Plant and Microbial Responses to Sorghum-Soybean Cropping Systems and Fertility Management

Monoculture production of soybeans (Glycine max (L.) Merr.) and grain sorghum (Sorghum bicolor (L.) Moench) generally results in declining grain yields. To better understand biological and chemical interactions causing yield declines with continuous cropping, microbial biomass, crop root dry weight, soil organic matter, and total N content were measured in a cropping system experiment on a Sharpsburg silty clay loam (Typic Argiudoll). The cropping treatments included continuous soybeans, continuous sorghum, and sorghum-soybean or soybean-sorghum rotations. These treatments were initiated 5 y prior to taking the reported measurements. Fertilizer treatments consisted of no amendment (control), N (45 kg ha⁻¹ on soybean plots and 90 kg ha⁻¹ on sorghum plots), and manure applied at 15.8 Mg dry matter ha⁻¹ yr⁻¹. In summer 1986, the microbial biomass C for the 0–30-cm layer of soil averaged across fertility treatments was 1.37, 1.49, 1.43, and 1.58 Mg ha⁻¹ for continuous soybeans, rotated soybeans, rotated sorghum and continuous sorghum, respectively. Soil microbial biomass C and soil organic matter contents for manured treatments were 11 to 14% and 6 to 16% greater, respectively than those of unfertilized controls. Root dry weights for the 0–30-cm depth were 26 and 77% higher for soybeans and sorghum if the previous crop was sorghum instead of soybeans. In summer 1986, microbial biomass in the 0–15-cm depth was correlated with root density and water-filled pore space in sorghum plots and with bulk density in soybean plots. Previous crop, present crop, and fertilizer treatment affected dry matter partitioning between above- and below-ground plant parts and microbial biomass. With sorghum as a previous crop, a higher proportion of the total production occurred below ground as roots and microbial biomass. Partial financial support was provided by the Int. Sorghum and Millet Collaborative Research Program (INTSORMIL), U. S. AID grant DAN-1254-G-SS-5065-00. Joint contribution of Nebraska Agric. Res. Div. and USDA-ARS. Nebraska Agric. Res. Div. Journal No. 8445.