Nitrogen mineralization of manure-amended soil: Effects of salinity and moisture content

Application of manure has been used to increase the nitrogen status of many arid and semi-arid agricultural soils, however, organic nitrogen contained in animal manures must be mineralized prior to utilization by crops. Nitrogen mineralization is, like other biological processes, affected by soil water status. This can be due to the direct influence of soil moisture and/or salt interactions on the nitrogen dynamics. It is critical to understand the interactive effects of salt and water on nitrogen in soils amended with organic fertilizer. Such knowledge may be used for improving and quantifying nitrogen use efficiency. Objectives of this study were (i) to examine the relative and interactive influences of soil osmotic and matric water potentials on nitrogen transformations in manure-amended and non-manured soils, (ii) to determine the extent of nitrogen mineralization over a range of soil water potentials, and (iii) to study the effect of manure addition on nitrogen dynamics in an agricultural desert soil. Gila fine sandy loam soil was treated by addition of varying amounts of distilled water, NaCl, and dairy manure and incubated at depth of 20 cm in 0.025 mm thickness Ziploc® bags. Ammonium nitrogen, nitrate nitrogen, gravimetric soil moisture content, and total soil water potential were measured weekly for 16 weeks. Our study showed a decrease in the amount of inorganic N released from both the non-manured and manure-amended soils at total soil water potentials of approximately -55 bars or less. Maximal amounts of inorganic-N were released at potentials of -23 to -3.5 bars. There was an accumulation in the amounts of NH₄-N released at potentials of -55 bars or less, presumably because of a reduction in net nitrification. The manure caused nitrogen immobilization especially during the early part of our study. Immobilization also occurred in non-manured soil, but this generally lasted only a few weeks. Immobilization was prolonged in soils with lower water potentials. Net N mineralization in the manure-amended soils was higher than in the non-manured soils when soil moisture content was at field capacity