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(P0.05). 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(P0.05),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.