Fate of applied biosolids nitrogen in a cut and remove forage system on an alluvial clay loam soil.

摘要

The fate of nitrogen (N) applied in biosolids was investigated in a forage production system on an alluvial clay loam soil in south-eastern Queensland, Australia. Biosolids were applied in October 2002 at the rates of 6, 12, 36, and 54 dry t/ha for aerobically digested biosolids (AE) and 8, 16, 48, and 72 dry t/ha for anaerobically digested biosolids (AN). Rates were based on multiples of the nitrogen limited biosolids application rate (0.5, 1.0, 3.0, and 4.5NLBAR) for each type of biosolid. The experiment included an unfertilized control and a fertilized control that received multiple applications of synthetic fertilizer. Forage sorghum was planted 1 week after biosolids application and harvested 4 times between December 2002 and May 2003. Dry matter production was significantly greater from the biosolids-treated plots (21-27 t/ha) than from the unfertilized (16 t/ha) and fertilized (18 t/ha) controls. The harvested plant material removed an extra 148-488 kg N from the biosolids-treated plots. Partial N budgets were calculated for the 1NLBAR and 4.5NLBAR treatments for each biosolids type at the end of the crop season. Crop removal only accounted for 25-33% of the applied N in the 1NLBAR treatments and as low as 8-15% with 4.5NLBAR. Residual biosolids N was predominantly in the form of organic N (38-51% of applied biosolids N), although there was also a significant proportion (10-23%) as NO₃-N, predominantly in the top 0.90 m of the soil profile. From 12 to 29% of applied N was unaccounted for, and presumed to be lost as gaseous nitrogen and/or ammonia, as a consequence of volatilization or denitrification, respectively. In-season mineralization of organic N in biosolids was 43-59% of the applied organic N, which was much greater than the 15% (AN)-25% (AE) expected, based on current NLBAR calculation methods. Excessive biosolids application produced little additional biomass but led to high soil mineral N concentrations that were vulnerable to multiple loss pathways. Queensland Guidelines need to account for higher rates of mineralization and losses via denitrification and volatilization and should therefore encourage lower application rates to achieve optimal plant growth and minimize the potential for detrimental impacts on the environment.