Nitrogen mineralization under perennial grasslands as a predictive tool in assessing nitrogen needs

Quantification of available nitrogen (N) under perennial grasslands has been consistently difficult. Ammonium is rapidly converted to nitrate (NO 3), and both are taken up by developed grass root systems. Nitrate is very mobile and can be lost from the system through leaching or runoff. Adequate N greatly affects biomass production and therefore it is essential to supplement insufficient resources, or reduce inputs if excess is available. ^ The purpose of this research was to develop a model whereby routine soil tests could assess a critical NO3 level for reaching optimum perennial grass yield. This model was achieved by supplying known incremental N treatments to two perennial grassland sites over two years and plotting the NO3 gradient found in the soil against relative yield. These values were informed by long term aerobic laboratory incubations which established the N supplying capacity of the soil, and the effect of N fertilizer on the rate and amount of N mineralization. In situ incubations were conducted in the field using a buried bag technique to quantify N availability under existing field temperature conditions without loss due to uptake or leaching. Tissue samples were also considered both for NO3 and total N concentrations and these values were used in conjunction with the soil sample data to assess the adequacy of the treatments applied. ^ Using three different models, the Cate-Nelson, Linear-Response Plateau, and Quadratic Plateau, a range of critical NO3 levels was found for each of the first two weeks of each growth period. An alternate in situ technique was introduced during the second year using anion exchange membranes (AEM\u27s). They were easily prepared, inserted into the soil where they incubated for 12 days, and extracted for nitrate. The AEM\u27s had many mechanical advantages over soil sampling, but most importantly they evaluated NO3 present over a period of time, not simply the point-in-time information furnished by a particular soil sample. Their results correlated well when compared with the soil samples taken at the same time, and also established a range of critical levels which could supply an alternative to soil data.