Soil nitrogen dynamics following herbicide kill and tillage of manured and unmanured grasslands

Chantigny, M. H., MacDonald, J. D., Angers, D. A., Rochette, P., Royer, I. and Gasser, M.-O. 2013. Soil nitrogen dynamics following herbicide kill and tillage of manured and unmanured grasslands. Can. J. Soil Sci. 93: 229–237. Grassland soils accumulate N, which could be lost following land-use change. Adjacent grassland sites, with and without liquid swine manure applied annually for 28 yr, were subdivided and left undisturbed (Control), or killed by herbicides with and without full inversion tillage (FIT) in the autumn or spring. We monitored hot-water extractable organic N (HWEON), and mineral N forms in KCl extractions and soil solutions (tension lysimeters) for 1 yr. Mean soil mineral N increased by 1 to 2.8 g m⁻² in the weeks following herbicide kill and FIT of the unmanured soils, and by 2.6 to 3.0 g m⁻² in the manured soil. These increases corresponded to declines in soil HWEON (−0.4 to −1.9 g m⁻² unmanured site; −2.4 to −4.9 g m⁻² manured site), suggesting that HWEON comprised N that is rapidly mineralized following grassland termination. More than 80% of N mineralized in the weeks following termination accumulated as NH₄ in the unmanured soils, compared with >70% as NO₃ in the manured soils. As a result, more mineral N (mainly NO₃) was found in the soil solution of manured soils. Manured grassland soils may represent a high risk of N loss following termination with herbicide in combination with FIT in the autumn, because of the rapid nitrification of mineralized N. For spring FIT, however, the rapid mineralization of soil N may represent a substantial nutrient source to the following crop.