退耕年限与方式对土壤团聚体稳定性及有机碳分布的影响

以黄土高原南部退耕还林年限6 a(FL06)和15 a(FL15)刺槐林地、退耕还草年限6 a(GL06)和15 a(GL15)紫花苜蓿草地为研究对象,以临近长期耕作坡耕地(CK)作为对照,采用湿筛法,分离出2 mm、1~2 mm、0.5~1 mm、0.25~0.5 mm和0.25 mm 5个粒级的水稳性团聚体,研究了退耕年限与方式对团聚体稳定性和不同粒径团聚体有机碳分布的影响。结果表明:在0~20 cm土层,退耕还林还草与未退耕相比能显著提高2 mm和1~2 mm粒径团聚体含量,显著减少0.25 mm粒径团聚体含量,其中对于2 mm和1~2 mm粒径团聚体在不同退耕年限与方式下含量表现为GL15GL06FL06FL15CK和GL15FL06GL06FL15CK;退耕还林和还草增加了两个土层的团聚体稳定性,GL15的平均重量直径(MWD)值和几何平均直径(GMD)值均最大,土壤结构最稳定,其次为GL06;不同退耕年限,2 mm粒径下退耕还林地和还草地、1~2 mm粒径下退耕还草地团聚体有机碳含量均随退耕年限的延长而增加。20~40 cm土层中,团聚体含量均值随粒径的减小而增加;MWD和GMD值均小于0~20 cm层;各粒径范围内退耕还林与还草后的团聚体有机碳含量与坡耕地相比总体表现出减小的趋势。研究结果表明,退耕改善了土壤结构,对各粒径团聚体有机碳含量分布的影响随退耕年限与方式不同效应各异,且GL15相较于其它退耕年限和方式下的样地有更好的土壤团聚体稳定性和更多的团聚体有机碳积累。

Effects of age and type of conversion from cropland to forest land and grassland on stability and organic carbon in soil aggregates

The purpose of this study was to elucidate the effect of plantation type and age on aggregate stability and aggregate-associated organic carbon content.Soil samples were collected from five types of land in the southern part of the Loess Plateau including:artificial pure locust forest land converted from slope cropland with plantation ages of 6 years (FL06) and 15 years (FL15),artificial alfalfa grassland converted from slope cropland with plantation ages of 6 years (GL06) and 15 years (GL15) and the neighboring conventional slop cropland (CK).The results revealed that:in 0 ～ 20 cm soil layer,land use conversion from cropland to forest land and grassland could significantly increase the fractions for ＞ 2 mm and 1 ～ 2 mm aggregate but decrease for ＜ 0.25 mm compared with the cropland.The fractions for ＞ 2 mm and 1 ～ 2 mm aggregate under different plantation type and age was in the order of GL15 ＞ GL06 ＞ FL06 ＞ FL15 ＞ CK and GL15 ＞ FL06 ＞ GL06 ＞ FL15 ＞ CK,respectively.Land use conversion increased aggregate stability in the two soil layers.GL15 had the maximum mean weight diameter (MWD) and geometric mean diameter (GMD) values,followed by GL06.Soil organic carbon in ＞ 2 mm aggregate under forest land and grassland,and that in 1 ～ 2 mm aggregate under grassland all increased with plantation age.In 20 ～ 40 cm soil layer,the mean of aggregate content increased with aggregate size decreasing,both the MWD and GMD value of this layer were less than those of 0 ～ 20 cm soil layer.There was an trend that all aggregate fractions under forest land and grassland had the less organic carbon content than that of cropland.The findings suggested that land use conversion from cropland to forest land and grassland improved soil structure,and its effect on organic carbon content in every aggregate fractions change along with plantation type and age.GL15 had better aggregate stability and more aggregate-associated organic carbon.