Microbial Resource Limitation in Aggregates in Karst and Non-Karst Soils

Zhenke Zhu,Yunqiu Wang,Tida Ge,Yuhong Li,Anlei Chen,Muhammad Shahbaz,Yehia Galal Mohamed Galal,Mostafa Zhran

doi:10.3390/agronomy11081591

Abstract

Karst is a widespread ecosystem with properties that affect the microbial activity and storage and cycling of soil organic carbon. The mechanisms underlying microbial resource availability in karst, which limit the microbial growth and activity in soil aggregates, remain largely unknown. We assessed the microbial resource limitations using exoenzymatic stoichiometry and key extracellular enzyme activities in bulk soil and aggregates in karst and non-karst forest soils. Soil organic carbon, total nitrogen, and microbial biomass carbon and nitrogen were significantly higher in bulk soil and the aggregate fractions in karst forests. However, the microbial biomass accumulation was higher in finer aggregates than in macroaggregate fractions. This may be attributed to the surface area of finer aggregates that increase the microbial C accumulation. In karst forests, the activity of extracellular enzymes β-d-glucosidase, β-N-acetylglucosaminidase, α-glucosidase, and α-d-1,4-cellobiosidase was two to three times higher in microaggregates (0.053–0.25 mm) and mineral fractions (<0.053 mm) than in macroaggregates. This coincided with the distribution of microbial biomass carbon and phosphorus in finer aggregate fractions. The microorganisms in bulk soil and aggregates in karst forests were largely co-limited by carbon and phosphorus and rarely by nitrogen and only by phosphorus in non-karst soils. The microbial phosphorus limitation in non-karst soils was alleviated in finer soil aggregates, while these fractions reflected slightly higher. microbial C limitations than bulk and other aggregates in karst forests. The patterns of microbial resource limitations in the bulk and aggregate fractions in karst ecosystems reflected the regulation of enzyme activity and soil organic carbon accumulation in finer aggregate fractions but not in other aggregates.

Highlights

IntroductionKarst ecosystems cover a considerable amount of the world’s land area and are significant reservoirs for soil inorganic and organic carbon (SOC) [1,2,3,4,5]
We studied the bulk soil and aggregate fractions in a typical karst ecosystem in one of the largest karst areas in the world located in Southwest China
The soil inorganic and organic carbon (SOC), MBC, acid phosphatase (AP), and MBP were significantly higher in the bulk soils and all soil aggregate fractions in karst forests

Summary

Introduction

Karst ecosystems cover a considerable amount of the world’s land area and are significant reservoirs for soil inorganic and organic carbon (SOC) [1,2,3,4,5]. SOC is concentrated in soils under changing global environmental conditions, which become more vulnerable to microbial decomposition, leading to flux emissions of CO2 [2] and further impacting the climate change. Information related to the mechanism for microbial SOC decomposition in karst soils is lacking. Soil microorganisms regulate the nutrient cycle (C, N, and P); playing crucial roles in nutrient availability; and immobilization [6,7,8,9,10]. Microorganisms in the soil function optimally when the ratio of essential elements, such as N or P, relative to C

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