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Aims: We aimed at assessing the influence of above- and below-ground environmental conditions over the performance of Centaurea jacea L., a drought-resistant grassland forb species. Methods: Transpiration rate, CO2 assimilation rate, leaf water potential, instantaneous and intrinsic water use efficiency, temperature, relative humidity, vapor pressure deficit and soil water content in one plant and root length density in four plants, all grown in custom-made columns, were monitored daily for 87 days in the lab. The soil water isotopic composition in eleven depths was recorded daily in a non-destructive manner. The isotopic composition of plant transpiration was inferred from gas chamber measurements. Vertical isotopic gradients in the soil column were created by adding labeled water. Daily root water uptake (RWU) profiles were computed using the multi-source mixing model Stable Isotope Analysis in R (Parnell et al. PLoS ONE 5(3):1–5, 2010). Results: RWU occurred mainly in soil layer 0–15 cm, ranging from 79 to 44%, even when water was more easily available in deeper layers. In wet soil, the transpiration rate was driven mainly by vapor pressure deficit and light intensity. Once soil water content was less than 0.12 cm3 cm− 3, the computed canopy conductance declined, which restricted leaf gas exchange. Leaf water potential dropped steeply to around − 3 MPa after soil water content was below 0.10 cm3 cm− 3. Conclusion: Our comprehensive data set contributes to a better understanding of the effects of drought on a grassland species and the limits of its acclimation in dry conditions. [ABSTRACT FROM AUTHOR]

Fairy rings promoting circular greening belts in the vegetation can shape soil microbial communities by altering soil conditions. Knowledge about soil variables involved in this process is incomplete. We characterised the soil microbial communities of six fairy rings in a montane grassland using phospholipid fatty acid (PLFA) profiling, and studied if changes in soil properties corresponded to changes in soil microbial PLFA patterns. Exchangeable potassium (K) decreased inside the current rings, while soil moisture increased in the zones where the greening belts were two years before sampling (R2015). Fairy ring associated changes in PLFA composition were highly related to soil K. Gram‐negative bacteria were associated with the zones outside the ring with the highest K content, whereas Gram‐positive bacteria proportions increased inside the ring‐affected zones. An environmental stress indicator, the iso to anteiso ratio of PLFA 17:0, decreased in the R2015 zones, coinciding with the highest soil moisture contents. Our findings highlight the unreported importance of soil K in fairy ring dynamics affecting microbial communities. This common omission could lead to incorrect conclusions. Hence, the effects of fairy rings on soil should be further tested. Highlights: Exchangeable potassium (K) decreased inside the current fairy rings.Fairy ring associated changes in PLFA microbial composition were present related to soil K.Soil moisture increased in the zones where the rings were 2 years before sampling.An environmental stress indicator decreased in those areas. [ABSTRACT FROM AUTHOR]

In situ field measurements as well as targeted laboratory studies have shown that freeze-thaw cycles (FTCs) affect soil trace gas fluxes. However, most of past laboratory studies adjusted soil moisture before soil freezing, thereby neglecting that snow cover or water from melting snow may modify effects of FTCs on soil trace gas fluxes. In the present laboratory study with a typical semi-arid grassland soil, three different soil moisture levels (32 %, 41 %, and 50 % WFPS) were established (a) prior to soil freezing or (b) by adding fresh snow to the soil surface after freezing to simulate field conditions and the effect of the melting snow on CO, CH, and NO fluxes during FTCs more realistically. Our results showed that adjusting soil moisture by watering before soil freezing resulted in significantly different cumulative fluxes of CH, CO, and NO throughout three FTCs as compared to the snow cover treatment, especially at a relatively high soil moisture level of 50 % WFPS. An increase of NO emissions was observed during thawing for both treatments. However, in the watering treatment, this increase was highest in the first thawing cycle and decreased in successive cycles, while in the snow cover treatment, a repetition of the FTCs resulted in a further increase of NO emissions. These differences might be partly due to the different soil water dynamics during FTCs in the two treatments. CO emissions were a function of soil moisture, with emissions being largest at 50 % WFPS and smallest at 32 % WFPS. The largest NO emissions were observed at WFPS values around 50 %, whereas there were only small or negligible NO emissions from soil with relatively low soil water content, which indicates that a threshold value of soil moisture might exist that triggers NO peaks during thawing. [ABSTRACT FROM AUTHOR]

Grasslands are one of the most common biomes in the world with a wide range of ecosystem services. Nevertheless, quantitative data on the change in nitrogen dynamics in extensively managed temperate grasslands caused by a shift from energy- to water-limited climatic conditions have not yet been reported. In this study, we experimentally studied this shift by translocating undisturbed soil monoliths from an energy-limited site (Rollesbroich) to a water-limited site (Selhausen). The soil monoliths were contained in weighable lysimeters and monitored for their water and nitrogen balance in the period between 2012 and 2018. At the water-limited site (Selhausen), annual plant nitrogen uptake decreased due to water stress compared to the energy-limited site (Rollesbroich), while nitrogen uptake was higher at the beginning of the growing period. Possibly because of this lower plant uptake, the lysimeters at the water-limited site showed an increased inorganic nitrogen concentration in the soil solution, indicating a higher net mineralization rate. The N2O gas emissions and nitrogen leaching remained low at both sites. Our findings suggest that in the short term, fertilizer should consequently be applied early in the growing period to increase nitrogen uptake and decrease nitrogen losses. Moreover, a shift from energy-limited to water-limited conditions will have a limited effect on gaseous nitrogen emissions and nitrate concentrations in the groundwater in the grassland type of this study because higher nitrogen concentrations are (over-) compensated by lower leaching rates. [ABSTRACT FROM AUTHOR]

Soils are the dominant source of atmospheric nitrous oxide (N2O), especially agricultural soils that experience both waterlogging and intensive nitrogen fertilization. However, soil heterogeneity and the irregular occurrence of hydrological events hamper the prediction of the temporal and spatial dynamics of N2O production and transport in soils. Because soil moisture influences soil redox potential, and as soil N cycling processes are redox-sensitive, redox potential measurements could help us to better understand and predict soil N cycling and N2O emissions. Despite its importance, only a few studies have investigated the control of redox potential on N2Oemission from soils in detail. This study aimed to partition the different microbial processes involved in N2O production (nitrification and denitrification) by using redox measurements combined with isotope analysis at natural abundance and 15N-enriched. To this end, we performed long-term laboratory lysimeter experiments to mimic common agricultural irrigation and fertilization procedures. In addition, we used isotope analysis to characterize the distribution and partitioning of N2O sources and explored the 15N-N2O site preference to further constrain N2O microbial processes. We found that irrigation, saturation, and drainage induced changes in soil redox potential, which were closely related to changes in N2O emission from the soil as well as to changes in the vertical concentration profiles of dissolved N2O, nitrate (NO3−) and ammonium (NH4+). The results showed that the redox potential could be used as an indicator for NH4+, NO3−, and N2O production and consumption processes along the soil profile. For example, after a longer saturation period of unfertilized soil, the NO3− concentration was linearly correlated with the average redox values at the different depths (R2 = 0.81). During the transition from saturation to drainage, but before fertilization, the soil showed an increase in N2O emissions, which originated mainly from nitrification as indicated by the isotopic signatures of N2O (δ15N bulk, δ18O and 15N-N2O site preference). After fertilization, N2O still mainly originated from nitrification at the beginning, also indicated by high redox potential and the increase of dissolved NO3−. Denitrification mainly occurred during the last saturation period, deduced from the simultaneous 15N isotope analysis of NO3− and N2O. Our findings suggest that redox potential measurements provide suitable information for improving the prediction of soil N2O emissions and the distribution of mineral N species along the soil profile under different hydrological and fertilization regimes. [ABSTRACT FROM AUTHOR]

Regional greenhouse gas (GHG) budgets in vast grasslands may be changing due to overgrazing and grassland types. However, the comprehensive effects of grazing patterns, environmental factors and grassland types on soil carbon dioxide (CO 2), methane (CH 4) and nitrous oxide (N 2 O) exchanges have been poorly studied. This study investigates the effects of long-term grazing on the soil-atmosphere exchanges of CO 2 , CH 4 and N 2 O in important processes within different grasslands in Inner Mongolia, China. Using manual static chamber and gas chromatography, we measured the fluxes of CO 2 , CH 4 and N 2 O from intact soil cores of paired grazed/ungrazed sites collected from two typical steppes (S tipa grandis and Leymus chinensis): one wetland in a flood plain and one desert steppe in the region of the Xilin River catchment, Inner Mongolia. Soil gas flux and concentration measurements were conducted in four simulated conditions (i.e. , drought, dry-wet, intense rainfall and freeze-thaw), which represent important processes in annual GHG exchanges. Extreme drought significantly inhibited CO 2 and N 2 O emissions in all plots but did not change the CH 4 uptake by typical steppes. Dry-wet transition and intense rainfall could remarkably promote soil CO 2 emission pulses at different types, significantly decrease CH 4 uptake by typical steppes, and arouse N 2 O emission pulses at all plots with different times of occurrence. During the freeze-thaw simulation, temperature-induced soil CO 2 emission and CH 4 uptake/emission presented a clear alternative variation, while soil thaw only slightly increased (<15 μg N m−2 h−1) in the steppes and sand dunes and was significantly higher in the wetland (11–96 μg N m−2 h−1). Long-term grazing significantly inhibited soil respiration rates at all grassland types, significantly decreased CH 4 uptake by the Leymus chinensis steppes, and did not show significant influence on N 2 O emission due to large spatial variations for all types. Compared to the ungrazed Leymus steppes, Stipa steppes, sand dune and wetland, continuously grazed sites were significantly reduced by 22%, 38%, 48% and 47% in total GHG emissions, respectively. Our results indicate that the potential of the steppe soil CH 4 sink function can be offset by N 2 O emission, especially in over-grazed plots. Furthermore, N 2 O emissions should be considered in wetland rangelands with significantly higher N 2 O emission potential (range: 0–343 μg N m−2 h−1) more than steppes (range: 0–132 μg N m−2 h−1) and sand dunes (range: 0–49 μg N m−2 h−1). Nevertheless, comprehensive evaluation of the grazing effect on ecosystem GHG emissions merits consider in both field observation and incubation experiments because soil properties and environmental factors could be changed by vegetation growth in different grazing practices. [ABSTRACT FROM AUTHOR]

• Black nitrogen represented by pyrrole was formed from nitrite-SOM reactions. • Amides were the main products of chemical nitrogen fixation in this study. • The formation of black nitrogen increased the stability of soil organic matter. • Lignin and phenols were important reactants in chemical N immobilization. Nitrogen (N) immobilization controls the N availability in soil, however, mechanisms involved in the chemical N fixation into soil organic N (SON) through reactions of reactive N compounds with soil organic matter (SOM) is not clear. Knowledge about the composition and stability of chemically produced SON is limited, which impedes understanding of the interplay of N and carbon (C) cycles at both the local and global scale. Here, we studied the chemical N immobilization of nitrite in soils from grassland, cropland, and forest with 15N labelling technique. And solid state 15N- and 13C NMR spectroscopies were applied to further explore the structure of chemically immobilized SON. We found that the chemical retention rate of nitrite did not differ significantly between land-uses, while the fulvic acid fraction was the SOM component most reactive to nitrite. In contrast to the common assumption that amides are mainly of biological origin and that black N compounds are formed from organic N compounds at high temperature during fires, our study revealed that amides and black N in the form of pyrroles were the main products of chemical reactions of nitrite with SOM. These findings indicate that chemical processes play a key role in biogeochemical N cycling, and provide new insight into the mechanisms of C N interactions in soil. [ABSTRACT FROM AUTHOR]