Abstract: Four typical ecosystems, i.e., maize-sweet potato rotational cultivated land (KMS), grazing grassland burned annually in winter (KGB), natural restoration land (KNR), and primary forest land (KPF), in Karst region of northwest Guangxi were selected to investigate the responses of soil nutrients (C, N and P), soil microbial biomass, and soil structure to the degradation of ecosystem. The contents of soil organic C, total N and P, and soil microbial biomass C, N, and P were significantly higher in KPF than in KMS, KGB, and KNR (P<0.01). In the latter three degraded ecosystems, the contents of soil organic C and total N were in the sequence of KNR>KGB>KMS but the difference was not significant, soil total P content in KMS (0.87 g·kg^-1) was 2.07 and 9.67 times of that in KNR and KGB, respectively (P<0.01), and soil microbial biomass C, N and P contents were significantly higher in KGB and KNR than in KMS (P<0.05). The soil microbial biomass C was significantly higher in KGB than in KNR (P<0.05), but there were no significant differences in soil microbial biomass N and P between the two ecosystems. These results illustrated that the reduction of human activity could induce a slight increase of soil organic C in Karst degraded ecosystems, and proper grazing and natural restoration could be the feasible modes for the restoration of degraded ecosystem. Soil microbial biomass was more sensitive in response to the change of ecosystem, being able to be used as a sensitive indicator to reflect the change of degraded ecosystem in Karst region. In KPF, KNR, and KGB, soil water-stable macro-aggregates (>0.25 mm) accounted for more than 70%, and dominated by >2 mm aggregates; while in KMS, soil water-stable macro-aggregates only occupied 40.34%, and dominated by 2-0.25 mm aggregates. The destruction rate of soil structure in KMS, KGB, KNR, and KPF was 51.62%, 23.48%, 9.09%, and 9.46%, respectively (P<0.05), indicating that human disturbance or farming practice destroyed soil macro-aggregates, and made the destruction rate of soil structure increased. To reduce human disturbance and implement natural rehabilitation would be the suitable ecological restoration strategy in Karst region.

Key words: Karst ecosystem, soil organic C, total N, total P, microbial biomass, soil aggregate, riparian buffer, best management practice, level spreader, concentrated flow path.