Distribution of nitrogen fractions in grazed and ungrazed fescue grassland Ah horizons

Abstract

Grazing affects the plant ecology and adds excreta, thereby influencing soil N relationships. Consequently, total N, mineralizable N, exchangeable N, hydrolyzable N, and urease activity were assessed at the Agriculture Canada Research Substation, Stavely, Alberta, in the Ah horizons on rough fescue (Festuca scabrella Torr.) grasslands stocked at either light (0.8 ha/AUM) or very heavy (0.2 ha/AUM) fixed rates for 38 years and in exclosures located within each field for an equal period of time. Even though total N expressed as t/ha per Ah horizon remained the same, changes in various N fractions were nevertheless evident. Grazing resulted in more NH4 +/- N and NO3-N in both fields at the time of sampling and each was greater at the higher stocking rate. Although soil N was less mineralizable, it was more acid-hydrolyzable at the higher stocking rate. Urease activity also increased. The effect on soil N characteristics of increased excreta loads is complex and still not well understood.