Effects of osmotic and matric potentials on nitrogen mineralization in unamended and manure-amended soils

Application of manure is used to increase the N and C status of arid and semiarid agricultural soils. Organic N contained in animal manures must be mineralized before crop use, a process which is affected by soil water status either by the direct influence of soil moisture and/or salt interactions on the N dynamics. The objectives of this work were: i) to examine the influences of soil osmotic and matric water potentials on N mineralization, ii) to determine the extent of N mineralization over a range of total soil water potentials, and iii) to study the effect of manure addition on N dynamics in an agricultural desert soil. Gila fine sandy loam soil was treated with varying amounts of water, sodium chloride, and composted dairy manure and incubated at a depth of 20 cm in Ziploc® bags. Inorganic N, soil moisture content, and total soil water potential were measured for 14 weeks. N mineralization was maximal at total soil water potentials of -2.3 to -0.35 MPa and decreased rapidly as potentials declined below -5.5 MPa. Regression analysis indicated that matric potential had twice the inhibitory affect on N mineralization compared with osmotic potential. Manure addition resulted in net N immobilization especially during the early part of the study. Net N immobilization also occurred in unamended 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 unamended soils when soil moisture content was at field capacity.