Soil Respiration Tests as Predictors of Nitrogen Mineralization Potential

Estimation of seasonal N availability via a soil test has proved difficult. This information gap has lead to fertilizer recommendations based upon soil inorganic N levels found before planting and or preside dress. Complicating matters, as famers adopt new irrigation technologies, the results from older fertilizer rate trails may also be less applicable. In general, most when not all recommendations do not account for the contribution of soil N mineralization during the growing season, which has been shown to contribute up to 50% of total plant N uptake. A variety of soil tests, both lab-based and in situ, have been used to estimate growing season N mineralization. Biologically based tests allow for an estimation of available soil N by incubating soil samples at temperatures and moisture contents that facilitate N mineralization, providing a proxy to estimate N mineralization potential. However, these incubations are lengthy and not well suited for routine analysis in a soil test lab setting. The close coupling of the C and N cycles theoretically should allow for biologically based estimation of long-term N mineralization using short-term cumulative CO2 evolution, as shown in some previous studies. This has been demonstrated in dairy-amended soils, which showed a correlation between 24-hour CO2 production and 28-day N mineralization. Other studies have shown a correlation between 72-hour CO2 and 28-day N mineralization in unamended soils. Generally, better correlations can be made in soils with higher organic matter content or consistent additions of organic based fertilizers. Overall, regardless of soil management, soil respiration based tests to estimate growing season soil N mineralization are variable and their use requires more testing to develop relationships to accurately predict in season soil N mineralization. INTRODUCTION Crops require sufficient nitrogen (N) to attain maximum yield potential. The unpredictable nature of soils to supply N cause it to be the most limiting nutrient for crop production (Foth and Ellis, 1997). Although many productive mineral soils contain several thousand kilograms of N per hectare, greater than 90% of soil N is unavailable in the form of organic matter, with the remainder existing mostly as fixed ammonium (NH4) in clays. Only a small fraction of the N in soils, generally less than 0.1%, exists in plant-available mineral forms, such as nitrate (NO3) and exchangeable NH4, at any one time, and no more than 1-2% of the total soil N will be available to plants during a growing season (Stevenson and Cole, 1999). However, the large size of the soil N pool can be significant source of plant available N during the growing season and