[Activity and Stoichiometry of Soil Enzymes in Aggregates of Different Soil Types in Subtropical Forests].

Abstract

To reveal the differences in soil microbial nutrient limitation and nutrient availability at the aggregate scale of soil types in a subtropical forest in China, the O/A horizon （0-3 cm and 0-7 cm, respectively） and AB horizon （below O/A horizon to 20 cm depth） of typical acidic red soil and neutral limestone soil in Northern Guangxi were studied. The activities of carbon-acquiring enzymes （sucrase, amylase, β-1,4-glucosidase （BG））, nitrogen-acquiring enzymes （urease, β-1,4-N-acetyl-glucosaminidase （NAG）, leucine amino peptidase （LAP））, and phosphorus-acquiring enzymes （phytase and acid phosphatase （AP）） in different aggregate sizes were determined to reveal the variation in soil enzyme stoichiometry characteristics. The results showed that the limestone soil had higher nitrogen-acquiring enzyme activity compared to that in the red soil, with urease, NAG, and LAP activities being 38.84%, 123.89%, and 4.06% higher, respectively. The differences in carbon- and phosphorus-acquiring enzyme activities between the two soils were not significant. The overall soil enzyme activities in the O/A horizon were higher than in the AB horizon for both red soil and limestone soil. Soil total organic carbon and pH were identified as key factors influencing soil enzyme activities. Most enzyme activities （BG, urease, NAG, and phytase） were higher in micro-aggregates （0.1-0.25 mm and <0.1 mm）, emphasizing the role of micro-aggregates in promoting organic matter decomposition and accelerating soil nutrient cycling. Compared to those in red soil, limestone soil exhibited greater differences in NAG and phytase activities among aggregates. The mean values of ln（BG）∶ln（NAG+LAP）∶ln（AP） of red soil and limestone soil as well as their aggregates were 1.02∶1∶1.04 and 0.95∶1∶0.93, respectively, generally conforming to the global average ratio of 1∶1∶1. The ln（BG）∶ln（NAG+LAP） and vector lengths were higher in red soil than in limestone soil, indicating stronger carbon limitation in the red soil. The ln（NAG+LAP）∶ln（AP） ratio was lower in red soil than in limestone soil, with the former having a larger vector angle, suggesting stronger phosphorus limitation. Soil pH and total phosphorus were identified as the primary influencing factors of enzyme activity stoichiometry characteristics, suggesting that the bioavailability of organic matter and phosphorus in acidic soil was lower than in limestone soil, prompting microbes to secrete carbon and phosphorus enzymes to enhance acquisition efficiency. The high pH of limestone soil may lead to nitrogen limitation due to nitrate leaching caused by higher mineralization and nitrification rates. Microbes in the large aggregates （1-2 mm and 0.5-1 mm） of both soils experienced stronger phosphorus limitation, whereas carbon limitation was stronger in micro-aggregates, especially in the 0.1-0.25 mm aggregates. The microbial nutrient pattern in limestone soil aggregates shifted from phosphorus limitation to nitrogen limitation from large to micro-aggregates. This study reveals the influence of soil type and aggregate on the soil microbial nutrient limitation patterns of soil microbial elements in subtropical forest soils of China.