Recovery and Relative Influence of Root, Microbial, and Structural Properties of Soil on Physically Sequestered Carbon Stocks in Restored Grassland

Abstract

Core Ideas We found that cultivated and never‐cultivated grassland soils had 12 and 59% of the total C pool sequestered in the micro‐within‐macro aggregate fraction, respectively. The micro‐within‐macro aggregate C increased at a rate of 27 g C m–2 yr–1 across the restoration chronosequence; 50% of total C was in this physically protected fraction after 35 yr of restoration. This is the first study to use structural equation modeling to explain soil C stocks in the micro‐within‐macro aggregate fraction. The model explains 98% of the variance in sequestered soil C, with microbial composition representing the strongest causal influence, followed by soil structure. Managing soil to sequester C can help mitigate increasing CO2 in the atmosphere. To maximize this ecosystem service, more knowledge of factors influencing C sequestration is needed. The objectives of this study were to (i) quantify recovery of the roots, microbial biomass and composition, and soil structure across a chronosequence of grassland restorations and (ii) use a structural equation model to develop a data‐based hypothesis on the relative influence of physical and biological soil properties on the soil C aggregate fraction diagnostic of sequestered C. Belowground plant biomass and tissue quality (C/N ratio), soil microbial biomass C, phospholipid fatty acid (PLFA) concentrations, soil structure, and soil C stocks in the bulk soil and each aggregate fraction were quantified from a cultivated field, prairies restored for 1 to 35‐yr (n = 6), and a never‐cultivated (native) prairie. Root biomass, microbial biomass C, arbuscular mycorrhizal fungi (AMF) PLFA biomass across the chronosequence increase to resemble native prairie following 35 yr of restoration. Many aspects of soil structure (i.e., bulk density, proportional mass of aggregate fractions, and aggregate mean weighted diameter) and the distribution C among soil fractions, including C in the micro‐within‐macro aggregate fraction (sequestered C), also became representative of native prairie within 35 yr of restoration. Total soil C stock and physically protected C increased at a similar rate (23 and 27 g C m‐2 yr‐1) respectively, across the chronosequence. After 35 yr of restoration, 50% of the total C pool was physically protected. The structural equation modeling developed by these data hypothesizes that microbial biomass C and AMF biomass (microbial composition) have the strongest causal influence on physically protected C. This model needs to be tested using independent sites to achieve greater inference.