PREDICTING PLANT-AVAILABLE NITROGEN FROM ORGANIC AMENDMENTS IN THE SECOND YEAR AFTER APPLICATION

Abstract

First-year plant-available N (PAN) release from organic amendments has received considerable study, but longer-term data are lacking. Our objectives were to: (i) measure the percentage of total N available to a sweet corn crop in the second year after organic amendment application, (ii) determine whether Yr 2 PAN predictions need to be amendment-specific, and (iii) evaluate laboratory incubation methods for PAN estimation vs. field PAN. PAN released from 8 composts and 4 uncomposted organic amendments was measured at two field locations (sandy loam and silt loam soils). Organic amendments were applied at field sites in the spring of Yr 1, supplying an average of 570 kg N/ha at an application rate of 30 Mg/ha. Data reported here were collected in Yr 2. Sweet corn (Zea mays ‘Golden Jubilee’) was planted at field sites to estimate PAN using an N fertilizer equivalency approach. In the laboratory, PAN was measured as inorganic N accumulated during a 7-d anaerobic incubation at 40°C, or a 110-d aerobic incubation at 22°C. Laboratory PAN was calculated using the difference method (PAN for amended soil vs. PAN for no amendment control soil). As a percentage of Yr 1 total N input, Yr 2 PAN was 7% in field trials, 7% for aerobic incubation (110-d, 22°C), and 3% for anaerobic incubation (7-d, 40°C). In the 110-d aerobic incubation, PAN release was linear across time, with 3% of total N input mineralized per thousand Nmin degree days (0°C base). The linear rate of PAN accumulation observed in aerobic lab incubations, coupled with the finding that Yr 2 PAN was strongly related to quantity of Yr 1 N input (across all amendments) suggests that relatively simple computer simulation models can be used to predict Yr 2 PAN for a wide variety of organic inputs.