Researchers from the Institute of Applied Ecology of the Chinese Academy of Sciences have found that soil microorganisms living in different-sized soil aggregates adopt contrasting strategies to cope with increasing nitrogen inputs, shedding new light on how microbial communities regulate carbon and nitrogen cycling in grassland ecosystems.

Their findings were published in Catena.

Atmospheric nitrogen deposition, largely driven by fossil fuel combustion and agricultural activities, has increased substantially in recent decades and has become an important component of global environmental change. Additional nitrogen can alter the balance between carbon and nitrogen resources in soils, affecting the activity of microorganisms that play a central role in decomposing organic matter and recycling nutrients. Microbial carbon use efficiency (CUE) and nitrogen use efficiency (NUE), which describe the proportion of absorbed carbon and nitrogen that microbes allocate to growth rather than loss, are considered key indicators of these ecological processes. However, soils are composed of aggregates of different sizes that create distinct microhabitats, and whether microbial communities within these aggregates respond similarly to nitrogen enrichment remains unclear.

To address this question, Dr. WANG Zhirui and LI Hui collaborated with researchers from Yangzhou University and Hebei University. They analyzed soils from a long-term nitrogen addition experiment conducted at the Erguna Forest-Steppe Ecotone Ecosystem Research Station.

The researchers found that microbial communities inhabiting different soil aggregates adopted contrasting adaptive strategies when nitrogen addition altered the balance between available carbon and nitrogen resources. In macroaggregates, which are soil particles larger than 250 μm and generally contain relatively fresh organic matter, microorganisms responded to increasing carbon limitation by raising their CUE while lowering their NUE. This adjustment allowed the microbial community to maintain a relatively constant biomass stoichiometry, a phenomenon known as stoichiometric homeostasis.

By contrast, microorganisms living in microaggregates smaller than 250 μm displayed a non-homeostatic strategy. Instead of changing their CUE and NUE, these microbial communities adapted to nitrogen-rich conditions by altering their composition and storing excess nitrogen in their biomass.

The researchers also found that microbes in microaggregates generally had a higher CUE than those in larger aggregates, reflecting their long-term adaptation to environments where available substrates are limited.

These results imply that soil microorganisms' response to nitrogen enrichment hinges heavily on the soil's physical structure. Adaptive strategies based on maintaining internal chemical balance or flexibly adjusting microbial composition can influence the movement of carbon and nitrogen through terrestrial ecosystems.