RESUMO
In this study, desert grassland, grassland edge, shrubland edge, shrubland were selec-ted as four transition sites in a nearly 30 years typical desert grassland-shrubland mosaic formed by anthropogenic shrub introduction. Soil properties and soil microbial characteristics under vegetation patches and bare interspace in each site were investigated to examine the responses of soil nitrogen to the desert grassland-shrubland state transition. It was shown that the aboveground biomass increased with transition from desert grassland to shrubland. Annual herbs increased largely with the introduction of shrubs. Soil moisture, microbial biomass and total nitrogen and carbon decreased with the transition. The abundance of microogranisms was lower in grassland edge and shrubland edge, and then increased in shrubland, which was slightly higher than that of desert grassland. With respect to nitrogen, nitrate content reached the highest level of 28.45 mg N·kg-1 and ammonium reached the lowest level of 4.81 mg N·kg-1 in shrubland, which were significantly increased by 52.3% and decreased by 10.4% compared with desert grassland. In addition, soil moisture and microbial biomass nitrogen was positively correlated across all sites. The relationship between mine-ralized nitrogen and soil moisture was non-linear, as they were positively correlated in desert grassland and grassland edge, but negatively correlated in shrubland edge and shrubland. During the 30-year transition from desert grassland to shrubland, our results showed that soil total nitrogen and microbial biomass nitrogen were significantly decreased, but mineralized nitrogen, especially for nitrate, significantly increased over time, indicating that soil nitrification was inhibited in desert grassland but accelerated in shrubland.
Assuntos
Nitrogênio , Solo , Carbono/análise , China , Ecossistema , Pradaria , Nitrogênio/análiseRESUMO
To understand the responses of soil extracellular enzyme activities to the desert grassland-shrubland anthropogenic transition, we examined soil properties and six extracellular enzyme activities, in soil under vegetation patches and bare interspaces in desert grassland, grassland edge, shrubland edge, shrubland in a typical anthropogenic desert grassland-shrubland mosaic in desert steppe of eastern Ningxia, China. The six measured enzymes included cellobiohydrolase, ß-1,4-xylosidase, ß-1,4-glucosidase, ß-1,4-N-acetylglucosaminidase, leucine aminopeptidase and alkaline phosphatase. We found that soil moisture, soil organic carbon, total nitrogen, total phosphorus, microbial biomass carbon and microbial biomass nitrogen decreased significantly by 26.0%-88.5% with the process of the desert grassland-shrubland anthropogenic transition. All soil properties, except soil organic carbon and soil moisture in grassland edge site, were 3.9%-82.3% higher under vegetation patches than those in bare interspaces in each site. The six extracellular enzyme activities also decreased by 22.1%-82.4% in the transition process, especially for leucine aminopeptidase and alkaline phosphatase, showing significant decrease by 82.4% and 75.5%, respectively. All extracellular enzyme activities but ß-1,4-N-acetylglucosaminidase in shrubland were significantly higher by 10.7%-42.7% under vegetation patches than those in bare interspaces in each site. The activities of six extracellular enzymes were all positively correlated with each other. All of them were positively correlated with soil properties. Moreover, activities of these soil extracellular enzymes responded more positively to the changes of microbial biomass carbon, microbial biomass nitrogen, and total nitrogen in the process of anthropogenic transition.
