CARBON AND NITROGEN MINERALIZATION OF ADDED PARTICULATE AND MACROORGANIC MATTER

Particulate organic matter (POM) is more sensitive than total SOM to c hanges in management practices and, accordingly, may indicate changes in soil quality. A soil incubation study was conducted to determine th e effects of added POM (75 to 250 mu m size fraction), or macroorganic matter (MOM, 250 to 2000 mu m size fraction) on C and N mineralizatio n and microbial and C and N content. A 1 kg composite made from 16 pre dominantly silt loam soils was amended with 10 g of POM, MOM or MOM gr ound to a reduced size of 75 to 250 mu m (GMOM). The MOM amendment equ aled 4.55-fold and POM equaled 1.60-fold of total MOM and POM found in the composite soil. Carbon mineralization of MOM and POM after 8 week s was approximately 9 and 4%, respectively of the total MOM and POM-C added. Reducing the size of MOM to 75 to 250 mu m did not affect miner alization. Nitrogen mineralization was slightly greater in the amended soils after 8 weeks and equaled 5 to 6% of the MOM or POM-total N add ed. Contribution of POM to total mineralized N from soil organic matte r (SOM) in the composite soil was proportional to the POM content in S OM or approximately 12%. Amended soils had 25 to 42% more biomass-C th an the control soil 2 weeks after amendment application. After 8 weeks , the amended soils contained about 32% more biomass-C. This increase in biomass-C at 8 weeks accounted for approximately 2% of the added C. At 8 weeks, microbial biomass-N in GMOM-, MOM- and POM-amended soils was about 56, 46 and 14% higher, respectively, than in the control soi l. These increases were approximately 8% of the MOM-N added and 2% of the POM-N added. Increases in POM resulted in increases in soil respir ation and microbial biomass-C and N, which also are suggested indicato rs of soil quality. Therefore, POM may be a suitable soil quality indi cator that provides similar information as soil respiration or microbi al biomass determinations. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.