To clarify responses of plant and soil carbon (C) and nitrogen (N) pools in grassland ecosystem to N addition, a field experiment was performed in a grassland in Keerqin Sandy Lands, Northeast China. We investigated vegetation composition and C and N pools of plant and soil (0-30 cm) after five consecutive years of N addition at a rate of 20 g N m -2 y -1 . Vegetation composition and species diversity responded dramatically to N addition, as dominance by C 4 perennials was replaced with C 3 annuals. Carbon in aboveground pool increased significantly (over two-fold), mainly due to the increase of the C in aboveground living plants and surface litter, which increased by 98 and 134%, respectively. Although soil C did not change significantly, the root C pool decreased in response to 5 years of N addition. The total ecosystem C pool was not significantly impacted by N addition because the large soil pool did not respond to N addition, and the increase in aboveground C was offset by the decrease in root C pool. Moreover, N addition significantly increased the aboveground N pool, but had no significant effects on belowground and total ecosystem N pools. Our results suggest that in the mid-term N addition alters the C and N partitioning in above-and belowground pools, but has no significant effects on total ecosystem C and N pools in these N-limited grasslands.
We measured soil chemical and microbial properties at a depth of 0-20 cm among mine tailings, abandoned mined land, contaminated cropland, and uncontaminated cropland around a magnesite mine near Haicheng City, Liaoning Province, China. The objective was to clarify the impact of Mg on the soils. We found that soluble Mg 2þ concentration and pH were significantly higher in contaminated soils (266-345 mg kg À1 and 9Á9-10Á3, respectively) than in uncontaminated soils (140 mg kg À1 and 7Á1, respectively). Soil nutrients (total N, total P, mineral N, available P and soluble Ca) and microbial biomass C and N decreased as pH and soluble Mg 2þ concentration increased. In addition, an increase of microbial metabolic quotient and a decrease of N mineralization rate were found in contaminated soils. Soluble Mg 2þ /Ca 2þ ratios in contaminated soils were 3Á5-8Á9-times higher than in uncontaminated soils. Our results indicate that soil contamination in such magnesite mine regions is characterized by high pH, Mg 2þ concentration and soluble Mg 2þ /Ca 2þ ratio, and low microbial activity and N and P availability. Future soil amelioration in the magnesite regions should consider applying acid ameliorants to neutralize high pH and applying calcareous ameliorants to increase Ca 2þ concentration.
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