Straw application coupled with N and P supply enhanced microbial biomass, enzymatic activity, and carbon use efficiency in saline soil

Seasonality of soil microorganisms plays a critical role in terrestrial carbon (C) and nitrogen (N) cycling. The asynchrony of immobilization by microbes and uptake by plants may be important for N retention during winter, when plants are inactive. Meanwhile, the known warming effects on soil microbes (decreasing biomass and increasing growth rates) may affect microbial seasonal dynamics and nutrient retention during winter.We sampled soils from a geothermal warming site in Iceland (www.forhot.is) which includes three in situ warming levels (ambient, +3 &#176;C, +6 &#176;C). We harvested soil samples at 9 time points over one year and measured the seasonal variation in microbial biomass carbon (Cmic) and nitrogen (Nmic) and microbial physiology (growth and carbon use efficiency) by an 18O-labelling technique.We observed that Cmic and Nmic peaked in winter, followed by a decline in spring and summer. In contrast growth and respiration rates were higher in summer than winter. The observed biomass peak at lower growth rates, suggests that microbial death rates must have declined even more than growth rates. Soil warming increased biomass-specific microbial activity (i.e., growth, respiration, and turnover rates per unit of microbial biomass), prolonging the period of higher microbial activity found in summer into autumn and winter. Microbial carbon use efficiency was unaltered by soil warming. Throughout the seasons, warming reduced Cmic and Nmic, albeit with a stronger effect in winter than summer and restrained winter biomass accumulation by up to 78% compared to ambient conditions. We estimated a reduced microbial winter N storage capacity by 45.5 and 94.6 kg ha-1 at +3 &#176;C and +6 &#176;C warming respectively compared to ambient conditions. This reduction represents 1.57% and 3.26% of total soil N stocks, that could potentially be lost per year from these soils.Our results clearly demonstrate that soil warming strongly decreases microbial C and N immobilization when plants are inactive, potentially leading to higher losses of C and N from warmed soils over winter. These results have important implications as increased N losses may restrict increased plant growth in a future climate.

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How relevant are microbial traits to understand soil biogeochemical cycles?

10.5194/egusphere-egu21-11186 ◽

2021 ◽

Author(s):

Tessa Camenzind ◽

Johannes Lehmann ◽

Anika Lehmann ◽

Carlos A. Aguilar-Trigueros ◽

Matthias C. Rillig

Keyword(s):

Enzymatic Activity ◽

Fungal Growth ◽

Biogeochemical Cycles ◽

Model Organisms ◽

Soil System ◽

Microbial Physiology ◽

Carbon Use Efficiency ◽

N Supply ◽

Use Efficiency

Our knowledge about the role of microbial organisms as drivers of soil biogeochemical cycles is mainly based on soil analyses, and the physiological information that exists for few microbial model organisms. In soil, measurements of process rates and element contents can be related to the apparent activity of the microbial community, though conclusions are often indirect - actual microbial physiology and diversity remains hidden. By contrast, analyses of microbial physiology under controlled conditions are hardly representative of the vast diversity of microorganisms in soil, and a transfer of these findings to complex soil systems is challenging. Thus, we argue that a better exchange among these ecological disciplines will lead to a valuable transfer of relevant questions, knowledge and improved understanding of the role of microbes in soil and its responses to environmental change. Here, we provide examples of an evaluation of microbial parameters relevant in soil biogeochemical cycles, analysing traits in a collection of 31 saprobic fungi in response to varying substrate conditions. The large dataset allowed to test several assumptions and conclusions derived from soil system analyses exemplarily for soil fungi. Specifically, we (1) evaluated the optimum C:N:P (carbon:nitrogen:phosphorus) substrate ratio for fungal growth and activity, (2) assessed the responses in carbon-use efficiency and enzyme activity to N deficiency, (3) analyzed the relevance of C versus N supply for fungal growth and activity under varying substrate conditions and (4) tested the assumption of microbial stoichiometric homeostasis, that represents a basic principle in soil ecological stoichiometry. Fungal responses to changes in N and C availability were partly consistent with expectations, e.g. regarding general nutrient demands, though as often discussed C availability appeared more relevant for growth especially in complex substrates. Enzymatic activity and respiration also positively correlated with N availability, resulting in decreased carbon-use efficiency at high N supply. These findings, for example, contradict certain conclusions in soil analyses, namely that N limitations will result in &#8220;N mining&#8221; (high enzymatic activity), while the excess of C causes &#8220;overflow respiration&#8221; and reduced CUE. Regarding fungal C:N:P ratios, those were only related to nutrient demands when growing in simple media, while in soil substrate such relations seem more complex. Contradicting the assumption of microbial homeostasis in soil, fungal individuals showed more flexible C:N:P ratios than expected, though the degree of flexibility varied among isolates. In general, the results also reveal a large trait variation among different isolates, with several traits showing a phylogenetic signal, indicating variations in microbial activity depending on community composition. Finally, we want to raise and discuss several emerging questions: How relevant is a deeper understanding of microbial physiology to understand soil biogeochemical processes? How do we include the variability of traits in diverse soil communities &#8211; are average values informative, or can we proceed with useful categories? And how can methods in soil science and microbial ecology be merged best to allow fruitful knowledge transfer?

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Impacts of conservation agriculture on soil nitrogen pools and microbial physiology

10.5194/egusphere-egu2020-4706 ◽

2020 ◽

Author(s):

Jörg Schnecker ◽

Gernot Bodner

Keyword(s):

Microbial Biomass ◽

Organic Carbon ◽

Soil Nitrogen ◽

Cover Crops ◽

Organic N ◽

Agricultural Fields ◽

Total N ◽

Carbon Use Efficiency ◽

Regenerative Agriculture ◽

Use Efficiency

Conservation or regenerative agriculture, i.e. reduction of mechanical soil disturbance, introduction of crop rotations, and especially cover crops as a form of natural soil amendment, has been shown to increase soil organic matter contents as well as soil health. One mechanism behind the increase in organic carbon under regenerative agriculture could be an increase in microbial biomass, as well as an enhanced carbon use efficiency (CUE) of the soil microorganisms in these systems. Such changes in microbial biomass and activity could also influence soil nitrogen (N) cycling. Here we show first results of on-farm research at four sites in Austria comparing crop fields under regenerative agriculture practices with conventional practices, and nearby perennial grasslands at each site. The four sites span different climate gradients, soil types and textures.Soil organic carbon (SOC) content ranged from 1 to 2.3% in the agricultural soils and was significantly higher under regenerative management compared to conventional practices in two out of four sites. SOC contents in perennial grasslands were up to 5% and always higher than in agricultural fields. Extractable organic carbon was similar in the two agricultural fields of the respective site, while grasslands diverged. Microbial biomass carbon was highest in grasslands at all sites and significantly higher in fields under regenerative agriculture compared to conventional agriculture at three out of four sites.Total nitrogen was highest in perennial grasslands at all sites, and similar in regenerative and conventional fields. The form of N however differed between soils under conventional and regenerative agriculture. Dissolved N, expressed per g total N was significantly higher or tended to be higher in conventional compared to regenerative agricultural fields. From this dissolved pool a higher proportion was in inorganic N forms that are more prone to leaching and gaseous loss compared to organic N forms. In soils from regenerative agricultural fields a higher proportion of the total N was found in the microbial biomass. This pool is considered to be highly dynamic, but also protected against losses. Less N in dissolved and inorganic form as well as a higher proportion of N in the microbial biomass indicates that the N cycle is more closed in soils managed regeneratively versus conventional.A greater importance of the microbial biomass could also have effects on soil C cycling. Higher microbial biomass is often related to increased carbon use efficiency, which in turn could indicate increased soil carbon sequestration. The already mentioned results will thus be discussed with further measurements of microbial respiration, growth and CUE.

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Assessing microbial residues in soil as a potential carbon sink and moderator of carbon use efficiency

Biogeochemistry ◽

10.1007/s10533-020-00720-4 ◽

2020 ◽

Vol 151 (2-3) ◽

pp. 237-249

Author(s):

Kevin Geyer ◽

Jörg Schnecker ◽

A. Stuart Grandy ◽

Andreas Richter ◽

Serita Frey

Keyword(s):

Microbial Biomass ◽

Carbon Sink ◽

Meta Analysis ◽

Clay Content ◽

Tracer Experiment ◽

Carbon Use Efficiency ◽

Sequestration Potential ◽

Use Efficiency ◽

Microbial Products

AbstractA longstanding assumption of glucose tracing experiments is that all glucose is microbially utilized during short incubations of ≤2 days to become microbial biomass or carbon dioxide. Carbon use efficiency (CUE) estimates have consequently ignored the formation of residues (non-living microbial products) although such materials could represent an important sink of glucose that is prone to stabilization as soil organic matter. We examined the dynamics of microbial residue formation from a short tracer experiment with frequent samplings over 72 h, and conducted a meta-analysis of previously published glucose tracing studies to assess the generality of these experimental results. Both our experiment and meta-analysis indicated 30–34% of amended glucose-C (13C or 14C) was in the form of residues within the first 6 h of substrate addition. We expand the conventional efficiency calculation to include residues in both the numerator and denominator of efficiency, thereby deriving a novel metric of the potential persistence of glucose-C in soil as living microbial biomass plus residues (‘carbon stabilization efficiency’). This new metric indicates nearly 40% of amended glucose-C persists in soil 180 days after amendment, the majority as non-biomass residues. Starting microbial biomass and clay content emerge as critical factors that positively promote such long term stabilization of labile C. Rapid residue production supports the conclusion that non-growth maintenance activity can illicit high demands for C in soil, perhaps equaling that directed towards growth, and that residues may have an underestimated role in the cycling and sequestration potential of C in soil.

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