农牧交错区草地利用方式导致的土壤颗粒组分变化及其对土壤碳氮含量的影响

VARIATIONS OF SOIL PARTICLE SIZE DISTRIBUTION WITH LAND-USE TYPES AND INFLUENCES ON SOIL ORGANIC CARBON AND NITROGEN

以位于农牧交错带的内蒙古多伦县6种主要草地利用方式为研究对象,对0~30cm土层的土壤颗粒组分、植物地下根系生物量、土壤有机碳（Soil organic carbon,SOC）和土壤全氮（Total nitrogen,TN）含量进行了测定分析。6种草地利用方式分别为自由放牧、刈割、围封禁牧、弃耕、人工牧草（紫花苜蓿（Medicago sativa））和青储作物（玉米（Zeamays））栽培。对土壤粒径分布的分维分析表明,自由放牧样地的土壤分维数明显低于其余5种类型的草地利用样地,表现出较大的土壤质地异质性。在土壤颗粒组分方面,弃耕样地和人工牧草样地的土壤粘粒（〈0.005mm）和粉粒（0.005~0.05mm）含量要明显高于其它草地利用方式,而自由放牧样地呈现相反的变异趋势。除人工牧草样地和自由放牧样地之外,代表其余4种类型草地利用方式的样地的粘、粉粒含量均随土层加深而降低。6种草地利用方式中,土壤砂粒（0.05~1mm）含量均与土壤碳氮含量呈负相关关系;除自由放牧样地和人工牧草样地外,其余4种草地利用方式的土壤粘粒与土壤碳氮含量均表现为显著正相关;植物根系生物量与土壤碳氮含量和粘、粉粒含量均呈显著正相关;地上生物量和凋落物仅与粘粒含量存在显著正相关关系。以上差异说明草地利用变化有可能通过影响植被而改变土壤物理性状,同时土壤碳氮含量和土壤颗粒组分的分布特征以及它们之间的相关关系受土地利用方式的影响较大。根系生物量和土壤粘粒含量可以共同解释土壤碳氮含量变异的70%,自变量的独立解释量约为20%。回归方程为：SOC=1.08×[粘粒含量]＋0.01×[根系生物量]-19.45,TN=0.079×[粘粒含量]＋0.001×[根系生物量]-1.143。

Aims In this study, variations of soil particle size distribution and correlations with soil C and N were studied on sites representing six different land-use types in the steppe grasslands of Inner Mongolia, Northern China. The six land-use types are grazing exclusion by fencing （GE）, mowing （MW）, free grazing （FG）, fallow （FL）, alfalfa pasture （AP）, and corn plantation （CP）. Our objectives were to： （a） assess variability of soil particle size distribution across different land-use types in the steppe grasslands of northern China and （b） examine correlations of soil particle size distribution with soil C and N as affected by land-uses. Methods Twenty-four sampling plots, each 30 m × 30 m, were established on sites representing the six landuse types. Measurements were made on soil particle size distribution, aboveground biomass, root biomass, litter, soil organic carbon （SOC） and soil total nitrogen （TN） in the 0 - 10, 10 - 20 and 20 - 30 cm soil layers. Data were evaluated by one-way analysis of variance and correlation analysis using SPSS. Fractal dimension of soil particle size distribution was calculated using the method of Yang et al. （1993）. Important findings Fractal dimension of soil particle size distribution was lowest in the FG among the six land-use types. Soil clay （ 〈0.005 mm） and silt （0.005 -0.05 mm） content of the FL and AP were consis-tently higher than the other four land-use types; whereas those of the FG were consistently lower. Clay and silt content decreased with soil depth except for the AP and FG. Soil sand content was significantly negatively cor- related with SOC and TN for all land-use types, while soil clay content was significantly positively correlated with SOC and TN except for FG and AP. Regardless of the land-use types, root biomass was found to be significantly positively correlated with SOC, TN and soil clay and silt content. Aboveground biomass and litter were significantly positively correlated with only clay content. Root biomass and clay content together explained 70% of the variance in SOC and soil TN, and separately only 20% of the variance each. The relationships were best described as. SOC = 1.08 × clay% ＋ 0.01 × biomassroots - 19.45, TN = 0. 079 × clay% ＋ 0. 001 ×biomassroots - 1. 143. Findings indicate that land-use types can have significant effects on soil physical properties as well as SOC and soil TN and consequently alter the relationships of soil particle size distribution with SOC and soil TN.