Soil organic matter diagenetic state informs boreal forest ecosystemfeedbacks to climate change

The fate of soil organic carbon (SOC) in boreal forestsis dependent on the integrative ecosystem response to climate change. Forexample, boreal forest productivity is often nitrogen (N) limited, andclimate warming can enhance N cycling and primary productivity. However, thenet effect of this feedback on the SOC reservoir and its longevity withclimate change remain unclear due to difficulty in detecting smalldifferences between large and variable carbon (C) fluxes needed to determinenet changes in soil reservoirs. The diagenetic state of SOC - resultingfrom the physicochemical and biological transformations that alter theoriginal biomolecular composition of detrital inputs to soil over time - isuseful for tracing the net response of SOC at the timescales relevant toclimate change not usually discernible from fluxes and stocks alone. Here,we test two hypotheses using a mesic boreal forest climate transect: (1) the SOCdiagenetic state is maintained across this climosequence, and (2) themaintenance of the SOC diagenetic state is a consequence of coupled soil C and Ncycling, signifying the role of enhanced N cycling supporting SOC inputsthat maintain SOC stocks within the warmer-climate forests. Shifts innonvascular to vascular plant inputs with climate observed in these andother boreal forests highlighted the need to carefully separatebiogeochemical indicators of SOC source from those signifying diageneticalteration. We thus evaluated and applied lignin biomarkers to assess thediagenetic alteration of SOC in these boreal forest organic soils anddirectly compared the lignin diagenetic state with that of soil organicnitrogen (SON) assessed through amino acid composition. The lignindiagenetic state remained constant across the climate transect, indicating abalance between the input and removal of lignin in these mesic borealforests. When combined with previous knowledge of these forest ecosystems,including the diagenetic state of SON and direct measures of C fluxes andstocks, the results indicate a coupled increase in C and N cycling withclimate warming that supports forest productivity and maintains SOC stocks.This balance could markedly shift as other factors begin to limit forestproductivity (e.g., trace nutrients, water) with further climate change oraffect forest nutrient allocation (e.g., forest age or compositionalchange). Further application of the approach presented here could be used todetect the limits of this and other ecosystem-climate feedbacks, byproviding a tractable and parameterizable index of the lignin state across largespatial scales, necessary for ecosystem-scale parameterizations.

Automated summary

The fate of soil organic carbon in boreal forests depends on the ecosystem response to climate change. It is unclear how climate change affects the net effect of nitrogen cycling and primary productivity on soil organic carbon reservoirs. The diagenetic state of soil organic carbon is a useful indicator of the net response to climate change. A study conducted in boreal forests shows that the diagenetic state of soil organic carbon remains constant across different climate zones, suggesting the role of coupled soil carbon and nitrogen cycling in maintaining soil organic carbon stocks.