Sewage Sludge Proteins as Labile Carbon and Nitrogen Sources

The study of specific, organic sewage sludge constituents is necessary to augment our knowledge of C and N mineralization in sludge‐amended soils. A laboratory incubation study of seven sewage sludges was initiated to test the hypothesis that sewage sludge proteins are labile C and N sources. Sewage sludge proteins were extracted with H 2 O, 10% (v/v) Triton X‐100, and 1.0 M NaOH and determined by the Lowry assay. Sewage sludges were mixed with Bresser sandy loam soil (fine‐loamy, mixed, mesic Aridic Argiustoll) at a rate of 10 g dry sludge kg −1 dry soil and incubated at 25 °C and 0.111 kg kg −1 soil water content for 12 wk to determine sludge C and N mineralization. Extractable sludge proteins were highly correlated to C mineralization ( r 2 = 0.94‐0.96), but they were poorly correlated to N mineralization ( r 2 = 0.40‐0.41). This supported the hypothesis that sludge proteins were a labile C source but not a labile N source. However, low molecular weight primary amines (assumed to be predominately protein degradation products) combined with the sludge C/N ratios were highly correlated to sludge N mineralization rates ( r 2 = 0.91). Nitrogen mineralization of sludge‐amended soil followed either zero‐ or first‐order kinetics. Kinetic models of the first‐order systems showed that N mineralization was best described as the decomposition of two distinct organic‐N pools. Sewage sludge proteins appear to be significant sources of labile C, and their degradation products apparently are critical N sources.