Influence of paper mill sludges on corn yields and N recovery

Mixed paper mill sludges are an important source of N for crop production. An estimate of direct and residual N recovery is necessary for their efficient management. A 3-yr field study (1997-1999) was conducted in central Quebec, Canada, to evaluate mixed paper mill sludges (PMS) effects on corn (Zea mays L.) yields and N nutrition, N recovery and N efficiency. The effects of PMS on soil NO3-N and total N levels were also determined. The study was situated on a silt loam Baudette soil (Humic Gleysol). The treatments included 3 PMS rates (30, 60 and 90 t ha(-1) on wet basis) applied alone or in combination with N fertilizer (90 and 135 kg N ha(-1), respectively, for 60 and 30 t ha(-1)). Treatments also included a control without PMS or N fertilizer, and a complete mineral N fertilizer (180 kg N ha(-1)) as recommended for corn. The previous plots were split beginning with the second year of the experiment, for annual and biennal PMS applications. Similar treatments as above were made on an adjacent site to evaluate N recovery under climatic conditions in 1999. In all years, PMS applied alone significantly increased corn yields by 1.5-5 t ha(-1), compared to the unfertilized control. However, corn yields and N uptake were highest from the application of PMS in combination with N fertilizer. Biennial PMS applications at 60 to 90 hr(-1) significantly increased corn yields and N uptake, which suggest high PMS residual effect; however, these increases were lower than those obtained with annual PMS applications. The N efficiency varied in 1997 from 13.0 to 15.4 kg grain kg N-1 for mineral N fertilizer and ranged from 3 to 13.7 kg grain kg N-1 for PMS, decreasing proportionally to increasing PMS rates. Apparent N recovery ranged from 13 to 19% in 1997 and from 10 to 14% in the residual year (1998), compared to 30 and 49%, respectively, for mineral N fertilizer. Depending on the PMS rate, N recovery varied from 13 to 21% in 1999. The results indicate high N supplying capacity and high residual N effects of PMS, which probably influenced corn yields and N nutrition. Annual PMS applications alone or combined with mineral N fertilizer had no significant effect on soil NO3-N and total N levels. This study demonstrates that application of low PMS rate (30 t ha(-1)) combined with mineral N fertilizer could achieve high agronomic, economic and environmental benefits on farms.