Evaluation of Nitrogen Availability Indexes for a Sludge Compost Amended Soil

Abstract

AbstractSeveral N availability methods were evaluated on a sludge amended soil, since sludge applications are usually based on N availability and a soil N evaluation to optimize loading rates. Yearly amendments of composted, lime‐stabilized, raw sludge were applied, at 0, 56, 112, and 224 Mg ha−1, to an Elkton silt loam soil (Typic Ochraquults), in Beltsville, MD, as follows: whole plots (1975–1977), half plots (1978), and no plots (1979–1981). In 1978 and 1981, surface and subsurface soil samples were collected and maize (Zea mays L.) total N uptake was determined. The biological method (73‐week aerobic incubation [NT]) of assessing soil N availability revealed an initial rapid miner. alization phase (NE) over the first 28 weeks and a constant phase thereafter. The chemical methods were total soil N (TN), acid KMnO4 oxidizable N (P), autoclave extractable N (A), and 2 M KCl extractable NH+4‐N (K). Curvilinear regression analyses relaling maize N uptake to these N indexes gave the following R2 values: NE, 0.93; NT, 0.94; TN, 0.93; P, 0.92; A, 0.95; and K, 0.92. All methods were equally sensitive on sludge treated soils; however, the incubation procedure was best on untreated soil. Including second zone soil N and expressing the results on a root‐zone volume basis improved N uptake predictions with unfertilized controls, but was unnecessary for sludge treated plots. These results indicate that chemical N availability indexes can be used to assess soil N availability on sludge treated land; provided they are calibrated to local soil‐crop‐climate conditions.