Controls on Soil Organic Matter Content and Stable Isotope Signatures in a New Mexico Lithosequence

Abstract

Arid and semiarid rangelands currently occupy approximately 30% of the earth’s surface and contain a wide variety of soil types and vegetation. The purpose of our study was to investigate the C and N dynamics of a soil lithosequence with parent material ranging from schist to limestone to shale in central New Mexico. Although the soil forming above the shale contained comparatively higher combined amounts of silt and clay (42.2–60.5%) through most of the full depth of the profile than the limestone (26.0–74.9%) and schist (20.7–41.1%), it had the lowest contents of soil organic C and soil organic N of the three soils (2.2–8.4% and 0.2–0.6%, respectively). This may be the result of higher rates of organic matter removal caused by increased runoff because of the higher potential for surface sealing of these soils. Vertical profiles of both δC and δN values in the soils revealed an increasing trend in both the schist (ΔδC, +3.46; ΔδN, +3.33)- and limestone (ΔδC, +1.67; ΔδN, +1.55)-dominated soils from top to bottom but showed a maximum increase of only 0.48‰ for δC and an increase of 1.01‰ for δN with depth in the shale-dominated soil. In addition, there is a positive correlation between percent clay and percent C in the schist-dominated soil and a negative correlation in the shale-dominated soil. This study suggests that while the increasing percentages of small particle sizes may play a role in restricting decomposition in this environment, the soils above the three different parent materials here did not behave in the same manner.