Soil respiration is not limited by reductions in microbial biomass during long-term soil incubations

Abstract

Declining rates of soil respiration are reliably observed during long-term laboratory incubations. However, the cause of this decline is uncertain. We explored different controls on soil respiration to elucidate the drivers of respiration rate declines during long-term soil incubations. Following a long-term (707 day) incubation (30°C) of soils from two sites (a cultivated and a forested plot at Kellogg Biological Station, Hickory Corners, MI USA), soils were significantly depleted of both soil carbon and microbial biomass. To test the ability of these carbon-and biomass-depleted ("incubationdepleted") soils to respire labile organic matter, we exposed soils to a second, 42 day incubation (30°C) with and without an addition of plant residues. We controlled for soil carbon and microbial biomass depletion by incubating field fresh ("fresh") soils with and without an amendment of wheat and corn residues. Although respiration was consistently higher in the fresh versus incubation-depleted soil (2 and 1.2 times higher in the fresh cultivated and fresh forested soil, respectively), the ability to respire substrate did not differ between the fresh and incubation-depleted soils. Further, at the completion of the 42 day incubation, levels of microbial biomass in the incubation-depleted soils remained unchanged, while levels of microbial biomass in the field-fresh soil declined to levels similar to that of the incubation-depleted soils. Extra-cellular enzyme pools in the incubation-depleted soils were sometimes slightly reduced and did not respond to addition of labile substrate and did not limit soil respiration. Our results support the idea that available soil organic matter, rather than a lack microbial biomass and extracellular enzymes, limits soil respiration over the course of long-term incubations. That decomposition of both wheat and corn straw residues did not change after major changes in the soil biomass during extended incubation supports the omission of biomass values from biogeochemical models.

Global pattern and controls of soil microbial metabolic quotient

“…, Birge et al. ). Basal respiration was measured without substrate addition, because substrate addition enhances microbial activity, but not necessarily microbial biomass, thereby altering MMQ (Lu et al.…”

“…S1). Many studies set 35-42 d as the threshold for short-term vs. long-term incubations when studying labile carbon limitation to microbial respiration (Shi et al 2006, Birge et al 2015. Basal respiration was measured without substrate addition, because substrate addition enhances microbial activity, but not necessarily microbial biomass, thereby altering MMQ (Lu et al 2014).…”

Carbon control on terrestrial ecosystem function across contrasting site productivities: the carbon connection revisited

“…Long-term laboratory incubations are able to detect changes to the soil microbial community composition and function in response to decreased SOC quality and quantity over time, while controlling for multiple physiochemical conditions. After a 707-d incubation, respiration and microbial biomass, but not extracellular enzyme activity, declined in concert with a 20% decrease in organic C (Birge et al 2015). During an 18-month incubation, Frosteg ard et al (1996) detected a decrease in the fungal : bacterial ratio, which they attributed to a loss of ectomycorrhizal fungi.…”

“…After a 707‐d incubation, respiration and microbial biomass, but not extracellular enzyme activity, declined in concert with a 20% decrease in organic C (Birge et al. ). During an 18‐month incubation, Frostegård et al.…”

“…Soil microbial functions are also directly controlled by SOC availability. For instance, rates of microbial respiration are often related to SOC availability (Birge et al 2015). As such, microbial respiration rates are strongly related to net primary productivity (NPP; Raich and Schlesinger 1992), and experimentally reduced C inputs have been shown to decrease microbial respiration (Bowden et al 1993, Rey et al 2002, Li et al 2004, Wang et al 2013).…”