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 experiments initiated in 1978 at the Northeast Research Center (NERC) and in 1954 at the Clarion-Webster Research Center (CWRC), both in Iowa. Surface soil samples were taken in 1996 and 1997 from plots of corn (Zea mays L.), soybeans (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago 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) values were determined by the chloroform-fumigation-extraction method and the chloroform-fumigation-incubation method, respectively. The C(mic) and N(mic) values 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 rotations) taken in oats or meadow plots, and the lowest values were found in continuous 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 significantly 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 less 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 fertilization showed no effect at either site. In general, multicropping systems resulted in greater C(mic):C(org) (1.1%) and N(mic):N(tot) (2.6%) ratios than monocropping systems (0.8% and 2.1%, respectively).