Measurement Of Microbial Biomass Research Articles

Changes in litter and nitrogen deposition differentially alter forest soil organic matter biogeochemistry

Global climate change has altered forest productivity across the globe. Although extreme drought and temperature have lowered forest productivity in some regions, increased productivity has been observed in many forests over the last few decades due to increases in atmospheric carbon dioxide, temperature, and nitrogen (N) deposition. These factors can lead to changes in both the quantity and quality of litterfall and root inputs to soil. Additionally, few studies have investigated multifactorial treatments (e.g., detrital changes and N deposition), on soil carbon (C) sequestration. To investigate the long-term compositional changes to soil organic matter (SOM) in response to litter and N inputs, soil samples were collected from the University of Michigan Biological Station (UMBS) Detrital Input and Removal Treatment (DIRT) site after 15 years of experimental treatments. The samples were characterized using elemental analysis, targeted SOM compound analyses, nuclear magnetic resonance spectroscopy and microbial biomass and community composition measurements. The exclusions of C inputs (litter and/or roots) resulted in molecular-level biogeochemical changes, however, the soils at UMBS are seemingly more resistant to losses in soil C compared to other DIRT studies. Although the addition treatments (Double Litter, Double Wood, Double Litter + N, and N) led to increased soil C concentrations at UMBS, we found evidence for enhanced SOM decomposition occurring with litter additions. Notably, the observations made from the concurrent treatment of Double Litter + N demonstrated that the responses of individual treatments are not representative of simultaneous applications. Collectively, these results demonstrate that soil C biogeochemistry is sensitive to fluctuations in C and N deposition and overall, these processes are dictated by site-specific ecological properties.

Seasonal Relationships of Alpine Plants and Microbes through a Stoichiometric and Enzymatic Lens

Alpine biomes experience harsh environmental conditions and short growing seasons, which necessitate interspecific and intraspecific interactions to ensure the stability of diversity and ecosystem multifunctionality. The relationship between plants and microbes in this environment is equally dynamic, with seasonal pulses of nutrients and the phenology of plants creating specific "hot moments" of biogeochemical activity. As a crucial zone of interaction between plant roots and microbial communities, the rhizosphere serves as a "hot spot" of biogeochemical cycling where the mineralization of nutrients, such as carbon, nitrogen, and phosphorus, allows for the transfer of nutrients between trophic levels. However, the nature of these interactions depends on edaphic and climatic conditions, potentially leading to cooperation or competition to meet the stoichiometric demands of organisms. Elevation gradients within alpine ecosystems provide dramatic shifts in temperature, precipitation, and soil development that allow for the study of these interactions over short geographical distances. In conjunction with seasonal sampling, this approach can provide a wide environmental context to observe the relationship between specific plants and microbial communities. By investigating the C/N ratios of plants, microbes, and soil, as well as microbial enzymatic potential, we can infer nutrient limitations, temporal niche partitioning, and biological responses to abiotic conditions. Within the Austrian Alps, we studied a selection of herbaceous plants and their associated microbial communities across an elevation gradient spanning 2200-2800 m (Fig. 1). The primary aims of the study were to assess the seasonal changes in C/N stoichiometry from both trophic levels, microbial enzymatic potential, and rhizosphere diversity of bacterial and fungal communities. To fulfill these aims, four locations were selected based on the two present biomes (alpine meadow and sub-nival zone) and the transition between them. Four to five plant species were collected during each season in 2023, including the often-neglected snow-covered winter season, along with rhizosphere and bulk soil for microbial biomass measurements and soil chemistry. Plant leaf tissue samples were analyzed using Isotope-ratio mass spectrometry for plant C/N ratios, while soil and microbial C/N ratios were calculated using chloroform fumigation extraction. Microbial enzymatic potential was assessed using hydrolase enzymatic assays for five fluorophore-labeled substrates. 16S-rRNA and 18S-rRNA genes were sequenced using an Illumina MiSeq platform from the fine roots of collected plant individuals to quantify the relative abundances of bacterial and fungal taxa. The findings of our study indicate that the higher microbial biomass (Cmic) in alpine meadow locations leads to increased enzymatic activity compared to sub-nival zones. However, specific plant species were found to enhance microbial biomass and enzymatic potential in different seasons, suggesting that plants promote microbial interaction and biogeochemical cycling during different seasons as a form of temporal niche partitioning. Most plant species demonstrated an increase in C/N ratios throughout the season, sometimes increasing by more than 200%. However, two Poa spp. showed the highest C/N ratios during the summer, which further correlated with higher microbial C/N ratios. By observing changes in stoichiometric ratios of organisms that interact and share nutrients, we propose that these relationships vary between host plants depending on their temporal niche and abiotic factors (soil and environmental conditions). Furthermore, the positive or negative correlation of plant and microbial C/N ratios may indicate the relative cooperation or competition between trophic levels.

Revisiting soil microbial biomass: Considering changes in composition with growth rate

Soil microbial biomass is assumed to have stable chemical composition. Various components of the biomass such as DNA, ATP, or chloroform-labile organic carbon are measured in soil and converted into total microbial biomass using experimentally derived conversion factors, which are also assumed to be constant. However, several observations suggest the opposite. The composition of soil microbial biomass is likely changing with specific growth rate as observed in pure cultures of single microbial species. In this study, we define a “sub-Microbial” model that explicitly represents changes in composition of soil microbial biomass associated with changes in specific growth rate. We calibrate the model with published data and compare its performance with the simpler Monod and Pirt models, which consider microbial biomass as a single pool with invariant chemical composition. The model explains well the variability in chloroform-labile content of microbial biomass following organic substrate additions as well as variability in ratios of different components of microbial biomass. Changes in composition of soil microbial biomass are quantitatively significant and occur over hours and days resulting in our sub-Microbial model outperforming both the Monod and Pirt models. Our results further indicate that the composition of soil microbial biomass changes consistently with growth rate across various soils. Here, we provide a methodological recommendation how to determine total soil microbial biomass and its physiological characteristics such as growth rate, turnover rate and substrate use efficiency as accurately as possible. In light of the presented results, we would like to initiate a discussion about the methodological issues associated with measurement of soil microbial biomass as these measurements are expected to inform a new generation of microbially-explicit soil biogeochemical models predicting development of terrestrial ecosystems under various scenarios.

No evidence that conifer biochar impacts soil functioning by serving as microbial refugia in boreal soils

AbstractIt is well established that application of biochar to soils can promote soil fertility, which ultimately may enhance plant growth. While many mechanisms have been proposed to explain this, one specific mechanism, the “microbial refugia hypothesis,” suggests that biochar may provide physical protection for soil microbe from soil microfauna that otherwise exert top‐down control on microbial biomass and activity. We tested the microbial refugia hypothesis by incubating two boreal soils with and without biochar derived from a wood mixture of boreal tree species (Picea abies and Pinus sylvestris), and with and without soil nematodes. We measured phospholipid fatty acids (PLFA) as a relative measure of microbial biomass, and several variables indicative of microbial activity, including extractable nutrient concentrations (NH4+, NO3−, and PO4−), heterotrophic N2‐fixation, and soil respiration. Contrary to our expectations, we found that biochar by itself did not stimulate microbial biomass or activity. Furthermore, we found that nematode addition to soil stimulated rather than depressed the biomass of several bacterial PLFA groups. Finally, interactive effects between the nematode treatment and biochar never worked in a way that supported the microbial refugia hypothesis. Our findings suggest that a typical boreal biochar applied to boreal soils may not have the same stimulatory effect on microbial biomass and activity that has been shown in some other ecosystems, and that enhanced plant growth in response to biochar addition sometimes observed in boreal environments is likely due to other mechanisms, such as direct nutrient supply from biochar or amelioration of soil pH.

Short-Term Effect of Nitrogen Fertilization on Carbon Mineralization during Corn Residue Decomposition in Soil

The effect of N fertilization on residue decomposition has been studied extensively; however, contrasting results reflect differences in residue quality, the form of N applied, and the type of soil studied. A 60 d laboratory incubation experiment was conducted to ascertain the effect of synthetic N addition on the decomposition of two corn (Zea mays L.) stover mixtures differing in C:N ratio by continuous monitoring of CO2 emissions and periodic measurement of microbial biomass and enzyme activities involved in C and N cycling. Cumulative CO2 production was greater for the high than low N residue treatment, and was significantly increased by the addition of exogenous N. The latter effect was prominent during the first month of incubation, whereas N-treated soils produced less CO2 in the second month, as would be expected due to more rapid substrate depletion from microbial C utilization previously enhanced by greater N availability. The stimulatory effect of exogenous N was verified with respect to active biomass, microbial biomass C and N, and cellulase and protease activities, all of which were significantly correlated with cumulative CO2 production. Intensive N fertilization in modern corn production increases the input of residues but is not conducive to soil C sequestration.

Nitrous oxide fluxes from long-term limed soils following P and glucose addition: Nonlinear response to liming rates and interaction from added P

Liming of acidic soils to regulate pH for crop growth may decrease emissions of nitrous oxide (N2O) due to direct effects of pH on the synthesis of N2O reductases by denitrifying bacteria. However, liming also changes general pH-dependent soil properties, including availability of phosphorus (P), with a feedback on N2O fluxes that remains largely unknown. Here we used a mesocosm approach to study the combined role of liming and P in regulating N2O fluxes from denitrification in an arable coarse sandy soil where N2O emissions under field condition coincided with rainfall events and irrigation, which facilitated anoxia. Soils from three long-term liming treatments (0, 4, and 12 Mg ha-1) with resulting pH(CaCl2) of 3.6, 4.7 and 6.3 were incubated at original bulk density first at 60% water filled pore space (WFPS) and successively at 75% WFPS with added nitrate, inorganic P (0 and 10 μg P g-1 soil) and glucose as labile carbon. N2O fluxes were measured during 28 days and were supplemented with measurements of CO2 fluxes, microbial biomass, potential denitrification, and acid phosphatase activity. The results showed a nonlinear response of N2O fluxes to liming rates, with highest fluxes at the intermediate liming level (4 Mg ha-1). Furthermore, inorganic P stimulated N2O fluxes only at the intermediate liming level. Assays of potential denitrification indicated that the N2O/(N2O + N2) product ratio decreased consistently with increasing liming rates, but total N2O fluxes responded nonlinearly likely due to combined effects on N2O/(N2O + N2) product ratios and total denitrification rates. The results suggest that liming and P addition interact on microbial properties and N2O emissions from acidic arable soils and may not follow linear trends. This makes it uncertain to predict and model the resulting net effect, which may depend on the actual pH range and P availability from the unlimed to the limed treatments.

Spatio-temporal variability in soil CO2 efflux and regulatory physicochemical parameters from the tropical urban natural and anthropogenic land use classes

Urban ecosystems, the heterogeneous and rapidly changing landscape, showed a considerable impact on the global C cycle. However, studies encompassing the spatial differences in urban land uses on soil C dynamics are limited in tropical ecosystems. In this study, seasonal and temporal variability in soil CO2 efflux (SCE) and its regulatory physicochemical variables under five urban land use classes viz., Bare (BAR), Agriculture (AGR), Plantation (PLT), Grassland (GRA) and Lawns (LAW) were assessed from 2014 to 2016. Bare land use was considered as the reference for observing the variation for different land uses. Seasonal measurements of SCE, soil temperature, moisture content, pH, ammonium-N, nitrate-N and microbial biomass C (MBC) were performed whereas soil organic C (SOC), soil N, and soil physical properties were measured annually. Our results showed a significant (P < 0.01) increase in SCE by 89%, 117%, 132% and 166% for land use types from BAR to AGR, PLT, GRA and LAW, respectively. The results revealed a two-fold increase in SCE from anthropogenically managed urban lawns as compared to bare soil. PLT and LAW land use classes showed higher SOC and N contents. SCE was found positively correlated with temperature, moisture, SOC, soil N and MBC whereas negatively correlated with ammonium-N and nitrate-N (at P < 0.05) for the overall dataset. Soil moisture, temperature, SOC, porosity and pH were identified as the major determinant of urban SCE by explaining 63% of the variability in overall SCE. Further, temperature for BAR and LAW; moisture for PLT; ammonium-N for GRA; and nitrate-N for AGR were identified as the major regulators of SCE for different land use classes. The findings revealed that the interaction of soil temperature and moisture with nutrient availability regulates overall and seasonal variability in SCE in an urban ecosystem. Since these variables are highly affected by climate change, thus, the soil C source-sink relationships in tropical urban ecosystems may further change and induce a positive global warming potential from urban ecosystems.

Medium-term influence of tetracyclines on total and specific microbial biomass in cultivated soils of Galicia (NW Spain)

This work examines the results of a soil incubation experiment in the laboratory, under controlled conditions of humidity and temperature. The purpose was to determine the medium-term influence of the presence of antibiotics on the total and specific microbial biomass, determined by means of the phospholipid fatty acids (PLFAs) analysis (total microbial biomass, and specific fungal, bacterial, actinobacterial, Gram-negative bacterial and Gram-positive bacterial biomass), as well as the relationship between some of these groups (fungal biomass/bacterial biomass, Gram-negative-bacterial /Gram-positive bacterial). The experiment was performed with four different cultivated soils with a similar pH but different organic matter (OM) content, to which eight doses of three antibiotics of the tetracycline group (tetracycline, oxytetracycline and chlorotetracycline) were added. Microbial biomass measurements (total and specific groups) were performed after 42 days of incubation. As expected, the total and specific microbial biomass values were different in the four soils studied. Both the total and the specific microbial biomass showed a similar response to the presence of antibiotics, although in several cases the data were inconsistent and difficult to interpret. In general, in all soils the addition of chlorotetracycline and tetracycline slightly modified or increased, to a greater or lesser extent, the values of both total and specific microbial biomass, particularly at higher doses. However, in certain cases, biomass values decreased due to the addition of the highest dose of oxytetracycline. With regard to fungal/bacterial and Gram&lt;sup&gt;-&lt;/sup&gt;bacteria/Gram&lt;sup&gt;+&lt;/sup&gt; bacterial biomass ratios, values slightly changed after the addition of the antibiotics.

Partitioning sources of CO2 emission after soil wetting using high-resolution observations and minimal models

Wetting dry soil triggers a pulse of microbial respiration. Respiration pulses after wetting can be complex, evolving through several stages before subsiding—possibly reflecting contributions from multiple carbon (C) sources. We hypothesized that respiration after wetting combines C from cellular sources (notably trehalose, a cellular osmo-protectant) and soluble extracellular C made available by the physical effects of wetting, with cellular sources contributing early during the pulse and extracellular C dominating later. We tested this hypothesis by making measurements of soil respiration, microbial biomass, trehalose, and water extractable organic C after wetting dry soil in the laboratory. We then devised several alternative minimal models to evaluate the potential contributions of trehalose and extracellular C to the respiration pulse. We found that the chloroform-labile fraction of the microbial biomass was comprised of 50% trehalose in dry soil. Both trehalose and water extractable organic C pools declined rapidly after wetting; trehalose disappeared within 3 h of water addition. Chloroform-labile glucose increased 3 h after water addition and then declined, suggesting rapid hydrolysis of trehalose and consumption of the resulting glucose. Respiration dynamics after wetting could be best explained by models that included two C sources, combining rapid metabolism of trehalose with slower consumption of extracellular soluble C. These results suggest that pulses of respiration after wetting reflect both osmolyte consumption within the microbial biomass and enhanced extracellular C availability; hence, models of microbial drought response must account for both processes to fully resolve soil C dynamics.

Soil microbial activity is resistant to recreational camping disturbance in a Prosopis dominated semiarid savanna

Recreational camping has been shown to suppress plant cover and expand bare ground area. These shifts have important implications for soil health. We used campsites in a semiarid savanna at the Santa Rita Experimental Range (SRER) in Arizona to test the hypotheses that 1) recreational camping is a disturbance that limits plant cover and soil microbial activity, and 2) the presence of Prosopis, which is known to encourage a fertility island effect, increases soil microbial activity within campsites. Camping disturbance did not influence any sampled measures of edaphic properties, plant cover, or soil microbial biomass and exoenzyme activities. However, the presence of Prosopis resulted in elevated litter, total dissolved nitrogen (TDN), and dissolved organic carbon (DOC). Multiple linear regression models suggest that observed resistance of soil microbial activities to camping disturbance may be due to both increased availability of organic C and N substrates beneath Prosopis and heightened seasonal water availability.

Robustness of the autochthonous microbial soil community after amendment of cattle manure or its digestate

In the last two decades, a change in land use has taken place in Europe. Manures that used to be applied to agricultural soils are now used in biogas reactors, and instead, digestate is applied. Here, we simulated soil amendment with either fresh or anaerobically digested cattle manure. The aim was to investigate the resilience of the resident microbiota and detect differences in the microbial biomass and activity after fertilizer amendment. Furthermore, the physiological community profile and the role of the indigenous microbial community was elucidated. In a microcosm experiment, two kinds of agricultural soil (γ-irradiated versus non-irradiated) were amended with either treatment. The effect of amendments on the community composition and physiological activity was tested immediately, after 1 and 3 months of incubation through 16S rRNA gene amplicon sequencing (initial soil community), genetic and physiological profiling (PCR-DGGE and MicroResp™), and measurement of basal respiration and microbial biomass. Either fertilizer did not affect the community composition of dominant fungi and bacteria in non-irradiated soils. This indicates the ability of the indigenous microbiota to outcompete allochthonous microorganisms. Soil microbial biomass was not changed, whereas basal respiration was significantly higher after amendment, especially when using fresh manure. MicroResp™ revealed slightly higher respiration for some substrates after 1 month; this finding was, however, not persistent and similar for manure and digestate. Generally, after 1 month, treatments returned to control levels for all parameters. In conclusion, amendment with anaerobically digested manure did not have a greater impact on soil microbial properties.

Measuring Soil Respiration with a Gas Chromatograph

Measuring Soil Respiration with a Gas Chromatograph

Interactive effects of liming and nitrogen management on carbon mineralization in grassland soils

Grassland management has the potential to modify soil carbon (C) mineralization, but the relative importance of combined soil improvers or fertilizers and land use intensity on C mineralization remains unclear. We used laboratory incubations to examine the interactive effects of lime addition, mineral N inputs and grassland management intensity on soil C mineralization potential over 84 days. Monitoring of CO2 production and O2 consumption was coupled with measurements of soil pH and microbial biomass for soils obtained from grasslands with contrasting levels of land management intensity (extensive versus intensive N management) at each of three upland sites. Lime addition increased soil pH, cumulative CO2 production and O2 consumption across all N treatments and soils. These positive effects of liming either partly or fully compensated the observed negative effects of N on CO2 and O2 fluxes. Responses to combined liming and N addition varied depending on management intensity; N addition had no effect on liming response ratios for O2 consumption at intensively-managed sites, but increased the magnitude of positive liming response ratios for O2 consumption at extensively-managed sites. Overall, our results suggest that the magnitude of liming-induced increases in C mineralization is mediated by effects of both past and present N management on the soil microbial community. This highlights the importance of considering agricultural practices when assessing the net contribution of agricultural liming to soil-atmosphere feedbacks on climate change.

Rapid estimation of microbial biomass in acid red soils with and without substrate incorporation

A rapid and alternative measurement of microbial biomass in acid red soils with and without substrate incorporation is proposed for soil quality evaluation. Soil microbial biomass C (SMBC) and N (SMBN) in 24 typical red soil samples developed from two parent materials (granite and arenaceous shale) were measured using fumigation-extraction followed by dry combustion method in comparison with ultraviolet (UV) spectrophotometry (increase in absorbance at 280 nm, ΔUV280). The reliability of microbial biomass estimation by UV spectrophotometry was verified using six representative red soils amended with biochar (0, 1, 3 and 5%) and glucose (0, 100, 500 and 1000 mg kg−1) separately. ΔUV280 was strongly correlated with SMBC and SMBN measured by dry combustion, regardless of biochar/glucose incorporation. Validated conversion equations from unamended soil data were dependent on confounding effects of organic C and particle size and can be described as follows: SMBC = 27.08 × ΔUV280 (R2 = 0.67, n = 24) and SMBN = 3.62 × ΔUV280 (R2 = 0.69, n = 24). Regression models for rapid estimation of microbial biomass in red soils from different parent materials had to be calibrated separately in case of amendments. In most cases, SMBC (R2 of 0.75–0.76 and root mean square error (RMSE) of 22.2–29.3 mg kg−1) and SMBN (R2 of 0.74–0.80 and RMSE of 2.60–14.2 mg kg−1) can be predicted from ΔUV280 in biochar/glucose-amended soils using these equations. The slope of the regression of SMBC against ΔUV280 shifted in biochar-amended granite soils, mainly due to uncoordinated changes of SMBC in response to the difference in parent material-induced nutrient availability, while shifts of SMBC (or SMBN) against ΔUV280 in glucose-amended arenaceous shale soils were attributed to particle size distribution. Soil microbial biomass (SMBC and SMBN) in red soils can be rapidly predicted by fumigation-extraction with UV spectrophotometry detection and corresponding correction of calibration curves, depending on soil nutrient availability, particle size distribution and organic C levels.

Shift from dormancy to microbial growth revealed by RNA:DNA ratio

Although soil microorganisms spend most of their lifetime at dormant or resting states, they are quickly activated by substrate input and easily switch to growth. Under steady-state, the double-stranded DNA (dsDNA) content is considered as a measure of total microbial biomass while the RNA content mainly indicates the active microbial fraction. In growing population, however, an increase in DNA is solely related to replication of microbial cells, while the RNA, besides growth, is also involved in non-growth processes. Therefore, the dsDNA and RNA content increase differently during microbial growth, applying the RNA:dsDNA ratio as promising indicator of growth-related and non-growth microbial activity. To what extend the ratio can be used to comparatively infer investment in microbial biomass production versus maintenance-related synthesis following substrate induction remains to be studied.We measured the RNA:dsDNA ratios of representative soil types of four different ecozones before and after glucose addition to prove the prediction capacity of physiological status of soil microorganisms. The RNA:dsDNA ratio remained stable after activation of microorganisms in Retisol and Luvisol indicating balanced growth. In contrast, very moderate increase in DNA in sandy Calcisol was accompanied by disproportionally high increase in RNA. As a result, the RNA:dsDNA ratio in Calcisol with low nutrient status increased by 36-fold after glucose amendment, indicating strong non-growth related processes. The RNA recovery decreased exponentially with increasing clay content, indicating the strong association to the textures of the soil types. This suggests, that the underestimation of RNA yields in clayey soils biased the RNA:dsDNA ratio, and subsequently the physiological state of the microbial community is not adequately represented in soils clay contents exceeding 30%. Monitoring the relative changes in dynamics are required to overcome the restricted applicability of RNA:dsDNA ratio in soils with high clay content.

Examination of residual chloroform interference in the measurement of microbial biomass C by fumigation-extraction

Using the chloroform fumigation-extraction (CFE) method for determining microbial biomass in soils, a presumption is followed, i.e., that chloroform becomes fully evacuated from the fumigated soil before extraction. Otherwise, the C-containing fumigant may comprise an unknown part of the extractable C flush, resulting in certain overestimation of soil microbial biomass. A quantitative assessment of extractable residual chloroform levels in fumigated soils was performed, testing soils varying in clay and organic matter contents. The tests were performed over a broad range of soil moisture conditions as to facilitate a mechanistic interpretation of possible chloroform interactions in the studied soils (i.e., dissolution, sorption). The results support the validity of the CFE method's application in agricultural soils, demonstrating the insignificance of biomass overestimation arising from residual chloroform.

A decadal-scale nutrient loading study in a coastal wetland: Impacts on soil microbial processes

Fertilizer use in agricultural lands and treated wastewater disposal have increased bioavailable nitrogen (N) and phosphorus (P) loading to wetlands and aquatic systems with potential implications for nutrient cycling and soil microbial processes. Recently, conflicting results of N loading impacts on microbial-mediated organic matter decomposition in coastal wetland soils have been reported. However, to date, the majority of studies have investigated short-term nutrient loading effects while few have reported long-term integrated impacts. Here, we present results from an 11-year soil nutrient loading study in an oligohaline coastal wetland treated with 0, 50, 200, or 1200kgNha−1yr−1 in combination with 0 or 131kgPha−1yr−1. We measured soil (0–10cm) physiochemical characteristics, soil microbial biomass N, and measures of biogeochemical cycling of N, including potentially mineralizable nitrogen and denitrification rates. Our results show that soil total P increased with P additions, but no significant differences in measures of soil microbial biomass or activity occurred with N, P, or N×P loading. We conclude that long-term N loading at rates equivalent to Mississippi River diversions had no persistent lasting effects on total N or examined soil microbial measures.

Soil carbon characterization and nutrient ratios across land uses on two contrasting soils: Their relationships to microbial biomass and function

Soil carbon (C) plays a central role in the global biogeochemical cycles of most major nutrients, but the degree to which the quality versus quantity of C controls microbial abundance and function across land uses is still somewhat uncertain. We measured soil organic matter (SOM) concentration and composition as well as nutrient ratios and other soil characteristics on two contrasting soil types across three land uses (forest, pasture, maize cropping), to determine their relationships to microbial abundance and specific measures of microbial activity (e.g. qCO2, the ratio of respiration rate to microbial biomass, and net laboratory N mineralization). Although there was significant variability in bulk SOM composition (by 13C NMR spectroscopy), we could detect differences between broad-leaved forest and pasture/maize systems on a landscape scale, primarily attributable to differences in aryl C content. Variability in O-alkyl C between sites correlated strongly with the soil C:N ratio, but variability in alkyl C (which was particularly evident in pasture sites) could not be adequately explained by measured environmental or soil characteristics. Soil C:P and N:P ratios followed similar patterns with forest > pasture > maize. Bulk soil C:N, hot-water extractable C:N and particulate C:N all followed similar patterns with forest > pasture ≈ maize cropping. Microbial biomass C:N followed a different pattern, however, with forest ≈ pasture > maize. Despite the differences in SOM composition and nutrient ratios, anaerobically mineralizable N and hot-water extractable C (as a measures of available C) best explained the variation in microbial biomass and function across sites. Anaerobically mineralizable N generally explained the most variation for microbial biomass and qCO2 and had the smallest soil or land use effect. Hot-water extractable C explained the most variance for net N mineralization. Addition of stoichiometric measures and other soil attributes (e.g. soil C:N, C:P, δ15N) in a multiple regression model explained more of the variation than a single factor plus the land use effect (though soil order still explained a small, but significant amount of variance for measures of microbial biomass). A measure of available C, however, was needed to explain the maximum amount of variance in microbial biomass and function across sites (i.e. total C plus nutrient status and other soil attributes could not explain as much of the variance). Our data suggest that nutrient content/stoichiometry does assist in defining the quality of SOM, but a measure of available C (similar to the “active” pool in C models) is also needed. Anaerobically mineralizable N and/or hot water–water extractable C appear to be adequate measures of available C that relate to “active” C, but C functional groups (from 13C NMR spectroscopy) were not particularly useful for this purpose.

Sustainable viticulture: The carbon-sink function of the vineyard agro-ecosystem

In addition to food production, tree crop-based agrosystems can provide other ecosystem services (ES) such as soil fertility maintenance, soil and water retention and landscape preservation. Tree crops can also act as carbon (C) storage facilities and climate change mitigating systems. We focus on the nature of viticulture as a provider of ES, in particular C storage. This study has been carried out focusing on two different wine-grape growing areas in central Italy (Latium region; natural and peri-urban hilly areas) where the vines (Vitis vinifera sativa L., cv Merlot) were cultivated according to two different management techniques (conventional vs organic farming). Grapevine C storage levels were analyzed in the two main vine C pools during the 2011 and 2012 growing seasons, i.e., above-ground and below-ground biomass, in accordance with the Net Ecosystem Production (NEP) methodology. In order to quantify C sequestration at the vineyard level, we determined soil C sequestration and soil functionality through the measurement of Total Organic Carbon (TOC), microbial biomass and soil and grapevine root respiration. To conclude, the net C balance was assessed at both the grapevine and the vineyard scale.Although the highest dry matter partitioning in the grapevines was measured in the above-ground organs, the root systems contributed to between 9 and 26% of the total vine C fixation. The soil’s C fixation was maximized in the organically managed vineyards (73.35tCha−1). The CO2 sequestration of one hectare of vineyard ranged between 5.72 (±0.07) and 7.23(±1.11)tCha−1year−1. Soil respiration represented the main (99%) CO2 emission source in the vineyard agro-ecosystems.The Principal Component Analysis (PCA) indicated that the soil physical characteristics, the grapevine’s biological properties, and the vineyard’s management techniques, like those handling the inter-row space that highly differ in the organic and conventional farming, turned out to be the main factors influencing the soil’s C storage and consequently the vineyard’s C balance.Together, these findings prove that vineyards can act as C sinks, if properly managed. Furthermore, vineyards could represent a crucial cropping system able to provide pivotal ecological services such as carbon dioxide sequestration. Viticulture can also contribute to the preservation and regulation of natural resources –, such as soil and agricultural landscapes, according to the new European Commune Agricultural Policy (CAP).

Nitrogen uptake efficiency of maize in monoculture and intercropped with Brachiaria humidicola and Panicum maximum in a dystrophic Red-Yellow Latosol of the Brazilian Cerrado

No-tillage systems associated with intercropping practices of grains and forages as cover crops are increasing in the Cerrado agricultural areas. The aim of this study was to quantify the nitrogen (N) uptake efficiency of maize (Zea mays L.) grown as monoculture and intercropped with tropical forages under a no-tillage system by using the 15N isotope tracer in conjunction with measurements of soil microbial biomass N and available soil N. The experiment was conducted in the 2010–11 growing season, in a Dystrophic Red-Yellow Latosol (Typic Haplustox) in the Cerrado. The experiment was established in a complete randomised block design with three replicates with the following treatments: maize monoculture; maize intercropped with Panicum maximum Jacq. cv. Aruana; and maize intercropped with Brachiaria humidicola (Rendle) Schweick. Nitrogen was applied as ammonium sulfate at a rate of 100 kg ha–1 (30 kg N ha–1 was applied at planting and 70 kg N ha–1 as a side-dressing). The N-fertiliser uptake efficiency in maize and grain yield was not affected by the presence of the intercropped forages. The intercropped B. humidicola and P. maximum recovered 2.08% and 3.71% of the N fertiliser applied, respectively. The soil was the main N source for maize. Maize intercropped with P. maximum showed higher values of microbial biomass N and available N in the soil.