宁夏东部荒漠草原-灌丛地转变过程土壤氮素响应

选取近30年荒漠草原灌丛引入形成的典型草地-灌丛镶嵌体内部的荒漠草地、草地边缘、灌丛边缘、灌丛地为研究样地,对各样地及其微生境(植丛与空斑)相关土壤指标进行测定,以了解荒漠草地向灌丛地转变过程中土壤氮素的响应特征。结果表明: 随草地灌丛化转变,草本与灌丛生物量均增加,其中一年生草本随灌丛引入增加明显;土壤水分、全碳、全氮及微生物生物量均呈降低趋势,微生物数量在草地边缘与灌丛边缘降低,而在灌丛地增加并略高于荒漠草地;从荒漠草地转变至灌丛地,硝态氮显著升高52.3%,最高为28.45 mg N·kg^-1,铵态氮显著降低10.4%,最低为4.81 mg N·kg^-1。微生物生物量氮与土壤水分呈显著正相关,而矿化氮与土壤水分响应关系则随着植被转变呈非线性变化,即硝态氮和铵态氮与土壤水分在荒漠草地和草地边缘呈正相关,而在灌丛边缘和灌丛地呈负相关。荒漠草地向灌丛地近30年的转变过程中,土壤全氮、微生物生物量氮呈降低趋势,而矿化氮显著增加,其中硝态氮尤为明显,表现出从荒漠草地土壤氮素的硝化抑制向灌丛地硝化加速转变。

Responses of soil nitrogen to the transition from desert grassland to shrubland in eastern Ningxia,China

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.