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

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 organic nitrogen and carbon after cultivating grass-clover pastures

Leaching of organic nitrogen (DON) and carbon (DOC) was measured after cultivating grass-clover of different age. It was found that DON and escpecially DOC was lost in considerable amounts, and that the leaching depends upon crop and management. The highest concentrations of DON were measured in the bare soil treatment, whereas concen-trations in catch crop treatments were between 1.2 and 3.2 mg N L-1. The leaching of DOC showed opposite trends compared to leaching of DON with higher values in the catch crop treatments (296 - 310 kg DOC ha-1) than in bare soil treatments (174 - 217 kg DOC ha-1)

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

Transport and reduction of nitrate in clayey till underneath forest and arable land.

Transport and reduction of nitrate in a typically macroporous clayey till were examined at variable flow rate and nitrate flux. The experiments were carried out using saturated, large diameter (0.5 m), undisturbed soil columns (LUC), from a forest and nearby agricultural sites. Transport of nitrate was controlled by flow along the macropores (fractures and biopores) in the columns. Nitrate reduction (denitrification) determined under active flow mainly followed first order reactions with half-lives (t1/2) increasing with depth (1.5–3.5 m) from 7 to 35 days at the forest site and 1–7 h at the agricultural site. Nitrate reduction was likely due to microbial degradation of accumulated organic matter coupled with successive consumption of O2 and NO3− in the macropore water followed by reductive dissolution of Fe and Mn from minerals along the macropores. Concentrations of total organic carbon measured in soil samples were near identical at the two study sites and consequently not useful as indicator for the observed differences in nitrate reduction. Instead the high reduction rates at the agricultural site were positively correlated with elevated concentration of water-soluble organic carbon and nitrate-removing bacteria relative to the forest site. After high concentrations of water-soluble organic carbon in the columns from the agricultural site were leached they lost their elevated reduction rates, which, however, was successfully re-established by infiltration of new reactive organics represented by pesticides. Simulations using a calibrated discrete fracture matrix diffusion (DFMD) model could reasonably reproduce the denitrification and resulting flux of nitrate observed during variable flow rate from the columns

Udvaskning af organisk kulstof efter kløvergræs

Opløst organisk stof danner grundlaget for mange af de biogeokemiske processer der foregår i jorden. Der er foretaget talrige undersøgelser af dannelse og tab af organisk stof fra skovjorde, hvorimod kun få er gennemført i landbrugsjord. Det vides at græsmarker øger jordens pulje af organisk stof, men kendskabet til det organiske stofs skæbne efter ompløjning af græsmarker er mangelfuldt. Formålet med denne undersøgelse var at bestemme størrelsesordner for udvaskning af opløst organisk kulstof (DOC) og den samlede mængde mobilt organisk kulstof (MOC) i dyrkningssystemer med kløvergræs, med særlig fokus på perioden efter ompløjning af græsmarken. Undersøgelserne er gennemført dels i Foulum i et markforsøg anlagt med henblik på at måle kvælstofudvaskning og eftervirkning af kløvergræs af forskellig alder, og dels i Sønderjylland i et markforsøg anlagt for at afprøve forskellige strategier til reduktion af N-udvaskningen. Jordvand fra forskellige dybder blev udtaget med sugeceller og analyseret for C-indhold, hvorefter udvaskningen blev beregnet vha. modelleret afstrømning. Målingerne i Foulum viste, at koncentrationen af DOC aftog med dybden: 21 – 31 mg C l-1 i 30 cm, 7 – 12 mg C l-1 i 60 cm og 6 - 11 mg C l-1 i 90 cm. Værdierne var højere i en eksisterende 9. års kløvergræs end efter ompløjning af 1. års eller 8. års kløvergræs. Med en afstrømning på 218 mm blev den samlede udvaskning til en meters dybde i Foulum i ompløjet 1. års og 8. års, og i en eksisterende 9. kløvergræs beregnet til henholdsvis 15, 15 og 27 kg DOC ha-1 i løbet af en 4-måneders periode fra november 2002 til marts 2003. I Sønderjylland varierede koncentrationen af DOC i 70-100 cm’s dybde mellem 9 og 40 mg C l-1 og koncentrationen af MOC mellem 16 og 63 mg C l-1. Den samlede afstrømning her var 596 - 645 mm, afhængig af forsøgsbehandling, hvilket resulterede i en udvaskning af DOC på 95 – 192 kg C ha-1 år-1 og af MOC på 174 – 310 kg C ha-1 år-1

Ældre marker med kløvergræs kan beskytte grundvandet mod nitrat

Ændringen i EU's landbrugspolitik (CAP) indebærer en afkobling af landbrugsstøtten og en afvikling af den nuværende hektarstøtte. Derved øges græsmarkernes værdi fordi græs vil konkurrere bedre med de øvrige grovfoderemner rent prismæssigt. I fremtiden forventes derfor en betydelig højere græsandel i kvægbrugenes sædskifter end det er tilfældet i dag. Samtidig betyder strukturudviklingen, at kvæggårdene bliver større og et stigende antal brug ikke kan afgræsse alle marker i sædskiftet, da afgræsning langt fra gården er besværlig eller helt uforeneligt med de daglige rutiner. Disse forhold forventes at bevirke, at der på kvægbrugene fremover vil være græsmarker, som får lov til at blive betydeligt ældre end de 2-3 år, der hidtil har været mest udbredt. Og denne udvikling kan få betydning for både produktion og miljø

Stor produktion af grovfoder og lav nitratudvaskning ved dyrkning af grønbyg med italiensk rajgræs efter kløvergræs

Har man problemer med kløvertræthed i sine marker kan en løsning være, at dyrke grønbyg (byg høstet ved begyndende skridning) med udlæg af italiensk (ital.) rajgræs. Nye forsøg beskrevet her viser, at dyrkning af grønbyg med udlæg af ital. rajgræs giver mulighed for at reducere udvaskningen af kvælstof (N) til et minimum og samtidig opnå en stor produktion af grovfoder. I modsætning hertil er der normalt stor risiko for udvaskning af kvælstof ved ompløjning af kløvergræs på sandjorde

High yield and low N leaching with barley as a green crop for silage after grass-clover

Danish organic farmers experience increasing difficulties with clover soil fatigue due to continuous cultivation of clover. A solution could be to undersow barley with Italian ryegrass after ploughing of grass-clover where the barley could be used as a green crop for silage (i.e., barley harvested at the beginning of heading), and Italian ryegrass could be used for cutting in autumn. This method offers advantages not only for the farmer but also for the environment. When grass-clover is ploughed there is a high risk of N leaching. However, new field experiments show, that with the suggested method it is possible to reduce leaching to a minimum and simultaneously achieve a high production of roughage

Øget udnyttelse af kvælstof efter ompløjning af afgræsset kløvergræs

Ompløjning af afgræsset kløvergræs frigiver store mængder kvælstof (N), som let udvaskes og går tabt. En øget udnyttelse af kvælstof kræver, at udvaskning af nitrat reduceres. Etablering af alm. rajgræs som efterafgrøde kan være en effektiv metode til at mindske udvaskningen, men den mest effektive metode har vist sig at være dyrkning af grønkorn (byg, der høstes ved begyndende skridning) med udlæg af ital. rajgræs. Derved kan N-udnyttelsen øges betydeligt, og udvaskningen nedbringes til et minimum svarende til vedvarende græs på naturarealer