Effects of precipitation changes on soil heterotrophic respiration and microbial activities in a switchgrass mesocosm experiment

Authors: DAI, WEI - Tennessee State University; LI, JIANWEI - Tennessee State University; PARAJULI, MADHAV - Tennessee State University; JIAN, SIYANG - University Of Oklahoma; HUI, DAFENG - Tennessee State University; FAY, PHILIP - Retired ARS Employee

Submitted to: European Journal of Soil Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Climate change is anticipated to increase regional variability in precipitation. The consequences of changing precipitation for natural and managed ecosystems remain uncertain, in part because of lack of information on the impacts on the accumulation and retention of soil carbon, which is linked to atmospheric CO2 concentration. This is an important question in cropping systems, which are depleted in soil carbon by might help buffer rising atmospheric CO2 concentration if practices can be devised to increase soil carbon storage. To address this, a three year study was conducted on the bioenergy crop switchgrass (Panicum virgatum L.) aimed at understanding the mechanisms by which soil microbes regulate gains and losses in soil carbon. Switchgrass established in large pots filled with soils collected from nearby switchgrass cropland were subjected to five levels of annual precipitation amounts; a control (P0) representing long-term average precipitation, two wetter amounts (33% and 50% higher than control) and two drier amounts (33% and 50% less than control). Each treatment was replicated five times. Soils were analyzed for carbon and nitrogen contents, biomass of soil microbes, rates of release of CO2 produced through decomposition, and the activities of various microbially-produced enzymes important in microbial processing of soil carbon. With increasing precipitation amount, soils were higher in soil moisture, more CO2 was released, and activities increased for certain enzymes associated with microbial carbon acquisition, but lower microbial growth efficiency. However, precipitation treatments had no effect on soil carbon or nitrogen contents or on activities of microbial enzymes associated with nitrogen acquisition. These findings demonstrate key points in the microbial processing of soil carbon that may impact the loss of soil CO2 from switchgrass cropland back to the atmosphere. This knowledge can help target processes in switchgrass cropalnds to decrease CO2 losses to help mitigate continuing atmospheric CO2 increases likely from increasing precipitation.

Technical Abstract: Precipitation regimes altered soil respiration but the underlying microbial mechanisms that likely mediate the effects remain rarely studied, particularly in a bioenergy cropland such as switchgrass (Panicum virgatum L.). In this study, a three-year long mesocosm experiment with five levels of precipitation amounts representing ambient precipitation as a control (P0), two wet conditions (P+33 and P+50: 33% and 50% enhancement relative to P0), and two drought conditions (P-33 and P-50: 33% and 50% reduction relative to P0) was conducted to investigate switchgrass soil respiratory responses to altered precipitation. Soil samples (0-15 cm) were collected and soil organic carbon (SOC), soil total nitrogen (TN), microbial biomass carbon (MBC), soil CO2 respiration rate (Rs), microbial biomass-specific respiration (Rss: respiration per unit of microbial biomass as a reciprocal index of microbial growth efficiency) and extracellular enzymes activities (EEAs) were quantified. Results showed that no impact of precipitation amount on SOC and TN contents despite significantly different soil moisture content among treatments. Relative to P0, P+33 significantly increased soil Rs by more than 3-fold and little changed MBC, leading to significantly higher Rss (P < 0.05). P+33 also significantly increased by 115.09% hydrolytic enzyme activities associated with labile carbon acquisition (Cacq). On the other hand, drought treatments (P-33 and P-50) caused insignificant changes in Rs, MBC, or Rss, and the only significant effect of drought treatments lied in decreased ß-D-cellobiosidase (CBH) and peroxidase (PEO) under P-33. The enzyme activities associated with nitrogen acquisition (Nacq) were insignificantly affected by either increased or decreased precipitation. Collectively, these results demonstrated that the enhanced precipitation amount stimulated soil respiratory carbon loss likely driven by both elevated hydrolase activities and reduced microbial growth efficiency (MGE). However, the less sensitive drought treatment effects suggested potential microbial tolerance to water deficiency in the switchgrass mesocosm experiment. This informed the key control of microbial function and physiology in mediating soil respiratory response to altered precipitation in the future.