Controls on microbial CO₂ production: a comparison of surface and subsurface soil horizons

Abstract

Although a significant amount of the organic C stored in soil resides in subsurface horizons, the dynamics of subsurface C stores are not well understood. The objective of this study was to determine if changes in soil moisture, temperature, and nutrient levels have similar effects on the mineralization of surface (0–25 cm) and subsurface (below 25 cm) C stores. Samples were collected from a 2 m deep unsaturated mollisol profile located near Santa Barbara, CA, USA. In a series of experiments, we measured the influence of nutrient additions (N and P), soil temperature (10–35°C), and soil water potential (−0.5 to −10 MPa) on the microbial mineralization of native soil organic C. Surface and subsurface soils were slightly different with respect to the effects of water potential on microbial CO2 production; C mineralization rates in surface soils were more affected by conditions of moderate drought than rates in subsurface soils. With respect to the effects of soil temperature and nutrient levels on C mineralization rates, subsurface horizons were significantly more sensitive to increases in temperature or nutrient availability than surface horizons. The mean Q10 value for C mineralization rates was 3.0 in surface horizons and 3.9 in subsurface horizons. The addition of either N or P had negligible effects on microbial CO2 production in surface soil layers; in the subsurface horizons, the addition of either N or P increased CO2 production by up to 450% relative to the control. The results of these experiments suggest that alterations of the soil environment may have different effects on CO2 production through the profile and that the mineralization of subsurface C stores may be particularly susceptible to increases in temperature or nutrient inputs to soil.

Factors influencing mineralizable carbon in a landscape with variable topography

“…This ratio did not vary significantly with depth (e.g., the average for 0-15 cm depth was 3.9 and for > 15 cm depth was 4.1) and was not correlated with total whole soil OC content or the OC content of either fraction. Although Fierer et al (2003) found that surface soil exhibited greater sensitivity of mineralization to drought (potentials of -1 and -5 MPa), their results may not be comparable with the results obtained in this study for several reasons. The corresponding water contents were much drier than those used in this study.…”

“…Although soil temperature and water content change with depth, there is little information on whether the sensitivity of mineralization rates to these variables changes with depth in the profile. Fierer et al (2003) observed that the relation between OC mineralization rates and water potential did not change appreciably with soil depth, although mineralization in surface soils (0-25 cm) was more affected by drought (water potentials of -1.5 to -5.0 MPa) than deeper soils. Mineralization rates at lower depths (25-200 cm) were particularly susceptible to increases in temperature or nutrient availability, possibly due to differences in OC chemical structure and the composition of microbial communities through the profile.…”

“…For instance, soil OC buried in depressional sites below the plough depth may be physically or chemically protected, and not readily mineralized because of decreased aeration and lower temperatures (Lomander et al 1998;Stallard 1998;Fierer et al 2003;Lal 2005).…”

“…Whereas some studies suggest that erosion results in a net flux of C to the atmosphere due to increased rates of mineralization, other studies suggest that erosion is favourable for C sequestration in depositional landscape positions (Polyakov and Lal 2004;Lal 2005). For instance, soil OC buried in depressional sites below the plough depth may be physically or chemically protected, and not readily mineralized because of decreased aeration and lower temperatures (Lomander et al 1998;Stallard 1998;Fierer et al 2003;Lal 2005).…”

Persistence of soil organic matter as an ecosystem property

“…Microbial activity may be reduced by suboptimal environmental conditions, nutrient limitation or energy scarcity, and organic matter may be less accessible because of its sparse density or association with reactive mineral surfaces. Microbial biomass decreases with soil depth 58 , and community composition changes to reflect an increase in substrate specialization 59 . Recent studies suggest that energy limitation, or the converse-'priming' (see below) by root exudates or dissolved organic carbon-is an important factor in the subsurface 48,49 .…”