Abstract
Rising carbon dioxide (CO2) concentrations and temperatures are expected to stimulate plant productivity and ecosystem C sequestration, but these effects require a concurrent increase in N availability for plants. Plants might indirectly promote N availability as they release organic C into the soil (e.g., by root exudation) that can increase microbial soil organic matter (SOM) decomposition (“priming effect”), and possibly the enzymatic breakdown of N-rich polymers, such as proteins, into bio-available units (“N mining”). We tested the adjustment of protein depolymerization to changing soil C and N availability in a laboratory experiment. We added easily available C or N sources to six boreal forest soils, and determined soil organic C mineralization, gross protein depolymerization and gross ammonification rates (using 15N pool dilution assays), and potential extracellular enzyme activities after 1 week of incubation. Added C sources were 13C-labelled to distinguish substrate from soil derived C mineralization. Observed effects reflect short-term adaptations of non-symbiotic soil microorganisms to increased C or N availability. Although C input promoted microbial growth and N demand, we did not find indicators of increased N mobilization from SOM polymers, given that none of the soils showed a significant increase in protein depolymerization, and only one soil showed a significant increase in N-targeting enzymes. Instead, our findings suggest that microorganisms immobilized the already available N more efficiently, as indicated by decreased ammonification and inorganic N concentrations. Likewise, although N input stimulated ammonification, we found no significant effect on protein depolymerization. Although our findings do not rule out in general that higher plant-soil C allocation can promote microbial N mining, they suggest that such an effect can be counteracted, at least in the short term, by increased microbial N immobilization, further aggravating plant N limitation.
Highlights
Plant primary production in temperate, boreal, and arctic ecosystems is often limited by low soil N availability (Vitousek and Howarth 1991)
Most soil N is contained within soil organic matter (SOM) polymers that are too large for immediate uptake by plants or microorganisms, and soil N availability depends on the breakdown of these polymers into smaller units (Schimel and Bennett 2004)
Principal Component 2 (PC2) separated Podzol mineral horizons from Podzol organic layers and Histosols irrespective of location, and this separation was driven by higher concentrations of dissolved organic C, total dissolved N, as well as activities of oxidative and N-acquiring hydrolytic enzymes (N-acetyl-b-Dglucosaminidase, leucine-aminopeptidase) in mineral horizons when normalized by soil organic C (SOC) content
Summary
Plant primary production in temperate, boreal, and arctic ecosystems is often limited by low soil N availability (Vitousek and Howarth 1991). Most soil N is contained within SOM polymers that are too large for immediate uptake by plants or microorganisms, and soil N availability depends on the breakdown of these polymers into smaller units (Schimel and Bennett 2004). This process is catalyzed by extracellular enzymes that are released into the soil by microbial decomposers. Microorganisms take up available N and invest it in growth and enzyme synthesis, and only N in excess is released to the soil solution as ammonium (Schimel and Bennett 2004) This process is referred to as ‘‘ammonification’’ or, more generally, as ‘‘N mineralization’’, when other mineral N forms are considered
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