Publications

ABSTRACT Aim Progress has been made in understanding the relationship between biodiversity and ecosystem functioning (BEF) in both experimental and real‐world ecosystems. Yet, we have a limited understanding of the extent to which biodiversity affects ecosystem functioning in heterogeneous environments and whether variation in ecosystem functioning between communities is related to variation in species richness or turnover. Here, we quantify the relative contribution of variation in species richness and species turnover to variation in ecosystem functioning between communities (i.e., the diversity effect) along two tropical elevational gradients. Location Andes (Ecuador) and Mt. Kilimanjaro (Tanzania). Taxa Studied Woody plants, springtails, soil arthropods, ants and frugivorous birds. Methods We collected data on seven ecosystem functions, including biomass and process rates, across six ecosystem types along the two elevational gradients. We then combine the ecological Price equation with the concept of β‐diversity to quantify how the diversity effect is shaped by environmental heterogeneity within and across ecosystem types, and whether the effect of environmental heterogeneity is primarily mediated by variation in species richness or species turnover. Results The diversity effect on ecosystem functioning increased consistently with environmental heterogeneity on both mountains. Species richness and turnover, on average, contributed similarly to the diversity effect on ecosystem functioning in both mountain regions, but effect sizes varied across functions. The increase in the diversity effect with environmental heterogeneity was primarily mediated by species richness, while species turnover played a secondary role in mediating the effects of environmental heterogeneity. Main Conclusions Our study reveals that the diversity effect on ecosystem functioning increases with environmental heterogeneity and that species richness, rather than species turnover, primarily drives this relationship. The dominant role of species richness in mediating the effect of environmental heterogeneity indicates that BEF relationships along environmental gradients are strongly influenced by environmental filters that limit local species coexistence.

Nitrous oxide (N 2 O) emissions from soil are an important contributor to global warming, particularly from intensively fertilized croplands. Biochar is commonly applied to reduce N 2 O emissions and raise soil fertility by regulating soil structure, microbial processes, and crop nitrogen use efficiency. However, the effects of biochar on N 2 O emissions from fertilized croplands depend on its sources and production conditions, including feedstocks, pyrolysis temperatures, properties and application rates. To generalize findings from individual studies, we synthesized 550 observations that simultaneously measured N 2 O emissions, nitrification enzyme activity (NEA), denitrification enzyme activity (DEA), and relevant functional genes ( AOA , AOB , narG , nirK , nirS, and nosZ ) to assess their responses to biochar production conditions, properties and application rates across cropland ecosystems. Wheat straw biochar increased the abundances of all functional genes related to N 2 O emissions and DEA, while pyrolysis temperatures exceeding 450 ℃ decreased DEA. Low-temperature pyrolysis biochar was particularly effective in reducing N 2 O emissions. The abundance of denitrifiers and DEA-related genes increased with the pH, ash content, and cation exchange capacity (CEC) of biochar. As biochar application rates increased, N 2 O emissions were reduced, largely due to an increase in nosZ gene abundance and soil pH. A common biochar application rate of 20 t ha −1 decreased N 2 O emissions by 19%, primarily through reduced denitrification, while 50 t ha −1 reduced N 2 O emissions by 48%. Biochar preparation conditions, and property changes had no significant effects on N 2 O emissions at application rates below 20 t ha −1 . When application rates exceeded 20 t ha −1 , biochar pyrolysis temperature and properties influenced N 2 O emissions, indicating a threshold application rate, beyond which biochar affects N 2 O emissions. Biochar regulates the soil N cycle and N 2 O emissions primarily through denitrification, with effects dependent on the biochar application rate. These findings underscore the crucial potential of increased biochar application to reduce N 2 O emissions from fertilized soils globally, thereby contributing to climate change mitigation. Graphical Abstract

Soil from Free-Air Carbon dioxide Enrichment (FACE) plots (FAL, Braunschweig) under ambient air (375 ppm; delta(13)C-CO(2)-9.8 per thousand) and elevated CO(2) (550 ppm; for six years; delta(13)C-CO(2)-23 per thousand), either under 100% nitrogen (N) (180 kg ha(-1)) or 50% N (90 kg ha(-1)) fertilisation treatments, was analysed by thermogravimetry. Soil samples were heated up to the respective temperatures and the remaining soil was analysed for delta(13)C and delta(15)N by Isotope Ratio Mass Spectrometry (IRMS). Based on differential weight losses, four temperature intervals were distinguished. Weight losses in the temperature range 20-200 degrees C were connected mostly with water volatilisation. The maximum weight losses and carbon (C) content were measured in the soil organic matter (SOM) pool decomposed at 200-360 degrees C. The largest amount of N was detected in SOM pools decomposed at 200-360 degrees C and 360-500 degrees C. In all temperature ranges, the delta(13)C values of SOM pools were significantly more negative under elevated CO(2) versus ambient CO(2). The incorporation of new C into SOM pools was not inversely proportional to its thermal stability. 50% N fertilisation treatment gained higher C exchange under elevated CO(2) in the thermally labile SOM pool (200-360 degrees C), whereas 100% N treatment induced higher C turnover in the thermally stable SOM pools (360-500 degrees C, 500-1000 degrees C). Mean Residence Time of SOM under 100% N and 50% N fertilisation showed no dependence between SOM pools isolated by increasing temperature of heating and the renovation of organic C in those SOM pools. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C.

A better understanding of the fate of fertilizer nitrogen (N) is critical to design appropriate N management strategies in plastic-mulched croplands. We evaluated the effects of plastic mulch on urea-N recovery by crops and loss from soil in furrow-ridge plots, with and without maize (Zea mays L.) cropping, in a semi-arid rain-fed site in China. We applied the same rate of urea-N (281kgha−1) to all treatments during the preparation of the furrow-ridges in 2011 and 2012 but 15N-labeled the urea in 2011 only. We used transparent film to cover all soil surfaces in the mulched treatments and seeded maize in furrows in treatments with crop. In 2011, plastic mulch increased the total N uptake in the aboveground biomass of maize by 53%, whereas it decreased the in-season labeled-N uptake by 19%, compared to non-mulched treatment. At harvest in 2011, in mulched treatments the total labeled-N remaining in the 0−170cm soil layer was 25% greater whereas unaccounted labeled-N was 69% less, than in non-mulched treatments, regardless of whether maize was cropped. In 2012 the effect of mulch on total maize N uptake was comparable to that in 2011, but the residual soil labeled-N uptake by maize was 63% higher in mulched compared to non-mulched treatment. At harvest in 2012, plastic mulch increased total labeled-N remaining in the 0−170cm depth in cropped soils and unaccounted labeled-N in non-cropped soils, compared with no mulch. Our results indicate that plastic mulch profoundly changes the fate of urea-N in maize production in cold and dry croplands.

No abstract is provided for this article.

The ability of roots to extract water and nutrients from soil depends on the biophysical properties of the rhizosphere, which are strongly influenced by mucilage secretion. The aim of this study was to introduce the concept of rhizoligands to engineer the biophysical properties of the rhizosphere. A rhizoligand is defined as an additive that increases the wettability of the rhizosphere and links the mucilage network to main intimate contact with the root surface. We hypothesize that rhizoligands: i) facilitate the rewetting of the rhizosphere during repeated drying and wetting cycles; ii) enhance rhizosheath formation; iii) increase enzyme activities in the rhizosphere; and iv) increase plant biomass. A commercial surfactant was selected as the prototype rhizoligand to test the effect on the rhizosphere biophysical properties of white lupin grown in quartz sand and subjected to six drying-rewetting cycles. Half of the plants were irrigated with water and the other half with the rhizoligand solution. When plants were 50 days old, we measured: i) soil water content; ii) rhizosheath mass; iii) activity of selected enzymes; iv) carbon content in the rhizosphere; and v) plant biomass. Rhizoligand increased rewetting rate of the rhizosphere after drying and subsequent rewetting, resulting in a greater soil water content. Rhizosheath formation was improved in plants irrigated with rhizoligand and sand particles attached to the roots increased by 1.64 times compared to plants irrigated with water. Activity of the enzymes chitinase, sulfatase, and β-glucosidase were 4, 7.9, and 1.5 times greater in the rhizosphere of plants irrigated with rhizoligand than in the rhizosphere of plants irrigated with water. Plant biomass was 1.2 fold greater in samples irrigated with rhizoligand solution than in samples irrigated with water. We conclude that application of rhizoligand improves plant performance by influencing the water dynamics in the rhizosphere and the plant, increasing the mechanical stability of the rhizosheaths and increasing the enzyme activities in the rhizosphere. Such effects are probably triggered by the interaction between mucilage and the applied rhizoligand, which reduces mucilage swelling (possibly by cross-linking mucilage polymers) and thus by increasing its viscosity keeps the mucilage close to the root surface. We propose the rhizoligand concept as a strategy to engineer the rhizosphere properties and to improve plant tolerance to water shortage.

Microorganisms govern soil nutrient cycling. It is therefore critical to understand their responses to human-induced increases in N and P inputs. We investigated microbial community composition, biomass, functional gene abundance, and enzyme activities in response to 10-year N and P addition in a primary tropical montane forest, and we explored the drivers behind these effects. Fungi were more sensitive to nutrient addition than bacteria, and the fungal community shift was mainly driven by P availability. N addition aggravated P limitation, to which microbes responded by increasing the abundance of P cycling functional genes and phosphatase activity. In contrast, P addition alleviated P deficiency, and thus P cycling functional gene abundance and phosphatase activity decreased. The shift of microbial community composition, changes in functional genes involved in P cycling, and phosphatase activity were mainly driven by P addition, which also induced the alteration of soil stoichiometry (C/P and N/P). Eliminating P deficiency through fertilization accelerated C cycling by increasing the activity of C degradation enzymes. The abundances of C and P functional genes were positively correlated, indicating the intensive coupling of C and P cycling in P-limited forest soil. In summary, a long-term fertilization experiment demonstrated that soil microorganisms could adapt to induced environmental changes in soil nutrient stoichiometry, not only through shifts of microbial community composition and functional gene abundances, but also through the regulation of enzyme production. The response of the microbial community to N and P imbalance and effects of the microbial community on soil nutrient cycling should be incorporated into the ecosystem biogeochemical model.

Moderate grazing can sustain high species diversity and productivity. However, nitrogen enrichment often reduces species richness while promoting primary productivity, which contradicts the traditional understanding of the positive effect of plant diversity on productivity. Whether the responses of diversity and productivity to N enrichment on a long-term scale conform to those on short-term scale. Furthermore, chemical N forms may alter species richness or functional diversity because of species preferences for different N forms. We used data from a 19-year field experiment with manipulated livestock exclusion and the addition of N in three chemical forms (ammonium, nitrate, and their mixture) in an alpine grassland to test their effects on diversity and aboveground productivity. Productivity in no N without grazing plots (Ctr) initially increased by 66 % during the first five years after livestock exclusion (0.53 kg m−2), following which productivity fluctuated at a similar level as that in winter livestock grazing (0.32 kg m−2) for the next eight years and ultimately decreased to the level of 0.24 kg m−2, below that in Gr with continued species loss. The addition of N forms in grazing exclusion increased productivity, accompanied by the loss of rare species in the first five years. Afterward, productivity decreased, and it took longer than the control to reach the same level as that seen in winter grazing. Then, productivity continued to decline with more species loss. The functional diversity of leaf traits and plant growth forms decreased. Ammonium favored grasses with guerrilla and phalanx rhizomes, nitrate favored sedges and forbs with guerrilla rhizomes, and ammonium-nitrate sustained a relatively higher diversity of plant growth forms. Our general conclusion is that grasslands with moderate grazing are beneficial for sustaining diverse species and high productivity. N enrichment reduces plant diversity, and the increase in productivity driven by N is only transitional, as productivity declines in the long run, with continuous N addition.

As the primary microbial substrate after shoot cutting, the element stoichiometry of root-detritus (dying or dead roots) influences the enzyme activity in root-detritusphere. However, the effect of the C/P ratio of root-detritus on the dynamics and distribution of enzyme activities is little revealed. We hypothesised that P fertilisation would decrease the C/P ratio of root-detritus, therefore affecting the hotspot areas and hot moments of C-acquiring and P-acquiring enzyme activities, as well as their activity ratio (C/P acquisition ratio). Root-detritus of low (59.0) and high (170.8) C/P ratios was produced in P-poor soil with and without P fertilisation, respectively. In situ soil zymography showed that the distribution of C-acquiring enzymes (β-glucosidase and cellobiohydrolase) was more associated with root-detritus than P-acquiring enzymes (acid and alkaline phosphomonoesterase). P fertilisation increased the hotspot areas of C-acquiring enzyme activities over the experiment, without influencing their temporal dynamics. However, its effect on phosphomonoesterase activities depended on the decomposition and delayed the appearance of the highest hotspot areas. P supply met the microbial demand in P-fertilised soil, with high C/P acquisition ratio and constant stoichiometry of microbial biomass C (MBC)/microbial biomass P (MBP). A low C/P acquisition ratio and high MBC/MBP in non-fertilised soil was observed, indicating P limitation for microorganisms. After the 150-day incubation, Olsen P significantly increased in P-fertilised soil (P < 0.05), whereas it decreased in the root-detritusphere of non-fertilised soil. We conclude that the decomposition of root-detritus with a low C/P ratio has potential to improve soil P availability; however, C-P imbalance may increase during the decomposition of root-detritus with a high C/P ratio.

ABSTRACT The drying‐rewetting of soil can increase the release of greenhouse gases over a short time period and is one of the key pathways for greenhouse gas emissions in many terrestrial ecosystems, particularly in drylands. The mechanisms underlying this pulse of greenhouse gas emissions remain nearly unknown. Here, we conducted simulated soil rewetting experiments using typical cover of dryland soils (bareland, cyanobacteria/lichen‐covered soil, and moss‐covered soil). The 13 C, 15 N, and 18 O labeling techniques allowed to explore the intrinsic mechanisms of rapid carbon (C) and nitrogen (N) release from the soils following rewetting. We found that the hydroxyl radical (˙OH) was produced after soil rewetting via the rapid activation of microorganisms. The carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) production strongly decreased after ˙OH removal, whereas the methane (CH 4 ) production was not affected. The synergistic action between ˙OH oxidation and microbial enzymatic reactions increased CO 2 production. The ˙OH also stimulated the oxidation of NH 4 + to NO 3 − and dominated the N 2 O production. Our results confirm the role of ˙OH in the production of greenhouse gases and indicate that microbially mediated ˙OH oxidation mechanisms are an overlooked key pathway for the emission of greenhouse gases during the soil rewetting. In the context of climate change, the extreme weather‐induced drying‐rewetting cycles in soils are becoming more frequent, making greenhouse gas emissions via the ˙OH oxidation pathway increasingly important.

Tropical forests, where the soils are nitrogen (N) rich but phosphorus (P) poor, have a disproportionate influence on global carbon (C) and N cycling. However, whether P addition can accelerate soil microbial C and N sequestrations and reduce greenhouse gas emissions (CO 2 and N 2 O) under increasing N deposition remains unclear. We investigated soil microbial taxonomy and functional traits involved in C and N cycling in response to 10-year independent and interactive effects of N and P additions in a primary and a secondary tropical forest in Hainan Island. In the primary forest, N addition boosted oligotrophic bacteria and phosphatase and enriched genes responsible for C-, P-mineralization, nitrification and denitrification, suggesting aggravated P limitation while N excess. This might stimulate P excavation via organic matter mineralization, and enhance gaseous N release, leading to 86-110% increases in soil CO 2 and N 2 O emissions. Phosphorus and NP additions reduced phosphatase activity and microbial network keystone species capable of P excavation in both forests, suggesting mitigation of P limitation. This further stimulated fungal growth in the secondary rather than primary forest mainly associated with lower C availability in primary forest. Soil CO 2 and N 2 O emissions declined by 25-82% following P and NP additions in the secondary forest, which could be ascribed to the alleviated P-mining from organic matter and increased microbial C and N immobilization with fungal growth. Overall, N addition accelerates all microbial processes for C and N release in tropical forests. Long-term P addition alleviates C and N release and reduces soil CO 2 and N 2 O emissions via reducing P mining from organic matter and promoting microbial C and N immobilizations in the secondary rather primary forest, mainly because of strong C limitation to microbial growth in primary forest.

Questions How do community‐weighted means of traits ( CWM ) and functional dispersion ( FD is), a measure of trait variability, change in response to gradients of temperature, precipitation, soil nutrients and disturbance? Is the decrease in trait similarity between plots continuous or discontinuous? Is species turnover between plots linked to trait turnover? Location Mount Kilimanjaro, Tanzania, Africa. Methods Sixty plots were established in 12 major vegetation types on Mount Kilimanjaro, covering large gradients of temperature, precipitation, soil nutrients and anthropogenic disturbance representing the dominant ecosystems in East Africa. Environmental data, plant abundances and plant traits were recorded for each plot. Trait CWM and FD is were related to environmental factors with partial least squares regressions. Trait similarity between pairs of plots was assessed with a null model approach. Results Both CWM and FD is of most traits responded strongly to environmental factors, particularly to precipitation and disturbance. FD is of traits associated with growth and reproduction mostly increased with temperature and precipitation, and decreased with disturbance. Pair‐wise plot comparisons revealed an inverse relationship of trait similarity with differences in temperature, precipitation and anthropogenic disturbance. However, changes in similarity were often discontinuous rather than continuous. Several vegetation types differed strongly in species composition but not in traits. Conclusions Trait dispersion indicating functional niches increased with productivity and temperature. Conversely, low‐productivity conditions were characterized by trait convergence. Discontinuous changes in trait similarity between plots suggested tipping points at which trait expressions change strongly to adjust to environmental conditions. Large sections of the temperature gradient were characterized by species turnover with only minor changes in traits, indicating that the functional composition may be resilient to gradual environmental changes until a tipping point is reached.

Soil pH is the master variable of soil properties and understanding its spatiotemporal changes in situ is key to unveiling numerous biogeochemical processes. The development of non-invasive imaging techniques provides the possibility to visualize and localize soil pH changes depending on various factors, e.g. fertilization and climate. Herein, the optodes pH mapping system was used to study the effects of eight fertilizer types including chicken manure, Ca(NO3)2, Mg(NO3)2, KNO3, NH4NO3, (NH4)2SO4, NH4H2PO4, and urea on the spatiotemporal distribution of soil pH with and without liming at 10 °C and 25 °C. Ammonium-based fertilizers, especially NH4NO3, (NH4)2SO4, and NH4H2PO4 strongly decreased soil pH by a maximum of 1.4 ± 0.16 units at both temperatures. The 0–2 cm, where fertilizers were applied, had the highest pH decreases, from where the acidity rapidly diffused to depth. The acidified depth extended down to 4.5 ± 0.14 cm over 60 d. Chicken manure increased the pH within 5 d, but the pH decreased again after 60 d. Soil temperature was a strong controller of acidity generation and transport to depth after fertilization: pH decreased by 0.1 ± 0.07–0.3 ± 0.07 units more at 25 °C than 10 °C due to increased activity of nitrifying microorganisms, and higher temperature accelerated the spatiotemporal dynamics of soil acidity. Although pH increased shortly after liming compared to unlimed soils, it decreased after adding ammonium-based fertilizers. Therefore, N fertilizer types and temperature should be considered for having a more efficient fertilization management with less consequences for soil acidification. The planar optode is a powerful non-invasive imaging technique that enables in situ visualization of the spatiotemporal changes of soil pH profile after fertilization.