Cropping and tillage management can increase atmospheric CO2, N2O, and CH4 concentrations, and contribute to global warming and destruction of the ozone layer. Fluxes of these gases in vented surface chambers, and water‐filled pore space (WFPS) and temperature of survace soil were measured weekly from a long‐term winter wheat (Triticum aestivum L.)‐fallow rotation system under chemical (no‐tillage) and mechanical tillage (noninversion subtillage at 7 to 10 cm or moldboard plowing to 15 cm) follow management and compared with those from “native” grass sod at Sidney, NE, from March 1993 to July 1995. Cropping, tillage, within‐field location, time of year, soil temperature, and WFPS influenced net greenhouse gas fluxes. Mean annual interrow CO2 emissions from wheat‐fallow ranged from 6.9 to 20.1 kg C ha−1 d−1 and generally increased with intensity and degree of tillage (no‐till least and plow greatest). Nitrous oxide flux averaged <1.2 g N ha−1 d−1 for sod and 1 to 2 g N ha−1 d−1 for wheat‐fallow. Tillage during fallow increased N2O flux by almost 100%. Nitrous oxide emissions were 1.5 to 3.7 times greater from crop row than interrow locations with greatest differences occurring during periods of highest N2O emission. Mean annual N2O flux over the 3 yr of study were 1.54 and 0.76 g N ha−1 d−1 for row and interrow locations. Methane uptake ranged from 5.9 to 9.9 g C ha−1 d−1 and was not influenced by row location. Seasonal CO2 and N2O flux, and CH4 uptake ranked as spring ≥ summer > autumn > winter. Winter periods accounted for 4 to 10% and 3 to 47% of the annual CO2 and N2O flux, respectively, and 12 to 21% of the annual CH4 uptake. Fluxes of CO2 and N2O, and CH4 uptake increased linearly with soil temperature. No‐till fallow exhibited the least threat to deterioration of atmospheric or soil quality as reflected by greater CH4 uptake, decreased N2O and CO2 emissions, and less loss of soil organic C than tilled soils. However, potential for increased C sequestration in this wheat‐fallow system is limited due to reduced C input from intermittent cropping.
A.R. M o s i e r I , J.M. D u x b u r y 2, J.R. F r e n e y 3, O. H e i n e m e y e r 4 and K. M i n a m i 5 I USDA/ARS, AbstractIn this paper we discuss three topics concerning N20 emissions from agricultural systems. First, we present an appraisal of N20 emissions from agricultural soils (Assessment). Secondly, we discuss some recent efforts to improve N20 flux estimates in agricultural fields (Measurement), and finally, we relate recent studies which use nitrification inhibitors to decrease N20 emissions from N-fertilized fields (Mitigation).To assess the global emission of N20 from agricultural soils, the total flux should represent NzO from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N from crop residues, N20 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 N20 production. The assessment of direct and indirect N20 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 N20-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 N20 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 N20 fluxes. These studies reveal that it is not the measurement technique that is providing much of the uncertainty in N20 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 N20 emissions. Using nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N20 emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N20 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.
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