Examining the C Saturation Concept in a Temperate Agricultural System.
Haegeun Chung1, Johan W. Six1, and John H. Grove2. (1) Dept of Plant Sciences, Univ of California, Davis, One Shields Avenue, Davis, CA 95616, (2) Plant and Soil Science Dept, N-122 ASCN, Univ of Kentucky, Lexington, KY 40546
Most of the soil organic matter models assume a linear relationship between soil C content and C input rate. However, in some soils, soil C may not increase even when the C input rate increases. This indicates that there may be an upper limit to how much C can be sequestered in soil. Previous studies on soil C dynamics have suggested soil C saturation based on the silt and clay content of the soil, but other mechanisms of soil C protection that leads to C saturation are yet to be explored. Soil can act as a sink of C, and storing C in soil can be an effective option to mitigate CO2 emission to the atmosphere. Moreover, higher organic matter content can improve the fertility of the soil. However, we are uncertain about limits in soil C sequestration, and through which mechanism this limit is imposed. We defined C saturation as the maximum capacity of soil to store C when C input rate at steady-state increases and when soil disturbance is minimized. The objective of our study was to test the C saturation concept in a temperate agroecosystem where there is gradient of C input to soil. We tested the C saturation concept in a long-term corn cultivation experiment established in Lexington, Kentucky. Because this site has been maintained for 35 years, we assumed that soil C content approximates equilibrium. This experiment is a randomized complete block design, and the treatment factors are tillage (no tillage and conventional tillage) and N fertilization. Nitrogen fertilizer was applied at the rate of 0 kg N ha-1, 84 kg N ha-1, 168 kg N ha-1, and 336 kg N ha-1. In this experiment, corn productivity increased with higher fertilization rate under both tillage systems, producing a range of C inputs over which to examine the C saturation concept. If C sequestration is governed by the saturation processes, C content in whole-soil and each soil fraction would reach an asymptote as C input increases. We used soil size fractionation through wet-sieving and C analysis to test our hypothesis. Analysis of the relationship between soil C input and soil C content showed that as cumulative C input to soil increases, the C content in whole-soil, macroaggregates and microaggregates increases. Because both linear and curvilinear regression models explained the relationships between the soil C input and C content, it remains to be determined whether C sequestration dynamics are influenced by the saturation processes in these soil fractions. However, regression analysis showed that the C content in silt + clay fraction does not increase (soil C content (g C/ kg soil fraction) = 0.01(cumulative C input (Mg C ha-1) + 7.6, r2 = 0.15) even when cumulative C input increased from 75 to 175 Mg C ha-1, indicating that C may be saturated in this soil pool. Therefore, it is likely that C saturation determined by the physicochemical properties of soil is more obvious in soil pools with slower C turnover rates.