基于最小限制水分范围评价不同耕作方式对土壤有机碳的影响

以2009年吉林省德惠市中层黑土上进行了8a的田间定位试验小区土壤为研究对象,对免耕和秋翻两种耕作方式及玉米-大豆轮作和玉米连作两种种植方式下耕层有机碳进行分析,分别采用加权平均和分层两种方法计算最小限制水分范围(LLWR),用其评价不同耕作方式对土壤有机碳的影响.结果表明,免耕在玉米-大豆轮作和玉米连作下0-5 cm土壤有机碳含量分别比秋翻增加了15.2％和11.5％ (P＜0.05).采用加权平均法计算的LLWR值为0.148-0.166 cm3/cm3,不同耕作方式下玉米-大豆轮作的LLWR高于玉米连作且在两种种植方式下均表现出免耕小于秋翻的特点；利用分层法计算得到的LLWR值介于0.130-0.173 cm3/cm3之间,玉米-大豆轮作和玉米连作下免耕0-5 cm LLWR均显著小于秋翻,而5-30 cm LLWR数值免耕大于秋翻(P＞0.05)；玉米-大豆轮作下0-30 cm各层LLWR均高于玉米连作.由于LLWR可以评价不同耕作方式对土壤有机碳的影响,因此采用加权平均法计算的LLWR可以客观的反映不同耕作处理尤其是种植方式对土壤有机碳的影响；而采用分层法计算的LLWR则更清晰的刻画了土壤表层与亚表层固碳能力的差异.

Evaluating tillage practices impacts on soil organic carbon based on least limiting water range

The least limiting water range (LLWR) is the range in soil water content within which limitations to plant growth associated with water potential, aeration and mechanical resistance to root penetration are minimal. It has been proposed as an index of the structural quality of soils for crop growth. In many studies, soil CO₂ evolution rates are well explained by changes in soil temperature and moisture using a Q₁₀ equation. Even though soil temperature is one of the most important factors influencing soil respiration, tillage-based differences in soil CO₂ evolution are not fully explained by changes in soil temperature because differences in soil temperature under different tillage practices are generally small and not significant compared with much larger seasonal temperature fluctuations. Conversely, differences in soil moisture content under different tillage systems are more often reported to be large enough to influence soil CO₂ evolution rates. This is probably related to the fact that soil structure, which is changed by tillage practices, partially determines soil moisture content. Even though bulk density and penetration resistance are commonly used measures of soil structure, they do not describe its interactions with moisture. A multi-factor parameter might better represent the complex relationships that exist between soil structure and moisture. The LLWR, which integrates several soil physical parameters, has been proposed as an index of soil structural quality. The LLWR is the range of volumetric soil water contents (cm³/cm³) within a soil where biological processes are not limited by soil water or O₂ availability. Although the concepts of LLWR have been applied to processes in plants, they have rarely been applied to microbial processes such as C mineralization and soil organic carbon (SOC). The purpose of this study is to evaluate the effect of tillage on SOC based on the least limiting water range. Soil samples were collected from a tillage trial established in Dehui County, Jilin Province, Northeast China, in fall 2001. Under different tillage practices (no tillage, NT and moldboard plow, MP) with two rotations (corn-soybean and continuous corn), LLWR was calculated using weighted average and stratification methods. The results showed that compared with MP, NT significantly increased the SOC content in the top 0-5 cm of soil under the corn-soybean rotation and continuous corn system by 15.2% and 11.5%, respectively (P<0.05). The values of LLWR under different tillage and rotation practices ranged from 0.148 to 0.166 cm³/cm³ using the weighted average method, and were lower under NT than MP. The LLWR under different tillage and rotation practices ranged from 0.130 to 0.173 cm³/cm³ using the stratification method. LLWR in the 0-5 cm soil layer were significantly lower under NT than MP (P<0.05), whereas the difference in the 5-30 cm layer was not significant (P>0.05). The LLWR was higher under the corn-soybean rotation than under the continuous corn system in each of the 0-30 cm layers. Because the LLWR can evaluate the impacts of different tillage practices on SOC, LLWR calculated using the weighted average method can generally reflect the impact of different tillage practices, especially crop rotation practices on SOC, while LLWR calculated from the stratification method more clearly indicated differences in SOC sequestration between the soil surface and subsurface.