INCREASING ORGANIC C AND N IN SOIL UNDER BROMEGRASS WITH LONG-TERM N FERTILIZATION

Two field experiments were conducted on bromegrass (Bromus inermis Ley ss.) on a thin Black Chernozem (Typic Boroll) at Crossfield, Alberto, Canada to determine the long-term effects of N fertilization on change s in concentration and mass of organic C and N in soil. In both experi ments, bromegrass was harvested for hay each year. In the experiment w here ammonium nitrate (AN) was applied annually at 0 to 336 kg N/ha fo r 27 consecutive years from 1968 to 1994, the concentration of total C in the 0-5 cm soil layer increased from 50.33 g/kg in the zero-N trea tment to 61.64 g/kg with 56 kg N/ha and to 64.15 g/kg with the 112 kg N/ha rate. Total C in soil also increased in the 5-10, 10-15 and 15-30 cm layers but to a lesser extent. The mass of total C in the 0-30 cm soil layer was increased by 18.46 Mg/ha with 56 kg N/ha and by 23.38 M g/ha with the 112 kg N/ha rate as compared to the zero-N treatment. To tal N in soil followed a similar trend as total C. In the experiment w hich received four N sources [ammonium nitrate (AN), urea, calcium nit rate (CN) and ammonium sulphate (AS)] applied annually at 168 and 336 kg N/ha for 15 years from 1979 to 1993, the total C in soil was greate r where N fertilizer was applied, but the increase in total C varied w ith N source. The concentration of total C in soil in the 0-5 cm layer tended to be greater with AN and AS than with CN, with the smallest i ncrease from urea. The mass of total C in soil (average of four N sour ces) at the 168 kg N/ha rate was increased by 18.98 Mg/ha in 0-30 cm a nd by 43.48 Mg/ha in the 0-60 cm layer as compared to the check treatm ent. The concentration of total C in soil also increased in the deeper layers to a depth of 60 cm, but the increases were much smaller than in the 0-5 cm layer. The changes in total N in soil followed a similar pattern as total C. In conclusion, long-term annual additions of fert ilizer N to bromegrass resulted in a marked increase in total C and N in soil and the increases were influenced by both rate and source of N fertilizer. The implications of these results are that grasslands can be managed to lessen the increase in atmospheric CO2 concentration, w hile also improving fertility (N-supplying capacity) and tilth of soil .