Effects of soil organic matter properties and microbial community composition on enzyme activities in cryoturbated arctic soils

Enzyme-mediated decomposition of soil organic matter (SOM) is controlled, amongst other factors, by organic matter properties and by the microbial decomposer community present. Since microbial community composition and SOM properties are often interrelated and both change with soil depth, the drivers of enzymatic decomposition are hard to dissect. We investigated soils from three regions in the Siberian Arctic, where carbon rich topsoil material has been incorporated into the subsoil (cryoturbation). We took advantage of this subduction to test if SOM properties shape microbial community composition, and to identify controls of both on enzyme activities. We found that microbial community composition (estimated by phospholipid fatty acid analysis), was similar in cryoturbated material and in surrounding subsoil, although carbon and nitrogen contents were similar in cryoturbated material and topsoils. This suggests that the microbial community in cryoturbated material was not well adapted to SOM properties. We also measured three potential enzyme activities (cellobiohydrolase, leucine-amino-peptidase and phenoloxidase) and used structural equation models (SEMs) to identify direct and indirect drivers of the three enzyme activities. The models included microbial community composition, carbon and nitrogen contents, clay content, water content, and pH. Models for regular horizons, excluding cryoturbated material, showed that all enzyme activities were mainly controlled by carbon or nitrogen. Microbial community composition had no effect. In contrast, models for cryoturbated material showed that enzyme activities were also related to microbial community composition. The additional control of microbial community composition could have restrained enzyme activities and furthermore decomposition in general. The functional decoupling of SOM properties and microbial community composition might thus be one of the reasons for low decomposition rates and the persistence of 400 Gt carbon stored in cryoturbated material

Soil organic matter quality exerts a stronger control than stoichiometry on microbial substrate use efficiency along a latitudinal transect

A substantial portion of soil organic matter (SOM) is of microbial origin. The efficiency with which soil mi-croorganisms can convert their substrate carbon (C) into biomass, compared to how much is lost as respiration, thus co-determines the carbon storage potential of soils. Despite increasing insight into soil microbial C cycling, empirical measurements of microbial C processing across biomes and across soil horizons remain sparse. The theory of ecological stoichiometry predicts that microbial carbon use efficiency (CUE), i.e. growth over uptake of organic C, strongly depends on the relative availability of C and nutrients, particularly N, as microorganisms will either respire excess C or conserve C while mineralising excess nutrients. Microbial CUE is thus expected to increase from high to low latitudes and from topsoil to subsoil as the soil C:N and the stoichiometric imbalance between SOM and the microbial biomass decrease. To test these hypotheses, we collected soil samples from the organic topsoil, mineral topsoil, and mineral subsoil of seven sites along a 1500-km latitudinal transect in Western Siberia. As a proxy for CUE, we measured the microbial substrate use efficiency (SUE) of added sub-strates by incubating soil samples with a mixture of 13 C labelled sugars, amino sugars, amino acids, and organic acids and tracing 13 C into microbial biomass and released CO 2 . In addition to soil and microbial C:N stoichio-metry, we also determined the potential extracellular enzyme activities of cellobiohydrolase (CBH) and phenol oxidase (POX) and used the CBH:POX ratio as an indicator of SOM substrate quality. We found an overall decrease of SUE with latitude, corresponding to a decrease in mean annual temperature, in mineral soil horizons. SUE decreased with decreasing stoichiometric imbalance in the organic and mineral topsoil, while a relationship of SUE with soil C:N was only found in the mineral topsoil. However, contrary to our hypothesis, SUE did not increase with soil depth and mineral subsoils displayed lower average SUE than mineral topsoils. Both within individual horizons and across all horizons SUE was strongly correlated with CBH:POX ratio as well as with climate variables. Since enzyme activities likely reflect the chemical properties of SOM, our results indicate that SOM quality exerts a stronger control on SUE than SOM stoichiometry, particularly in subsoils were SOM has been turned over repeatedly and there is little variation in SOM elemental ratios

Microbial nitrogen use efficiency along a latitudinal gradient in Western Siberia, Russia

Ökosysteme weißen nicht nur Unterschiede in ihren klimatischen Bedingungen und ihrer Vegetation auf sondern auch in ihrer Bodenzusammensetzung in Form von organischen und mineralischen Bodensubstanzen. Hohe Variation ist vor allem in der Verfügbarkeit diverser Nährstoffe, wie beispielsweise Kohlenstoff (C) und Stickstoff (N), zu beobachten. Vor allem variiert die Nährstoff-Verfügbarkeit in Böden stark entlang latitudinaler Gradiente. Dies setzt eine hohe physiologische Anpassungsfähigkeit der bodenbewohnenden Mikroorganismen voraus, um eine hohe Konkurrenzstärke gegenüber Pflanzen aber auch anderen Bodenorganismen zu gewährleisten. Hierbei spielen vor allem Prozessraten eine grundlegende Rolle. Mikroorganismen können Aminosäuren direkt aufnehmen, um ihren Stickstoffbedarf zu decken. Liegen Aminosäuren allerdings als längere Peptide, Proteine oder anderwärtige Polymere vor, so geht eine obligate Protein-Depolymerisierung der mikrobiellen Aufnahme voraus. Ist die Stickstoff-Verfügbarkeit im Boden gering, werden die depolymerisierten Aminosäuren von der mikrobielle Bodengemeinschaft augenblicklich aufgenommen und als Biomasse assimiliert (mikrobielle Aminosäuren-Immobilisierung). Aufgenommene Aminosäuren können auch wieder ausgeschieden und in Form von Ammonium (NH4+) in den Boden freigesetzt werden (Stickstoff-Mineralisierung). Dieser freigesetzte Stickstoff kann folglich wieder aufgenommen, assimiliert (mikrobielle NH4+ Immobilisierung) und wieder freigesetzt werden. All diese Transformationsraten variieren in ihrer Höhe je nach dominanter Stickstoff Quelle sowie genereller Stickstoff Verfügbarkeit des jeweiligen Bodens. Die daraus resultierende mikrobielle Anpassungsfähigkeit kann mittels Stickstoff Nutzungseffizienz (NUE) angegeben werden. Die NUE drückt aus wieviel des aufgenommen Stickstoffs in die mikrobielle Biomasse eingebaut beziehungsweise als NH4+ wieder freigesetzt wird. Je höher sie ist, umso mehr wird der N von der mikrobiellen Gemeinschaft genützt und als Biomasse assimiliert, d.h. die mikrobielle Aminosäuren-Immobilisierung überwiegt gegenüber der Stickstoff-Mineralisierung. Ziel dieser Arbeit war die Analyse des Einflusses der Stickstoff-Verfügbarkeit auf mikrobielle Prozessraten in Böden. Hierbei standen Raten der Protein-Depolymerisierung, der Stickstoff-Mineralisierung sowie der mikrobiellen Immobilisierung von Aminosäuren und Ammonium (NH4+) im Fokus des Forschungsinteresses. Alle Raten wurden mittels „15N pool dilution“ analysiert. Diese Methode erlaubt die Quantifizierung von Flüssen indem betreffende Pools mit dem schwereren Stickstoff Isotop 15N markiert werden und Konzentrationsveränderungen sowie Veränderungen in der Isotopen Anreicherung über die Zeit gemessen werden. So werden beispielsweise 15N-Aminosäuren zur Bestimmung der Protein-Depolymerisierung und Immobilisierung von Aminosäuren, beziehungsweise 15NH4+ zur Bestimmung der Stickstoff-Mineralisierung und Immobilisierung von NH4+ eingesetzt. Hierfür wurden organische und mineralische Bodenproben entlang eines 1400km langen latitudinalen Transekts in West Sibirien, Russland, gezogen. Dadurch wurde sichergestellt, dass alle wesentlichen Ökosysteme abgebildet wurden: die von Permafrost beherrschte Tundra, der boreale Nadelwald der Taiga, der laubwerfende Mischwald und die Steppe. Zusammenfassend lässt sich feststellen, dass N-Transformationsraten von Klimavariablen wie Temperatur und Bodenfeuchtigkeit, aber auch von C und N Konzentrationen in Böden sowie der Menge organischer Bodensubstanzen abhängig sind. Die höchsten Protein Depolymerisierungsraten und N Mineralisierungsraten wurden im borealen Nadelwald gemessen, begleitet von den höchsten C und N Konzentrationen in diesen Böden. Tundra und Steppe waren hingegen durch niedrigere Transformationsraten gekennzeichnet. Transformationsraten waren generell in allen organischen Horizonten hoch. Eine Abnahme in der Protein Depolymerisierung aber eine Zunahme in der N Mineralisierung mit steigender Bodentiefe lässt auf eine reduzierte Konkurrenz zwischen Pflanzen und Mikroorganismen in tieferen Bodenschichten schließen. Im Gegensatz zu unserer Annahme, wurde die höchste Stickstoff Nutzungs Effizienz (NUE) in den südlichen Ökosystemen der Steppe und Waldsteppe gemessen, während eine signifikante Abnahme in der mikrobiellen NUE mit zunehmendem geographischen Breitengrad beobachtet werden konnte. Die NUE in der Tundra konnte aufgrund der geringen Mineralisierungsraten, welche unter das Detektionslimit fielen, nicht kalkuliert werden. Die vorgefundene hohe NUE im Süden weißt auf eine starke mikrobielle Adaptierung an das hohe C:N Verhältnis von Streu- und Laubfall hin, während Ökosysteme mit mittleren C:N Verhältnissen durch niedrige NUE Werte gekennzeichnet waren. Die ursprüngliche chemische Zusammensetzung des Streueintrags scheint demnach der bestimmende Faktor der mikrobiellen NUE zu sein. Dennoch sind unzählige weitere Faktoren, wie beispielsweise die Limitierung anderer Nährstoffe, als wesentliche Faktoren in der Entwicklung und Veränderung der mikrobiellen NUE zu berücksichtigen.Ecosystems show differences in climatic conditions, vegetation and soil organic matter (SOM) content, especially differing in soil N availability along latitudinal gradients. These circumstances require high physiological adaptation of the soil microbial community to compete successfully for nutrients with plants but also with other soil microbial organisms. In this study we aimed at determining the influence of soil N availability on soil microbial transformation rates, focusing on protein depolymerization and N mineralization rates, all based on 15N pool dilution techniques. Organic and mineral soil samples were taken along a 1,400 km latitudinal transect in Western Siberia, Russia, covering all major ecosystems of tundra, boreal forest, deciduous forest and steppe. N transformation rates seemed to be highly influenced by soil moisture and soil C and N concentrations. Highest protein depolymerization and N mineralization rates occurred in the boreal forest, being accompanied by peaking soil C and N concentrations and high water content, whereas lowest transformation rates were found in tundra and steppe soil. Reduced plant microbial competition for N in deep soil layers was considered to stimulate N mineralization and lower protein depolymerization rates. Highest microbial nitrogen use efficiency (NUE) was found in the southern steppe environment, while values were respectively low at all taiga sites. Unfortunately, NUE could not be calculated for the southern tundra as mineralization rates were under detection limit. High NUE suggested microbial adaptation to high litter C:N ratios, whereas lowest NUE occurred where intermediate litter C:N could be observed. We suggest that initial litter chemistry highly defines microbial NUE, but certainly, there are numerous other factors influencing and changing NUE, e.g. limitations of other nutrients, that should be reconsidered

Microbial nitrogen dynamics in organic and mineral soil horizons along a latitudinal transect in western Siberia

Soil N availability is constrained by the breakdown of N-containing polymers such as proteins to oligopeptides and amino acids that can be taken up by plants and microorganisms. Excess N is released from microbial cells as ammonium (N mineralization), which in turn can serve as substrate for nitrification. According to stoichiometric theory, N mineralization and nitrification are expected to increase in relation to protein depolymerization with decreasing N limitation, and thus from higher to lower latitudes and from topsoils to subsoils. To test these hypotheses, we compared gross rates of protein depolymerization, N mineralization and nitrification (determined using (15)N pool dilution assays) in organic topsoil, mineral topsoil, and mineral subsoil of seven ecosystems along a latitudinal transect in western Siberia, from tundra (67°N) to steppe (54°N). The investigated ecosystems differed strongly in N transformation rates, with highest protein depolymerization and N mineralization rates in middle and southern taiga. All N transformation rates decreased with soil depth following the decrease in organic matter content. Related to protein depolymerization, N mineralization and nitrification were significantly higher in mineral than in organic horizons, supporting a decrease in microbial N limitation with depth. In contrast, we did not find indications for a decrease in microbial N limitation from arctic to temperate ecosystems along the transect. Our findings thus challenge the perception of ubiquitous N limitation at high latitudes, but suggest a transition from N to C limitation of microorganisms with soil depth, even in high-latitude systems such as tundra and boreal forest.\n\nKEY POINTS: We compared soil N dynamics of seven ecosystems along a latitudinal transectShifts in N dynamics suggest a decrease in microbial N limitation with depthWe found no decrease in microbial N limitation from arctic to temperate zones

Microbial community composition shapes enzyme patterns in topsoil and subsoil horizons along a latitudinal transect in Western Siberia

Soil horizons below 30cm depth contain about 60% of the organic carbon stored in soils. Although insight into the physical and chemical stabilization of soil organic matter (SOM) and into microbial community composition in these horizons is being gained, information on microbial functions of subsoil microbial communities and on associated microbially-mediated processes remains sparse. To identify possible controls on enzyme patterns, we correlated enzyme patterns with biotic and abiotic soil parameters, as well as with microbial community composition, estimated using phospholipid fatty acid profiles. Enzyme patterns (i.e. distance-matrixes calculated from these enzyme activities) were calculated from the activities of six extracellular enzymes (cellobiohydrolase, leucine-amino-peptidase, N-acetylglucosaminidase, chitotriosidase, phosphatase and phenoloxidase), which had been measured in soil samples from organic topsoil horizons, mineral topsoil horizons, and mineral subsoil horizons from seven ecosystems along a 1500km latitudinal transect in Western Siberia. We found that hydrolytic enzyme activities decreased rapidly with depth, whereas oxidative enzyme activities in mineral horizons were as high as, or higher than in organic topsoil horizons. Enzyme patterns varied more strongly between ecosystems in mineral subsoil horizons than in organic topsoils. The enzyme patterns in topsoil horizons were correlated with SOM content (i.e., C and N content) and microbial community composition. In contrast, the enzyme patterns in mineral subsoil horizons were related to water content, soil pH and microbial community composition. The lack of correlation between enzyme patterns and SOM quantity in the mineral subsoils suggests that SOM chemistry, spatial separation or physical stabilization of SOM rather than SOM content might determine substrate availability for enzymatic breakdown. The correlation of microbial community composition and enzyme patterns in all horizons, suggests that microbial community composition shapes enzyme patterns and might act as a modifier for the usual dependency of decomposition rates on SOM content or C/N ratios

Adjustment of microbial nitrogen use efficiency to carbon:nitrogen imbalances regulates soil nitrogen cycling

Microbial nitrogen use efficiency (NUE) describes the partitioning of organic N taken up between growth and the release of inorganic N to the environment (that is, N mineralization), and is thus central to our understanding of N cycling. Here we report empirical evidence that microbial decomposer communities in soil and plant litter regulate their NUE. We find that microbes retain most immobilized organic N (high NUE), when they are N limited, resulting in low N mineralization. However, when the metabolic control of microbial decomposers switches from N to C limitation, they release an increasing fraction of organic N as ammonium (low NUE). We conclude that the regulation of NUE is an essential strategy of microbial communities to cope with resource imbalances, independent of the regulation of microbial carbon use efficiency, with significant effects on terrestrial N cycling

Amino acid production exceeds plant nitrogen demand in Siberian tundra

Arctic plant productivity is often limited by low soil N availability. This has been attributed to slow breakdown of N-containing polymers in litter and soil organic matter (SOM) into smaller, available units, and to shallow plant rooting constrained by permafrost and high soil moisture. Using N-15 pool dilution assays, we here quantified gross amino acid and ammonium production rates in 97 active layer samples from four sites across the Siberian Arctic. We found that amino acid production in organic layers alone exceeded literature-based estimates of maximum plant N uptake 17-fold and therefore reject the hypothesis that arctic plant N limitation results from slow SOM breakdown. High microbial N use efficiency in organic layers rather suggests strong competition of microorganisms and plants in the dominant rooting zone. Deeper horizons showed lower amino acid production rates per volume, but also lower microbial N use efficiency. Permafrost thaw together with soil drainage might facilitate deeper plant rooting and uptake of previously inaccessible subsoil N, and thereby promote plant productivity in arctic ecosystems. We conclude that changes in microbial decomposer activity, microbial N utilization and plant root density with soil depth interactively control N availability for plants in the Arctic

Plant-derived compounds stimulate the decomposition of organic matter in arctic permafrost soils

Arctic ecosystems are warming rapidly, which is expected to promote soil organic matter (SOM) decomposition. In addition to the direct warming effect, decomposition can also be indirectly stimulated via increased plant productivity and plant-soil C allocation, and this so called "priming effect" might significantly alter the ecosystem C balance. In this study, we provide first mechanistic insights into the susceptibility of SOM decomposition in arctic permafrost soils to priming. By comparing 119 soils from four locations across the Siberian Arctic that cover all horizons of active layer and upper permafrost, we found that an increased availability of plant-derived organic C particularly stimulated decomposition in subsoil horizons where most of the arctic soil carbon is located. Considering the 1,035 Pg of arctic soil carbon, such an additional stimulation of decomposition beyond the direct temperature effect can accelerate net ecosystem C losses, and amplify the positive feedback to global warming