Integrated evaluation of greenhouse gas emissions (CO2, CH4, N2O) from two farming systems in southern Germany

Agriculture, Ecosystems and Environment 91 (2002) 175–189

Integrated evaluation of greenhouse gas emissions (CO2 , CH4 , N2 O) from two farming systems in southern Germany H. Flessa a,∗ , R. Ruser b , P. Dörsch c , T. Kamp b , M.A. Jimenez b , J.C. Munch b , F. Beese a b

a Institute of Soil Science and Forest Nutrition, University of Göttingen, Büsgenweg 2, D-37077 Göttingen, Germany GSF-National Research Center for Environment and Health, Institute of Soil Ecology, D-85764 Neuherberg, Germany c Department of Soil and Water Sciences, Agricultural University of Norway, P.O.B. 5028, NO-1432 Aas, Norway

Received 7 August 2000; received in revised form 2 May 2001; accepted 15 May 2001

Abstract Agricultural practices contribute to emissions of the greenhouse gases CO2 , CH4 and N2 O. The aim of this study was to determine and discuss the aggregate greenhouse gas emission (CO2 , CH4 and N2 O) from two different farming systems in southern Germany. Farm A consisted of 30.4 ha fields (mean fertilization rate 188 kg N per ha), 1.8 ha meadows, 12.4 ha set-aside land and 28.6 adult beef steers (year-round indoor stock keeping). Farm B followed the principles of organic farming (neither synthetic fertilizers nor pesticides were used) and it consisted of 31.3 ha fields, 7 ha meadows, 18.2 ha pasture, 5.5 ha set-aside land and a herd of 35.6 adult cattle (grazing period 6 months). The integrated assessment of greenhouse gas emissions included those from fields, pasture, cattle, cattle waste management, fertilizer production and consumption of fossil fuels. Soil N2 O emissions were estimated from 25 year-round measurements on differently managed fields. Expressed per hectare farm area, the aggregate emission of greenhouse gases was 4.2 and 3.0 Mg CO2 equivalents for farms A and B, respectively. Nitrous oxide emissions (mainly from soils) contributed the major part (about 60%) of total greenhouse gas emissions in both farming systems. Methane emissions (mainly from cattle and cattle waste management) were approximately 25% and CO2 emissions were lowest (circa 15%). Mean emissions related to crop production (emissions from fields, fertilizer production, and the consumption of fossil fuels for field management and drying of crops) was 4.4 and 3.2 Mg CO2 equivalents per hectare field area for farms A and B, respectively. On average, 2.53% of total N input by synthetic N fertilizers, organic fertilizers and crop residues were emitted as N2 O–N. Total annual emissions per cattle unit (live weight of 500 kg) from enteric fermentation and storage of cattle waste were about 25% higher for farm A (1.6 Mg CO2 equivalents) than farm B (1.3 Mg CO2 equivalents). Taken together, these results indicated that conversion from conventional to organic farming led to reduced emissions per hectare, but yield-related emissions were not reduced. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cattle; Carbon dioxide; Greenhouse gas; Methane; Nitrogen fertilizer; Nitrous oxide; Organic farming; Southern Germany

1. Introduction

∗ Corresponding author. Tel.: +49-551-39-3507; fax: +49-551-39-3310. E-mail address: [email protected] (H. Flessa).

Atmospheric concentrations of the greenhouse gases CO2 , CH4 and N2 O are increasing at a rate of approximately 0.4, 0.6 and 0.25% per year, respectively (IPCC, 1996). Since these increases contribute

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to changes in the earth’s climate, there is a growing interest in quantifying the significant sources and sinks of these trace gases and the international community has taken steps to reduce these emissions. Agricultural practice is assumed to be one of the major sources of greenhouse gas emissions, particularly of N2 O and CH4 , and it accounts for approximately one-fifth of the annual increase in radiative forcing (IPCC, 1996). Nitrous oxide emissions from agriculture are estimated to account for more than 75% of the total global anthropogenic emission (Duxbury et al., 1993; Isermann, 1994), the major part being produced in soils as an intermediate during nitrification and denitrification (Hutchinson and Davidson, 1993). The primary reasons for enhanced N2 O release from cultivated soils are increased N inputs by mineral fertilizers, animal wastes and biological N fixation (IPCC, 1996). A constant emission factor of 1.25% for the amount of N applied to agricultural land is currently recommended for calculating global and national emissions from fertilized soils (IPCC, 1997). Agriculture and related activities account for about two-thirds of all anthropogenic CH4 emissions (Duxbury et al., 1993). Biological CH4 production in anaerobic environments (e.g. enteric fermentation in ruminant animals, flooded rice fields, and animal waste processing) is the principle source of CH4 from agriculture (IPCC, 1996). In addition, agricultural practices may influence atmospheric concentration of CH4 by affecting its consumption in aerated soils. This biological CH4 oxidation in aerobic soils is estimated to comprise 3–9% of the global atmospheric CH4 sink (Prather et al., 1995). Even in highly industrialized countries such as Germany, the agricultural sector belongs to the most important national sources of N2 O and CH4 emissions. Estimates of the contribution of the agricultural sector to the total Germany’s N2 O emissions vary between 39 and 52% (Federal Ministry for Environment/Nature Conservation and Nuclear Safety, 1997; Isermann and Isermann, 1998). The contribution of agriculture to German CH4 emissions is estimated to be 34% (Federal Ministry for Environment/Nature Conservation and Nuclear Safety, 1997). A reduction of CO2 emissions by 25% by the year 2005 (based on the reference year 1990) is the objective of the climate protection program in Germany. Further, goals for reducing N2 O and CH4 emissions are currently being

developed. The magnitude of atmospheric loading due to agricultural production may be strongly influenced by the type of farming and land management system used. Greenhouse gas emissions originating from both biotic and abiotic processes have to be considered in the complete emission inventory of a farm. Considerable contributions to total emissions may originate from soils, livestock, animal wastes, consumption of fossil fuels and production of fertilizers (Adger et al., 1997; Kramer et al., 1999). In this study, the aggregate greenhouse gas emission by two farming systems in southern Germany was evaluated. Both systems included plant and animal production. However, one system conformed to the principles of integrated farming (recommended by the official agricultural advisory service) and the other system followed the principles of organic farming (neither synthetic fertilizers nor pesticides were used). Multi-annual data (1992–1998) from field experiments on the two adjacent farming systems were analyzed and additionally mean greenhouse gas emission factors described in the literature were used to obtain the integrated assessment of the greenhouse gas emission. The aims of this study were: (1) to determine the relationship between N input and N2 O emission from differently cultivated soils; (2) to quantify total CO2 , CH4 and N2 O emissions by the different farming systems; (3) to show the relative contribution of these trace gas species and their single sources to the total greenhouse gas release; and (4) to discuss the possible impact of the results on future emission assessments and on strategies for future mitigation of emissions.

2. Materials and methods 2.1. Farming systems The inventory of greenhouse gas emissions from the different farming systems was performed at the Research Station Scheyern of the FAM Research Network on Agroecosystems, 40 km north of Munich in southern Germany (48◦ N30.0 , 11◦ E20.7 ). The research station is located 445–498 m above sea level, in a hilly landscape derived from tertiary sediments partly covered by loess. There is a high variability of soil types and soil properties, however most of the soils have a loamy soil texture and are classified as

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Fig. 1. Mean monthly air temperature (open circles) and mean cumulative monthly precipitation (bars) measured in Scheyern from 1994 to 1998.

Cambisols (Sinowski, 1995). The mean annual precipitation is 833 mm and the mean annual temperature is 7.4◦ C. The average monthly air temperature and the mean cumulative monthly precipitation during the experimental years 1994–1998 are shown in Fig. 1. Two independent farming systems have been established at the research station and plant and animal production under both systems is summarized in Table 1. The farm with integrated crop production (farm A) followed a four-phase crop rotation consisting of potato (Solanum tuberosum L.), winter wheat (Triticum aestivum L.), maize (Zea mays L.) and winter wheat. Potato and most of the wheat were grown as cash crops and the maize (for silage production) and grassland were used to feed beef steers, whose slurry was in turn

applied primarily (approximately 80%) to the fields in farm A. The remaining slurry (about 20%) was applied to fields in another farm. Cover crops (mustard, Sinapis alba L.) were cultivated to reduce nitrate leaching and soil erosion during winter. Nitrogen was applied in the form of mineral fertilizers and slurry. The mean N application rate (synthetic fertilizer and slurry) was 250 kg per ha for winter wheat, 150 kg per ha for maize and 100 kg per ha for potato. The mean livestock number was 28.6 animal units, with one animal unit having a live weight of 500 kg. The organic farming system (farm B) followed a seven-field crop rotation, alternating summer and winter crops as follows: (1) grass–clover mixture (harvested as forage); (2) seed potato; (3) winter wheat with undersowing of

Table 1 Plant and animal production of the investigated farming systems in southern Germany Farm A (integrated crop cultivation)

Farm B (organic farming)

Fields (ha) Meadows (ha) Pasture (ha) Set-aside land (ha) Fertilization

30.4 1.8 – 12.4 Mineral fertilizers (f), cattle slurry (s)

Crop rotation and mean N application (kg N per ha, fertilizer type)

Potato (100, f), winter wheata (250, f and s), maize (150, f and s), winter wheata (250, f and s)

Livestock

Bull fattening, 28.6 animal unitsc

31.3 7 18.2 5.5 Farmyard manure (m), cattle slurry (s), N fixation by legumes Grass–clover mixture, potato (105, m), winter wheatb (92, m and s), sunflower, clover–grass mixture, winter wheat (84, m and s), winter rye (37, s) Cattle herd, 35.6 animal unitsc

a

Cover crop mustard. White clover undersowing. c One animal unit corresponds to a live weight of 500 kg. b

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clover; (4) sunflower (Helianthus annuus L.) with undersowing of a (5) grass-clover mixture (forage); (6) winter wheat; and (7) winter rye (Secale cereale L.) with undersowing of grass–clover mixture. Approximately 40% of the farm area consisted of pastures and meadows, which were used to feed the cattle herd for meat production. The mean livestock number of this herd was 35.6 animal units. This system utilized no mineral N fertilizers and pesticides. Fertilization was achieved by manure and slurry application, and symbiotic N fixation (cultivating legumes).

greenhouse effect. The GWP index is defined as the cumulative radiative forcing between the present and a selected time in the future, caused by a unit mass of gas emitted now (IPCC, 1996). The GWP (with a time span of 100 years) of CO2 , CH4 and N2 O is 1, 21 and 310, respectively. The emissions of CO2 , N2 O, CH4 and CO2 equivalents were determined from the individual sources in the two farming systems.

2.2. Inventory analysis

The mean annual N2 O and CH4 flux rates in soils under different management were estimated from a large number of flux measurements conducted on the fields and on the field trials area of the experimental farms between 1992 and 1998. The analysis is based on the integrated evaluation of year-round measurements including different crops, field management, fertilization rates, fertilizer types and soils (Table 2). The 25 annual fluxes listed in Table 2 (except for No. 24) are based on flux measurements performed at least weekly for a period of 1 year. A detailed description and analysis of the temporal and spatial variation of N2 O and CH4 fluxes at each site is presented with their respective references. All fluxes were measured using the same gas sampling (closed chambers, five replicates) and analysis method. A detailed description of the soil covers and the gas sampling procedure is given by Flessa et al. (1995). The gas analysis by an automated gas chromatographic system was described by Loftfield et al. (1997). The relationship between the N input and annual N2 O emission was analyzed by regression analysis following the revised International Panel on Climate Change (IPCC) guidelines for estimating N2 O emissions from agricultural soils (IPCC, 1997). Total N input was calculated for each site considering mineral N fertilizers, animal wastes (slurry and farmyard manure) and the incorporation or mulching of crop residues (Table 2). The pasture soil (Table 2, No. 25) was excluded as it contained a considerably higher organic carbon content (25 g kg−1 ) than the crop soils (10–19 g kg−1 ) and emissions might have been also affected by grazing in the previous years. Total N inputs by animal wastes and crop residues were provided by the farm manager (Kainz, personal communication, 2000) and by Stenger (1996). The sites

At the onset of the greenhouse gas inventory, it became necessary to define the boundaries of the systems under analysis. The following sources of greenhouse gas emissions were included in the estimation of the total emissions from the two farming systems: (1) N2 O and CH4 emissions from soils; (2) CH4 emissions from enteric fermentation; (3) N2 O and CH4 emissions from animal waste management; (4) CO2 , N2 O and CH4 emissions related to production of synthetic fertilizer; and (5) CO2 , N2 O and CH4 emissions related to consumption of fossil fuels. Emissions related to other agricultural inputs, such as pesticides and seeds, were not included in the analysis as these were considered to be negligible (Kramer et al., 1999). Emissions occurring from the production of investment goods (machines and buildings) were not considered. Furthermore, it was assumed that soil organic carbon stocks remained unchanged, and we did not consider indirect N2 O emissions induced by leaching of NO3 − or NH3 volatilization. The data used to calculate the greenhouse gas emissions from the different sources were based on farm-specific results on land use, fertilization, animal production and fossil fuel consumption, and source-specific emission factors for CO2 , N2 O and CH4 . The emission factors were either derived from the present field experiments (e.g. emissions from soils, excrement of grazing cattle) or from data on greenhouse gas emissions described in the literature (e.g. animal production, fertilizer production, consumption of fossil fuels). All emissions were converted to CO2 equivalents using the global warming potential (GWP), which determines the relative contribution of a gas to the

2.3. N2 O and CH4 fluxes in fields and unfertilized soils

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where N input resulted primarily from the incorporation of legumes (Table 2, Nos. 21–24) were analyzed separately, because no data on the N input from N fixation existed. 2.4. Emission of N2 O and CH4 from pasture soils Emission factors calculated for fields were not used to estimate N2 O and CH4 emissions from pastures, since urine and dung from grazing cattle are voided in discrete patches comprising high concentrations of soluble N and easily available organic compounds. These nutrient-enriched excretal patches are hot spots of N2 O and CH4 formation. In a previous study (Flessa et al., 1996) N2 O and CH4 emissions were quantified from fresh urine and dung patches on a pasture in farm B (organic farming system) over a period of 78 days (Table 3). Based on results of production of dung and urine deposits, amount of N excreted with dung and urine and measured N2 O and CH4 emissions rates, N2 O and CH4 emission factors were estimated for dung and urine voided during grazing. The N2 O and CH4 fluxes on the pasture soil not directly affected by cattle deposits (cattle was excluded from this plot) have been studied over a period of 1 year by Dörsch (2000). The annual N2 O and CH4 fluxes obtained from the results of this study are included in Table 2 (site No. 25). All flux measurements were carried out using the soil covers and gas analysis system as described previously. 2.5. Emission of CH4 and N2 O from cattle and cattle waste storage The amount of CH4 that is directly emitted from cattle by enteric fermentation was determined from the number and age of animals, and from mean CH4 emis-

sion factors reported for cattle production in Germany. The mean annual CH4 emission from young cattle in Germany has been reported to be 52 kg per animal for the age group 6–12 months, and 60 kg per animal for the age group 12–24 months (Ahlgrimm and Gädeken, 1990). A mean emission factor of 58 kg CH4 for a beef weighing 500 kg (live weight) was assumed in the present study. Direct N2 O emissions from cattle were excluded from this study, as these were considered to be negligible (Tiedje, 1988). Emissions from manure storage are affected by the type of waste management and the storage duration (Woodbury and Hashimoto, 1993). Substantial CH4 emission may occur when manure decomposes in an anaerobic environment. The manure from cattle of farm A (bull fattening, housed year-round in a stable) was stored as slurry in tanks for about 6 months. Following the revised IPCC guidelines for national greenhouse gas inventories (IPCC, 1997), CH4 emissions from slurry storage were estimated, considering the excretion of degradable organic material (volatile solids, VS) per animal (estimated to 3 kg dry matter per animal unit and day), the maximum CH4 producing capacity for the VS excreted (0.17 m3 CH4 per kg) and the mean emission factor when the VS was treated as liquid slurry. The latter is highly variable, ranging from about 5 to 65% (Heyer, 1994). The IPCC (1997) assumes a mean emission factor of 35%, but Heyer (1994) has pointed out that mean emissions from slurry storage systems used in Germany might be smaller (about 15%). This value (15%) was used in our study to estimate CH4 emission from slurry storage on farm A. The management of cattle manure produced on farm B depended on the season. During the growing period the cattle were fed by grazing. Emissions from urine and dung from grazing cattle are discussed separately

Table 3 Amount of N excreted with dung and urine by one adult beef during a grazing period of 1 day, and the total N2 O–N and CH4 –C (mean ± S.D.) evolved from these droppings (calculated from Flessa et al., 1996)

Dung patches Urine patches Urine and dung a

Total N excreted (g per day)

N2 O–N emission (g)

N2 O–N emission per N excreted (%)

CH4 –C fluxa (g)

19.9 90.0 109.9

0.09 ± 0.03 3.45 ± 0.77 3.54 ± 0.77

0.45 3.83 3.20

0.778 ± 0.065 −0.014 ± 0.009 0.764 ± 0.066

Positive CH4 flux values are emission rates; negative values are uptake rates.

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Table 4 Emissions of CO2 , CH4 , N2 O and the calculated total emission of CO2 equivalents during the production of synthetic nitrogen fertilizer and during the production and use of diesel fuel (emissions during production and combustion) (Kaltschmitt and Reinhardt, 1997)

Synthetic N fertilizer (1 kg N) Diesel fuel (1 kg) a

CO2 (g)

CH4 (g)

N2 O (g)

Total CO2 equivalents (g)

2468 (45.2)a 3534 (93.8)

0.54 (0.2) 0.64 (0.4)

9.63 (54.6) 0.71 (5.8)

5465 3767

The data in brackets are percentages of the total CO2 equivalents.

in Section 3.2. The cattle was housed in a stable for 6 months per year and excrements produced in this period were stored as solid manure. The CH4 emission from storage of solid manure was estimated assuming an emission factor of 1.5% (Heyer, 1994; IPCC, 1997) of the maximum CH4 producing capacity of the VS excreted (0.17 m3 CH4 per kg).

3. Results and discussion 3.1. N2 O and CH4 fluxes in fields and unfertilized soils

2.6. Emissions from fertilizer production and use of diesel fuel and heating oil

The fields in farms A and B represent the most intensively studied agricultural sites with regard to the emission of N2 O in southern Germany. The analysis of N2 O emission is based on the data presented in Table 2 (see Section 2.3).

The production of synthetic N fertilizers is associated with the emission of greenhouse gases, which are primarily caused by the consumption of fossil energy (mainly CO2 emission) for NH3 synthesis and N2 O release during production of nitric acid. Kaltschmitt and Reinhardt (1997) estimated total CO2 , CH4 and N2 O emissions during the production and transport of N fertilizers used in Germany. Their results are summarized in Table 4 and were used in the present study to estimate emission of greenhouse gases from fertilizer production. Synthetic N fertilizers were not used on farm B. Neither of the two farming systems utilized mineral P and K fertilizers in recent years, due to the high P and K reserves of the loamy soil. Another contribution to the emission of greenhouse gases from farming is the consumption of diesel fuel and heating oil. These emissions were estimated from the mean annual fuel consumption in farms A and B using the emission factors for CO2 , CH4 and N2 O listed in Table 4. These factors include emissions during production, transport and burning (Kraus et al., 1999). The total greenhouse gas emission (3767 g CO2 equivalents per kg) is comprised primarily of CO2 production during burning that accounts for more than 90% of the total emissions.

3.1.1. N2 O emission from fertilized fields The mean background N2 O–N emission from sites with no N input (Table 2, Nos. 13, 14 and 20) was 0.5 kg per ha per year. This value is within the range of background emissions (−0.6 to 3.2 kg N2 O–N per ha per year) reported by Bouwman (1996). The total N input from synthetic fertilizer, organic fertilizer and crop residues at the experimental sites varied between 0 and 275 kg per ha per year (Table 2). The annual N2 O–N emission from fertilized soils ranged from 1.3 to 16.8 kg per ha per year (Table 2). Emissions of N2 O–N per unit N input (subsequent to a background emission correction of 0.5 kg N2 O–N per ha per year) were highly variable, ranging between 5.9 (Table 2, No. 2) and 0.7% (Table 2, No. 11). Despite this variability a highly significant relationship existed between the annual N2 O emission and total N input (Fig. 2, one outlier excluded), which could be explained best by the function y = 0.63+0.0253x (R 2 = 0.61). The standard error of the slope of this function was 0.005. These results agree with those summarized by Bouwman (1996), who found that annual N2 O emission from cultivated soils was decisively influenced by N supply. However, his regression analysis, that was based on 20 observations from grassland and maize fields, indicated a mean emission factor of 1.25 ± 1% of the amount of N applied. The emission

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Fig. 2. Relation between total N input (by synthetic fertilizer, cattle waste and crop residues) and annual N2 O emission measured on differently managed fields at the research station in Scheyern. The regression analysis is based on published data listed in Table 2 (one data point (䊏) was excluded).

factor of 1.25% is also currently recommended by the IPCC for estimating direct N2 O emissions from agricultural soils (IPCC, 1997). MacKenzie et al. (1998) found in Quebec, Canada, that about 1.0–1.6% of the added N under maize was emitted as N2 O. Dobbie et al. (1999) summarized data on N2 O emission from fertilized soils in Scotland and concluded that the factors for grassland and potato may be higher than 1.25%, depending on the distribution and total amount of precipitation. In a German field experiment, Kaiser et al. (1998) found that relative N2 O losses from applied N-fertilizer ranged between 0.7 and 4.1%. They suggested that crop specific emission factors should be determined. Recently, Kaiser and Ruser (2000) proposed the long-term nitrogen in/out balance as a predictor of N2 O emissions from fertilized fields. In the present study, the N input from the incorporation of crop residues was included in determining the N2 O emission factor, according to the revised IPCC guidelines (IPCC, 1997). This method takes into account that N2 O formation associated with crop residue decomposition may significantly contribute to the total N2 O emission from cultivated soil. The high mean emission factor in the present study (2.53% of the N input) can probably be attributed to local soil properties (fine, silty texture), soil management (organic fertilizers, incorporation of cover crops and main crop residues) and climatic conditions (intensive precipitation events, freeze-thaw) that favor denitrification. The topsoil at all experimental sites had a high water hold-

ing capacity with a water-filled pore space of about 80–90% at field capacity. This resulted in high water saturation following precipitation events as well as in generally high soil moisture contents (about 50–60% water-filled pore space) during most of the duration of the study (Flessa et al., 1995; Ruser et al., 2001). Additionally, application of organic fertilizers and incorporation and mulching of crop residues may have likely contributed to N2 O losses by denitrification. Another important factor contributing to the high emissions observed were the N2 O losses during the winter. Increased emissions induced by freezing and thawing events occurred each year and accounted for a substantial part of the annual emissions. Further these events contributed considerably to the large variation of N2 O emission factors (emission related to the N input). An example for this was the maximum N2 O emission of 16.8 kg N2 O–N per ha per year measured for the wheat field in 1992–1993 (Table 2, No. 2). Nearly half of this emission occurred during the 4 months (December to March) when exceptionally harsh and frequent freezing and thawing events took place. The results show that measurements extending over several years are required to obtain sufficiently reliable estimates of mean annual N2 O fluxes from differently managed fields. Although this study involved comprehensive series of measurements, determination of the mean annual emission was tainted with considerable uncertainty. A similar conclusion was recently also drawn by Dobbie et al. (1999) from the inter annual variation

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in N2 O emissions from agricultural soils in Scotland. Dobbie and coworkers found that the large differences between annual fluxes at the same site could be partly explained by the amount of rainfall following fertilizer application. 3.1.2. N2 O emission from leguminous crops Four fields were studied where the N input was primarily attributed to the incorporation of legume residues (Table 2, Nos. 21–24). These sites showed high annual N2 O emissions (7.4–12.9 kg N2 O–N per ha per year). The plowing of clover in autumn resulted in increased emissions during the entire cropping period of the subsequent year, even if no N fertilizer was applied during this period (Table 2, Nos. 23 and 24). Markedly increased emissions were also observed in sunflower fields following incorporation of legume cover crops (Table 2, Nos. 21 and 22). In these cases farmyard manure application may have additionally contributed to the large N2 O losses. In contrast, direct N2 O emissions during the cropping period of legumes were rather small (Table 2, Nos. 3 and 4). The mean N2 O emission induced by the incorporation of legumes was determined, considering the background emission (0.5 kg N2 O–N per ha per year) and assuming that the emission factor for N applied with manure was the same as the one for mineral N fertilizers and slurry (2.53%). The mean N2 O emission induced by the incorporation of legumes was 9.1 kg N2 O–N per ha per year if all emissions not related to the incorporation of legumes (e.g. background and manure induced emissions) were disregarded from the annual fluxes (Table 2, Nos. 21–24). These results agree with the observations of Wagner-Riddle et al. (1997), who found markedly increased N2 O emission rates the year subsequent to the incorporation of alfalfa (Medicago sativa L.) residues. The large emissions were probably due to the combined mineralization of biologically fixed residue N with enhanced microbial respiration during residue decay. The latter may induce anaerobic microsites that favors N2 O formation (Flessa and Beese, 1995). 3.1.3. Mean N2 O emission from the crop rotations The results shown in Fig. 2 and Table 2 were used to determine the mean N2 O emission for each crop of the two crop rotations described in Table 1. The total mean N2 O emission per crop rotation was then obtained by

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averaging across four-crop phases in the rotation of farm A, and seven-crop phases in the rotation of farm B. The mean total N inputs by synthetic fertilizer, slurry and farmyard manure are shown in Table 1. The mean N inputs from main crop residues per hectare and year were: for potato 70 and 45 kg for farms A and B, respectively; for cereals 24 kg for farm A (straw was collected and removed from the fields of farm B); and for sunflower 50 kg. The mean residue N from mustard cultivated as cover crop on farm A was 63 kg per ha. Emissions of N2 O from fields of farm A were determined by applying the emission factor of 2.53% to all N inputs (synthetic fertilizer, slurry, crop residues). For farm B, emissions were calculated by applying both this emission factor (2.53% to N input by manure, slurry and crop residues) and the mean emission for the incorporation of legumes (9.1 kg N2 O–N per ha per year). The obtained mean annual N2 O–N emission for the complete rotation cycle was 6.3 and 5.4 kg per ha for farms A and B, respectively. Unfertilized soils (meadows and set-aside land) were assumed to have low background emissions (0.5 kg N2 O–N per ha per year) and were excluded from the assessment of total greenhouse gas emissions from the farming systems. However, these sites were included when greenhouse gas emissions from agricultural production were related to the total farm area. About 20% of the slurry produced by the cattle of farm A (approximately 335 kg N) was applied to fields of another farm. Emissions induced by this slurry were attributed to the agricultural production of farm A using the emission factor of 2.53% of the applied N. 3.1.4. CH4 fluxes in fields The annual CH4 –C uptake of the differently managed sites ranged from 313 to 567 g per ha. The mean uptake rate was 396 g CH4 –C per ha per year. There was no effect of fertilization rate on the annual CH4 uptake. Also, cultivation effects on CH4 uptake could not be determined. Due to its negligible contribution to the total greenhouse gas emission from the farms, CH4 uptake by soils was excluded from the present analysis. 3.2. Emission of N2 O and CH4 from pasture soils The results on N2 O and CH4 emissions from urine and dung patches on the pasture of farm B have been described extensively by Flessa et al. (1996). Emission

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factors for dung and urine deposits excreted on pastures are summarized in Table 3. The N2 O–N emission factor was 8.5-fold higher for N excreted with urine (3.8%) than with dung (0.45%). The farm-specific emission factor for total N excreted (3.2%) was higher than the mean emission factor (2%) proposed by the IPCC (1997), but it was within the range (0.1–5% for urine) reported in other studies summarized by Oenema et al. (1997). In the present study, the total annual N2 O and CH4 fluxes from pasture soils were determined considering the mean cattle stocking density (Table 1), the mean grazing period (182 days per year), the N2 O and CH4 emissions from cattle deposits (Table 3), and the N2 O and CH4 fluxes measured on a pasture plot that was not affected by cattle deposits (cattle were excluded from this plot). This control site had an annual N2 O–N emission of 2.2 kg per ha and a CH4 –C uptake of 706 g per ha per year (Table 2, No. 25). The total annual N2 O–N emission from the pasture area (18.2 ha) of farm B consisted of emissions from deposits of the grazing cattle (22.9 kg) and from pasture soil not directly affected by these deposits (40 kg). Emissions from the ungrazed control plot were considerably higher than background emissions from unfertilized fields. These emissions are probably affected by the N supply and soil compaction resulting from cattle grazing during the previous years. Since no data on background emission from long-term unfertilized grassland existed, the mean emission from the unfertilized former fields (vegetated fallow and grass, Table 2, Nos. 13, 14 and 20) was used as background flux from grassland (0.5 kg N2 O–N per ha per year). The total net CH4 –C flux on the pasture area resulted from the total emissions from cattle deposits (5 kg per year) and the total uptake by pasture soil not directly affected by dung (13 kg per year). All N2 O emissions exceeding the background flux of 0.5 kg N2 O–N per ha per year, and all CH4 emissions from cattle droppings were included in our assessment of the total greenhouse gas emissions. 3.3. Emission of CH4 and N2 O from cattle and cattle waste storage The beef steers of farm A (Table 1), which were kept indoors for the whole year reached a live weight of 500 kg at the age of about 16 months. The total direct CH4 emission from the 28.6 animal units (500 kg live

weight per animal unit) of farm A was 1244 kg CH4 –C per year assuming a CH4 emission factor of 58 kg CH4 per animal unit (see Section 2.5). The same CH4 emission factor (58 kg CH4 per animal unit) was used for the cattle of farm B (Table 1), which were allowed to graze during the growing period and were housed in a stable during the winter. The total direct CH4 –C emission from the cattle herd of 35.6 animal units of farm B amounted to 1549 kg per year. Additional CH4 and N2 O emissions may occur from livestock manure. Here, only those emissions occurring prior to manure application on soils were considered. Slurry and manure-induced soil emissions have been taken into account in the Sections 3.1 and 3.2. Based on the emission factors described in the Section 2.5, the total CH4 –C emission from the storage of slurry (farm A) produced by one animal unit was approximately 14 kg per year. This was equal to about one-third of the direct CH4 emission from enteric fermentation. The total CH4 –C emission from solid manure (farm B) produced by one animal unit was 0.7 kg per year (see Section 2.5). This yielded a total CH4 emission from solid manure storage of farm B of 25 kg CH4 –C per year. These data indicate that CH4 emissions (per animal unit) related to waste management were approximately 17-fold higher for farm A (liquid slurry) than farm B (solid manure and grazing). Only a few studies have examined N2 O emission during storage of animal wastes. They indicated that, in contrast to CH4 emissions from manure, release of N2 O tends to increase with increasing manure aeration. Emissions of N2 O–N from slurry storage ranged between 0.01 and 0.08% of the slurry N (Oenema and Velthof, 1993; Heinemeyer et al., 1997). Losses of N2 O–N from dung heaps were about 0.1–0.8% of the manure N (Sibbesen and Lind, 1993; Heinemeyer et al., 1997). In the present study, N2 O emissions from cattle waste storage were included, assuming emission factors of 0.05 and 0.5% for storage of slurry and solid manure, respectively. The N2 O emission from cattle waste storage was determined by assuming that one animal unit produced 50 kg N as slurry or solid manure per year. The storage of cattle slurry in farm A resulted in a total emission of 0.7 kg N2 O–N per year (for 28.6 animal units). The solid manure storage in farm B (35.6 animal units, solid manure production for 6 month per year) gave rise to emissions of 4.5 kg N2 O–N per year.

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3.4. Emissions from fertilizer production and use of diesel fuel and heating oil

34.4 Mg CO2 equivalents for farms A and B, respectively (Table 5).

The mean greenhouse gas emission from the production of 1 kg synthetic fertilizer N applied in Germany was 5465 g CO2 equivalents, of which N2 O emissions comprised nearly 55% (Table 4). Similar results have been reported recently for the production of synthetic N fertilizers in The Netherlands (Kramer et al., 1999), but the release of CO2 equivalents per kg N produced were even greater (6847 g) and N2 O represented nearly 61% of the total greenhouse gas emission. The results shown in Table 2 suggest that the incorporation of legume residues may result in higher N2 O emissions than those induced by the application of synthetic N fertilizers. However, abiotic emissions produced during fertilizer production have to be taken into account when comparing emissions resulting from the application of mineral fertilizers with those from symbiontic N fixation. Total greenhouse gas emission resulting from synthetic N fertilizer use was 17.45 kg CO2 equivalents per kg N if emissions from fertilizer production and fertilized soils were considered together. About 70% of these emissions originated from fertilized soils. The mean amount of synthetic N fertilizers applied to the fields (30.4 ha fertilized fields) of farm A was 152 kg N per ha per year and total greenhouse gas emission resulting from the production of synthetic N fertilizers used was 25.25 Mg CO2 equivalents per year. The mean amount of diesel fuel consumed (mainly used for cultivation) at farms A and B was 3365 and 8412 kg per year, respectively. The greater consumption rate observed for farm B was not only due to the larger area of farmed soils (Table 1; fields and grassland), but also to greater consumption per hectare (105 and 149 kg per ha for farms A and B, respectively), owing to the more intensive soil and pasture management of farm B. The mean annual consumption of heating oil (mainly used for ventilation and drying of crops) was 1307 kg for farm A and 723 kg for farm B. The emissions from the annual fuel consumption were calculated from the emission factors listed in Table 4. The contribution of fossil fuel consumption of to the total annual greenhouse gas emissions from the two farming systems was 17.6 Mg CO2 equivalents and

3.5. Total greenhouse gas emission from the two farming systems Table 5 summarizes the results of the integrated evaluation of greenhouse gas emissions from the two farming systems. It provides insights into the main sources of greenhouse gas emissions and shows their contribution to the total atmospheric loading, expressed as aggregate CO2 equivalent. The total emissions from both farms were nearly identical (186 and 188 Mg CO2 equivalents for farms A and B, respectively). Related to the total farm area (Table 1, farm A: 44.6 ha; farm B: 62 ha) the emission of CO2 equivalents were approximately one-third higher for farm A (4.2 Mg per ha) than farm B (3.0 Mg per ha). The relative contribution of the various sources to the total greenhouse gas emission was similar in both farming systems. Emissions from soils contributed the major part (52% farm A, 57% farm B), cattle production and storage of cattle waste made up approximately one-fourth, and fossil fuel consumption and fertilizer production together contributed 23 (farm A) and 18% (farm B, no mineral fertilizer was used). The relative importance of the emissions from soils depended primarily on the intensity of animal production. The farming systems investigated here were not specialized in animal production and the livestock number per hectare farmed area was low (0.9 and 0.6 animal units per hectare for farms A and B, respectively). The results suggest that in systems with more intensive cattle production (2–3 animal units per hectare) that are quite common in southern Germany, CH4 emission from enteric fermentation and waste management is the major contribution to the total emission of greenhouse gases. In the present study, nitrous oxide contributed the major part (about 60%) to the total greenhouse gas emissions in both farming systems (Fig. 3); methane contributed approximately 25% and CO2 emissions of were the lowest (about 15%). All emission estimates for the biotic processes have considerable uncertainty. This is especially true for N2 O emissions from soils since these emissions may vary by orders of magnitude, both spatially and temporally. The estimates of N2 O emissions from soils are probably the main factor of uncertainty in

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Fig. 3. Total CO2 , CH4 and N2 O emissions from the agricultural production of two farming systems (A and B) in southern Germany and the contribution of different sources to theses emissions. The farming systems are described in Table 1 (cattle, emissions from enteric fermentation and cattle waste storage; fossil fuel, emissions from production and consumption of fossil fuel; fertilizer, emissions from fertilizer production; pasture, emissions from pasture including dung and urine deposits; fields, emissions from arable soils).

this integrated assessment of greenhouse gas emissions from the two farming systems. An important reference unit for comparison of greenhouse gas emissions from agricultural production is crop yield. However, comparison of crop-specific yield-related emissions was difficult in this study, since the two farming systems were based on different rotation phases. Additionally, the results show that the assignment of measured N2 O emissions to specific crops is problematic, because emissions can be considerably influenced by the crop rotation, in particular by the type of crop and the management during the previous year. An exact comparison of yield-related emissions was also complicated by differences of mean soil fertility mainly caused by the thickness of loess deposits, which were on average greater for farm A than farm B. Since a detailed crop-specific comparison of yield-related emissions was hardly possible, the study focused on a more general assessment of the farming systems. The mean emission related to crop production (emissions from fields, fertilizer production and the consumption of fossil fuels for field management and drying of crops) was about 38% larger for farm A (4.4 Mg CO2 equiv-

alents per ha) than farm B (3.2 Mg CO2 equivalents per ha). However, the crop yield was 47% lower for farm B than farm A, if the mean dry matter yield of wheat (5590 kg per ha for farm A and 2960 kg per ha for farm B) was we used as an indicator of the mean yield level of the farming systems. In spite of the mentioned restrictions, these data indicated that organic farming did not result in reduced emissions per unit crop yield. Total emissions per cattle unit from enteric fermentation and waste storage were about 25% higher for farm A (1.6 Mg CO2 equivalents per cattle unit) than farm B (1.3 Mg CO2 equivalents per cattle unit). This difference was attributed to higher emissions from waste management on farm A. However, emissions were nearly identical if related to beef production, as animal productivity (mean increase in weight per day) was about 20% higher for farm A (≈1200 g per day) than farm B (≈950 g per day). If greenhouse gas emission from feed production is included, total emissions per unit beef are probably higher for the more extensive cattle management than the intensive bull fattening. Similar conclusions were drawn by Subak (1997), who compared greenhouse gas emis-

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sions from extensive and intensive beef production in Europe. 4. Conclusions The emission of greenhouse gases related to agricultural production occurs from various steps within the production chain. Hence, the assessment of different management systems requires an integrated analyses of greenhouse gas emission covering the complete production chain and including the life cycle of agricultural inputs. The results from this study demonstrate the important contribution of N2 O emissions from soils to the total greenhouse gas emission from agricultural production. The importance of N2 O was even underestimated here, since indirect emissions induced by NO3 − leaching and NH3 volatilization were not considered. In this study, site-specific N2 O emission factor (2.5%) was markedly higher than the default value (1.25%) recommended by the IPCC (1997). The conversion from conventional to organic farming resulted in reduced emissions per hectare farm and field area, but yield-related emissions were not reduced. Thus, the conversion to organic farming may contribute to the reduction of greenhouse gas emissions from agriculture, if agrarian policy strives to reduce the intensity of agricultural production. From a more general perspective (considering the globally increasing demand for food), emissions should also be assessed with regard to production of protein for human nutrition. This approach clearly shows that the extent of livestock husbandry is a key factor determining greenhouse gas emission related to food production. This is due to the direct emissions from animals and animal wastes and, more importantly, to the low N efficiency of meat production, since about 80–95% of the N intake with feed is excreted as dung and urine. Therefore, the reduction of crop production for animal husbandry in favor of human nutrition represents one of the most efficient measures for mitigating greenhouse gas emissions from agriculture. Acknowledgements This research was supported by the GSF-Forschungs-zent-rum für Umwelt und Gesundheit, the

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