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.

Understanding how carbon, nitrogen, and key soil attributes affect gas emissions from soil is crucial for alleviating their undesirable residual effects that can linger for years after termination of manure and compost applications. This study was conducted to evaluate the emission of soil CO2, N2O, and CH4 and soil C and N indicators four years after manure and compost application had stopped. Experimental plots were treated with annual synthetic N fertilizer (FRT), annual and biennial manure (MN1 and MN2, respectively), and compost (CP1 and CP2, respectively) from 1992 to 1995 based on removal of 151 kg N ha(-1) yr(-1) by continuous corn (Zea mays L.). The control (CTL) plots received no input. After 1995, only the FRT plots received N fertilizer in the spring of 1999. In 1999, the emissions of CO2 were similar between control and other treatments. The average annual carbon input in the CTL and FRT plots were similar to soil CO2-C emission (4.4 and 5.1 Mg C ha(-1) yr(-1), respectively). Manure and compost resulted in positive C and N balances in the soil four years after application. Fluxes of CH4-C and N2O-N were nearly zero, which indicated that the residual effects of manure and compost four years after application had no negative influence on soil C and N storage and global warming. Residual effects of compost and manure resulted in 20 to 40% higher soil microbial biomass C, 42 to 74% higher potentially mineralizable N, and 0.5 unit higher pH compared with the FRT treatment. Residual effects of manure and compost on CO2, N20, and CH4 emissions were minimal and their benefits on soil C and N indicators were more favorable than that of N fertilizer.

Little is known about the relative contributions of episodic tillage and precipitation events to annual greenhouse gas emissions from soil. Consequently, we measured carbon dioxide (CO2), nitrous oxide (NzO), and methane (CH4) fluxes from soil in wh eat-fallow cr opping system in western Nebraska using vented surface chambers, before and immediately after tillage and wetting with 5.1 cm of water, during the fallow period in 199511996. Replicated fallow management treatments included no-tillage, snbtillage, and plow representing a wide range in degree of soil disturbance. Soil bulk density, water-filled pore space, electrical conductivity (ECru), nitrate (NO3), and pH within top 30.5 cm soil, and soil temperature at 0 to 7.6 cm were measured to assess their correlation with variations in gas flux and tillage and wetting. Atmospheric concentrations above the soil (at ~40 cm) increased by 15% for COz and 9 to 31% for NzO and 6 to 16% for CH4 within I min after tillage and returned to background concentrations within 2 h. Except immediately after tillage, net CH~ flux was negative, from the atmosphere into soil, and is referred to as CH4 uptake. Overall, increases (1.5-4-fold) in CO~ and NzO losses from soil, and CH4 uptake by soil were short lived and returned to background levels within 8 to 24 h after tillage. Losses of CO~ and N~O increased to 1.7 and 5 times background emissions, respectively, for 24 h following wetting, while CH4 uptake declined by about 60% for 3 to 14 d after wetting. Water-filled pore space in the surface soil fell below 60% within 24 h after saturation and exhibited an inverse relationship (R z = 0.66) with CH4 uptake. A significant decline in soil NO3 and ECI:I in the top 7.6 cm occurred following wetting. Under our experimental conditions, and the expected frequency of tillage and wetting events, failure to include these short-lived episodic gas pulses in annual flux estimations may underestimate annual CO2 and N20 loss up to 13 and 24%, respectively, and overestimate CH 4 uptake by up to 18% in this cropping system. E NG-TERM EFFECTS of intensive soil and crop management practices have been known to influence COz, N20, and CH4 fluxes from soils (IPCC, 1996). But little is known about the immediate effects of agronomic practices on short-term COz, NzO, and CH4 pulses, and the magnitude of their contributions to annual flux. Because greenhouse gas emissions from soil occur due to complex biological, chemical, and physical processes, it is probable that short-term changes in soil properties immediately following tillage and irrigation or precipitation influence greenhouse gas fluxes. Significant changes in soil structure, porosity, and microhabitat occur immediately following tillage (Doran and Linn, 1994; Reicosky, 1997). This in turn may influ

rye cover crop following soybean to increase the amount of winter and spring surface residue cover prior to plant-Use of a winter rye (Secale cereale L.) cover crop following soybean ing and during the seedbed and establishment phase of [Glycine max (L.) Merr.] has been shown to reduce the soil erosion potential in a corn (Zea mays L.)-soybean rotation system, but little the corn year. They demonstrated that regardless of is known about the effect of rye on residual soil NO 3 -N (RSN). An tillage management or rate of N application, planting a irrigated field study was conducted for 4 yr on a Sharpsburg silty clay winter rye cover crop following soybean increased the loam (fine, smectitic, mesic Typic Argiudoll) to compare crop rotation total surface residue cover by 30% over soybean alone. and winter rye cover crop following soybean effects on RSN underThe combined surface cover obtained from soybean and several tillage practices and N fertilization rates. Treatments each year rye residues was equivalent in cover and persistence to were (i) tillage: no-till or disk; (ii) rotation: corn following soybean/rye that provided by corn residue. Increased residue cover (Cbr) or soybean/rye following corn (BRc), corn following soybean reduced the risk of soil erosion until development of a (Cb) or soybean following corn (Bc), and corn following corn (Cc); protective corn canopy, without adversely affecting corn and (iii) N rate: 0, 100, and 300 kg N ha Ϫ1 (applied to corn). Rye in grain yield. the Cbr/BRc rotation was planted in the fall following soybean harvest and chemically killed in the spring of the following year prior to corn Research indicates that in addition to affecting surplanting. Each spring, before tillage and N application, RSN was face residue cover, a winter rye cover crop can also determined to a depth of 1.5 m, at 30-cm intervals. The net springinfluence the dynamics of N cycling (Ditsch and Alley, to-spring change in RSN between subsequent spring seasons was 1991). Planting a winter rye cover crop following corn computed for each plot, and annual aboveground N uptake for rye,in a no-till system reduced spring NO 3 -N accumulation corn, and soybean were determined. Rye, rotation, N rate, and tillage in soil due to N uptake by rye (Ditsch et al., 1993; significantly influenced RSN in the top 1.5 m of soil. The presence Shipley et al., 1992). Early-spring rye is usually killed of rye (BRc) reduced total spring RSN between 18 and 33% prior and its residues returned to the soil before planting corn.to corn planting in 2 of the 3 yr, compared with the no-rye system When these residues decompose, significant changes in (Bc), as rye immobilized from 42 to 48 kg N ha Ϫ1 in aboveground soil NO 3 -N can occur due to mineralization of N from dry matter. Recycling of N in high-yielding rye cover crop residues led to an increase in RSN accumulation after corn in the succeeding rye residue. This may cause a build-up of NO 3 -N in soil spring. Up to 277 kg RSN ha Ϫ1 accumulated at high rates of N following 643

rye cover crop following soybean to increase the amount of winter and spring surface residue cover prior to plant-Use of a winter rye (Secale cereale L.) cover crop following soybean ing and during the seedbed and establishment phase of [Glycine max (L.) Merr.] has been shown to reduce the soil erosion potential in a corn (Zea mays L.)-soybean rotation system, but little the corn year. They demonstrated that regardless of is known about the effect of rye on residual soil NO 3 -N (RSN). An Abbreviations: RSN, residual soil nitrate. Rotation sequences: Bc,

Nitrate leaching from agricultural fields into groundwater has caused environmental and health concerns. A study was conducted during 1992-1993 in the Central Platte Valley of Nebraska to assess the nitrate leaching potential under recommended center-pivot irrigation and fertilizer best management practices for continuous corn (Zea mays L.). At time of planting corn, potassium bromide (KBr) and double-labeled ISN ammonium nitrate (I0 atom% ISNH415NO3) tracers were applied at rates of 200 kg Br ha-i and 30 kg N ha-i to four 6.1 by 3.7 m plots that were representative of major soil types on the 32.l-ha field. Soil and plants sampled 7 wk after planting and at harvest in 1992, and in the spring of 1993, were analyzed for Br and N content and a mass balance was determined. At corn harvest, 41% (81.9 kg ha-I) of the applied Br loss from the top 1.2 m of soil was attributed to leaching. Also, 54% (16.2 kg ha-i) of the tracer ap plied was lost from the system; 41% (12.3 kg ha-I) through leaching; and 13% (3.9 kg ha-i) through denitrification and volatilization. By time of planting in the spring of 1993, 70% (139.3 kg ha-I) of the applied Br and 46% (13.8 kg ha-i) of the fertilizer N leached below 1.2 m. High negative correlations were found between soil clay and silt contents, and Br or NO3 leaching. Despite use of best management practices for irrigation water and N applications, large amounts of nitrate can be lost through leaching under irrigated corn in this subhumid climate on fine-to medium-textured soils.

pools, soil nutrients, and microbial environments and activities, which are some of the controlling factors in Understanding how carbon, nitrogen, and key soil attributes affect the emission of CO 2 , N 2 O, and CH 4 to the atmosphere gas emissions from soil is crucial for alleviating their undesirable re-(Intergovernmental Panel on Climate Change, 1996; Considual effects that can linger for years after termination of manure and rad, 1996). At the end of a 10-yr experiment, application compost applications. This study was conducted to evaluate the emission of soil CO 2 , N 2 O, and CH 4 and soil C and N indicators four years of cattle manure and red clover (Trifolium pratense L.) after manure and compost application had stopped. Experimental plots hay resulted in accumulation of the most labile and were treated with annual synthetic N fertilizer (FRT), annual and bibiologically active soil organic C pools (Wander et al., ennial manure (MN1 and MN2, respectively), and compost (CP1 and 1994; Wander and Traina, 1996). Biologically active C CP2, respectively) from 1992 to 1995 based on removal of 151 kg N promotes microbial activities and CO 2 emissions from ha Ϫ1 yr Ϫ1 by continuous corn (Zea mays L.). The control (CTL) plots soils. Emission of N 2 O depends on the form of N found received no input. After 1995, only the FRT plots received N fertilizer in manure (Intergovernmental Panel on Climate Change, in the spring of 1999. In 1999, the emissions of CO 2 were similar be-1996) and rate of manure application. Lessard et al. tween control and other treatments. The average annual carbon input (1996) reported that total N 2 O emission from soil over in the CTL and FRT plots were similar to soil CO 2-C emission (4.4 a 185-d period increased from 0.7 to 1.0 kg N 2 ON ha Ϫ1 and 5.1 Mg C ha Ϫ1 yr Ϫ1 , respectively). Manure and compost resulted when dairy cow manure application was increased from in positive C and N balances in the soil four years after application. 170 to 339 kg total N ha Ϫ1. Agronomic practices in gen-Fluxes of CH 4-C and N 2 ON were nearly zero, which indicated that eral reduce CH 4 oxidation capacity of soils (Mosier et the residual effects of manure and compost four years after application al., 1991; Kessavalou et al., 1998). In northeastern Colohad no negative influence on soil C and N storage and global warming. Residual effects of compost and manure resulted in 20 to 40% higher rado, CH 4 oxidation capacities of tilled and irrigated soil microbial biomass C, 42 to 74% higher potentially mineralizable soils cropped to winter wheat (Triticum aestivum L.) N, and 0.5 unit higher pH compared with the FRT treatment. Residand corn were 90% lower than grassland soils (Bronson ual effects of manure and compost on CO 2 , N 2 O, and CH 4 emissions et al., 1992; Bronson and Mosier, 1993). were minimal and their benefits on soil C and N indicators were more Continued elevated emissions of greenhouse gases favorable than that of N fertilizer.

No abstract