Biologically active soil organic matter fractions in sustainable cropping systems

We sampled corn (Zea mays L.), soybean (Glycine max L.), and wheat (Triticu m aestivum L.) rotations, corn monocultures, and plant successional experim ents in Southwest MI over a 2-year period to study the effects of alternati ve 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 measur ed using the chloroform fumigation, incubation method. POM was defined as t he organic C and N content of primary soil particles in the 53-2000 mum siz e 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 inc ubations. 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 l ower in July and November (6.0) than in April and September (7.3), and lowe r in successional treatments without tillage (5.2) than in agronomic treatm ents (6.7). Changes in microbial N were large enough to affect short term N availability, but tended to be transitory. POM increased after compost add itions, and was greater in successional plots without tillage than in tille d treatments, but did not increase immediately after the incorporation of p lant 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 ot her soils. The C:N ratio was 17.0 on average in the 250-2000 mum size fract ion, compared to 15.5 in the 53-250 mum fraction. There was a strong correl ation between POM and N mineralization in 70- and 150-day aerobic incubatio ns, but there was greater N mineralization per unit POM in April and Novemb er than in September or October in both years of the study. These results s uggest 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.