NITROGEN MINERALIZATION IN DENSITY FRACTIONS OF SOIL ORGANIC-MATTER FROM MAIZE AND LEGUME CROPPING SYSTEMS

Biologically-mediated nutrient availability in the soil is largely dep endent on soil organic matter (SOM) decomposition and mineralization p rocesses. The light fractions of SOM obtained by floatation of the san d-size fraction in a silica suspension (LL, 150-3000 mu m and <1.13 g cm(-3)) and floatation of whole soil in sodium iodide (NAL, <1.7 g cm( -3)) were previously found to be strongly affected by maize and legume cropping systems. The objective of this study was to assess the anaer obic and aerobic N mineralization rates of LL, NAL and whole soil afte r the eighth cropping season (4 y) in seven maize and legume cropping systems. Significantly greater (P < 0.05) N mineralization in whole so il, LL and NAL resulted from the seasonal additions of Gliricidia sepi um prunings to continuous maize than from continuous maize with remova l of crop residues and from a gliricidia stand with prunings removed. Aerobic and anaerobic N mineralization in whole soil from the seven cr opping systems correlated (P < 0.05) with aerobic N mineralization in LL and NAL, which in turn correlated with the N concentration and amou nt of N in the respective fraction. Amount of LL-N but not LL-C or dry weight of LL correlated with anaerobic and aerobic N mineralization o f whole soil. Dry weight, amount of N and amount of C in NAL were not correlated with whole soil mineralization. These results suggest that amount of N in light fraction SOM merits further examination as a sens itive measure of biologically-mediated N availability. Copyright (C) 1 996 Elsevier Science Ltd