Soil carbon responses to past and future CO2 in three Texas prairie soils

Abstract

Changes in soil carbon storage could affect and be affected by rising atmospheric CO 2 . However, it is unlikely that soils will respond uniformly, as some soils are more sensitive to changes in the amount and chemistry of plant tissue inputs whereas others are less sensitive because of mineralogical, textural, or microbial processes. We studied soil carbon and microbial responses to a preindustrial-to-future CO 2 gradient (250-500 ppm) in a grassland ecosystem in the field. The ecosystem contains three soil types with clay fractions of 15% to 55%: a sandy loam Alfisol, a silty clay Mollisol, and a black clay Vertisol. Soil and microbial responses to atmospheric CO 2 are plant-mediated; and aboveground plant productivity in this ecosystem increased linearly with CO 2 in the sandy loam and silty clay. Although total soil organic carbon (SOC) did not change with CO 2 treatment after four growing seasons, fast-cycling SOC pools increased with CO 2 in the two clay soils. Microbial biomass increased 18% and microbial activity increased 30% across the CO 2 gradient in the black clay (55% clay), but neither factor changed with CO 2 in the sandy loam (15% clay). Similarly, size fractionation of SOC showed that coarse POM-C, the youngest and most labile fraction, increased four-fold across the CO 2 gradient in the black clay, but increased by only 50% across the gradient in the sandy loam. Interestingly, mineral-associated C, the oldest and most recalcitrant fraction, declined 23% across the gradient in the third soil type, a silty clay (45% clay). Our results provide evidence for priming in this soil type, as labile C availability and decomposition rate (measured as soil respiration and soil C mineralization) also increased across the CO 2 gradient in the silty clay soil.In summary, CO 2 enrichment in this grassland increased the fast-cycling SOC pool as in other CO 2 studies, but only in the two high-clay soils. Priming in the silty clay could limit SOC accumulation after prolonged CO 2 exposure. Because soil texture varies geographically, including data on soil types could enhance predictions of soil carbon and microbial responses to future CO 2 levels.

Enhanced priming of old, not new soil carbon at elevated atmospheric CO2

“…We can therefore add evidence to support previous statements that elevated CO 2 induces decomposition of older soil C ( van Groenigen et al, 2005;Xie et al, 2005;Niklaus and Falloon, 2006). Likewise, elevated CO 2 enhanced the formation of coarse particulate SOM (fresh SOM) and decreased the fraction of physically protected SOM (old SOM) in forest soil (Hofmockel et al, 2011) and in prairie soil (Procter et al, 2015). Given that old SOM pools contain significant, yet (to a large extend) physically and chemically protected N stocks, this lends support to the hypothesis that priming in response to labile C supply is a mechanism by which (some) microorganisms gain access to a reservoir of N to meet their enhanced N demand under conditions of ample C supply (Dijkstra et al, 2013;Chen et al, 2014).…”

Faster turnover of new soil carbon inputs under increased atmospheric CO2

“…Several factors have been suggested to affect the response of plant growth and soil C dynamics to CO 2 enrichment: (i) type of vegetation (Ainsworth & Long, ), (ii) the CO 2 fumigation technology used (De Graaff, Van Groenigen, Six, Hungate, & van Kessel, ), (iii) experiment duration (Norby Warren, Iversen, Medlyn & McMurtrie ), (iv) soil texture (Procter, Gill, Fay, Polley, & Jackson, ), (v) age of the vegetation (Körner et al., ), and (vi) N availability (Van Groenigen et al., ). To test whether these factors affected CO 2 responses, we categorized each study based on plant type (that is, woody vs. herb), experimental facility (greenhouse, GH, and growth chamber, GC vs. open‐top chamber, OTC, and free air CO 2 enrichment, FACE), and study duration (<1 year vs. 1–4 years).…”

Bacterial community response to a preindustrial‐to‐future CO2 gradient is limited and soil specific in Texas Prairie grassland

“…Members of Micromonosporaceae have previously been linked to enhanced cellulose degrading capability (Yeager et al, ). Gaiellaceae are known to be chemoorganotrophic (Albuquerque et al, ), possibly favored by environments rich with organic C. The increase in relative abundance of Micromonosporaceae and Gaiellaceae could be attributed to higher labile C availability with increasing CO 2 concentration in clay soils as shown in a previous LYCOG study (Procter et al, ). We did not measure microbial biomass, but results from the fourth year of CO 2 treatment at LYCOG showed that active soil microbial biomass increased most with CO 2 in the clay soils (Procter et al, ).…”

“…We hypothesized that CO 2 , soil type, season, and their interactive effects would determine bacterial community structure and taxonomic composition. We also expected increased bacterial diversity due to increase in labile C availability at higher CO 2 concentrations, specifically in clay soils (Procter, Gill, Fay, Polley, & Jackson, ). Contrary to our expectations, CO 2 gradient had no significant effect on taxonomic and phylogenetic diversity.…”