荒漠草原土壤微生物矿化对灌丛引入过程及水分的响应

Responses of soil microbial mineralization to the anthropogenic introduced shrub encroachment and water gradients in the desert steppe

以宁夏荒漠草原封育草地(封育14年)、放牧地为对照,开展不同年限(3a、12a、22a)和间距(40 m、6 m、2 m)柠条地(Caragana korshinskii)灌丛引入过程土壤微生物矿化及对不同水分梯度(自然降雨、前期饱和、持续干旱)响应特征研究。结果表明:灌丛地0—20 cm和0—100 cm土层土壤全氮含量均高于放牧地且随年限和密度增加呈增高趋势,分别为0.28—0.42、0.27—0.42 g/kg和0.28—0.51、0.27—0.51 g/kg,但在22年且密度增至2 m间距的灌丛地与封育草地相比差异不显著或呈降低趋势;微生物量氮变化分别为0.041—0.057、0.041—0.081 mg/kg,且22年灌丛地呈显著降低,2 m间距灌丛地呈显著增加,但均显著低于封育草地0.079 mg/kg和放牧地0.103 mg/kg(P<0.05);真菌/细菌比值随灌丛地年限和密度增加变化为0.06—0.33,均高于封育草地0.04和放牧地0.03(P<0.05),微生物变化改变了灌丛地的矿化过程,相较封育草地0.003 mg kg^(-1)d^(-1)和放牧地-0.068 mg kg^(-1)d^(-1)的净矿化速率,不同年限/间距灌丛地虽分别降低为-0.155—-0.084、-0.116—-0.061 mg kg^(-1)d^(-1),但均表现为前期加速硝化过程,使得硝态氮则随年限和密度增加呈增加趋势显著高于两样地,而铵态氮变化则较为稳定;封育草地对干旱具缓冲作用,持续矿化,而放牧地和灌丛地净矿化速率对水分降低响应积极,分别降低280.3%和440.2%,其中放牧地和灌丛地以硝化作用变化尤为显著,特别是灌丛地则随干旱加剧净硝化速率均呈显著降低趋势(P<0.05),其净矿化速率变化以硝化过程占主导。

With the 14-year enclosed grassland and the grazed grassland as controls,the impacts of anthropogenic shrublands(Caragana korshinskii)with different planted years(3 a,12 a,and 22 a)and planted intervals(40 m,6 m,and 2 m)on the soil microbial mineralization and the responses of these lands to the water gradients(embient rainfall,initial watered saturation,drought)were investigated in the desert steppe of Eastern Ningxia,China.It was founded that the soil total nitrogen content in the 0—20 cm and 0—100 cm soil layers of the shrubland increased with the year and density increase of shrub introdution which were higher than those of the grazing land,0.28—0.42,0.27—0.42 g/kg and 0.28—0.51,0.27—0.51 g/kg,but soil total nitrogen in 22 a-shrubland had no significant difference and even the 2 m-shrubland was lower than that of the enclosed grassland.The changes of soil microbial biomass nitrogen in shrublands with different years and densities were 0.041—0.057 mg/kg and 0.041—0.081 mg/kg,respectively,significantly lower than those in enclosed land 0.079 mg/kg and grazed land 0.103 mg/kg.The ratio of fungi/bacteria in shrublands with years and density varied from 0.06 to 0.33,which was significantly higher than that in enclosed land 0.04 and grazed lands 0.03.and showed a decrease trend with the years increase and an increase trend with the increase density,but nitrate nitrogen increased with year and density increase,wheares ammonia nitregon had no significant changes.Compared with the net mineralization rate of the enclosed land 0.003 mg kg^(-1)d^(-1)and grazed land-0.068 mg kg^(-1)d^(-1),respectively,those of the shrublands with different years and densities decrease to-0.155—-0.084 mg kg^(-1)d^(-1)and-0.116—-0.061 mg kg^(-1)d^(-1),respectively(P<0.05).The grassland had a certain buffering effect on rainfall changes and so the increased soil drought had no significant effect on the soil mineralization of enclosed grassland in the observation time,while the soil net mineralization rate in the grazed land and the introduced shrublands responded positively to water gradient decrease and decreased by 280.3%and 440.2%,respectively.And especially the net nitrification rate changed significantly in grazed land and shrublands with the increase of drought(P<0.05),and the net mineralization rate is dominated by the nitrification process in shrublands.