Biologically active soil organic matter fractions in sustainable cropping systems

We sampled corn (Zea mays L.), soybean (Glycine max L.), and wheat (Triticum aestivum L.) rotations, corn monocultures, and plant successional experiments in Southwest MI over a 2-year period to study the effects of alternative management practices on microbial biomass and particulate organic matter (POM) C and N in the top 20 or 25 cm of soil. Microbial biomass was measured using the chloroform fumigation, incubation method. POM was defined as the organic C and N content of primary soil particles in the 53-2000 mum size class. Microbial C decreased during a drought in 1994, and was greater in treatments receiving compost rather than inorganic fertilizer in 1995, but was only weakly correlated with N mineralization in aerobic laboratory incubations. Microbial biomass accounted for an average of 2.6% of soil C and 4.9% of soil N in all treatments sampled in 1995. Microbial C:N ratio was lower in July and November (6.0) than in April and September (7.3), and lower in successional treatments without tillage (5.2) than in agronomic treatments (6.7). Changes in microbial N were large enough to affect short term N availability, but tended to be transitory. POM increased after compost additions, and was greater in successional plots without tillage than in tilled treatments, but did not increase immediately after the incorporation of plant residues. POM accounted for 19.7% of C and 14.8% of N in 1995. Its C:N ratio was 20.8 in a never tilled successional treatment and 16.0 in all other soils. The C:N ratio was 17.0 on average in the 250-2000 mum size fraction, compared to 15.5 in the 53-250 mum fraction. There was a strong correlation between POM and N mineralization in 70- and 150-day aerobic incubations, but there was greater N mineralization per unit POM in April and November than in September or October in both years of the study. These results suggest that POM could be used to estimate N mineralization if combined with information about recently deposited plant residues. (C) 2001 Published by Elsevier Science B.V.