Abstract:Nitrogen (N) deposition has been increasing during recent decades and may affect supply of soil nutrients and resources acquired by organism. Soil enzyme activity is an important indicator for reflecting the nutrient acquisition of plants and microorganisms. To explore the effects of long-term N and phosphorus (P) additions on activities of N and P cycling enzymes in subtropical forest soil, two plantations of Acacia auriculiformis and Eucalyptus urophylla in south China were applied N and P fertilizers for 8 years, each with 50 kg/(hm2·a), and then the activities of soil enzymes, including P-cycling enzymes[phosphomonolipase (PME) and phosphodiesterase (PDE)] and N-cycling enzymes[β-1,4-acetylglucosaminidase (NAG) and l-leucine aminopeptidase (LAP)] were measured. The results showed that N addition had no significant effect on activities of soil N and P cycling enzymes. P and N+P additions had significant negative effects on activities of PME and PDE, but which had no effect on activities of NAG and LAP. The growth of soil microorganisms and plants of subtropical plantations in south China may be limited by P rather than by N, and P fertilization could alleviate soil P limitation on plants and microorganisms. Therefore, these would provide an important insight for forest management in the future.

Abstract:In order to reveal the enzymatic mechanism of soil nutrient transformation, the soil nutrient contents and enzyme activities in plantations, such as nitrogen-fixing tree species (Acacia crassicarpa, A. melanoxylon, A. cincinnata, A. auriculiformis, A. mangium) and non-nitrogen-fixing species (Eucalyptus urophylla×E. grandis) were studied, and their relationships were analyzed. The results showed that the pH in 40-60 cm soil of Acacia plantations were higher than that in E. urophylla×E. grandis plantation, which total P and K contents at all soil layers were lower than those in E. urophylla×E. grandis plantation, while total C and total N contents at 20-40 cm soil were high in Acacia plantations. Soil available nutrients contents were significantly higher in A. melanoxylon and A. mangium than those in E. urophylla×E. grandis plantation (P<0.05). Compared to E. urophylla×E. grandis plantation, the acid phosphatase and cellulase activities at 0-10 cm soil layer were high in Acacia plantations, which the activities of soil urease, sucrase, cellulase and aryl sulfatase were the highest in A. auriculiformis plantation (P<0.05); the activities of soil urease, cellulose, chitinase and amylase were the highest in A. cincinnata plantation (P<0.05). Correlation analysis revealed that the activities of soil urease, sucrase and chitinase were significantly negative to soil AP (P<0.05); the activities of soil sucrase and cellulase were signifi-cantly negative to soil NH4+-N; the activities of soil urease, cellulase and aryl sulfatase were significantly negative to soil TK; the activities of soil chitinase were significantly positively to soil TN; the activities of soil amylase were significantly positively to soil NH4+-N; the activities of soil catalase were significantly positively to soil TK. Therefore, compared to E. urophylla×E. grandis plantation, the introduced Acacia species to soil degraded mountains in southern China could improve key soil enzyme activities and soil available nutrients, which was beneficial to the eco-restoration of degraded soil and the maintenance of long-term productivity of plantations.

Abstract:Soil microorganisms, mainly bacteria, fungi and actinomycetes, under Eucalytus forest, Acacia forest and mixed forests were investigated during 1992 to 1993 in Huaxian county near Guangzhou. Soil enzyme activities, including cellulase, invertase, urease,proteinase and phosphate activities under different forest types were analysed. It was shown that mean annual amounts of soil dricrobes under forests are in order mixed forest>pure Acacia forest >pure Eucalyptus forest>barren and waste land. High activities of cellulase and invertase were found in dxed forest of Acacia and Eucalyptus, while urease,proteinase and catalase in pure acacia forest. Soil respiration intensity was high in Acacia mangium forest and in its dxed forest, and was low in Eucalyptus forest. Five years after forestation with tree species mentioned above on barren and waste land, the soil ndcroenvironment was shown to be greatly improved, which resulted in the increments of soil microbes under ground and of biomasses above ground.