Nitrogen mineralization in soil from perennial grassland measured through long-term laboratory incubations

Under perennial grasslands, nitrogen contained in organic matter becomes available at varying rates via mineralization throughout the growing season. The amount of N present at any given time indicates only the quantity immediately present, and does not include N which has already been removed either by leaching or uptake into the plant system, nor the N which will become available as organic matter breaks down over time. Long-term aerobic laboratory incubation methods have been used successfully to estimate potential N mineralization under various cropping conditions. They had not been used successfully, however, to estimate potential N availability under perennial grassland. In this research, soil samples from two long-term perennial grassland sites were taken before and after N fertilizer application at rates of 0, 175, 350 and 525 kg/ha. The soils were incubated in the laboratory at 35 degrees C and were eluted at 2, 4, 8, 12, 16, 22 and 30-week intervals, the length of time prescribed for determining N mineralization potential. Because a plateau had not been reached, incubation was allowed to continue for 198 weeks and 148 weeks for the pre- and post-N samples, respectively. Total N was high, as was soil organic matter in both sets of soil samples. Nitrogen mineralization potential was underestimated after 30 weeks of incubation, and overestimated after 148 weeks. The closest agreement between N measured and the estimated N mineralization potential, came after 198 weeks of incubation. This study confirmed the high N-supplying capacity of soil under long-term perennial grasslands. It also indicated that the recommended 30-week period needed to estimate N mineralization potential under other cropping systems was insufficient for a perennial grassland soil. Cumulative differences in N mineralization were found with varying rates of N fertilizer application, but these differences were rarely seen on an individual weekly basis, nor were they significant at the termination of the experiment. The response to N application differed by site.