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Advances in the contribution of microbial necromass to biological soil crust organic carbon accumulation from a research group led by Prof. Shaoshan An

Update time:2022-06-20

Recently, members of Shaoshan An group, together with researchers from the University of Goettingen and the Institute of Earth Environment, Chinese Academy of Sciences published a paper in Soil Biology and Biochemistry entitled "Initial soil formation by biocrusts: nitrogen demand and clay protection control microbial necromass accrual and recycling". Dr. Baorong Wang is the first author of the paper, prof. Shaoshan An is the corresponding author, and the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau is the first affiliation. This research work was funded by the National Natural Science Foundation of China (41877074 and 42077072).

Biological soil crusts (hereafter referred to as “biocrusts”) cover approximately 12% of Earth’s terrestrial surfac, biocrusts accelerate C and nutrient biogeochemical cycling in arid and semiarid region. Microbial necromass is increasingly considered to be the main source of organic carbon (C) sequestration in soils. Quantitative information on the contribution of microbial necromass to soil organic carbon (SOC) formation and the factors driving necromass accumulation, decomposition and stabilization during the initial soil formation in biological crusts (biocrusts) remain unavailable. This greatly limits our understanding of microbially mediated carbon cycling processes in desert ecosystems.

To address this knowledge gap, we investigated the composition of microbial necromass and its contributions to SOC sequestration in a biocrust formation sequence consisting of five stages: bare sand, cyanobacteria stage, cyanobacteria-moss stage, moss-cyanobacteria stage, and moss stage on sandy parent material on the Loess Plateau. Microbial necromass was an important source of SOC (Fig. 1), and was incorporated into the particulate and mineral-associated organic C (MAOC). Because bacteria have smaller and thinner cell wall fragments as well as more proteins than fungi, bacterial necromass mainly contributed to the MAOC pool, while fungal residues remained more abundant in the particulate organic C (POC). MAOC pool was saturated fast with the increase of microbial necromass, and POC more rapid accumulation than MAOC suggests that the clay content was the limiting factor for stable C accumulation in this sandy soil. The necromass exceeding the MAOC stabilization level was stored in the labile POC pool (especially necromass from fungi) (Fig. 2).

Fig.1 Dynamics of fungal necromass carbon (C) and bacterial necromass C, fungal:bacterial necromass ratio, microbial necromass C contribution to soil organic C, fungal and bacterial necromass C contribution to soil organic C, fungal necromass to total microbial necromass C ratio, necromass accumulation coefficient (microbial necromass C/living microbial biomass C: necromass accumulation per unit of microbial biomass C) in biological soil crusts (BSC) horizon and in the 0–2 cm of BSC-underlying mineral soil during 5 stages of BSC formation: bare sand (~ 0–1 years old), cyanobacteria stage (Cy, ～ 3–7 years old), cyanobacteria-moss stage (Cy-Mo, ～ 8–13 years old), moss-cyanobacteria stage (Mo-Cy, ～ 20–25 years old), and moss stage (Mo, ～ 30 years old).

Fig. 2 Regressions of microbial necromass C with particulate (POC) and mineral-associated organic C (MAOC), soil organic C (SOC) with the POC/MAOC ratio, and soil clay and silt content with bacterial and fungal necromass C.

Activities of four enzymes (i.e., β-1,4-glucosidase, β-1,4-N-acetyl-glucosaminidase, leucine aminopeptidase, and alkaline phosphatase) increased with fungal and bacterial necromass, suggesting that the increasing activity of living microorganisms associated with accelerated turnover and formation of necromass. Microbial N limitation raised the production of N acquisition enzymes (e.g., β-1,4-N-acetyl-glucosaminidase and leucine aminopeptidase) to break down necromass compounds, leading to further increase of the nutrient pool in soil solution (Fig. 3). The decrease of microbial N limitation along the biocrusts formation chronosequence is an important factor for the necromass accumulation during initial soil development. High microbial N demands and insufficient clay protection lead to fast necromass reutilization by microorganisms and thus, result in a low necromass accumulation coefficient, that is, the ratio of microbial necromass to living microbial biomass (on average, 9.6). Consequently, microbial necromass contribution to SOC during initial soil formation by biocrust is lower (12-25%) than in fully developed soil (33%-60%, literature data). Nitrogen (N) limitation of microorganisms and an increased ratio between N-acquiring enzyme activity and microbial N, as well as limited clay protection, resulted in a low contribution of microbial necromass to SOC by initial formation of biocrust-covered sandy soil. Summarizing, soil development leads not only to SOC accumulation, but also to increased contribution of microbial necromass to SOC, whereas the plant biomass contribution decreases (Fig. 4).

Fig. 3 Redundancy analysis (RDA) identifies the relationships between microbial biomass, extracellular enzyme activities, available nutrients, particulate organic C, mineral-associated organic C, clay content, and microbial necromass C.

Fig. 4 Conceptual framework for the preservation, decomposition and stabilization of microbial necromass in biological crust dominated sandy soils.

Paper link：https://www.sciencedirect.com/science/article/pii/S0038071722000645