Contributions to N mineralization from soil macroorganic matter fractions incorporated into two field soils

The contribution of soil macroorganic matter N (macro-OM, d < 1.0 g cm−3) to gross rates of N mineralization was examined in two grassland field soils in England using three 15N-labelling techniques, referred to as the difference, mirror image and net recovery methods. The difference method involved measuring gross mineralization rates in soils with or without incorporated unlabelled macro-OM, using isotope dilution of added 15NH4+. The mirror image approach involved measuring the isotopic enrichment of the ammonium pool in soil to which 15N-labelled macro-OM had been incorporated. The net recovery method was a net N mineralization estimate based upon the recovery of label in plants and soil to which 15N-labelled macro-OM had been incororated. The difference method provided the most accurate estimates of N mineralization from the incorporated macro-OM. Estimates made using the direct and net recovery methods were more variable and confounded by movement of the macro-OM away from the original site of incorporation. Approximately 2.4 and 13.7% of the N in the incorporated macro-OM isolated from the different soils was mineralized over the 66 d following incorporation. This represented approximately 3.4 and 2.3% of the cumulative gross N mineralization when corrected for background amounts of soil macro-OM. This low contribution suggests that most of the N mineralized in grazed grasslands is derived from other forms of soil organic matter associated with mineral particles.