Several methods for providing an index of the availability of soil nitrogen were evaluated by relating laboratory soil test values to yields of dry matter and uptake or nitrogen by orchardgrass in the greenhouse. Determination of soil organic matter or total soil nitrogen contents was relatively ineffective. Available nitrogen initially present in the soil at sampling time, extracted by several different methods, was superior to measurements of nitrogen released upon incubation. The nitrate form of nitrogen, either initially present or released upon incubation, served as a better index of soil nitrogen availability than the ammonium form. Inclusion of initial nitrates and nitrates released upon incubation as independent variables in a multiple regression resulted in the highest correlation with total nitrogen uptake. In the three incubation methods evaluated, discarding the initial nitrogen content of the soil, as is commonly done, severely reduced the validity of the tests as measures of nitrogen availability.
Root influence on soil nitrogen availability and microbial community dynamics results in contrasting rhizosphere priming effects in pine and spruce soil
