Nitrogen availability and decomposition of urban yard trimmings in soil

Application of yard trimmings from urban landscapes onto farmland is an emerging recycling alternative in the Pacific Northwest of the United States. Estimates of the plant-available nitrogen provided by yard trimmings are needed to meet grower demands and environmental regulations. Our objectives were to: (i) estimate the available N provided by yard trimmings containing grass clippings and woody plant materials, (ii) evaluate the impact of aging on the available N supplied by grass clippings, and (iii) identify laboratory analyses that are correlated with N mineralized from yard trimmings in soil. Yard trimmings were mixed with sandy loam soil and incubated at 25 [degrees]C to determine N availability and CO2 loss. Mixed grass clippings + woody trimmings with typical C:N ratios of 15 to 19 had an available N equivalent of 10 to 14% of total N; grass clippings alone had an available N equivalent of 21 to 37% of total N. Aging of grass clippings for 14 to 28 days in unmanaged piles increased NH4-N, ash, and lignin concentrations. Aging also reduced N and C mineralized from grass clippings in the soil. Nitrogen mineralized from yard trimmings in soil was correlated (R2 = 0.64 to 0.78) with the yard trimmings total N, C:N, lignin, carbohydrate + hemicellulose concentrations, and with CO2 evolution during a 7-day incubation in the soil. Based on the present study, it seems unlikely that excess N will be provided by typical application rates of mixed yard trimmings. Mixed yard trimmings with a typical moisture of 600 g kg-1, total N of 17 g kg-1 (dry matter basis), and N availability equal to 15% of total N supplied approximately 1 kg of available N per metric ton. [PUBLICATION ABSTRACT]