Nitrogen Phytoavailability of Composted Biosolids.

Composting of biosolids with plant bulking agents at a typical 1:1 dry weight mixing ratio often increases the carbon-to-nitrogen ratio and can decrease nitrogen (N) phytoavailability of biosolids. A greenhouse study was done to evaluate the relative N phytoavailability (RNP) of composted biosolids produced by composting biosolids with common yard waste mix at 1:1 (biosolids:yard waste) dry weight ratio. Composted and un-composted biosolids (applied at 400 mg N kg^-1 soil equivalent to 870 kg N ha^-1) and four chemical fertilizer (ammonium nitrate) treatments at 0, 35, 69, and 138 mg N kg^-1equivalent to 0, 75, 150, and 300 kg N ha^-1, respectively, were applied to a silty clay loam soil to assess the RNP of the amendments. Corn (Zea mays) and ryegrass (Lolium perenne) were grown in the amended soil. The bioassay was conducted with and without four months of pre-incubation of soil after application of the amendments. The response of the dry matter yield of corn and ryegrass to biosolids was not affected by composting, but N uptake by both corn and ryegrass was lower in composted biosolids as compared to that in the un-composted biosolids. The RNP of the composts was ~ 10%, which was less than that observed for un-composted biosolids. The pre-incubation showed minimal impacts on N release and availability from the organic amendments. We recommend the use of 10% for the composted biosolids and 20% for uncomposted biosolids as plant N availability in calculating the application rate of the organic amendments.