Nitrogen Mineralization Rates Research Articles

Synergistic impacts of co‐occurring invasive grasses cause persistent effects in the soil‐plant system after selective removal

AbstractHuman influence on the environment is so extensive that virtually all ecosystems on the planet are now affected by biological invasions. And, often, ecosystems are invaded by multiple co‐occurring non‐native species. Hence, it is important to understand the impacts these invasions are producing on biodiversity and ecosystem processes.Here, we present results of a 2‐year long field experiment where we tested the effects of co‐occurring invasive C4African grasses in a Cerrado area in central Brazil. We compared plant and arthropod communities, plant biomass, and soil nitrogen dynamics and soil chemical characteristics across five experimental treatments:Urochloa decumbensremoval;Melinis minutifloraremoval; bothU.decumbensandM.minutifloraremoval;U.decumbensandM.minutiflorainvaded plots; and uninvaded Cerrado. We hypothesized that selective removal of invasive grasses would have distinct effects on the native ecosystem structure and functioning. We expected that each invasive grass would produce a different type of impact on the native ecosystem and that their impacts would be synergistic when co‐occurring.Removal ofM.minutifloradoubled native plant diversity and biomass when compared to invaded plots, whereas removal ofU.decumbensdid not alter these parameters. Cerrado plots had four times more plant species than plots cleared of invasives. Removal of invasive grasses did not affect the species richness or community composition of soil epigeal fauna. Cerrado soils had lower fertility, organic matter content and pH than invaded soils. The effects were generally higher when both invasive grasses were removed, suggesting impacts were synergistic, butM.minutiflorahad greater effects on plants and soils thanU.decumbens. Both invasive species produced negative impacts, but a single species was the main driver. We also detected persistent effects of the invasive grass species on the ecosystem after 2 years of removal.We conclude that invasive species of the same functional group have similar types of effects in native ecosystems, but the magnitude of impact was largely dependent on invasive species biomass and cover. Where multiple invasive species are present, research and management of invaded ecosystems should tackle the interacting effects of co‐occurring invaders.

Gross nitrogen transformation rates in semiarid tropical soils under different salinity and vegetation conditions

AbstractExacerbated inundations and saltwater intrusions due to sea‐level rise can significantly alter the ecology of low‐lying coastal drylands by affecting their nutrient cycling and productivity. During the present study, controls on production and consumption of nitrogen in soils of semiarid tropical climate with different levels of vegetation–salinity–moisture condition were investigated using 15N isotope dilution experiments. For this purpose, soils were collected from five land types including two in salt flat and one each in grassland (GL), wetland (WL), and an agricultural field. Soils with no vegetation, high salinity, and high moisture (salt flats) showed significant decline in gross production of nitrogen (mineralization rate: 0.21 ± 0.10 mg N·kg−1·d−1; nitrification rate: 0.27 ± 0.17 mg N·kg−1·d−1) compared to soils with vegetation, low salinity, and low moisture (GL, WL, and agricultural) (mineralization rate: 2.47 ± 1.74 mg N·kg−1·d−1; nitrification rate: 1.43 ± 1.30 mg N·kg−1·d−1). Backward stepwise regression analysis of the results indicated salinity to be the most important factor controlling nitrogen transformation rates. It is likely that soils undergoing frequent seawater exposure or excessive evaporation may experience slowdown in nitrogen cycling due to salinization leading to reduced nutrient production and consumption. Due to climate change, the areal extent of coastal drylands undergoing such decline in nutrient production is likely to increase, eventually leading to loss of productivity and desertification.

The effect on soil nitrogen mineralization resulting from biochar and straw regulation in seasonally frozen agricultural ecosystem

To thoroughly explore the effects of different regulation measures on soil nitrogen cycling in seasonally frozen areas, four treatments, natural control (BL), biochar addition (BA), straw addition (SA) and combined of biochar and straw (CBS), were established on the Songnen Plain of Northeast China. Variations in soil moisture, temperature and urease activity were analyzed. In addition, the mineralization rate was calculated based on the changes of soil inorganic nitrogen content. Furthermore, the response functions of the soil nitrogen mineralization rate were constructed, and the response mechanism for the effect of different regulation patterns on soil nitrogen was determined. It showed that during the freezing period, straw mulch effectively inhibited the loss of soil water and heat, and hindered the reduction in soil urease activity; the average soil urease activity under the SA treatment increased by 0.390, 0.272, and 0.157 mg⋅(g−1·d−1) relative to those under the BL, BA, and CBS treatments, respectively. During the thawing period, the combined of bio char and straw had better heat preservation and water conservation, which greatly enhanced the activity of soil urease. The regulation of biochar and straw significantly affected the soil nitrogen mineralization rate its relationship with influencing factors. During the freezing period, the inorganic nitrogen content was the highest under the SA treatment. During the thawing period, the combined of biochar and straw enhanced the accumulation of soil nitrogen, and the response of the nitrogen mineralization rate to soil water, heat, and urease activity was strongest. In addition, the regulation abilities of biochar and straw decreased with increasing soil depth. These research results have important significance for understanding nitrogen transformations and efficiently guiding agricultural cleaner production.

Nitrogen mineralization rates of the soils incubated under different temperatures from different elevations along an environmental gradient on Yakushima Island

AbstractSoil nitrogen (N) mineralization is the product of the soil biochemical properties and temperature. It is known that in situ soil N mineralization monotonously decreases with increasing elevation along a mountain slope reflecting decreasing air temperature. However, inherent soil biochemical properties (quality of organic matter and soil microbe profile) may also affect the decreasing soil N mineralization. We analyzed how temperature and inherent soil organic matter quality affect soil N mineralization rates using the elevation gradient of Yakushima with a “common‐site” approach with multiple incubation temperatures. We collected topsoils from seven forests at seven different elevations along 170–1,550 m a.s.l and incubated them under each of four temperatures at 15, 20, 25 and 30°C for 10 days in the lab. We also measured soil microbial biomass and heterotrophic soil respiration rates. Net soil N mineralization rates incubated at 15, 25 and 30°C decreased significantly linearly with increasing elevation of the origin of the soil samples at the same temperature and the intercepts of the regression lines were similar among incubation temperatures 15, 20 and 25°C. These results indicated that in situ net N mineralization rates are regulated primarily by quality of soil organic matter in our sites. Soil phenolics/N ratios increased with increasing elevation reflecting the vegetation shift along elevation and significantly negatively correlated to net soil N mineralization rates incubated at 15°C. We conclude that elevationally differentiated vegetation and associated inherent soil properties exert strong influences on net soil N mineralization rates on Yakushima Island.

高寒草甸退化对祁连山土壤微生物生物量和氮矿化速率的影响

高寒草甸退化对祁连山土壤微生物生物量和氮矿化速率的影响

Elevated CO2 improved soil nitrogen mineralization capacity of rice paddy

Elevated CO2 would increase rice yields and may lead to nitrogen limitation and potentially influence the sustainability of agricultural production. Blindly increasing the amount of chemical fertilizer will damage the environment and is very unwise. Therefore, clarifying the response of soil nitrogen mineralization capacity to elevated CO2 is critical for both sustainable agriculture production and environmental protection. Here, we relied on Free-Air CO2 Enrichment (FACE) platform and used a waterlogged incubation method to investigate the effects of elevated CO2 on soil nitrogen mineralization capacity under different fertilization levels when planted different rice cultivars (strong and weak-CO2 response rice). According to the first-order kinetic equation fitting, compared with Ambient, elevated CO2 increased soil potential mineralized nitrogen (Np) by 16.18%. Path analysis indicated that fertilization status, rice cultivar, soil organic carbon and soil C: N ratio might affect Np. There was a significant positive correlation between soil nitrogen mineralization rate and Np. Under different fertilization conditions and rice cultivars, the improvement degree of soil nitrogen mineralization capacity (Np and soil nitrogen mineralization rate) by elevated CO2 was different. These findings suggest that more parameters and influencing factors should be taken into account when studying soil nitrogen cycle models under the condition of global change.

Investigation of in situ soil nitrogen mineralization in a Picea-Abies forest in Tibet Plateau: effects of increased nitrogen input

The main objective of this study was to quantify the dynamics of ammonium (NH4+) and nitrate (NO3-) in the humus (0-7cm) and the uppermost mineral layer (0-15cm) of a forest soil. The soil was treated annually from 2012 to 2013 with one single dose of nitrogen (0, 15, 30kg N ha-1yr-1 applied as (NH4)2SO4, NH4Cl, KNO3). Net N mineralization, including net ammonification and net nitrification was determined in four in situ incubation periods over 2 years in a Picea-Abies forest stand at the Qinghai-Tibet Plateau, Southwest China. Measurements were done using soil cores (7cm or 15cm deep) with a resin bag filled with combined anion and cation exchange resins placed at the base to collect the N leaching from the soil. The accumulation rate of N was corrected for both deposition and fertilizer N inputs. In all treatments, both the content and accumulation of the mineral N were dominated by NH4+ which accounts for about 76-89% of the net mineralization. The accumulation rate of N decreased to 64-83% in KNO3 treatments. The net N mineralization rate increased with nitrogen input, especially in NH4+-N treatments (p<0.05). However, this promoting role decreased over time. At the highest (NH4)2SO4 additions, the net ammonification and net mineralization rate increased notably in the humus (0-7cm) rather than in the uppermost mineral layer (0-15 cm). Previous studies that reported on soil net mineralization from forests under different environmental conditions were compiled and assessed for the effects of atmospheric N deposition and environmental factors, annual precipitation, and annual temperature on annual fluxes of net nitrogen mineralization in forest soils, worldwide. The results show that an increase in atmospheric N deposition significantly enhances the soil net nitrogen mineralization rate. Variation in atmospheric N deposition accounts for 48% of the variation in the rate of soil net nitrogen mineralization across the forests.

Below‐ground responses to insect herbivory in ecosystems with woody plant canopies: A meta‐analysis

Abstract Insect herbivory can have important consequences for the functioning of terrestrial ecosystems. Despite a growing recognition of the role of herbivores in above‐ground–below‐ground interactions, our current understanding is mainly restricted to studies of vertebrates in grassland and tundra ecosystems, while ecosystems with tree‐like canopies (termed forests below) and invertebrates remain understudied. Here, we assess the current state of knowledge of one key aspect of plant–herbivore interactions by conducting a meta‐analysis of the peer‐reviewed literature on the below‐ground consequences of above‐ground insect herbivory in forest ecosystems. Main results are reported as aggregated relative effect sizes (Cohen's d). We find that above‐ground insect herbivory reduced below‐ground carbon (C) allocation by plants to roots (−0.56) and root exudation (−0.85), causing shifts in root–symbiont communities, for example, a decrease (−0.67) in the abundance of ectomycorrhizal fungi. Microbial decomposer abundances showed no significant responses, while soil faunal abundances increased (0.50). C and nitrogen (N) mineralization rates (C: 0.48, N: 0.48) along with nutrient leaching (C: 0.30, N: 0.77) increased, with a stronger response to outbreak relative to background insect densities. The negative responses increased in strength in colder and dryer biomes while positive responses were reinforced in warmer and wetter biomes, thus extending previously shown effects for vertebrate herbivores to also include insect herbivory. The positive response by soil fauna to insect herbivory was the notable exception. This may be associated with the limited physical soil disturbance caused by insects compared to ungulates. Furthermore, we identified an under‐representation in the literature of large areas of boreal and tropical biomes calling for research priorities to fill these knowledge gaps. We present three recommendations for future research: addressing (a) biological drivers of biogeochemistry and response pathways, (b) knowledge gap from boreal and tropical forests, and (c) heterogeneity of herbivore disturbances. Synthesis. Insect herbivores significantly accelerate soil C and N cycling during outbreaks in forest ecosystems, but we lack knowledge on the underlying biological drivers. Overall, below‐ground responses to insect herbivory are similar to vertebrate herbivory responses, which may simplify implementing herbivory effects into ecosystem models. Nonetheless, we identify a few important differences and general knowledge gaps on which we base recommendations for future research.

Effect of simulated emerald ash borer infestation on nitrogen cycling in black ash (Fraxinus nigra) wetlands in northern Minnesota, USA

Black ash (Fraxinus nigra) wetlands are an important economic, cultural, and ecological resource in the northern Great Lake States, USA, and are threatened by the invasive insect, emerald ash borer (Agrilus planipennis Fairmmaire [EAB]). EAB-induced ash mortality can increase air temperatures, alter understory vegetation communities, and modify wetland hydrology by elevating the water table following canopy dieback, which may, in turn, alter nitrogen cycling in wetland soils and affect plant N uptake, N export to watersheds, and N emissions to the atmosphere. We sampled soils in replicated 1.6-ha plots six years after the following treatments were applied: girdling all ash greater than six cm diameter to simulate EAB-induced mortality, clearcutting with removal of all trees, and an unharvested control. In situ samples were collected from soil cores and from adjacent bulk areas influenced by vegetation to measure the concentration and rates of accumulation of inorganic nitrogen over the 2017 growing season. In the soil cores, the greatest nitrogen accumulation occurred in the unharvested control treatment and nitrate/nitrite were the dominant species of inorganic N. In contrast, in bulk soils, the clearcut treatment had greater overall nitrogen mineralization than both the control and girdle treatments, and ammonium had a greater relative abundance than nitrite/nitrate. Potential N mineralization (assessed with a lab incubation) was not affected by treatment, indicating that treatment effects observed in the field were likely due to changes in the microenvironment. Overall, the effect of simulated EAB on net nitrogen mineralization rates was minimal as mineralization was greatly constrained by ecosystem conditions compared to suitable, laboratory conditions, under which potential N mineralization was an order of magnitude greater than measured in the field. However, canopy treatment influenced water table levels, soil temperature, and vegetation communities, all of which influenced the relative abundance of inorganic N species. Our findings indicate limited indirect effects of EAB on net N mineralization in black ash wetlands.

Peanut residues supply minimal plant‐available nitrogen on a major soil series in the USA peanut basin

AbstractField observations have shown that a substantial portion of peanut leaves abscise in windrows during pod curing, leading to an uneven distribution of leaves and stems when intact residues are spread during harvest. Possible differences in nitrogen (N) mineralization rates between peanut leaf and stem residues may lead to spatial and temporal variability in available N during subsequent crops. The objective of this study was to quantify N mineralization in soil amended with different peanut residue components under simulated conventional and conservation tillage practices. A 252‐day microlysimeter incubation was conducted in which peanut leaves, stems and a 1:1 mixture of leaves:stems from three varieties were incorporated or placed on the soil surface to simulate conventional or conservation tillage, respectively. Soils were periodically leached to assess N mineralization compared with a soil‐only control. Nitrogen mineralization was only affected by residue component. Averaged over variety and residue placement, soil amended with leaves mineralized 10% more N relative to the control or soil containing stems. It was estimated that leaves supplied 25 kg N ha−1 over 252 days at 0–15 cm soil depth, which would likely be insufficient to induce a yield response by a subsequent crop. This study suggests that uneven distribution of peanut leaf and stem residues following harvest causes only minor spatial and temporal variability in available N during subsequent crop growth. These results support the growing body of evidence indicating that peanut residue N contributions to subsequent crops are negligible in the peanut basin of the south‐eastern USA.

Nitrogen Mineralization Rates in Long-Term Manured Fields Estimated using Lab Incubation Experiment

Land application of dairy manure is a common practice in confined dairy operations in the Pacific Northwest...

Decomposition characteristics of indigenous organic fertilisers and introduced quick compost and their short-term nitrogen availability in the semi-arid Ethiopian Rift Valley

Case studies on the assessment of local organic fertilisers (OFs) in their quality (decomposition characteristics and nutrient availability for crops) are few in sub-Saharan Africa (SSA). This study assessed the quality of local OFs from the Ethiopian Rift Valley. The decomposition characteristics were assessed by acid detergent fibre analysis methods, while the short-term nitrogen availability was assessed by a combination of laboratory incubations and inorganic nitrogen and acid detergent soluble nitrogen determinations. A commercial hand-held reflectometer (RQFlex) was used for determining nitrogen components. The mean acid detergent soluble organic matter contents exceeded 250 mg g−1, indicating the OF feedstock contained much of the readily decomposable organic matter. Some of the indigenous compost (kosi) samples showed net nitrogen immobilisation during the initial period of incubation, followed by net nitrogen mineralisation in one month of incubation with 4% of the nitrogen mineralisation rate. Kosi should be applied to a field one month before seeding. Short-term nitrogen availability from kosi was too low to substitute for inorganic fertilisers. The combination of the simple analysis methods shown in this study is a quick, cost-saving, and accurate quality assessment approach for OFs, which can be useful in the field and at experimental stations in SSA.

Improvement of subsoil physicochemical and microbial properties by short-term fallow practices.

Fallow management can improve the soil nutrients in the topsoil and upper subsoil. However, little is known about the effects of short-term (one year) fallowing with different treatments, such as vegetation and fertilization, on subsoil (20–40 cm) properties. We conducted field trials to explore the changes in subsoil properties in response to such treatments in the Yellow River Delta region in China. Different vegetation and fertilization treatments were applied, and we measured the carbon and nitrogen contents, microbial biomass and microbial community structure in the subsoil. Fallowing without manure resulted in the storage of more total nitrogen (16.38%) than fallowing with manure, and meadow vegetation improved the ammonium nitrogen content (45.71%) relative to spontaneous vegetation. Spontaneous vegetation with manure improved the microbial biomass nitrogen (P < 0.05). Although the impact of short-term fallowing on microbial community structure was low, an effect of management was observed for some genera. Blastopirellula, Lysobacter, and Acidobacteria Gp6 showed significant differences among fallow treatments by the end of the year (P < 0.05). Blastopirellula abundance was related to the microbial biomass nitrogen and nitrogen mineralization rate in the subsoil. Manure retained a high abundance of Lysobacter, which may strengthen soil-borne disease resistance. The response of Acidobacteria Gp6 showed that meadow vegetation without manure may not benefit future crops. Although the treatments did not significantly improve microbial community structure in the one-year period, annual fallowing improved certain subsoil properties and increased the number of functional genera, which may enhance crop productivity in the future.

Nitrogen Mineralization in Mississippi River Deltaic Plain Freshwater Floating Marshes

ABSTRACTThe mineralization of soil organic nitrogen in two floating marshes types in Louisiana, USA with different vegetation and mat thickness (and biomass) was measured by incubating peat soil in situ in polyethylene bottles at depths of 10 and 25 cm. The size of the extractable inorganic N (NH4+ – N) pool was related to mat thickness and soil bulk density. The N mineralization rates were twice as great at the thick mat site (1.5 mg N/kg soil/day) compared to the thin mat floating marsh (0.71 mg N/kg soil/day). The amount mineralized on a volume basis to 30 cm depth were 22 mg N/m2/day for the thick mat marsh and 6 mg N/m2/day for the thin mat marsh. The nitrogen mineralization rates observed in this study reflect plant biomass differences between the thick mat and thin mat marshes, with the former exhibiting approximately 7-fold greater biomass. These low overall mineralization rates suggest that nitrogen entering these freshwater floating marshes from Mississippi River water percolating through the root zone, would result in increased plant productivity-increasing marsh stability and also providing a natural mechanism contributing to the processing and removal of dissolved inorganic nitrogen in river water entering these wetlands.

Changes in soil properties rather than functional gene abundance control carbon and nitrogen mineralization rates during long-term natural revegetation

Agricultural abandonment has been taken as one of the most widely used strategies for ecological restoration. Abandoned croplands expanded worldwide since the middle of the twentieth century and will increase considerably in the future. Whether agricultural abandonment alters the abundances of soil carbon (C) and nitrogen (N) mineralization genes, and whether these functional genes are linked to alterations in C and N mineralization rates remain unknown. We took advantage of a well-established chronosequence of nature restoration sites on abandoned croplands which represent a century of vegetation succession in China’s Qinling Mountains, to determine soil C and N mineralization rates, and their relationships with vegetation succession, edaphic characteristics, soil microbial communities, and functional genes based on GeoChip 5.0. We found that soil C and N mineralization rates, microbial biomass and composition, and the number and abundances of functional genes were significantly affected by vegetation succession following agricultural abandonment. Soil C and N mineralization rates increased by sixfold and threefold, respectively, along the cropland to forest successional gradient. The NO3-N, NH4-N and soil available phosphorus (P) concentrations were important factors associated with the C and N mineralization genes. The shifts in edaphic environments (i.e., soil moisture, cation exchange capacity, and C/P ratio), vegetation-derived substrates (i.e., soil organic C and N), and soil microbial composition (i.e., Fungi/Bacteria ratio, amounts of Gram-negative bacteria, Actinomycetes, and arbuscular mycorrhizal fungi), rather than the abundances of functional genes involved in mineralization processes, controlled soil C and N mineralization rates along the long-term vegetation successional gradient. Abandoning cropland for natural succession could facilitate the recuperation of soil C and N processes and alter soil microbial composition and functional genes. The abundances of microbial functional genes are less important than previously expected in regulating soil C and N mineralization rates.

Effects of weeding and fertilization on soil nitrogen mineralization and leaching in the mixed forest of Dalbergia odorifera and Santalum album

The mixed plantation of precious tree species Dalbergia odorifera and Santalum album is an important model for the development of plantations in South China. Based on an experiment with four treatments, i.e., CK (without any treatment), weeding, fertilization, weeding+fertilization, we investigated the effects of weeding and fertilization on soil net mineralization rate, net nitrification rate, net ammonification rate, and nitrogen leaching rate. Among the four treatments, the maximum net nitrogen mineralization rate in the 0-10 cm soil layer was 18.92 and 18.13 mg·kg-1·month-1 in spring and autumn, respectively; the maximum nitrification rate was 20.35 and 18.85 mg·kg-1·month-1in spring and autumn, respectively; the maximum ammonium mineralization rate was 0.22 and 0.26 mg·kg-1·month-1 in summer and winter, respectively; the maximum nitrogen leaching was 15.98 mg·kg-1·month-1 in autumn and 86.69 mg·kg-1·year-1 in the whole year. The treatments of weeding, fertilization, weeding+fertilization reduced net mineralization rate and net nitrification rate. The annual nitrogen mineralization decreased by 26.2%, 16.1% and 6.3%, respectively, and the annual nitrogen nitrification decreased by 17.1%, 16.6%, and 1.4%, respectively. Moreover, the treatments of weeding, fertilization, weeding+fertilization inhibited soil ammonium accumulation and reduced annual nitrogen leaching by 25.2%, 8.6%, and 6.1%, respectively. Compared with weeding, fertilization and weeding+fertilization, seasonal factors had more significant effects on soil nitrogen mineralization and leaching. Weeding, fertilization and weeding+fertilization decreased soil nitrogen nitrification and ammonification to a certain extent, reduced soil nitrogen mineralization and leaching loss, which contribute to the preservation of soil fertility and nitrogen accumulation.

Douglas fir stimulates nitrification in French forest soils

Douglas fir trees presumable stimulate nitrification in the soil. We studied in 21 French Douglas fir forests if and how nitrification is modulated by soil properties, past land use and current forest management. Soil (0–10 cm depth) was collected and initial concentrations of N-NH4+ and N-NO3−, potential net nitrogen mineralization (PNM) and net nitrification (PNN) rates and microbial biomass were measured. At 11 of the 21 sites, annual nitrate fluxes in the soil were measured using anion exchange resin bags. Soils contained between 2.3 to 29.4 mg N-NO3− kg soil−1. About 86% (±14%) of mineral N was nitrate. The proportion of nitrate increased to almost 100% during incubation. PNN varied from 0.10 mg N kg soil−1 day−1 to 1.05 mg N kg soil−1 day−1 (21 sites). Neither the initial nitrate concentration nor PNN was related to soil chemistry (pH, % C, %N, P, CEC), microbial biomass, texture, past land use or thinning. In situ net nitrate accumulation (NNA) estimated with resins beds varied from 4 to 100 kg N-NO3− ha−1 yr−1 (11 sites). It was positively correlated with base saturation, clay content, ELLENBERG N, temperature and negatively with soil organic N, C/N ratio and precipitation.

Macrophytes and crabs affect nitrogen transformations in salt marshes of the Yangtze River Estuary

Nitrogen (N) cycling is important in determining ecosystem primary productivity and the succession of plant communities in coastal marshes. In order to examine the effects of biotic disturbances (i.e. vegetation change and crab burrowing) on N transformations, we conducted a field experiment in which crabs were either removed or left intact in three marsh types (bare flat, Phragmites australis marsh and Spartina alterniflora marsh) at Chongming Dongtan in the Yangtze River Estuary. The potential rates of soil gross nitrogen mineralization (GNM), gross ammonium immobilization (GAI), gross nitrification and gross nitrate consumption were determined using an ex situ incubation approach. Gross N mineralization was significantly lower in vegetated marshes than in the bare flat, probably because dissolved organic carbon (the main energy source for microbes) was lower in vegetated marshes. Less dissolved organic carbon in vegetated marshes may be attributable to high microbial demand for labile carbon relative to the supply of labile carbon in the rhizosphere. The presence of crabs increased the GAI:GNM ratio, indicating that crabs may increase the soil microbial demand for ammonium. Ammonium immobilization competed with nitrification for ammonium, and limited nitrification in crab-present plots. Furthermore, the lower nitrate production by nitrifies in crab-present plots resulted in lower rates of nitrate consumption. These results highlight the role of labile carbon and N in mediating the effects of macrophyte and soil fauna on N transformations.

Differential responses of net N mineralization and nitrification to throughfall reduction in a Castanopsis hystrix plantation in southern China

BackgroundMany regions in the world are experiencing changes in precipitation pattern, which likely impact soil nitrogen cycling and availability. However, we know little about how soil nitrogen processes respond to drought stress under climate change.MethodsA continuous 5-year experiment of throughfall reduction treatment (TRT) was conducted in a Castanopsis hystrix plantation in subtropical China to assess how soil nitrogen processes responded to a change in precipitation. Net nitrogen transformation, soil properties, microbial biomass carbon (MBC) and nitrogen (MBN), and microbial community phospholipid fatty acids (PLFAs as bacteria, fungi and arbuscular mycorrhizal fungi (AMF)) were investigated in the wet and dry seasons over the period of the manipulation experiment.ResultsTRT had no significant effects on net ammonification rate (NAR) and nitrogen mineralization rate (NMR), and the unchanged NAR and NMR were mainly caused by the no change of soil nitrogen availability (i.e. NH4+-N, NO3−-N and dissolved organic nitrogen). However, TRT significantly increased net nitrification rate (NNR) in the wet season primarily due to the increase in NO3− concentration, and might be further caused by reduced NO3− leaching, denitrification or NO3−-N uptake in the TRT. Differently, TRT significantly decreased the NNR in the dry season, which might be linked to the limited SWC that resulted in greater microbial nitrate immobilization than gross nitrate mineralization. In addition, TRT significantly influenced the soil microbial community composition in 0–10 cm soil layer in the wet season, primarily due to the variations in NO3−-N, DOC and DON.ConclusionsPrecipitation reduction affected the NNR rather than NAR and NMR, and consequently, further affected soil N availability and N uptake by Castanopsis hystrix.

Effects of salinity and moisture on sediment net nitrogen mineralization in salt marshes of a Chinese estuary

The interactive effects of salinity and moisture on the net nitrogen mineralization were seldom studied in coastal wetlands. To reveal the phenomenons and mechanisms of salinity and moisture effects, we conducted a 30-days laboratory experiment with different salt addition levels (0, ppt, 5 ppt and 35 ppt) and moisture levels (10%, 50% and 100% of water holding capacity (WHC)), and the key N transformation rates and enzymes activities were measured. Our results showed that during the incubation, the rates of soil net nitrogen mineralization (Rmin) and nitrification (Rnit) under all treatments, increased during 0–1 day, decreased during 1–3 days, then increased and kept around zero. As incubation time increased, urease activities increased, arylamidase activities decreased and fluorescein diacetate activities increased first and then decreased. Increasing salinity under high moisture (100% WHC) treatments and increasing moisture under high salinity (35 ppt) treatments would promote Rmin and Rnit. Whereas, increasing salinity under low moisture (10% WHC) treatments, and increasing moisture under low salinity (0 ppt) treatments would suppress Rmin and Rnit. The responses of enzyme activities to salinity and moisture gradients almost correspond with Rmin, indicating salinity and moisture could affect N transformation by the regulation of related microbial enzyme activities.