Soil samples archived over four decades were used in an incubation experiment to measure the effects of reforestation on nitrogen dynamics in old agricultural fields. Samples collected from 0 to 7.5 cm and 35 to 60 cm...Soil samples archived over four decades were used in an incubation experiment to measure the effects of reforestation on nitrogen dynamics in old agricultural fields. Samples collected from 0 to 7.5 cm and 35 to 60 cm depths in Calhoun,South Carolina USA at intervals ranging between five and nine years since 1962 were incubated aerobically at 30 ℃ for 30 days using a system specially designed to maintain constant soil moisture. Mineral N was measured in 2 mol·L -1 KCl extracts. The ratio of mineralized N to total N rapidly decreased in the first two decades of forest development. Within 20 years after planting, plenty of available N had been accumulated in the biomass, which had a significant negative correlation with soil total N ( r - top -0.828～-0.898; r - deep-0.848～-0.989). It indicated that agricultural inputs of N were important to early tree development. Significantly, by age 40, soil mineral N had increased to 50%of that in the beginning of tree planting. The accretion of mineralizable N suggests that forest floor is serving as an increasingly important source for this nutrient. Further, it indicates that forest managers have the opportunity to manipulate a large pool of forest organic matter to sustain soil N supply.展开更多
文摘Soil samples archived over four decades were used in an incubation experiment to measure the effects of reforestation on nitrogen dynamics in old agricultural fields. Samples collected from 0 to 7.5 cm and 35 to 60 cm depths in Calhoun,South Carolina USA at intervals ranging between five and nine years since 1962 were incubated aerobically at 30 ℃ for 30 days using a system specially designed to maintain constant soil moisture. Mineral N was measured in 2 mol·L -1 KCl extracts. The ratio of mineralized N to total N rapidly decreased in the first two decades of forest development. Within 20 years after planting, plenty of available N had been accumulated in the biomass, which had a significant negative correlation with soil total N ( r - top -0.828～-0.898; r - deep-0.848～-0.989). It indicated that agricultural inputs of N were important to early tree development. Significantly, by age 40, soil mineral N had increased to 50%of that in the beginning of tree planting. The accretion of mineralizable N suggests that forest floor is serving as an increasingly important source for this nutrient. Further, it indicates that forest managers have the opportunity to manipulate a large pool of forest organic matter to sustain soil N supply.
