Modelling and measuring the effect of nitrogen catch crops on the nitrogen supply for succeeding crops

Nitrogen catch crops are grown to absorb nitrogen from the rooting zone during autumn and winter. The uptake of N (N-upt) from the soil inorganic N pool (N-min) to a pool of catch crop nitrogen, will protect the nitrogen against leaching. After incorporation, a fraction (m) of the catch crop nitrogen is mineralized and becomes available again. However, not all available nitrogen present in the soil in the autumn is lost by leaching during winter. A fraction (r) of the nitrogen absorbed by the catch crop would, without a catch crop, have been retained within the rooting zone. The first year nitrogen beneficial effect (N-eff) of a catch crop may then be expressed by N-eff = m*N-upt - r*N-upt The soil-plant simulation model DAISY was evaluated for its ability to simulate the effects of catch crops on spring N-min and N-eff. Based on incubation studies, parameter values were assigned to a number of catch crop materials, and these parameter values were then used to simulate spring N-min. The model was able to predict much of the variation in the measured spring N-min (r(2) = 0.48***) and there was good agreement between the measured and the simulated effect of winter precipitation on spring N-min and N-eff. Scenarios including variable soil and climate conditions, and variable root depth of the succeeding crop were simulated. It is illustrated that the effect of catch crops on nitrogen availability for the succeeding crop depends strongly on the rooting depth of the succeeding crop. If the succeeding crop is deep rooted and the leaching intensity is low, there is a high risk that a catch crop will have a negative effect on nitrogen availability. The simulations showed that the strategy for the growing of catch crops should be adapted to the actual situation, especially to the expected leaching intensity and to the rooting depth of the succeeding crop.