黄土高原丘陵沟壑区土壤物理性质对苜蓿 种植年限的响应

西部黄土高原丘陵沟壑区是中国乃至世界上水土流失最严重的区域,以禾谷类作物单播为主的传统农业生产系统和过度耕作是引致水土流失的最主要原因。紫花苜蓿作为优良豆科牧草,在区域生态环境建设和产业结构调整中发挥着重要作用。因此,本研究通过设置在陇中黄土高原半干旱区的长期定位试验,以苜蓿草地(3 a、10 a、12 a)和农田(马铃薯地)为主要研究对象,探讨了土壤物理性质对于苜蓿种植年限的响应,为黄土高原雨养农业系统紫花苜蓿适宜种植年限的选择及苜蓿草地的可持续利用提供科学依据。结果表明,随着紫花苜蓿种植年限的加长,土壤表层呈容重降低、孔隙度增加的变化趋势,而下部土层变化不明显。苜蓿种植可以提高耕层0~30 cm土壤0.25 mm水稳性团聚体含量、平均重量直径(MWD)和几何平均直径(GMD),同时降低团聚体破坏率(PAD),且随种植年限的延长效果愈加明显。苜蓿种植一定年限后土壤总有机碳(TOC)和易氧化有机碳(ROOC)与农田差异明显,其中种植苜蓿土壤易氧化有机碳占总有机碳的比例为44%~57%,农田土壤易氧化有机碳比例占52%~68%,表明种植苜蓿不仅提高了土壤总有机碳含量,且改变了土壤有机碳的组成比例。与农田相比,苜蓿种植可改善土壤水分入渗性能,表现为随种植年限的延长呈现先增加后降低的趋势。黄土高原沟壑区种植苜蓿可以改善土壤有机质形态和物理结构,提高土壤渗透能力,但苜蓿种植年限以10 a为宜,10 a之后应该进行轮作换茬以维持雨养农业系统的可持续发展。

Response of soil physical properties to alfalfa growth years in the Western Loess Plateau

AbstractAs one of the poorest regions in China, the Loess Plateau has the severe erosion across large areas, which has limited agricultural development beyond subsistence farming. The traditionally cereal-dominant agricultural system driven by extensive tillage and cultivation on slopes has contributed significantly to erosion on the plateau. Alfalfa (Medicago sativa) was introduced in China and has since been cultivated for over 2 000 years now. It is one of the important perennial legumes cultivated on the Loess Plateau which increases livestock production, reduces soil erosion in the annual crop rotation system and improves soil fertility through alfalfa-based pasture crop rotation on slopes and annual cropping in upland plains. In a field experiment conducted in Dingxi City, Gansu Province (a typical semiarid area of the West Loess Plateau), different growth periods of alfalfa were investigated in relation to soil physical properties. The study also discussed the optimum growth years of alfalfa in the Loess Plateau in the central Gansu Province. Soil samples were taken from the 0?50 cm soil layers in alfalfa field plots with different growth years (3 years, 10 years and 12 years) and in potato fields. Measurements of soil bulk density, soil aggregate stability, saturated hydraulic conductivity, total organic carbon (TOC) and readily oxidized organic carbon (ROOC) were then recorded. The results showed that soil bulk density was significantly lower and total porosity significantly higher in potato fields than in alfalfa fields for topsoil, but not significantly different for deep soil layers. The content of > 0.25 mm water-stable macro-aggregates, mean weight diameter (MWD) andgeometric mean diameter (GMD) of soil aggregates in alfalfa fields increased with alfalfa growth years, and were significantly higher than those in cropland fields for the 0?30 cm soil depth. Then the percentage aggregate destruction (PAD) of soils in alfalfa fields was significant lower than that in potato fields, which decreased with increasing alfalfa growth years. TOC and ROOC improved growth conditions in alfalfa fields for a certain period of growth. The proportion of ROOC to TOC was 44%–57% and 52%–68% in alfalfa fields and in potato fields, respectively. This indicated that alfalfa fields improved soil organic carbon (SOC) content and as well decreased percent ROOC to TOC. SOC was significantly higher in alfalfa fields than in potato fields, where it initially increased and then decreased with increasing alfalfa growth period. Therefore alfalfa cultivation on the plateau was significantly beneficial under rainfed farming system, including the enhancement of soil structure, changes in SOC fraction, and soil permeability. It was, therefore concluded that the optimum growth period of alfalfa in the semi-arid areas of the Loess Plateau was 10 years. It was recommended to develop alfalfa-cereal rotation system after 10 years of alfalfa growth in order to increase crop production, reduce soil erosion and enhance the sustainability of the agricultural system.