Top-down effects on biological soil crust function

Biological soil crusts (BSCs) are communities of microorganisms that control ecosystem functions in drylands. Despite their importance, little is known about how trophic interactions affect BSC function. We conducted a series of mechanistic experiments to tease out the direct (i.e., consumption) and indirect (i.e., fecal and mucus deposition) pathways by which crustivore (i.e., consume BSCs) and detritivores affect BSC functions— complemented by a manipulative field experiment exploring the integrative effect of these pathways. We showed that detritivore feces, mucus, and grazing increased the BSCs CO2 efflux. Detritivore feces also increased BSC %N. Crustivorous snail feces increased BSC CO2 efflux, and their mucus decreased BSC %C and %N. In the field, both snail species increased BSC %C and did not affect BSC CO2 efflux. Combined, our findings highlight that macro-invertebrate consumers exert top-down regulation on BSC function, opening the door for a whole new avenue of trophic rese...生物土壤结皮（Biological soil crusts, BSCs）是调控干旱区生态系统功能的微生物群落。尽管其生态重要性不言而喻，但学界对营养级互作如何影响生物土壤结皮功能的认知仍较为匮乏。本研究开展了一系列机理实验，以厘清结皮取食者（crustivore，即取食生物土壤结皮的生物）与食腐动物通过直接（即取食）和间接（即粪便与黏液沉积）途径对生物土壤结皮功能产生的作用，并辅以一项控制性野外实验，探究上述两类途径的综合效应。研究结果显示：食腐动物的粪便、黏液以及牧食行为均可提升生物土壤结皮的CO₂排放通量；食腐动物粪便还可提高结皮的氮含量百分比。结皮取食性蜗牛的粪便可提升结皮CO₂排放通量，而其黏液则会降低结皮的碳含量百分比与氮含量百分比。在野外环境中，两种蜗牛均能提升结皮的碳含量百分比，且未对结皮CO₂排放通量产生影响。综合来看，本研究结果表明大型无脊椎动物消费者可对生物土壤结皮功能实施下行调控，为营养生态学研究的全新方向打开了大门。