Heinz Flessa

Seasonal variation of N2O and CH4 fluxes in differently managed arable soils in southern Germany

Agricultural practices are assumed to contribute significantly to the increase in atmospheric nitrous oxide (N 2 O) concentrations observed in the last decades, and they might influence the consumption of atmospheric methane (CH 4 ) by soil. The aim of this study was to quantify the effects of management intensity, soil type, and frost periods on the emission of N 2 O and the consumption of CH 4 in rotations in southern Germany. Fluxes of N 2 O and CH 4 were monitored over 12 months, using a closed chamber technique. The extensively managed system was cropped to sunflower and fertilized with farmyard manure (12 t ha −1 ). The intensively managed field was planted with spring wheat and fertilized with a total of 190 kg N ha −1 given as calcium ammonium nitrate (30 kg N) and urea-NH 4 NO 3 solution (160 kg N). Variation in the N 2 O emissions with time was extremcly high, with flux rates ranging from 0 to 2700 μg m −2 h −1 . The N 2 O fluxes were influenced by soil properties, management practices, and weather. The highest release rates were measured in the winter during thawing of the frozen soil. During the growing season, N 2 O emission was highest after heavy precipitation. No strong relationship was found between N 2 O emission rates and soil factors such as soil temperature, soil moisturc, and soil nitrate content. Annual fluxes of N 2 O from the extensively managed field were 9.4 and 12.9 kg N 2 O-N ha −1 yr −1 for a sandy soil and a clay soil, respectively. Total N 2 O-N losses from the intensively fertilized field amounted to 9.6 kg ha −1 yr −1 for a silty soil with a tendency to waterlogging during wintertime and to 16.8 kg ha −1 yr −1 for a loamy colluvial soil. Up to 46% of the annual N 2 O evolution was emitted during December and January when frost/thaw cycles induced extremely high N 2 O production. The application of urea-NH 4 NO 3 solution significantly increased N 2 O emission rates. Of the 160 kg N applied, 2.9 kg N or 1.8% was lost as N 2 O within a period of 8 weeks. Rates of CH 4 -C uptake varied from 0 to 17 μg m −2 h −1 . Soil temperature correlated positively and soil moisture correlated negatively with the CH 4 consumption. Stepwise multiple linear regression, including these soil factors, explained up to 44% of the variation in CH 4 fluxes. Annual CH 4 -C uptake was rather similar for the intcnsively and extensively managed Eutrochrepts, ranging from 348 to 395 g ha −1 . Significantly higher CH 4 -C consumption of 567 g ha −1 yr −1 occurred in the colluvial soil (Typic Udifluvent). N fertilizers had no effect on the CH 4 flux rates.

Effect of crop-specific field management and N fertilization on N2O emissions from a fine-loamy soil

Agricultural soils are a major source of atmospheric N2O. This study was conducted to determine the effect of different crop-specific field management and N fertilization rates on N2O emissions from a fine-loamy Dystric Eutrochrept. Fluxes of N2O were measured for two years at least once a week on plots cropped with potatoes (Solanum tuberosum) fertilized with 50 or 150 kg N ha−1 a−1, winterwheat (Triticum aestivum) fertilized with 90 or 180 kg N ha−1 a−1, corn (Zea mays) fertilized with 65 or 130 kg N ha−1 a−1, and on an unfertilized, set-aside soil planted with grass (mainly Lolium perenne and Festuca rubra). The mean N2O emission rate from the differently managed plots was closely correlated to the mean soil nitrate content in the Ap horizon for the cropping period (April to October, r2 = 0.74), the winter period (November to March, r2 = 0.93, one outlier excluded), and the whole year (r2 = 0.81). N2O emissions outside the cropping period accounted for up to 58% of the annual emissions and were strongly affected by frost-thaw cycles. There was only a slight relationship between the amount of fertilizer N applied and the annual N2O emission (r2 = 0.20). The mean annual N2O-N emission from the unfertilized set-aside soil was 0.29 kg ha−1. The annual N2O-N emission from the fertilized crops for the low and the recommended rates of N fertilization were 1.34 and 2.41 kg ha−1 for corn, 2.70 and 3.64 kg ha−1 for wheat, and 5.74 and 6.93 kg ha−1 for potatoes. The high N2O emissions from potato plots were due to (i) high N2O losses from the interrow area during the cropping season and (ii) high soil nitrate contents after the potato harvest. The reduction of N fertilization (fertilizer was applied in spring and early summer) resulted in decreased N2O emissions during the cropping period. However, the emissions during the winter were not affected by the rate of N fertilization. The results show that the crop-specific field management had a great influence on the annual N2O emissions. It also affected the emissions per unit N fertilizer applied. The main reasons for this crop effect were crop-specific differences in soil nitrate and soil moisture content.

Plant-induced changes in the redox potentials of rice rhizospheres

Redox potentials in microsites of the rhizosphere of flooded rice were continuously measured for several days. Close to the root tips redox potential markedly increased. The highest increase was measured in the rhizosphere of the tips of short lateral roots. Aerobic redox conditions were reached there, except in a very strongly reduced soil. Both the extension of the oxidation zone around the root tips and the maximum redox potential reached were influenced by the reducing capacity of the soil. The radius of the redox rhizosphere varied from less than 1 mm in a strongly reduced soil up to 4 mm in a weakly reduced one. The root-induced oxidation processes in the rhizosphere depended on the atmospheric oxygen supply to the roots.

N2O and CH4 fluxes in potato fields: automated measurement, management effects and temporal variation

Abstract The large temporal variation in nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) flux rates is a major source of error when estimating cumulative fluxes of these radiative active trace gases. We developed an automated system for near-continuous, long-term measurements of N2O, CH4 and CO2 fluxes from cropland soils and used it to study the temporal variation of N2O and CH4 fluxes from potato (Solanum tuberosum L.) fields during the crop periods of 1997 and 1998, and also to determine the effects of management practices and weather. Additionally, we evaluated the error of other common methods, namely, weekly or monthly measurements, used for estimating cumulative fluxes. The fluxes were quantified separately for the ridges, uncompacted interrows and tractor-compacted interrows. Total N2O–N emission from the potato field during the growing period (end of May to September) was 1.6 kg ha−1 in 1997 and 2.0 kg ha−1 in 1998; emissions were highest for the tractor-compacted soil. Periods of increased N2O losses were induced by heavy precipitation (in particular in compacted soil) and by the killing of potato tops (on the ridges) by herbicide application. The total CH4–C uptake in the potato field during the growing period was 295 g ha−1 in 1997 and 317 g ha−1 in 1998. The major fraction of the total CH4 uptake (≈86%) occurred on the ridges. Weekly measurements of N2O fluxes complemented by additional event-related flux determinations provided accurate estimates of total emissions. The monthly flux determination was not adequate for determining the temporal variation of the N2O emission rates. Weekly measurements were sufficient to provide reliable estimates of the cumulative CH4 uptake.

Isotopologue enrichment factors of N2O reduction in soils

Isotopic signatures can be used to study sink and source processes of N(2)O, but the success of this approach is limited by insufficient knowledge on the isotope fractionation factors of the various reaction pathways. We investigated isotope enrichment factors of the N(2)O-to-N(2) step of denitrification (epsilon) in two arable soils, a silt-loam Haplic Luvisol and a sandy Gleyic Podzol. In addition to the epsilon of (18)O (epsilon(18O)) and of average (15)N (epsilon(bulk)), the epsilon of the (15)N site preference within the linear N(2)O molecule (epsilon(SP)) was also determined. Soils were anaerobically incubated in gas-tight bottles with N(2)O added to the headspace to induce N(2)O reduction. Pre-treatment included the removal of NO(3) (-) to prevent N(2)O production. Gas samples were collected regularly to determine the dynamics of N(2)O reduction, the time course of the isotopic signatures of residual N(2)O, and the associated isotope enrichment factors. To vary reduction rates and associated fractionation factors, several treatments were established including two levels of initial N(2)O concentration and anaerobic pre-incubation with or without addition of N(2)O. N(2)O reduction rates were affected by the soil type and initial N(2)O concentration. The epsilon(18O) and epsilon(bulk) ranged between -13 and -20 per thousand, and between -5 and -9 per thousand, respectively. Both quantities were more negative in the Gleyic Podzol. The epsilon of the central N position (epsilon(alpha)) was always larger than that of the peripheral N-position (epsilon(beta)), giving epsilon(SP) of -4 to -8 per thousand. The ranges and variation patterns of epsilon were comparable with those from previous static incubation studies with soils. Moreover, we found a relatively constant ratio between epsilon(18O) and epsilon(bulk) which is close to the default ratio of 2.5 that had been previously suggested. The fact that different soils exhibited comparable epsilon under certain conditions suggests that these values could serve to identify N(2)O reduction from the isotopic fingerprints of N(2)O emitted from any soil.

Dual isotope and isotopomer signatures of nitrous oxide from fungal denitrification – a pure culture study

RATIONALE The contribution of fungal denitrification to the emission of the greenhouse gas nitrous oxide (N2O) from soil has not yet been sufficiently investigated. The intramolecular (15)N site preference (SP) of N2O could provide a tool to distinguish between N2O produced by bacteria or fungi, since in previous studies fungi exhibited much higher SP values than bacteria. METHODS To further constrain isotopic evidence of fungal denitrification, we incubated six soil fungal strains under denitrifying conditions, with either NO3(-) or NO2(-) as the electron acceptor, and measured the isotopic signature (δ(18)O, δ(15)Nbulk and SP values) of the N2O produced. The nitrogen isotopic fractionation was calculated and the oxygen isotope exchange associated with particular fungal enzymes was estimated. RESULTS Five fungi of the order Hypocreales produced N2O with a SP of 35.1 ± 1.7 ‰ after 7 days of anaerobic incubation independent of the electron acceptor, whereas one Sordariales species produced N2O from NO2(-) only, with a SP value of 21.9 ± 1.4 ‰. Smaller isotope effects of (15)Nbulk were associated with larger N2O production. The δ(18)O values were influenced by oxygen exchange between water and denitrification intermediates, which occurred primarily at the nitrite reduction step. CONCLUSIONS Our results confirm that SP of N2O is a promising tool to differentiate between fungal and bacterial N2O from denitrification. Modelling of oxygen isotope fractionation processes indicated that the contribution of the NO2(-) and NO reduction steps to the total oxygen exchange differed among the various fungal species studied. However, more information is needed about different biological orders of fungi as they may differ in denitrification enzymes and consequently in the SP and δ(18)O values of the N2O produced.

No general soil carbon sequestration under Central European short rotation coppices

Wood from short rotation coppices (SRCs) is discussed as bioenergy feedstock with good climate mitigation potential inter alia because soil organic carbon (SOC) might be sequestered by a land‐use change (LUC) from cropland to SRC. To test if SOC is generally enhanced by SRC over the long term, we selected the oldest Central European SRC plantations for this study. Following the paired plot approach soils of the 21 SRCs were sampled to 80 cm depth and SOC stocks, C/N ratios, pH and bulk densities were compared to those of adjacent croplands or grasslands. There was no general trend to SOC stock change by SRC establishment on cropland or grassland, but differences were very site specific. The depth distribution of SOC did change. Compared to cropland soils, the SOC density in 0–10 cm was significantly higher under SRC (17 ± 2 in cropland and 21 ± 2 kg C m−3 in SRC). Under SRC established on grassland SOC density in 0–10 cm was significantly lower than under grassland. The change rates of total SOC stocks by LUC from cropland to SRC ranged from −1.3 to 1.4 Mg C ha−1 yr−1 and −0.6 Mg C ha−1 yr−1 to +0.1 Mg C ha−1 yr−1 for LUC from grassland to SRC, respectively. The accumulation of organic carbon in the litter layer was low (0.14 ± 0.08 Mg C ha−1 yr−1). SOC stocks of both cropland and SRC soils were correlated with the clay content. No correlation could be detected between SOC stock change and soil texture or other abiotic factors. In summary, we found no evidence of any general SOC stock change when cropland is converted to SRC and the identification of the factors determining whether carbon may be sequestered under SRC remains a major challenge.

Properties and degradability of hydrothermal carbonization products.

Biomass carbonized via hydrothermal carbonization (HTC) yields a liquid and a carbon (C)-rich solid called hydrochar. In soil, hydrochars may act as fertilizers and promote C sequestration. We assumed that the chemical composition of the raw material (woodchips, straw, grass cuttings, or digestate) determines the properties of the liquid and solid HTC products, including their degradability. Additionally, we investigated whether easily mineralizable organic components adsorbed on the hydrochar surface influence the degradability of the hydrochars and could be removed by repetitive washing. Carbon mineralization was measured as CO production over 30 d in aerobic incubation experiments with loamy sand. Chemical analysis revealed that most nutrients were preferably enriched in the liquid phase. The C mineralization of hydrochars from woodchips (2% of total C added), straw (3%), grass (6%), and digestate (14%) were dependent on the raw material carbonized and were significantly lower (by 60-92%; < 0.05) than the mineralization of the corresponding raw materials. Washing of the hydrochars significantly decreased mineralization of digestate-hydrochar (up to 40%) but had no effect on mineralization rates of the other three hydrochars. Variations in C mineralization between different hydrochars could be explained by multiple factors, including differences in the O/C-H/C ratios, C/N ratios, lignin content, amount of oxygen-containing functional groups, and pH. In contrast to the solids, the liquid products were highly degradable, with 61 to 89% of their dissolved organic C being mineralized within 30 d. The liquids may be treated aerobically (e.g., for nutrient recovery).

Recent research progress on the significance of aquatic systems for indirect agricultural N2O emissions

Abstract A considerable fraction of N applied to agricultural soils is lost to adjacent systems via leaching and runoff. During the flow of this N-load through drainage systems, aquifers, riparian zones, streams and estuaries, N2O is produced, consumed, transported and emitted to the atmosphere. These ‘indirect’ agricultural N2O emissions are considered as a potentially important N2O source. However, their magnitude is still under debate. In this paper, (1) the processes causing ‘indirect’ emissions are summarized; (2) concepts of emission factors are discussed; (3) recent studies supplying indirect emission factors are reviewed; and (4) a potential new approach for evaluating emission factors is presented. The majority of recent data on N2O fluxes from aquifers and drainage systems support the assumption that the IPCC default emission factor for N2O from leached agricultural N in groundwater and drainage ditches (EF5-g) of 0.015 is too high. Recent reports of relatively high N2O emission from riparian areas suggest that future estimates of EF5 should explicitly account for these systems. In this paper, three different types of emissions factors (EF(A), EF(B) and EF(C)) are compared. Among the investigated systems, N2O emission in relation to NO3 −-loading (EF(A)) in riparian buffer zones ranged between 0.02 and 0.06 which is more than one order of magnitude above EF(A) of aquifers, rivers and drainage systems (0.00065 to 0.001). N2O emission in relation to NO3 − consumption within a specific system (emission factor EF(C)) is suitable for classifying the environmental impact of nitrate removal. EF(C) in the riparian buffer zones was one order of magnitude higher compared to aquifers and rivers. This suggests that restoration of riparian buffers to lessen NO3 − discharge to streams and oceans could increase global N2O emission. Isotopic signatures of N2O in aquatic systems are clearly distinct from the signatures of surface emitted N2O. This suggests that it might be possible to validate the contribution of aquatic systems to global N2O emission using isotopic budget calculations.

Direct nitrous oxide emissions from oilseed rape cropping – a meta-analysis

Oilseed rape is one of the leading feedstocks for biofuel production in Europe. The climate change mitigation effect of rape methyl ester (RME) is particularly challenged by the greenhouse gas (GHG) emissions during crop production, mainly as nitrous oxide (N2O) from soils. Oilseed rape requires high nitrogen fertilization and crop residues are rich in nitrogen, both potentially causing enhanced N2O emissions. However, GHG emissions of oilseed rape production are often estimated using emission factors that account for crop‐type specifics only with respect to crop residues. This meta‐analysis therefore aimed to assess annual N2O emissions from winter oilseed rape, to compare them to those of cereals and to explore the underlying reasons for differences. For the identification of the most important factors, linear mixed effects models were fitted with 43 N2O emission data points deriving from 12 different field sites. N2O emissions increased exponentially with N‐fertilization rates, but interyear and site‐specific variability were high and climate variables or soil parameters did not improve the prediction model. Annual N2O emissions from winter oilseed rape were 22% higher than those from winter cereals fertilized at the same rate. At a common fertilization rate of 200 kg N ha−1 yr−1, the mean fraction of fertilizer N that was lost as N2O‐N was 1.27% for oilseed rape compared to 1.04% for cereals. The risk of high yield‐scaled N2O emissions increased after a critical N surplus of about 80 kg N ha−1 yr−1. The difference in N2O emissions between oilseed rape and cereal cultivation was especially high after harvest due to the high N contents in oilseed rapes crop residues. However, annual N2O emissions of winter oilseed rape were still lower than predicted by the Stehfest and Bouwman model. Hence, the assignment of oilseed rape to the crop‐type classes of cereals or other crops should be reconsidered.