Nitrate leaching and nitrogen balances for integrated willow-poultry organic systems in Denmark

CONTEXT

Integrating outdoor poultry production with agroforestry promotes both production and animal welfare, but very little data exist on field nitrate leaching and soil nitrogen (N) balances.

OBJECTIVE

The objective of the study was to quantify the effect of feed (soybean portion was partly replaced with ‘green’ protein from refined local biomass) and stocking density (low, 6 m² hen⁻¹, high, 4 m² hen⁻¹) on nitrate leaching and soil N mass balance (inputs minus outputs) in outdoor poultry systems combined with willow and grass agroforestry on sandy soils.

METHODS

The experiment was conducted in “organic” settings in Denmark on coarse sand soil cultivated with perennial grass-clover ley and short-rotation coppice with willow trees. Nitrate leaching was determined from soil nitrate concentrations measured at 1 m depth and water fluxes simulated by the process-based model Daisy for one full hydrological year involving the grazing period of spring-summer-autumn, and the winter and following spring.

RESULTS AND CONCLUSIONS

Observed soil nitrate concentrations were considerably lower and less variable in the willow zone compared to the grass zone. Nitrate leaching was significantly lower in the willow (26 ± 14 kg N ha⁻¹) compared to the grass zone (154 ± 28 kg N ha⁻¹). At factor level, nitrate leaching was comparable and marginally lower at low- compared to high stocking density. Feed input and manure output were major N flows in the paddocks, resulting in large surface N balance of 1198–1241 kg N ha⁻¹ at high- and 843–875 kg N ha⁻¹ at low stocking density, the lower end in each range being for the green-protein feed. The soil N balances were 26 % lower than the surface balances and similar between feed factors, yielding 614–634 and 899–906 kg N ha⁻¹ for low and high stocking density, respectively.

SIGNIFICANCE

High spatial and temporal variability in nitrate destined for leaching from sandy soils remains challenging to control with short rotation coppiced willow in outdoor poultry paddocks on grass-clover ley. Reducing the stocking density of the hens by 33 % tightens the soil N balance for about 36 % (33–38 %), although further measures are necessary to reduce the N balance to levels environmentally suitable for agricultural soils.