Roland J. Buresh

Integrated Nitrogen Management in Irrigated Rice

More than 90% of the rice (Oryza sativa l.) in the world is produced in Asia (IRRI, 1986b). Within Asia, rice production has been increasing an average 2.7% annually—slightly faster than population growth. The increased production is primarily due to higher yields per hectare rather than increased land area under cultivation. The adoption of modern varieties, increased irrigation facilities, and more fertilizer use have been credited for the yield increases. An estimated 24% of the increase in Asian rice production from 1965 to 1980 was attributed to use of fertilizer, mainly N (Barker et al., 1985).

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.

Chapter 3 Crop Residue Management for Lowland Rice-Based Cropping Systems in Asia

Intensification of rice-based cropping systems in Asia has substantially increased production of food and associated crop residues. The interval between crops in these systems is often brief, making it attractive for farmers to burn residues in the field to hasten and facilitate tillage for the next crop. Open-air burning causes serious air quality problems affecting human health and safety, and it has been banned by many Asian governments. In this chapter, we evaluate for rice-based cropping systems existing and emerging in-field alternatives to burning residues based on criteria of productivity, profitability, environmental impact, and sustainability. In intensive rice monocropping systems, residue is typically managed under conditions of soil flooding and anaerobic decomposition during the rice crop. In systems, where rice is rotated with an upland (non-flooded) crop, there are two major categories: residue of upland crop managed during flooded rice and rice residue managed during the upland crop. One option during the flooded rice crop is incorporation of residues from the previous rice or upland crop into the soil. Many studies have examined incorporation of crop residue during land preparation for flooded rice. In the vast majority of cases there was no significant increase in yield or profit. Residue decomposition in anaerobic flooded soil substantially increases methane (CH 4 ) emission relative to residue removal. Surface retention of residue during rice cropping is challenging to implement because residue must be removed from the field during conventional tillage with soil flooding (i.e., puddling) and then returned. Alternatively, rice must be established without the traditional puddling that has helped sustain its productivity. Mulch is a good option for rice residue management during the upland crop, especially with reduced and no tillage. Mulch can increase yield, water use efficiency, and profitability, while decreasing weed pressure. It can slightly increase nitrous oxide (N 2 O) emission compared with residue incorporation or removal, but N fertilization and water management are typically more important factors controlling N 2 O emission than residue management. Long-term studies of residue removal have indicated that removing residue from continuous rice systems with near continuous soil flooding does not adversely affect soil organic matter (SOM). The use of crop residue as a mulch with reduced or no tillage for upland crops should be promoted in rice-based cropping systems. On the contrary, residues from the crop preceding rice on puddled and flooded soil can be considered for removal for off-field uses.

Managing native and legume-fixed nitrogen in lowland rice-based cropping systems

Lowlands comprise 87% of the 145 M ha of world rice area. Lowland rice-based cropping systems are characterized by soil flooding during most of the rice growing season. Rainfall distribution, availability of irrigation water and prevailing temperatures determine when rice or other crops are grown. Nitrogen is the most required nutrient in lowland rice-based cropping systems. Reducing fertilizer N use in these cropping systems, while maintaining or enhancing crop output, is desirable from both environmental and economic perspectives. This may be possible by producing N on the land through legume biological nitrogen fixation (BNF), minimizing soil N losses, and by improved recycling of N through plant residues. At the end of a flooded rice crop, organic- and NH4-N dominate in the soil, with negligible amounts of NO3. Subsequent drying of the soil favors aerobic N transformations. Organic N mineralizes to NH4, which is rapidly nitrified into NO3. As a result, NO3 accumulates in soil during the aerobic phase. Recent evidence indicates that large amounts of accumulated soil NO3 may be lost from rice lowlands upon the flooding of aerobic soil for rice production. Plant uptake during the aerobic phase can conserve soil NO3 from potential loss. Legumes grown during the aerobic phase additionally capture atmospheric N through BNF. The length of the nonflooded season, water availability, soil properties, and prevailing temperatures determine when and where legumes are, or can be, grown. The amount of N derived by legumes through BNF depends on the interaction of microbial, plant, and environmental determinants. Suitable legumes for lowland rice soils are those that can deplete soil NO3 while deriving large amounts of N through BNF. Reducing soil N supply to the legume by suitable soil and crop management can increase BNF. Much of the N in legume biomass might be removed from the land in an economic crop produce. As biomass is removed, the likelihood of obtaining a positive soil N balance diminishes. Nonetheless, use of legumes rather than non-legumes is likely to contribute higher quantities of N to a subsequent rice crop. A whole-system approach to N management will be necessary to capture and effectively use soil and atmospheric sources of N in the lowland rice ecosystem.

Inorganic nitrogen dynamics in fallows and maize on an Oxisol and Alfisol in the highlands of Kenya

Abstract Fallows with naturally regenerated or planted vegetation are important in many subsistence agricultural systems of tropical regions, but the underlying soil processes in fallows are not properly understood. We investigated N dynamics under different fallow vegetation on a Kandiudalfic Eutrudox (2372-mm rain in 16 months) and a Kandic Paleustalf (1266-mm rain in 15 months) in the Kenyan highlands. The treatments, which extended for three cropping seasons (15–16 months), were Zea mays (maize), natural regrowth of vegetation (natural fallow), planted Sesbania sesban (sesbania fallow) and uncultivated soil without vegetation (bare fallow). Inorganic N (nitrate+ammonium-N) to 2-m depth under bare fallow increased by 242 kg N ha −1 year −1 on the Oxisol and 54 kg N ha −1 year −1 on the Alfisol, indicating that N mineralization exceeded N losses. Subsoil inorganic N (0.5–2.0 m) remained relatively unchanged after three crops of unfertilized maize, which produced limited total biomass because of P deficiency. Inorganic N decreased during natural and sesbania fallows, and both fallows similarly depleted subsoil inorganic N. The fallows depleted inorganic N at 0.5–2.0 m by 75–125 kg N ha −1 year −1 down to a minimum N content between 40 and 80 kg N ha −1 . After slashing sesbania and incorporating the above-ground biomass with 154–164 kg N ha −1 , soil inorganic N increased within 2 months by 136 kg N ha −1 on the Oxisol and 148 kg N ha −1 on the Alfisol. Inorganic N decreased after cropping the bare fallow on the Oxisol with maize, indicating that inorganic N was prone to leaching during heavy rains when the maize was small. A considerable part of the N in biomass of the natural fallow was recycled. Much of the total N accumulated by the sesbania fallow was removed with the wood and the amount of N recycled was similar on the Oxisol and Alfisol. We conclude that sesbania fallows can retrieve considerable subsoil inorganic N on deep soils with high subsoil N and effectively cycle this N through its rapidly decomposable biomass to subsequent crops.

Sieve size effects on root length and biomass measurements of maize (Zea mays) and Grevillea robusta

The purpose of this study was to investigate the effects of different mesh sizes on the recovery of root length and biomass and to determine whether the degree of recovery was influenced by plant species and sample location. Sieves of 2.0, 1.0, 0.5 and 0.25 mm (4.0, 1.0, 0.25 and 0.06 mm2) mesh sizes were used to recover and measure the root length and biomass of Zea mays L. (maize) at 0–15 cm and 30–45 cm depths and of Grevillea robusta A. Cunn. ex R. Br. (grevillea) at the same depths 1.0 m and 4.5 m from a line of grevillea trees. At 0–15 cm, the coarser sieves (sum collected with 2.0 and 1.0 mm sieves) recovered approximately 80% of the total root biomass measured, but only 60% of the root length. The proportion of total maize root length and biomass recovered by the coarser sieves decreased with soil depth. The proportion of total grevillea root length recovered by the coarser sieves was similar at the two soil depths, but increased slightly with distance from the tree line. The ≥ 0.5 mm sieves recovered between 93 and 96% of grevillea and maize root biomass and between 73 and 98% of their root length, depending on the sample location. Roots passing through the 0.5 mm sieve, but recovered by the 0.25 mm sieve were about 20% of total maize root length and grevillea root length at 1.0 m from the tree line but < 5% of the total grevillea root length at 4.5 m from the tree. Roots passing through the 0.5 mm sieve but recovered by the 0.25 mm sieve contributed only slightly to root biomass. Although the ≥ 0.5 mm sieves provided adequate measurements of root biomass, the ≥ 0.25 mm sieves were required for accurate measurement of fine root length. There was no universal correction for root length and biomass underestimation when large sieve sizes were used because the proportions of length and biomass recovered depended on the plant species and on soil depth and distance from the plant.

Utilisation of soil organic P by agroforestry and crop species in the field, western Kenya

A field experiment in western Kenya assessed whether the agroforestry species Tithonia diversifolia (Hemsley) A. Gray, Tephrosia vogelii Hook f., Crotalaria grahamiana Wight & Arn. and Sesbania sesban (L) Merill. had access to forms of soil P unavailable to maize, and the consequences of this for sustainable management of biomass transfer. The species were grown in rows at high planting density to ensure the soil under rows was thoroughly permeated by roots. Soil samples taken from beneath rows were compared to controls, which included a bulk soil monolith enclosed by iron sheets within the tithonia plot, continuous maize, and bare fallow plots. Three separate plant biomass samples and soil samples were taken at 6-month intervals, over a period of 18 months. The agroforestry species produced mainly leaf biomass in the first 6 months but stem growth dominated thereafter. Consequently, litterfall was greatest early in the experiment (0–6 months) and declined with continued growth. Soil pH increased by up to 1 unit (from pH 4.85) and available P increased by up to 38% (1 μg P g−1) in agroforestry plots where biomass was conserved on the field. In contrast, in plots where biomass was removed, P availability decreased by up to 15%. Coincident with the declines in litterfall, pH decreased by up to 0.26 pH units, plant available P decreased by between 0.27 and 0.72 μg g−1 and Po concentration decreased by between 8 and 35 μg g−1 in the agroforestry plots. Declines in Po were related to phosphatase activity (R2=0.65, P<0.05), which was greater under agroforestry species (0.40–0.50 nmol MUB s−1 g−1) than maize (0.28 nmol MUB s−1 g−1) or the bare fallow (0.25 nmol MUB s−1 g−1). Management of tithonia for biomass transfer, decreased available soil P by 0.70 μg g−1 and Po by 22.82 μg g−1. In this study, tithonia acquired Po that was unavailable to maize. However, it is apparent that continuous cutting and removal of biomass would lead to rapid depletion of P stored in organic forms.

Improving Nitrogen Fertilization in Rice by Site-Specific N Management

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 − 1 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.

Nitrogen gas (N2+N2O) flux from urea applied to lowland rice as affected by green manure

A field study was conducted on a clay soil (Andaqueptic Haplaquoll) in the Philippines to directly measure the evolution of (N2+N2O)−15N from 98 atom %15N-labeled urea broadcast at 29 kg N ha−1 into 0.05-m-deep floodwater at 15 days after transplanting (DT) rice. The flux of (N2+N2O)−15N during the 19 days following urea application never exceeded 28 g N ha−1 day−1. The total recovery of (N2+N2O)−15N evolved from the field was only 0.51% of the applied N, whereas total gaseous15N loss estimated from unrecovered15N in the15N balance was 41% of the applied N. Floodwater (nitrate+nitrite)−N in the 5 days following urea application never exceeded 0.14 g N m−3 or 0.3% of the applied N. Prior cropping of cowpea [Vigna unguiculata (L.) Walp.] to flowering with subsequent incorporation of the green manure (dry matter=2.5 Mg ha−1, C/N=15) at 15 days before rice transplanting had no effect on fate of urea applied to rice at 15 DT. The recovery of (N2+N2O)−15N and total15N loss during the 19 days following urea application were 0.46 and 40%, respectively. Direct recovery of evolved (N2+N2O)−15N and total15N loss from 27 kg applied nitrate-N ha−1 were 20% and 53% during the same 19-day period. The failure of directly-recovered (N2+N2O)−15N to match total15N loss from added nitrate-15N might be due to entrapment of denitrification end products in soil or transport of gaseous end products to the atmosphere through rice plants. The rapid conversion of added nitrate-N to (N2+N2O)−N, the apparently sufficient water soluble soil organic C for denitrification (101 μg C g−1 in the top 0.15-m soil layer), and the low floodwater nitrate following urea application suggested that denitrification loss from urea was controlled by supply of nitrate rather than by availability of organic C.

Nitrogen contribution of cowpea green manure and residue to upland rice

Cowpea, Vigna unguiculata (L.) Walp., is well adapted to acid upland soil and can be grown for seed, green manure, and fodder production. A 2-yr field experiment was conducted on an Aeric Tropaqualf in the Philippines to determine the effect of cowpea management practice on the response of a subsequent upland rice crop to applied urea. Cowpea was grown to flowering and incorporated as a green manure or grown to maturity with either grain and pods removed or all aboveground vegetation removed before sowing rice. Cowpea green manure accumulated on average 68 kg N ha−1, and aboveground residue after harvest of dry pods contained on average 46 kg N ha−1. Compared with a pre-rice fallow, cowpea green manure and residue increased grain yield of upland rice by 0.7 Mg ha−1 when no urea was applied to rice. Green manure and residue substituted for 66 and 70 kg urea-N ha−1 on upland rice, respectively. In the absence of urea, green manure and residue increased total aboveground N in mature rice by 12 and 14 kg N ha−1, respectively. These increases corresponded to plant recoveries of 13% for applied green manure N and 24% for applied residue N. At 15 d after sowing rice (DAS), 33% of the added green manure N and 16% of the added residue N was recovered as soil (nitrate + ammonium)-N. At 30 DAS, the corresponding recoveries were 20 and 37% for green manure N and residue N, respectively. Cowpea cropping with removal of all aboveground cowpea vegetation slightly increased (p<0.05) soil (nitrate + ammonium)-N at 15 DAS as compared with the pre-rice fallow, but it did not increase rice yield. Cowpea residue remaining after harvest of dry pods can be an effective N source for a subsequent upland rice crop.