Bioassay of Plant Growth from Soils Varying in Active C and N Fractions.

Biologically active soil C and N fractions are important attributes of healthy soil. As a bioassay, we hypothesized that active soil C and N fractions would relate with plant dry matter production from unamended soil. Four replicate soil samples from 22 tall fescue fields in North Carolina and Virginia were collected in August 2015 for evaluation of soil C and N mineralization through aerobic incubation at 25°C and 50% water-filled pore space for up to 24 d. Triplicate subsamples of each soil sample were also utilized as a medium in the greenhouse for sorghum-sudan grass growth during 6 weeks. Net N mineralization varied from 40 to 240 mg kg-124 d-1. Cumulative C mineralization, as well as the flush of CO2 following rewetting of dried soil during the initial 3 d, were highly related to net N mineralization. Dry matter of sorghum-sudan grass plants varied from as low as 4.1 mgg-1 soil when C mineralization was 485 mg kg-124 d-1 and net N mineralization was 47mg kg -124 d-1 to as high as 23.5mgg-1 soil when C mineralization was 1053mg kg-124 d-1andnet N mineralization was 224 mg kg-124 d-1. Plant growth of sorghum-sudan grass was highly correlated with active C and N fractions, which validated our hypothesis.