EFFECTS OF OSMOTIC AND MATRIC POTENTIALS ON NITROGEN MINERALIZATION IN UNAMENDED AND MANURE-AMENDED SOILS

Abstract

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 j2.3 to j0.35 MPa and decreased rapidly as potentials declined below j5.5 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. (Soil Science 2008;173:203‐213)