Flow of nutrients and climate relevant gases in experimental rotations on three soil types

Within organic farming there is a continued effort to close nutrient cycles in order to improve nutrient use efficiency for crop production while also minimizing environmental impacts. In Denmark a long-term crop rotation experiment on three soil types, i.e., Jyndevad (4.5% clay, 2.0% C), Foulum (8.8% clay, 3.8% C) and Flakkebjerg (15.5% clay, 1.7% C), provides a platform for studies of agronomic and environmental aspects of organic farming. Selected results from investigations at these sites are presented. In 2007 and 2008 one study determined soil microbial biomass N and mineralizable N on all three locations during the growing season in four selected rotations, three organic and one conventional; the objective was to evaluate the role of soil organic N dynamics for crop N supply. Consistent effects of crop rotation were observed across the three sites, but there were dramatic differences between soils in the ability to store N in labile organic pools. The greenhouse gas balance of organic farming systems has become an important indicator of sustainability, and strategies to mitigate emissions are investigated. Nitrous oxide emissions were monitored in the field between October 2007 and September 2008 at Foulum and Flakkebjerg in the winter wheat crop included in all four rotations, which differed in manure strategy (livestock slurry, grass-clover green manure, or mineral N) and use of catch crops. At both sites fluxes were measured concurrently at least twice per month. Also, a second monitoring program of N2O emissions was conducted between March 2008 and May 2009 to investigate grass-clover co-digestion as a strategy to improve N use efficiency and reduce N2O emissions per unit product. Here, all crops in the rotation were represented in the monitoring program. Both studies indicate that N input rather than cropping system or soil management control the extent of N2O emissions. Our limited ability to control N2O emissions may be due to the fact that N2O production is caused by complex interactions between C/N transformations and soil properties, including moisture. This has been investigated using intact and repacked soil from Foulum and Flakkebjerg sites. Adjusting the soil to well-defined soil water potentials with or without amendments serve to illustrate these interactions and the microbial response to nutrient supply and soil aeration. Ammonia volatilization after livestock slurry application should be avoided as NH3 is an indirect source of N2O, and NH3 losses will reduce crop yields in N limited cropping systems. Livestock slurry treatment (anaerobic digestion, separation) and application methods (trailhose, direct injection) can influence the potential for NH3 volatilization by reducing the slurry-air contact, but with a possible trade-off with direct N2O emissions. Some results from a two-year field study, and their implications for manure management, are discussed

Short-term nitrous oxide emissions from pasture soil as influenced by urea level and soil nitrate

Nitrogen excreted by cattle during grazing is a significant source of atmospheric nitrous oxide (N2O). The regulation of N2O emissions is not well understood, but may vary with urine composition and soil conditions. This laboratory study was undertaken to describe short-term effects on N2O emissions and soil conditions, including microbial dynamics, of urea amendment at two different rates (22 and 43 g N m-2). The lower urea concentration was also combined with an elevated soil NO3- concentration. Urea solutions labelled with 25 atom% 15N were added to the surface of repacked pasture soil cores and incubated for 1, 3, 6 or 9 days under constant conditions (60% WFPS, 14°C). Soil inorganic N (NH4+, NO2- and NO3-), pH, electrical conductivity and dissolved organic C were quantified. Microbial dynamics were followed by measurements of CO2 evolution, by analyses of membrane lipid (PLFA) composition, and by measurement of potential ammonium oxidation and denitrifying enzyme activity. The total recovery of 15N averaged 84%. Conversion of urea-N to NO3- was evident, but nitrification was delayed at the highest urea concentration and was accompanied by an accumulation of NO2-. Nitrous oxide emissions were also delayed at the highest urea amendment level, but accelerated towards the end of the study. The pH interacted with NH4+ to produce inhibitory concentrations of NH3(aq) at the highest urea concentration, and there was evidence for transient negative effects of urea amendment on both nitrifying and denitrifying bacteria in this treatment. However, PLFA dynamics indicated that initial inhibitory effects were replaced by increased microbial activity and net growth. It is concluded that urea-N level has qualitative, as well as quantitative effects on soil N transformations in urine patches

Nitrous oxide emissions from arable soil: Effects of crop rotation, tillage and manure management

Soil carbon storage and nitrous oxide (N2O) emissions are both important for the greenhouse gas balance of agricultural soil, but difficult to verify under field conditions due to high spatial and temporal variability. Carbon stock changes are particularly elusive because they occur over decades, and future climate or management changes may revert current trends. Emissions of N2O from arable soil are derived mainly from the short-term (<1 yr) turnover of crop residues, fertilizers and manure, which indicates that mitigation options may be found with a better understanding of management effects on soil C and N cycling and N2O emissions. This presentation will describe N2O studies within long-term crop rotation experiments that allow side-by-side comparisons of contrasting management strategies. A tillage experiment on a sandy loam soil, established in 2002, has been used to study effects of residue management and tillage on N2O emissions. With removal of crop residues there was no difference between three tillage strategies, but with residue retention there was significantly higher N2O emission from ploughed soil compared to non-inversion tillage. Cover crops are particularly needed in organic farming systems where the N supply is limited. Another long-term experiment with eight four-crop rotations was established on three soil types in 1996 to investigate strategies to improve crop yields. Estimates of N2O emission from the same crop (winter wheat) in different rotations, and from all crops of a single rotation, suggest that emissions should be considered at the crop rotation level. Although short-term N2O emissions appear to be driven by organic inputs and fertilizers, there are also long-term effects of crop rotation that may interact with environmental drivers such as rainfall or freeze-thaw events. Laboratory results with intact soil cores from four rotations will be used to discuss the relative importance of carbon availability and soil gas diffusivity

Greenhouse gas emissions from animal manure

On-farm emissions from animals and manure must be taken into account when the GHG mitigation potential of grassland management strategies involving grazing are evaluated. Greenhouse gas (GHG) emissions from manure management include direct emissions of CH4 and N2O, as well as indirect emissions of N2O derived from NH3/NOx. Quantification of GHG emissions from manure are typically based on national statistics for manure production and housing systems combined with emission factors which have been defined by the IPCC or nationally. The quality of GHG inventories for manure management is critically dependent on the applicability of these emission factors. Animal manure is collected as solid manure + urine, as liquid manure (slurry) or as deep litter, or it is deposited outside in drylots or on pastures. These manure categories represent very different potentials for GHG emissions, as also reflected in the methane conversion factors and nitrous oxide emission factors, respectively. However, even within each category the variations in manure composition and storage conditions can lead to highly variable emissions in practice. This variability is a major source of error in the quantification of the GHG balance for a system. To the extent that such variability is influenced by management and/or local climatic conditions, it may be possible to improve the procedures for estimating CH4 and N2O emissions from manure. Manure (dung and urine) deposited during grazing influences fluxes of both CH4, N2O and NH3 from the pasture. In particular, urine patches are important point sources of NH3 and N2O, whereas the N input may locally reduce CH4 oxidation activity. Ammonia losses from pastures are not specifically represented in the IPCC methodology, which calculates NH3 volatilization as a fixed proportion of total N excreted. However, ammonia losses from excretal returns to the pasture increase with N surplus in the diet since this N is mainly excreted as urea in the urine. Also, several methodologies exist for mitigating NH3 losses from storage facilities. Hence, both optimized feeding and restricted access to grazing with collection of manure on the farm are available as NH3 mitigation options, though not identified by the IPCC methodology. Technical solutions to reduce NH3 volatilization from storages may reduce (slurry) or increase (solid manure) CH4 emissions, an aspect that must also be taken into account. The N2O emission factor for N deposited on pastures is higher than for N in manure collected during housing, indicating that restricted access to grazing is also a N2O mitigation option. Several studies have suggested that N2O emissions from excreta deposited during grazing interact with factors like feed composition, stock density, N fertilization, soil compaction and climate. However, there is presently little evidence to suggest that emissions of N2O can be consistently changed via management practices

Nitrous oxide emissions from grazed grassland: effects of cattle management and soil conditions

Traditionally, dairy cattle spend a substantial part of the year on pastures. For organic farming within EU it is specified that ”all mammals must have access to pasturage or an open-air exercise area” which they must be able to use whenever ”weather conditions and the state of the ground permits” (Council Regulation [EEC] No 2092/91 ). Dairy production systems are characterized by a considerable N surplus, and N deposited during grazing represents a significant risk for environmental losses, including N2O emissions. Excess N is excreted mainly in the urine, the composition of which is influenced by factors such as lactation stage, sward quality and intake of supplements. Resulting N concentrations in urine patches can range from 20 to 80 g N m-2, and soil environmental conditions associated with such a range of N inputs could affect the potential for N2O production via nitrification and denitrification. Soil properties and fertilization also influence N2O emissions. This presentation shows results from a work package within the MIDAIR project which aimed to describe known sources of variability within the grazing system, and their impact on N2O emissions. The objective was to evaluate if management changes can be proposed that will reduce the risk for N2O emissions associated with grazing. Field studies have addressed the heterogeneity of soil physical, chemical and microbiological properties, while plot-scale and laboratory experiments have examined the fate of urinary C and N and the microbial response to urine deposition

Afgrøderester og sædskifte har stor betydning for udledning af lattergas

Lattergas udgør en del af det atmosfæriske tab af kvælstof fra dyrkningsjorden. Målt i kg kvælstof er mængderne små, men fordi lattergas er en meget kraftig drivhusgas, er gassens betydning for landbrugets samlede udledninger stor

Efterafgrøder - godt eller skidt for klimaet?

Arealet med efterafgrøder vokser i disse år. De efterlader planterester i jorden om foråret, men også kvælstof. Vi ser på, hvad det betyder for kulstoflagringen og lattergasudledningen under og efter vækstperioden

Vær bevidst om risikoen for lattergas fra efterafgrøder

EFTERAFGRØDER INDGÅR OFTE i sædskifter for at samle kvælstof op, som frigives, når planterester fra hovedafgrøden nedbrydes. Der er positive sideeffekter for klimaet i form af kulstofbinding og opsamling af kvælstof, som ellers efter udvaskning er en indirekte kilde til drivhusgassen lattergas. Men efterafgrøder bliver også selv til planterester - enten på grund af frost, eller når de om foråret afsluttes gennem jordbearbejdning. Under den nedbrydning, som følger, frigives den kvælstof, som er samlet op om efteråret. Nedbrydning kan danne lattergas, idet jordens mikroorganismer er aktive længe før en ny afgrøde er klar til at optage kvælstof fra efterafgrøden, er . De står for processer som nitrifikation og denitrifikation, som begge kan føre til udledning af lattergas. Hvis lattergas udledes fra efterafgrøders planterester om foråret, er det dårligt for det samlede klimaaftryk. Kriterier, når du skal vælge efterafgrøde, er: • effektiv opsamling af kvælstof om efteråret • at kvælstoffet kan udnyttes af næste hovedafgrøde • at kvælstof frigives i et tempo, hvor en ny afgrøde kan udnytte denne eftervirkning

Kvælstofdynamik i økologiske sædskifter

Jordens indhold af "aktivt" kvælstof påvirkes af sædskiftet. Kendskab til mængder og dynamik kan fremme selvforsyningen på økologiske bedrifter

Effects of green manure storage and incorporation methods on greenhouse gas fluxes and N mineralization after soil application

Organic arable farming faces challenges with low crop yields, partly due to inefficient use of green manure-derived nitrogen (N). Under current farming practices, green manure leys are often cut and mulched during the growing season with the associated risk of environmental N losses, leading to eutrophication and global warming. In this 3-month incubation experiment, we tested a new green manure management strategy as part of the ICROFS project HighCrop. With the new strategy, green manure leys are instead harvested and preserved until the following spring either as compost mixed with straw (grass-clover:straw, 4:1, w:w) or as silage of harvested ley biomass. In spring, these two green manure materials can then be used for targeted fertilization of spring sown crops. The objectives of the study were to: • Assess how storage methods (compost vs. silage) affect N2O fluxes and soil respiratory CO2 emissions after soil application of preserved grass-clover green manure. • Determine whether the greenhouse gas fluxes are influenced by the incorporation method, more specifically harrowing (simulated by mixing the material into the top 5 cm soil layer) and ploughing (the material placed at 15 cm depth). • Compare composted and ensiled green manures concerning their abilities to provide plant-available N during a 3-month period. During the experiment, gas fluxes were measured at nine occasion followed by eight destructive soil harvests. In total, the study included 192 soil units that were incubated at 15 °C in darkness. Each unit consisted of a packed soil core (26 cm high × 10 cm diameter) with bulk density of 1.07 g cm-3 and gravimetric soil moisture of 20 %. The addition of compost and silage corresponded to a fertilization rate of 120 kg total N ha-1. A mineral fertilizer treatment was included as a reference and received 80 kg NH4-N ha-1. Compared to the more degraded compost, the silage material had a high content of labile compound. In addition, incorporation of green manure by harrowing was expected to improve soil microbes’ access to the materials, and thereby increase the decomposition rate. In line with this, cumulative CO2 emissions from the green manure treatments was lowest for compost incorporated by ploughing and highest for silage incorporated by harrowing. Between 32 and 54 % of the added green manure carbon was respired as CO2 during the 3-month experiment. Interestingly, mineral fertilizer suppressed soil respiratory CO2 emission. Generally, N2O emissions were higher from the silage-amended soils than from soils fertilized with compost. Especially, silage incorporated by ploughing gave rise to increased N2O effluxes, corresponding to 0.3 % of applied total N during the 3-month period. This could partly result from denitrification of initial soil nitrate, stimulated by high local oxygen consumption in the labile silage layer. In contrast, compost incorporated by harrowing caused a downwards N2O flux into the soil, presumably an effect of lacking mineral N availability in this treatment. Overall, our study showed that emissions of N2O can be reduced by incorporating green manure using harrowing instead of ploughing. Net mineralization of green manure-derived N was absent until more than three weeks after incorporation of the materials. Over the 3-month experiment, grass-clover silage provided the highest net release of inorganic N with preliminary results corresponding to 38-43 kg N ha-1, irrespective of the incorporation method used. In contrast, no increase in soil mineral N was observed for the composted grass-clover and straw mixture compared to the unfertilized control soil. In fact, soil incorporation of compost by harrowing caused immobilization of soil mineral nitrogen 1-2 months after experimental set-up