Nitrogen mineralization in soil layers, soil particles and macro-organic matter under grassland

A study was conducted to determine mineralization rates in the: field and i n different soil layers under three grassland managements (v-iz, a reseeded award, a permanent award with a conventional N management, and a long-term grass sward with 0 N (0-N) input). Potential mineralization rates of soil particles (sand, sill and clay) and macro-organic matter fractions of diffe rent sizes (i.e. 0.2-0.5, 0.5-2.0 and >2 mm) were also determined in the la boratory. In the reseeded plot-a, net mineralization was unchanged down to 40 cm depth. In the undisturbed conventional-N swards, mi neralization rate s were substantially higher in the top layer (0-10 cm) than in the deeper l ayers, In plots which had received no fertilizer N, mineralization was cons istently low in all the layers. There was more macro-organic matter (MOM) i n the 0-N plots (equivalent to 23 g kg(-1) soil for 0-40 cm) than in the tw o-fertilized. plots (i.e. conventional-N and reseeded) which contained simi lar amounts (e.g. 15 g kg(-1) sail). C and N contents of separated soil par ticles did not differ amongst the treatments, but there were large differen ces with depth. Potential mineralization in the bulk soil was greatest in t he 0-10 cm layers and gradually decreased with depth in all the treatments. Separated sand particles had negligible rates of potential mineralization and the clay component had the highest rates in the subsurface layers (10-4 0 cm). MOMs had high potential rate of mineralization in the surface layer and decreased with soil depth, hut there was no clear pattern in the differ ences between different size fractions.