John M. Duxbury

Global estimates of potential mitigation of greenhouse gas emissions by agriculture

Technologies to reduce net emissions of carbon dioxide, methane and nitrous oxide within the agriculture sector were reviewed to estimate the global potential for mitigation of these radiatively active greenhouse gases. Our estimates of the potential reduction of radiative forcing by the agricultural sector range from 1.15-3.3 Gt C equivalents per year. Of the total potential reduction, approximately 32% could result from reduction in CO2 emissions, 42% of carbon offsets by biofuel production on 15% of existing croplands, 16% from reduced CH4 emissions and 10% from reduced emissions of N2O. Agriculture encompasses large regional differences in management practices and rates of potential adoption of mitigation practices. Acceptability of mitigation options will depend on the extent to which sustainable production will be achieved or maintained and benefits will accrue to farmers. Technologies such as no-till farming and strategic fertilizer placement and timing are now being adopted for reasons other than concern for climate change issues.

Nitrous Oxide Emissions from Agricultural Fields: Assessment, Measurement and Mitigation

In this paper we discuss three topics concerning N2O emissions from agricultural systems. First, we present an appraisal of N2O emissions from agricultural soils (Assessment). Secondly, we discuss some recent efforts to improve N2O flux estimates in agricultural fields (Measurement), and finally, we relate recent studies which use nitrification inhibitors to decrease N2O emissions from N-fertilized fields (Mitigation).To assess the global emission of N2O from agricultural soils, the total flux should represent N2O from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N from crop residues, N2O from subsurface aquifers below the study area, and current N fertilization. Of these N sources only synthetic fertilizer and animal manures and the area of fields cropped with legumes have sufficient global data to estimate their input for N2O production. The assessment of direct and indirect N2O emissions we present was made by multiplying the amount of fertilizer N applied to agricultural lands by 2% and the area of land cropped to legumes by 4 kg N2O-N ha-1. No regard to method of N application, type of N, crop, climate or soil was given in these calculations, because the data are not available to include these variables in large scale assessments. Improved assessments should include these variables and should be used to drive process models for field, area, region and global scales.Several N2O flux measurement techniques have been used in recent field studies which utilize small and ultralarge chambers and micrometeorological along with new analytical techniques to measure N2O fluxes. These studies reveal that it is not the measurement technique that is providing much of the uncertainty in N2O flux values found in the literature but rather the diverse combinations of physical and biological factors which control gas fluxes. A careful comparison of published literature narrows the range of observed fluxes as noted in the section on assessment. An array of careful field studies which compare a series of crops, fertilizer sources, and management techniques in controlled parallel experiments throughout the calendar year are needed to improve flux estimates and decrease uncertainty in prediction capability.There are a variety of management techniques which should conserve N and decrease the amount of N application needed to grow crops and to limit N2O emissions. Using nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N2O emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N2O emissions from agricultural soils. Inhibitors may be selected for climatic conditions and type of cropping system as well as the type of nitrogen (solid mineral N, mineral N in solution, or organic waste materials) and applied with the fertilizers.

Assessing and Mitigating N2O Emissions from Agricultural Soils

Agricultural cropping and animal production systems are important sources of atmospheric nitrous oxide (N2O). The assessment of the importance of N fertilization from synthetic fertilizer, animal wastes used as fertilizers and from N incorporated into the soil through biological N fixation, to global N2O emissions presented in this paper suggests that this source has been underestimated. We estimate that agricultural systems produce about one fourth of global N2O emissions. Methods of mitigating these emissions are presented which, if adopted globally could decrease annual N2O emissions from cropped soils by about 20%.

Food chain aspects of arsenic contamination in Bangladesh: effects on quality and productivity of rice.

Abstract The total arsenic content of 150 paddy rice samples collected from Barisal, Comilla, Dinajpur, Kaunia, and Rajshahi districts, and from the BRRI experimental station at Rajshahi city in the boro and aman seasons of 2000 was determined by hydride generation-inductively coupled plasma emission spectroscopy (ICP). Arsenic concentrations varied from 10 to 420 µg/kg at 14% moisture content. Rice yields and grain arsenic concentrations were 1.5 times higher in the boro (winter) than the summer (monsoon) season, consistent with the much greater use of groundwater for irrigation in the boro season. Mean values for the boro and aman season rices were 183 and 117 µg/kg, respectively. The variation in arsenic concentrations in rice was only partially consistent with the pattern of arsenic concentrations in drinking water tube wells. There was no evidence from yield or panicle sterility data of arsenic toxicity to rice. Processing of rice (parboiling and milling) reduced arsenic concentrations in rice by an average of 19% in 21 samples collected from households. Human exposure to arsenic through rice would be equivalent to half of that in water containing 50 µg/kg for 14% of the paddy rice samples at rice and water intake levels of 400 g and 4 L/cap/day, respectively.

The significance of agricultural sources of greenhouse gases

The impact of development of land for agriculture and agricultural production practices on emissions of greenhouse gases is reviewed and evaluated within the context of anthropogenic radiative forcing of climate. Combined, these activities are estimated to contribute about 25%, 65%, and 90% of total anthropogenic emissions of CO2, CH4, and N2O, respectively. Agriculture is also a significant contributor to global emissions of NH3, CO, and NO. Over the last 150 y, cumulative emissions of CO2 associated with land clearing for agriculture are comparable to those from combustion of fossil fuel, but the latter is the major source of CO2 at present and is projected to become more dominant in the future. Ruminant animals, rice paddies, and biomass burning are principal agricultural sources of CH4, and oxidation of CH4 by aerobic soils has been reduced by perturbations to natural N cycles. Agricultural sources of N2O have probably been substantially underestimated due to incomplete analysis of increased N flows in the environment, especially via NH3 volatilization from animal manures, leaching of NO3-, and increased use of biological N fixation.The contribution of agriculture to radiative forcing of climate is analyzed using data from the Intergovernmental Panel on Climate Change (IPCC)(base case) and cases where the global warming potential of CH4, and agricultural emissions of N2O are doubled. With these scenarios, agriculture, including land clearing, is estimated to contribute between 28–33% of the radiative forcing created over the next 100yr by 1990 anthropogenic emissions of CO2, CH4, and N2O. Analyses of the sources of agriculturally generated radiative climate forcing show that 80% is associated with tropical agriculture and that two-thirds comes from non-soil sources of greenhouse gases. The importance of agriculture to radiative forcing created by different countries varies widely and is illustrated by comparisons between the USA, India, and Brazil. Some caveats to these analyses include inadequate evaluations of the net greenhouse effects of agroecosystems, uncertainties in global fluxes of greenhouse gases, and incomplete understanding of tropospheric chemical processes.Extension of the analytical approach to projected future emissions of greenhouse gases (IPCC moderate growth scenario) indicates that agriculture will become a less important source of radiative forcing in the future. Technological approaches to mitigation of agricultural sources of greenhouse gases will probably focus on CH4 and N2O because emissions of CO2 are essentially associated with the socio-political issue of tropical deforestation. Available technologies include dietary supplements to reduce CH4 production by ruminant animals and various means of improving fertilizer N management to reduce N2O emissions. Increased storage of C in soil organic matter is not considered to be viable because of slow accretion rates and misconceptions about losses of soil organic matter from agricultural soils.

A COMPARISON OF THE EFFECTS OF MICRONUTRIENT SEED PRIMING AND SOIL FERTILIZATION ON THE MINERAL NUTRITION OF CHICKPEA ( Cicer arietinum ), LENTIL ( Lens culinaris ), RICE ( Oryza sativa ) AND WHEAT ( Triticum aestivum ) IN NEPAL

SUMMARY Soil deficiencies of zinc (Zn) and boron (B) limit crop production in Nepal. Improving the micronutrient status of plants would increase yield and increase micronutrient content of the seeds, leading to better nutrition of the progeny crop and to improved human micronutrient nutrition. The primary micronutrient problem in grain legumes is B deficiency, while in rice (Oryza sativa), Zn deficiency is more important, and wheat (Triticum aestivum) suffers from both deficiencies. A series of field experiments was carried out over two seasons to compare soil fertilization and micronutrient seed priming as methods of improving Zn and B nutrition of each crop. Micronutrient treatments were evaluated for their effect on grain yield and grain micronutrient content. Soil B fertilization increased B content of the grain of lentil (Lens culinaris), chickpea (Cicer arietinum), and wheat by a factor of two to five, while increasing the yield of chickpea only. Soil fertilization with Zn had no effect on yield of any crop, but resulted in a small increase in Zn in wheat grain. Sowing micronutrient-primed seeds had no effect on yield or micronutrient content of the progeny seeds in most cases. During the first season, the primed chickpea seeds failed to emerge at either site, causing complete yield loss, but this negative effect was not observed in the second season with similar priming treatments at nearby sites, and no effect of priming on yield was observed with any other crop in either season.

Spatial variability of arsenic concentration in soils and plants, and its relationship with iron, manganese and phosphorus.

Spatial distribution of arsenic (As) concentrations of irrigation water, soil and plant (rice) in a shallow tube-well (STW) command area (8 ha), and their relationship with Fe, Mn and P were studied. Arsenic concentrations of water in the 110 m long irrigation channel clearly decreased with distance from the STW point, the range being 68-136 microg L(-1). Such decreasing trend was also noticed with Fe and P concentrations, but the trend for Mn concentrations was not remarkable. Concerning soil As, the concentration showed a decreasing tendency with distance from the pump. The NH(4)-oxalate extractable As contributed 36% of total As and this amount of As was associated with poorly crystalline Fe-oxides. Furthermore only 22% of total As was phosphate extractable so that most of the As was tightly retained by soil constituents and was not readily exchangeable by phosphate. Soil As (both total and extractable As) was significantly and positively correlated with rice grain As (0.296+/-0.063 microg g(-1), n=56). Next to drinking water, rice could be a potential source of As exposure of the people living in the As affected areas of Bangladesh.

Nitrous Oxide Emissions from Drained, Cultivated Organic Soils of South Florida

Due to the intense microbial oxidation of organic soils in the Florida Everglades, approximately 1400 kg N/ha are mineralized annually. Most of this nitrogen is lost to the atmosphere through denitrification in the soil. Nitrous oxide is one of the gaseous products of denitrification, therefore the objectives of this study were to determine the quantities of N2O emitted from these soils and to measure the effect of this N2O on ambient mixing ratios in the Everglades. Nitrous oxide fluxes from these soils ranged from 4 g N/ha/day, during dry periods, to 4500 g N/ha/day following rainfall events. Nitrous oxide emissions increased with increasing soil moisture. From April through the end of December 1979, a total of 165, 97, and 48 kg N2O-N/ha were emitted from fallow, St. Augustine grass, and sugarcane fields, respectively. There was a diurnal variation in the N2O mixing ratios of air 8 m above the soil in the Everglades. This diurnal fluctuation was affected by wind speed. There was a significant linear co...

Methane emissions associated with a green manure amendment to flooded rice in California

The goals of sustainable food production and mitigation of greenhouse gas emissions may be in conflict when green manures are used in flooded rice systems. A field study was initiated in early spring 1992 near Sacramento, California to quantify the potential for enhanced methane emissions following a green manure amendment to rice. Replicate flux measurements were made twice a day every 3–4 days throughout the growing season in four treatment plots: burned rice straw, spring incorporated rice straw, burned straw plus purple vetch and spring incorporated straw plus vetch. Seasonal methane emissions ranged from 66–136 g CH4 m−2 and were 1.5 to 1.8 times higher from the straw plus vetch treatments relative to the straw only treatments. No significant differences in emissions were found between the two straw only treatments or the straw plus vetch treatments. Methane fluxes were exponentially related to soil temperature, but no effect of redox potential or floodwater depth were observed. The potential impact of these results on the global methane budget is discussed.

Effect of water management, arsenic and phosphorus levels on rice in a high-arsenic soil-water system: II. Arsenic uptake.

Rice consumption is one of the major pathways for As intake in populations that depend on a rice diet in several countries of South and South-east Asia. Pot experiments were undertaken to investigate the effects of water management (WM), arsenic (As) contaminated soil-water and Phosphorus (P) rates on As uptake in rice plants. There were 18 treatments comprising of three each of As rates (0, 20 and 40 mg kg(-1) soil) and P rates (0, 12.5 and 25 mg kg(-1) soil) and two WM (aerobic and anaerobic) strategies on winter (boro var. BRRI dhan 29) and monsoon (aman var. BRRI dhan 32) rice at the Wheat Research Center (WRC), Nashipur, Dinajpur, Bangladesh. Arsenic concentrations in rice grain and straw increased significantly (P ≤ 0.01) with the increasing As rates in the soil. Arsenic availability in soil pore-water solution was less (58%) under aerobic WM (redox potential-Eh=+135 to +138 mV; pH-6.50 at 24.3 °C) as compared to anaerobic WM (flooded: Eh=-41 to -76 mV; pH-6.43 at 23 °C). The highest total grain As content 2.23 ± 0.12 mg kg(-1) and 0.623 ± 0.006 mg kg(-1) was found in T(6) (P(12.5)As(40)-anaerobic) and T(9) (P(25)As(40)-anaerobic) in BRRI dhan 29 and BRRI dhan 32, respectively, which was significantly higher (41-45%) than in the same As and P treatments for pots under aerobic WM. The As content in rice straw (up to 24.7 ± 0.49 ppm in BRRI dhan 29, 17.3 ± 0.49 mg kg(-1) in BRRI dhan 32 with the highest As level) suggested that As can more easily be translocated to the shoots under anaerobic conditions than aerobic condition. BRRI dhan 29 was more sensitive to As than BRRI dhan 32. Under aerobic WM, P soil amendments reduced As uptake by rice plants. The study demonstrated that aerobic water management along with optimum P amendment and selection of arsenic inefficient rice varieties are appropriate options that can be applied to minimize As accumulation in rice which can reduce effects on human and cattle health risk as well as soil contamination.