Crop production and nitrate leaching from grassland converted to cereal cropping
Jens Peter Mølgaard, Danish Institute of Agricultural Sciences, Ruth Grant, National Environment Research institute, Ib Sillebak Kristensen, Danish Institute of Agricultural Sciences
The grass-clover fields are said to be the motor of organic farming in Denmark. This means that the N-fixation of the clover, supplies nitrogen for the stock and crops of the farm as a whole. Currently, nitrate leaching from farmed land is of great concern from both an environmental and a health point of view. Often, it is assumed that organic farming leads to a reduced risk of N losses from the farm, but experimental data on this subject are scarce.
Two sites dairy farms have been selected for the investigation. One farm is on sandy soil (Brørup) and one on loamy sand (Assentoft). Both farms have grazed grass-clover in the rotation. On each site, a grass-clover field has been selected and this field has been split in two, giving two crop sequences, which are used for measurements (table 1).
The barley fields were split into three plots at each site, two plots received manure in normal (1N) and double normal rate (2N), respectively and one plot was not given manure (0N). The nitrate leaching is only measured in the plot given the normal manure rate.
The measurement programme has been set up in collaboration with the DINOG project under DARCOF II. This measurement programme can be outlined as:
- General farm data
- Manure
- Soil characterisation
- Climate
- Soil mineral nitrogen and microbial biomass
- Below-ground and above-ground biomass in grass-clover
- Above-ground crop biomass and yield in cereals
- Earthworms
- Nitrate leaching
- N-fixation and denitrification (DINOG project)
Nitrate leaching
In December 2000 sampling stations were established at each field, and samples were taken onto January 2001. Each sampling station consisted of 12 suction cups (teflon type) installed at a depth of 100 cm and covering an area of approximately 100 m2. At field 2, additional suction cups were installed in June 2001 because the grass growth did not recover sufficiently after the installation of the suction cups in the winter. Soil water was extracted by means of a continuous vacuum and samples were collected every second week. Samples from the individual suction cups were analysed for nitrate-N, whereas pooled samples from each station were analysed for NH4-N and total-N. Pooled samples were also analysed for ortho-phosphate and potassium as an indicator of preferential flow, and for total dissolved carbon with the purpose of describing organic matter conversion. Water percolation through the root zone was modelled by means of the water balance model EVACROP. Nitrate leaching was calculated on a daily basis using the modelled percolation values and concentration values obtained by flow-interpolation between the sampling points.
Results
The measured nitrate concentrations are shown in figure 1. Rainfall was high in 2001/02 and lower in 2002/03. This resulted in a marked difference in water percolation between the years (table 2).
At the loamy soils at Assentoft, the grass-clover ensured a low nitrate leaching (17 kg N/ha) during the run-off period 2001/02. Conversion of the grass sward resulted in a large increase in nitrate leaching (199 kg N/ha) during the same period. Nitrate leaching levelled off during the second year after ploughing. Ploughing of the grass-clover field 2 in 2002 also lead to increased nitrate leaching (92 kg N/ha).
At the coarse sandy soils at Brørup with high rainfall there was a considerable nitrate leaching (65 kg N/ha) from the grass-clover field in 2001/02 (table 3). Conversion of the grass sward resulted in a large increase in leaching (192 kg N/ha), and this high level of nitrate leaching continued during the following year after ploughing (250 kg N/ha). Conversion of the grass sward at field 2 in 2002 resulted in equally high leaching values (264 kg N/ha).
The grain yield showed no response to manure application in 2001 (table 3). The measurements of leaching have been ended in late spring 2003. The data have been compiled and stored for use within model validation. A refereed paper with the model results is in preparation.
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