Historical climate controls soil respiration responses to current soil moisture

Abstract

Ecosystem carbon losses from soil microbial respiration are a key component of global carbon cycling, resulting in the transfer of 40-70 Pg carbon from soil to the atmosphere each year. Because these microbial processes can feed back to climate change, understanding respiration responses to environmental factors is necessary for improved projections. We focus on respiration responses to soil moisture, which remain unresolved in ecosystem models. A common assumption of large-scale models is that soil microorganisms respond to moisture in the same way, regardless of location or climate. Here, we show that soil respiration is constrained by historical climate. We find that historical rainfall controls both the moisture dependence and sensitivity of respiration. Moisture sensitivity, defined as the slope of respiration vs. moisture, increased fourfold across a 480-mm rainfall gradient, resulting in twofold greater carbon loss on average in historically wetter soils compared with historically drier soils. The respiration-moisture relationship was resistant to environmental change in field common gardens and field rainfall manipulations, supporting a persistent effect of historical climate on microbial respiration. Based on these results, predicting future carbon cycling with climate change will require an understanding of the spatial variation and temporal lags in microbial responses created by historical rainfall.

Keywords: climate change; legacies; microbial; precipitation.

Fig. 1.

NMS of 454 pyrosequencing data from the gradient sites captured 93% of the variation in bacteria community composition, with 72% in the first axis and 21% in the second axis. (A) Along NMS axis 1, historical rainfall (MAP) explained 66% of the variation in bacteria community composition (P = 0.001). (B) On NMS axis 2, microbial biomass C (MBC) accounted for 40% of the variation (P = 0.027). No other environmental variables contributed to the regression models.

Fig. 2.

Soil respiration by region of origin and soil moisture for (A) control and (B) litter addition treatments and (C) moisture sensitivity of soil respiration as a function of increasing MAP and litter addition in the laboratory incubation reciprocal moisture experiment. There were significant interactions of region by moisture (P < 0.001) and moisture by litter (P < 0.001) (A and B). Soils originating from wetter eastern regions respired significantly more than drier central or western regions except at 7% moisture, where western soils respired more than eastern. Respiration was elevated with litter, except at 7% moisture. Means ±1 SE (n = 8) are plotted, with values averaged over 12 dates. MAP explained 61% of the variation in moisture sensitivity (C) (P < 0.001).

Fig. 3.

Respiration rates for soils originating from historically wetter eastern sites and drier western sites incubated at high and low soil moisture after 18 mo transplanted in the (A) eastern and (B) western common gardens. Region of soil origin was the only significant factor affecting respiration (P = 0.004). Data are means ±1 SE (n = 18), with values averaged over four time points taken across 8 wk in the laboratory. Note that the y axes do not start at zero.

Fig. 4.

The moisture response of soil respiration rates in the field rainfall manipulation experiment was significantly affected by current soil moisture (P < 0.001). Field rain treatments, which were maintained at high (1,331 mm⋅y⁻¹) or low (326 mm⋅y⁻¹) levels, had no effect on respiration. Means ±1 SE (n = 8) are plotted with values averaged over 8 wk in laboratory microcosms.