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

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)

Challenges in modelling dissolved organic matter dynamics in agricultural soil using DAISY

Because dissolved organic matter (DOM) plays an important role is terrestrial C-,N-and P-balances and transport of these three components to aquatic environments, there is a need to include it in models. This paper presents the concept of the newly developed DOM modules implemented in the DAISY model with focus on the quantification of DOM sorption/desorption and microbial-driven DOM turnover. The kinetics of DOM sorption/desorption is described by the deviation of the actual DOM concentration in solution from the equilibrium concentration, Ceq. The Ceq is soil specific and estimated from pedotransfer functions taking into account the soil content of organic matter, Al and Fe oxides. The turnover of several organic matter pools including one DOM pool are described by first-order kinetics. The DOM module was tested at field scale for three soil treatments applied after cultivating grass–clover swards. Suction cups were installed at depths 30, 60 and 90 cm and soil solution was sampled for quantification of dissolved organic C (DOC) and dissolved organic N (DON). In the topsoil, the observed fluctuations in DOC were successfully simulated when the sorption/desorption rate coefficient k was low. In the subsoil, the observed concentrations of DOC were steadier and the best simulations were obtained using a high k. The model shows that DOC and DON concentrations are levelled out in the subsoils due to soil buffering. The steady concentration levels were based on the Ceq for each horizon and the kinetic concept for sorption/desorption of DOC appeared aviable approach. If Ceq was successfully estimated by the pedotransfer function it was possible to simulate the DOC concentration in the subsoil. In spite of difficulties in describing the DOC dynamics of the topsoil, the DOM module simulates the subsoil concentration level of DOC well, and also — but with more uncertainty — the DON concentration level

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

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

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

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

Udvaskning af organisk bundet kvælstof og kulstof

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 afgrøde og dyrkningsmetode. Ved lav nitratudvaskning, som f.eks. under en veludviklet efterafgrøde, udvaskes forholdsvis mere org. N end ved høj nitratudvaskning, og der udvaskes mere org. C fra en plantedækket jord end fra bar jord