Six years of in situ CO₂ enrichment evoke changes in soil structure and soil biota of nutrient‐poor grassland

Abstract

Nutrient‐poor grassland on a silty clay loam overlying calcareous debris was exposed to elevated CO2 for six growing seasons. The CO2 exchange and productivity were persistently increased throughout the experiment, suggesting increases in soil C inputs. At the same time, elevated CO2 lead to increased soil moisture due to reduced evapotransporation. Measurements related to soil microflora did not indicate increased soil C fluxes under elevated CO2. Microbial biomass, soil basal respiration, and the metabolic quotient for CO2 (qCO2) were not altered significantly. PLFA analysis indicated no significant shift in the ratio of fungi to bacteria. 0.5 m KCl extractable organic C and N, indicators of changed DOC and DON concentrations, also remained unaltered. Microbial grazer populations (protozoa, bacterivorous and fungivorous nematodes, acari and collembola) and root feeding nematodes were not affected by elevated CO2. However, total nematode numbers averaged slightly lower under elevated CO2 (−16%, ns) and nematode mass was significantly reduced (−43%, P = 0.06). This reduction reflected a reduction in large‐diameter nematodes classified as omnivorous and predacious. Elevated CO2 resulted in a shift towards smaller aggregate sizes at both micro‐ and macro‐aggregate scales; this was caused by higher soil moisture under elevated CO2. Reduced aggregate sizes result in reduced pore neck diameters. Locomotion of large‐diameter nematodes depends on the presence of large enough pores; the reduction in aggregate sizes under elevated CO2 may therefore account for the decrease in large nematodes. These animals are relatively high up the soil food web; this decline could therefore trigger top‐down effects on the soil food web. The CO2 enrichment also affected the nitrogen cycle. The N stocks in living plants and surface litter increased at elevated CO2, but N in soil organic matter and microbes remained unaltered. Nitrogen mineralization increased markedly, but microbial N did not differ between CO2 treatments, indicating that net N immobilization rates were unaltered. In summary, this study did not provide evidence that soils and soil microbial communities are affected by increased soil C inputs under elevated CO2. On the contrary, available data (13C tracer data, minirhizotron observations, root ingrowth cores) suggests that soil C inputs did not increase substantially. However, we provide first evidence that elevated CO2 can reduce soil aggregation at the scale from µm to mm scale, and that this can affect soil microfaunal populations.

Elevated CO2 and Tree Species Affect Microbial Activity and Associated Aggregate Stability in Soil Amended with Litter

“…There are also some inconsistencies in reported responses of soil microbiota to varying CO 2 regimes. Thus, eCO 2 over six growing seasons did not significantly alter soil microbial biomass carbon, metabolic quotient and basal respiration in nutrient-poor grassland [11]; these findings were attributed to nitrogen limitations. However, eCO 2 over five years did not affect microbial biomass, even where nitrogen fertilizer was added [12], suggesting that microbial biomass is insensitive to changes in atmospheric CO 2 .…”

“…Litter quality effects on MBC in casts may vary with time; thus, high quality litter (lucerne) gave higher MBC at 14 and 28 days compared to wheat straw, but these amendments did not differ at 7 and 56 days [35]; data here for very recent casts showed higher respiration with better quality litter but in later soil aggregates these differences were inconsistent. Given the effects of eCO2 on N concentrations, a reduced microbial community might have been expected; other studies [11] have attributed the absence of any eCO2 effect on MBC in grassland soils to N limitation. If N is a key determinant of MBC responses to eCO2 [14], in our study, some variation between species in microbial eCO2 responses might have been expected given their very different litter N contents; only species' differences were observed and then only in casts.…”

Relationship between plant species diversity and soil microbial functional diversity along a longitudinal gradient in temperate grasslands of Hulunbeir, Inner Mongolia, China

“…Carney and Matson (2005) found that plant diversity had a significant effect on microbial community composition through alterations in microbial abundance rather than community composition. Other studies also reported that aboveground net primary productivity was expected to increase soil carbon input through enhancing the turnover of plant biomass and enhancing root exudation and may therefore influence carbon-limited microbial communities in the soil (Niklaus et al 2003;Zak et al 2003).…”

Temporal variation overshadows the response of leaf litter microbial communities to simulated global change

“…Second, the response of microbial communities to environmental change will depend on the time of year. Seasonal variation in the traits of the microbial community could alter its response to environmental change (Bardgett et al, 1999;Niklaus et al, 2003;Cregger et al, 2012). Third, season-dependent responses will be particularly strong when the environmental parameter also varies seasonally.…”