Soil microbial biomass carbon and nitrogen as affected by cropping systems

The impacts of crop rotations and N fertilization on microbial biomass C (C -mic) and N (N-mic.) were studied in soils of two long-term field experimen ts initiated in 1978 at the Northeast Research Center (NERC) and in 1954 at the Clarion-Webster Research Center (CWRC), both in Iowa. Surface soil sam ples were taken in 1996 and 1997 from plots of corn (Zen mays L.), soybeans (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Med icago sativa L,) that had received 0 or 180 kg N ha(-1) before corn and an annual application of 20 kg P and 56 kg K ha(-1). The C-mic and N-mic value s were determined by the chloroform-fumigation-extraction method and the ch loroform-fumigation-incubation method, respectively. The C-mic and N-mic va lues were significantly affected (P<0.05) by crop rotation and plant cover at time of sampling, but not by N fertilization. In general, the highest C- mic and N-mic contents were found in the multicropping systems (4-year rota tions) taken in oats or meadow plots, and the lowest values were found in c ontinuous corn and soybean systems. On average, C-mic made up about 1.0% of the organic C (C-org), and N-mic contributed about 2.4% of the total N (N- tot) in soils at both sites and years of sampling. The C-mic values were si gnificantly correlated with C-org contents (r greater than or equal to 0.41 **), whereas the relationship between C-mic and N-tot was significant (r le ss than or equal to 0.53***) only for the samples taken in 1996 at the NERC site. The C-mic:N-mic ratios were, on average, 4.3 and 6.4 in 1996, and 7. 6 and 11.4 in 1997 at the NERC and CWRC: sites, respectively. Crop rotation significantly (P<0.05) affected this ratio only at the NERC site, and N fe rtilization showed no effect at either site. In general, multicropping syst ems resulted in greater C-mic:C-org (1.1%) and N-mic:N-tot (2.6%) ratios th an monocropping systems (0.8% and 2.1%, respectively).