Soil water effects on the use of heat units to predict crop residue carbon and nitrogen mineralization

Abstract

Soil heat units (degree days) have previously been shown to predict net N mineralization from crop residues and papermil sludge. The present study was designed to identity the effects of soil water potential on predictions of mineralization with heat units and to compare field and laboratory results of white lupin (Lupinus albus L. cv. ‘Ultra’) N mineralization. Lupin-amended soil and unamended controls were incubated at factorial combinations of temperature (15, 20, and 25°C) and soil moisture (-0.30,-0.03, and-0.01 MPa) for 198 days. Incorporation of the lupin residue resulted in net N immobilization. No net N mineralization had been observed for any temperature at a soil moisture level of-0.30 MPa by the close of the incubation study. The number of heat units that accumulated until commencement of net N mineralization did not differ for five of the six remaining temperature x water treatment combinations.The number of heat units that accumulated until net N mineralization began (2058–2814 degree-days) in the present study were similar to those reported in a complementary field study (1990–2360 degree-days). Temperature and moisture interactively affected lupin-residue C mineralization. The cumulative substrate C that had evolved by the time of net N mineralization did not differ for a given temperature between soil moisture levels of-0.03 and-0.01 MPa. Heat units were not useful for describing crop-residue C mineralization in this study. Heat units appear to adequately predict net N mineralization from organic residues at soil water potentials within the-0.03 to-0.01 MPa range, but may not be valid for prolonged drier conditions.