice must therefore be based on the K input/output balance, the achievable yield target, and the effective K-supplying power of the soil.

Do organic amendments improve yield trends and profitability in intensive rice systems

Opinions differ as to the importance of organic amendments (OA) for sustaining crop productivity in the intensive, irrigated rice systems of Asia. Our objectives were to (1) quantify the effects of farmyard manure (FYM) and straw incorporation on yield trends in long-term experiments (LTEs) with rice–rice (R–R) (Oryza sativa L.) and rice–wheat (R–W) (Triticum aestivum L.) systems and (2) assess the potential effects of OA on profitability, taking into account long-term effects on yield. We analyzed yield trends in 25 LTE (seven R–R, 18 R–W systems) across a wide geographical range in Asia. Three main conclusions emerged from this analysis. First, application of either manure or straw did not improve grain yield trends in R–R and R–W cropping systems. Second, depending on socio-economic conditions, use of manure or straw in these cropping systems may be profitable, provided these OA are used as a complement to a recommended dose of inorganic NPK (i.e. organic materials should not be used as the primary nutrient source). Third, current experimental designs to assess the suitability of OA need to be improved in order to allow a better comparison of the relative advantages of inorganic and organic fertilizers. The major shortcoming of current designs is that they do not properly adjust mineral fertilizer rates in the inorganic treatments to account for the macronutrient input from OA. Thus, our tentative estimates of the profitability of OA may be overstated.

Cereal area and nitrogen use efficiency are drivers of future nitrogen fertilizer consumption

At a global scale, cereal yields and fertilizer N consumption have increased in a near-linear fashion during the past 40 years and are highly correlated with one another. However, large differences exist in historical trends of N fertilizer usage and nitrogen use efficiency (NUE) among regions, countries, and crops. The reasons for these differences must be understood to estimate future N fertilizer requirements. Global nitrogen needs will depend on: (i) changes in cropped cereal area and the associated yield increases required to meet increasing cereal demand from population and income growth, and (ii) changes in NUE at the farm level. Our analysis indicates that the anticipated 38% increase in global cereal demand by 2025 can be met by a 30% increase in N use on cereals, provided that the steady decline in cereal harvest area is halted and the yield response to applied N can be increased by 20%. If losses of cereal cropping area continue at the rate of the past 20 years (−0.33% per year) and NUE cannot be increased substantially, a 60% increase in global N use on cereals would be required to meet cereal demand. Interventions to increase NUE and reduce N losses to the environment must be accomplished at the farm-or field-scale through a combination of improved technologies and carefully crafted local policies that contribute to the adoption of improved N management; uniform regional or national directives are unlikey to be effective at both sustaining yield increases and improving NUE. Examples from several countries show that increases in NUE at rates of 1% per year or more can be achieved if adequate investments are made in research and extension. Failure to arrest the decrease in cereal crop area and to improve NUE in the world’s most important agricultural systems will likely cause severe damage to environmental services at local, regional, and global scales due to a large increase in reactive N load in the environment.