SimDen – A simple empirical model for quantification of N2O emission and denitrification

The denitrification process, including emission of nitrous oxide (N2O), is an important process both from an agricultural and environmental point of view. However, the process is, due to its high spatial and temporal variability, very difficult and resource demanding to quantify, and the only alternative is often to use simulation models. However, simulation models often need more input information than what is available. Therefore, a simple empirical model - SimDen - for quantifying the denitrification is presented, where only information about soil type and fertilisation are needed as input variables. Basically, the denitrification in SimDen is calculated as (N2O-emission) x (N2/N2O-ratio): - The N2O-emission is derived from input of N and emission factors, as used in the IPCC-methodology. SimDen use emission factors of 0.8% of applied N for inorganic fertiliser and 2.5% for animal manure/slurry and N2 fixation. Only N2 fixation below harvest or grazing height, i.e., fixation in stubble and roots, is considered as N-input. - N2/N2O-ratios are based on literature values. Soil moisture and content of organic matter affect both the N2O-emission and the N2/N2O-ratio, which made it possible to relate the denitrification to the Danish soil types, which differs in hydraulic properties and soil texture. SimDen is verified against a number of field measurements from Danish soils and has been compared to simulations with the soil-plant-atmosphere model Daisy. SimDen can be downloaded from: www.agrsci.dk/simde

Effect of age and cutting frequency on below-ground biomass in grass-clover

Significant amounts of plant biomass and fixed N are incorporated into soil as roots, nodules and root exudates during the growth of grass-clover, which consequently is exposed to soil microbial degradation when the field is ploughed. This has major implications for our understanding of the potential contributions of legumes in the N economics and losses in grass-clover cropping systems. We have studied the effects of grass-clover pasture age and cutting frequency (simulated grazing) on the development of the above ground as well as the below ground plant biomass, with emphasis on below-harvest contribution to the total N2 fixation

Effects of cutting frequency on plant production, N-uptake and N2 fixation in harvested and below-harvest plant biomass of grass clover

Nitrogen (N) accumulating in stubble, stolons and roots is an important component in N balances in perennial ryegrass-white clover swards, and the effects of cutting frequency on the biomass of above- and below-harvest height were studied during two consecutive years. Total dry matter (DM) and total N production, and N2 fixation, were measured at two cutting frequencies imposed in the summers of two years either by cutting infrequently at monthly intervals to simulate mowing or by frequent cutting at weekly intervals to simulate grazing. Total DM production harvested was in the range 3000 - 7000 kg DM ha-1 with lower DM production associated with the frequent cutting treatment, and it was significantly affected by the different weather conditions in the two years. The higher cutting frequency also reduced the biomass below harvest height, but the different weather conditions between years had less effect on stubble and, in particular, root biomass. The biomass of white clover roots was significantly lower than that of perennial ryegrass roots, and remained at a relatively constant level (200-500 kg DM ha-1) throughout the experiment, whereas the biomass of perennial ryegrass roots increased from 2 400 kg DM ha-1 in the year of establishment to 10 200 kg DM ha-1 in the infrequent cutting treatment and 6 650 kg DM ha-1 in the frequent cutting treatment by the end of the experiment, giving shoot: root ratios of 4.7 – 16.6 and 0.5 – 1.6 for white clover and perennial ryegrass, respectively. Annual N2 fixation was in the range 28 – 214 kg N ha-1, and the proportion of N fixed in stolons and roots was on average 0.28. However, since the weather conditions affect the harvested DM production and the shoot: root ratio, care must be taken when estimating total N2 fixation based on an assumed or fixed shoot: root rati

Nitrate leaching in grazed grasslands of different composition and age

In a field experiment at Research Centre Foulum a suction cup technique was used to investigate nitrate leaching from grassland depending on composition (grass-clover or perennial ryegrass), management (grazing or cutting) and age of the swards. In 1997-2001 was investigated the successive nitrate leaching from 4-7 year old grazed grass-clover and ryegrass with cut plots of similar age and spring barley as reference. In 2000-2001 the simultaneous nitrate leaching from newly established swards, swards grazed for 1 and 7 years and swards cut for 7 years was investigated. In the newly established swards nitrate leaching from grass-clover and ryegrass were similar but at increasing sward age nitrate leaching from the fertilized ryegrass increased dramatically compared to a constant low level from the unfertilized grass-clover. Apparently, the clover component of grass-clover was able to equalize differences in soil nitrogen availability in swards of different age. The results of N2 fixation studies in swards of different age in 2001 will be used in the interpretation of the nitrate leaching data

Hvor meget kvælstof henter kløvergræs fra luften?

Bælgplanter, herunder de forskellige kløverarter, kan ved knoldbakteriers hjælp (symbiose) optage luftens kvælstof (N2), og derved udnytte denne uudtømmelige kvælstofkilde. Processen er simpelt-hen forudsætningen for at kunne drive økologisk jordbrug, og det har derfor stor interesse at kende størrelsen af N-fikseringen og have kendskab til hvilke faktorer der har betydning for derved om muligt at kunne optimere produktiviteten og reducere eventuelle miljømæssige effekter af et givet dyrkningssystem

Ærter som kvælstofsamler i vinterafgrøder

I markforsøg blev det undersøgt hvorvidt samtidig såning af ærter og vinterkorn kunne øge kerneudbytter i kornet. Princippet i dyrkningsmetoden er at ærterne i løbet af efteråret via biologisk N-binding optager luftens kvælstof, som efter ærternes udvintring friggives til kornafgrøden i det tidlige forår. Sammenfattende kunne det konkluderes, at såfremt der ikke er større udgifter forbundet med anskaffelse af ærteudsæd (evt. ærter af egen avl), kan dyrkningssystemet med fordel anvendes. Specielt i år hvor de klimatiske forhold gør det vanskeligt at udbringe gødning tidligt, vil ”ærte-kvælstof” kunne sikre at kornafgrøden kommer godt i gang

N-fiksering - hvor meget og hvordan?

Denne biologiske proces er en forudsætning for at kunne drive økologisk jordbrug, og det har derfor stor interesse at kende størrelsen af N-fikseringen og have kendskab til hvilke faktorer der har betydning, for derved om muligt at kunne optimere produktiviteten og reducere eventuelle miljømæssige effekter af et givet dyrkningssystem. En lang række undersøgelser har vist, at der er en forholdsvis konstant relation mellem høstet kløvertørstof og N-fiksering. I kløvergræs udgør N-fikseringen således 30-35 kg N per tons høstet kløvertørstof. Dvs. at hvis der i en kløvergræs i løbet af vækstsæsonen er produceret f.eks. 8 tons tørstof pr. ha og den gennemsnitlige kløverandel har været 25%, bliver N-fikseringen 60-70 kg N/ha. Hertil kommer imidlertid en række andre bidrag, som ikke indgår i den høstede del: N-fiksering i stub, udløbere og rødder, overførsel af fikseret N fra kløver til græs, samt udskillelse af fikseret N til jordens N-pulje. Disse bidrag udgør, afhængig af bl.a. kløvergræssets alder og jordtype, op til 125% af N-fikseringen i den høstede del, således at den samlede N-fiksering i eksemplet bliver omkring 150 kg N/ha. Under praktiske forhold er det imidlertid ofte sådan at mængden af produceret kløvertørstof eller kløverandelen ikke kendes, og man må ty til andre mere usikre metoder for at beregne N-fikseringen. En af de faktorer der har størst betydning for N-fikseringen er tilførsel af gødnings-N. Ved gødskning reduceres N-fikseringen, og spørgsmålet er om ikke man "får mere for pengene" ved at anvende gødningen andre steder i sædskiftet, og lade kløveren klare gødskningen af kløvergræsset

Organisk kvælstof kan også udvaskes

Når vi taler om udvaskning af kvælstof mener vi i reglen nitratudvaskning til én meters dybde. Spørgsmålet er imidlertid om der også udvaskes organisk bundet kvælstof, og hvilken effekt det organiske stof eventuelt har på udvaskningens størrelse og på kvælstoffets videre skæbne i jordlagene under en meters dybde. I jorden findes en stor pulje af organisk stof, som ved mikrobiel aktivitet efterhånden nedbrydes til mindre organiske kulstof- og kvælstofforbindelser, der kan frigives til jordvæsken med potentiel risiko/mulighed for at blive udvasket. Nye forsøg har vist at der udvaskes organisk bundet kvælstof (org. N) og kulstof (org. C) i ikke helt ubetydelige mængder, og at udvaskningen af dette bl.a. afhænger af afgrøder og dyrkningsmetode

Nitrogen leaching following cultivation of grazed grass-clover on coarse sandy soil

Nitrogen leaching following cultivation of grazed grass-clover on coarse sandy soil Background and objectives When grass-clover is ploughed there is a high risk of nitrogen leaching. The objective of this study was to measure the effectiveness of an early catch crop in reducing nitrogen leaching from coarse sandy soil. Barley as a green crop for silage was undersown with Italian ryegrass in spring and harvested at the beginning of early heading, and the Italian ryegrass was subsequently used for roughage production in autumn. Material and methods Experiments were established in spring 2003 on a commercial organic farm with a coarse sandy soil. Two fields with grass-clover were ploughed. One field, a 3-year-old grass-clover, had formed part of a crop rotation dominated by cereals, and the other, a 5-year-old grass-clover, was part of a grass-intensive rotation grazed by dairy cows. After ploughing the grass-clover, the following treatments were established in each of the two fields: 1) spring barley, harvested at maturity and subjected to mechanical weed control in autumn (“Mature”) and 2) spring barley harvested early as a green crop for silage with an undersown catch crop of Italian ryegrass (“Green”), which was mowed twice in autumn. The treatments were fertilized with 0, 60 or 120 kg ammonium-N ha-1 in cattle slurry, injected in the spring following ploughing. The resultant treatments are called: Mature-0N, Mature-60N, Mature-120N, Green-0N, Green-60N and Green-120N. Leaching of nitrogen (nitrate and total-N) was measured from May 2003 to May 2004 by means of ceramic suction cups installed in treatments fertilized with 0 or 120 kg ammonium-N ha-1. Results and conclusions Nitrate leaching after Mature-0N was 174 and 240 kg N ha-1 in 3-year-old and the 5-year-old grass-clover, respectively, when the soil was kept bare by rotovating twice during autumn. In Mature-120N leaching was 302 and 316 kg N ha-1. In Green-0N and Green-120N leaching was only 7-9 kg N ha-1. This means that the ”Green” treatments reduced leaching by 166-309 kg N ha-1, corresponding to 95-98%. In addition to nitrate leaching, 10 and 30 kg N ha-1 was leached as other N-containing compounds with the highest amount from Mature-0N and Mature-120N after the 5-year-old grass-clover. Yields harvested in Mature-0N were 3.4 and 3.9 Mg dry matter ha-1, and yields harvested in Green-0N were 6.4 and 9.7 Mg dry matter ha-1. A comparison of the treatments Mature-0N and Manure-120N showed additional nitrate leaching of on average 102 kg N ha-1 when 120 kg ammonium-N ha-1 was applied. This is matched by a corresponding lack in yield increase when applying 120 kg N ha-1. In contrast to ”Mature” treatments, leaching from ”Green” treatments did not differ, irrespective of whether manure was applied or not. This can be explained by an additional N uptake of on average 127 kg N ha-1 in Green-120N compared with Green-0N. So in the “Green” treatments most of the manure N was taken up by the ryegrass instead of being leached. The experiments showed that barley used as a green crop for silage undersown with Italian ryegrass could reduce leaching to a minimum. This offers advantages not only for the environment but also for farmers, as a high production of roughage was possible. Besides, the increasing difficulties with clover soil fatigue experienced by Danish organic farmers could be avoided

Leaching of dissolved organic carbon (DOC) and nitrogen (DON) from grass-clover pastures after ploughing

In mixed arable grassland systems there can be a considerable build-up of nitrogen (N) caused by input through symbiotic N2 fixation and by the deposition from grazing animals. After ploughing of such grazed grassland large amounts of inorganic N may be released through mineralisation and subsequently leached to deeper soil layers or eventually to the groundwater. However, not much is known about the contribution of dissolved organic nitrogen (DON) to the total amount of N leached