The non-limiting and least limiting water ranges for soil nitrogen mineralization

A better understanding of factors controlling N mineralization would improve our ability to estimate fertilizer requirements more accurately. Net N mineralization approaches a small reaction rate at low and high water contents, giving rise to lower and upper limiting water contents and the least limiting water range (LLWR). Within the LLWR, there is a range in water contents in which mineralization is largely independent of water content, that is, the non-limiting water range (NLWR). An incubation study was conducted to determine the LLWR and NLWR for five soils with different properties, and two relative compaction levels with and without the addition of a legume crop residue. These soils were incubated for 1 and 3 mo at eight water contents. Net N mineralization increased with incubation time and legume addition and varied curvilinearly with water-filled pore space (WFPS). Logistic functions were generated to establish the relationships between net N mineralization and WFPS (%) and to calculate LLWR and NLWR. The mean NLWR was 32.2% after 1 mo and decreased to 18.1% after 3 mo whereas the mean LLWR was 55% after 1 mo and increased to 70.8% after 3 mo. The LLWR increased with organic C or total N but decreased with the addition of legume residue. The NLWR decreased with clay content and with the addition of legume after 3 mo. Emissions of N2O were greatest at water contents near the upper limit of the LLWR. Use of the NLWR to differentiate soils on the basis of the sensitivity of N mineralization to variation in water content is illustrated.