Seasonal nitrate levels in the soil solution of an organic pasture managed for nature conservation

Abstract

Bouman, O. T. 2008. Seasonal nitrate levels in the soil solution of an organic pasture managed for nature conservation. Can. J. Soil Sci. 88: 423Á428. Extensive management of pastures is considered as an option to enhance nature conservation in agricultural landscapes. Partial reduction of the grazing frequency from four to two rotations and discontinuing the tending of sod and sward have been tested for extensive management of a mixed pasture at the Nova Scotia College of Agriculture in Truro since 2004. It is not well known how extensification affects the N cycle of organic pastures without mineral fertilizer or manure applications. Moisture and temperature of the topsoil and nitrate content of the leachate extracted with ceramic suction cups at 0.5 m depth were monitored prior to grazing and during sward regrowth in the first and second rotation in spring of 2005 and 2006. In each year, more than 75% of all samples contained less than 2.5 mg NO 3 -N L (1 . A significant seasonal nitrate dynamic suggests that plant N uptake reduces nitrate leaching in the first rotation, while soil biological activity increases N supply due to soil warming in excess of plant uptake, thus raising the nitrate level in the second rotation. Extensification significantly reduced soil temperature with a concomitant albeit small decrease of nitrate levels in the soil leachate. The pasture cycled nitrogen very tightly with little room for leakages as a result of imposed treatments during the spring season. Mots clé s: Paˆturage, biologique, tempe´rature, azote, lessivage Nature conservation constitutes an important aspect of sustainable development in agriculture. Rotational grazing and intensive agronomic practices have reduced plant species richness of agricultural grasslands in temperate regions (Ellenberg 1996). Organic farming holds potential to restore some species richness in grasslands (Bakker and ter Heerdt 2005). On dairy farms, a reduced grazing frequency on some paddocks of a rotational grazing system would set pasture land temporarily aside for the purpose of nature conservation. This practice would effectively reduce the available pasture area without any re-zoning of agricultural landscapes, thus without any permanent loss of farm land.It is not well known how a decrease in pasture utilization and the discontinuing of sod and sward tending would affect the nutrient cycles of a pasture. In organic pasture management, the productive function and sustainability of a pasture depends on maintaining a floristic balance between leguminous plant species and grass species in the sward. This floristic balance is known to be sensitive to grazing frequency and pasture cultivation (Ellenberg 1996). Grassland species also differ in their effects on the nitrogen cycle (Personeni and Loiseau 2005) and the leguminous component of a sward tends to increase soil nitrogen leaching (Loiseau et al. 2001;Scherer-Lorenzen et al. 2003