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Mineralization of Nitrogen in Fermentation Residue from Citric Acid Production¹

ABSTRACT

One of the byproducts of commercial citric acid production is presscake: a mixture of microbial biomass, CaSO₄, and a filter aid such as perlite. Large production plants produce this byproduct at rates of over 50 metric tons/day. Since the material is often applied to agricultural land, it is important to know the rate of release of N and other plant nutrients so that appropriate application rates can be determined.

Wafers (1.5 cm diam by 0.6 cm thick) of presscake were incorporated into a loamy sand at rates supplying 223, 446, and 669 ppm N and surface-applied to supply 446 ppm N. Powdered presscake was also incorporated at the 446-ppm-N rate. The mixtures were incubated for 32 weeks.

Nitrate accumulated rather rapidly during the first 8 weeks, with little accumulation thereafter. At the end of the experiment, NO₃-N levels at the 446-ppm-N rate were 57% of the applied N with incorporated wafers, 32% with incorporated powder, and 28% with surface-applied wafers. Gaseous N losses were 31% of the applied N with incorporated wafers, 61% with powder, and 55% with surface-applied wafers. Addition of presscake increased levels of extractable P and Ca in most treatments.

Key Words: presscake • yeast • P • Ca • denitrification • ammonia volatilization • cotton seed meal

¹ Pap. no. 5472 of the J. Ser. of the North Carolina Agric. Exp. Stn., Raleigh, NC 27607. This work was supported in part by funds provided by Pfizer Chemicals Div., Pfizer, Inc., Groton, Conn.

² Assistant Professor and former senior honor student, Dep. of Soil Sci., North Carolina State Univ., Raleigh.

Received for publication December 5, 1977.