Net Nitrification Rates Research Articles

Different responses of soil fungal and bacterial communities to nitrogen addition in a forest grassland ecotone.

Continuous nitrogen deposition increases the nitrogen content of terrestrial ecosystem and affects the geochemical cycle of soil nitrogen. Forest-grassland ecotone is the interface area of forest and grassland and is sensitive to global climate change. However, the structure composition and diversity of soil microbial communities and their relationship with soil environmental factors at increasing nitrogen deposition have not been sufficiently studied in forest-grassland ecotone. In this study, experiments were carried out with four nitrogen addition treatments (0 kgN·hm-2·a-1, 10 kgN·hm-2·a-1, 20 kgN·hm-2·a-1 and 40 kgN·hm-2·a-1) to simulate nitrogen deposition in a forest-grassland ecotone in northwest Liaoning Province, China. High-throughput sequencing and qPCR technologies were used to analyze the composition, structure, and diversity characteristics of the soil microbial communities under different levels of nitrogen addition. The results showed that soil pH decreased significantly at increasing nitrogen concentrations, and the total nitrogen and ammonium nitrogen contents first increased and then decreased, which were significantly higher in the N10 treatment than in other treatments (N:0.32 ~ 0.48 g/kg; NH4+-N: 11.54 ~ 13 mg/kg). With the increase in nitrogen concentration, the net nitrogen mineralization, nitrification, and ammoniation rates decreased. The addition of nitrogen had no significant effect on the diversity and structure of the fungal community, while the diversity of the bacterial community decreased significantly at increasing nitrogen concentrations. Ascomycetes and Actinomycetes were the dominant fungal and bacterial phyla, respectively. The relative abundance of Ascomycetes was negatively correlated with total nitrogen content, while that of Actinomycetes was positively correlated with soil pH. The fungal community diversity was significantly negatively correlated with nitrate nitrogen, while the diversity of the bacterial community was significantly positively correlated with soil pH. No significant differences in the abundance of functional genes related to soil nitrogen transformations under the different treatments were observed. Overall, the distribution pattern and driving factors were different in soil microbial communities in a forest-grassland ecotone in northwest Liaoning. Our study enriches research content related to factors that affect the forest-grassland ecotone.

Nine years of low-dose biochar amendment suppresses nitrification rate in low-yield brown soil

Nitrification is the crucial process of nitrogen (N) transformation and ammonia oxidation is the first and the speed-limited step of nitrification. The one-time amendment of large quantities of biochar has been reported to alter soil fertility. However, it remains unclear how continuous low-dose biochar application affects the abundance and community composition of ammonia oxidizers and the subsequent nitrification and crop production. In this article, we calculate the soil net nitrification rate and the abundance and communities of ammonia-oxidizers across different growth stages of peanuts at 0–20 cm soil layer under four fertilization regimes, namely control with zero fertilization (CK), biochar application (BC), chemical fertilizer application (NPK) and biochar-based fertilizer application (BBF). Results showed that continuous application of low-dose biochar can significantly increase crop yield by 7.8 % compared to unfertilized soil indirectly through regulating the microbial mechanism in the nitrification process. The highest bacterial amoA gene copy numbers were found in the NPK treatment, 2.35-fold higher than the CK treatment. Compared to the single input of NPK chemical fertilizer, biochar-based fertilizer reduced the nitrification rate by reducing the relative abundance of bacteria, especially Nitrosomonas and Nitrosospira. In summary, our findings provide evidence that: (1) sustained low-dose biochar input could also increase the ammonia-oxidizing bacterial abundance, changing soil properties and increasing the crop yield; (2) biochar-based fertilizer could significantly reduce the nitrification rate compared to NPK application by reducing the AOB-amoA gene abundance and the relative abundance of Nitrosomonas and Nitrosospira; (3) soil C/N ratio was also important for the ammonia-oxidizing community.

Nitrate leaching and its susceptibility in response to elevated nitrogen deposition in Japanese forests

ABSTRACT Understanding the variability in nitrogen (N) leaching from forested ecosystems and its susceptibility to elevated atmospheric N deposition is essential for assessing the impact of atmospheric N deposition on forested ecosystems. This study explored the site variability in nitrate (NO3 −) leaching from forested areas and its susceptibility to elevated atmospheric N deposition from five sites in broadleaved forests grown on Andosols and Cambisols from northern to southern Japan. Site variability of NO3 − leaching and its possible contributing factors, including soil parent material and net nitrification rate, were evaluated using a generalized linear model with Akaike’s Information Criterion used for model selection. Three-year experimental N fertilization (50 kg N ha−1 yr−1 as ammonium nitrate) experiments were conducted at each site to evaluate the susceptibility of NO3 − leaching to elevated atmospheric N deposition. Soil solution NO3 − concentration below the rooting zone (50 cm below the ground surface) was variable among the five sites and was the highest in Shibecha, one of the Andosols sites in northern Japan where soil net nitrification tended to be higher than elsewhere. In model selection for NO3 − leaching, the soil parent material was not selected but soil net nitrification was selected, indicating that soil net nitrification could explain the variability of NO3 − leaching. A significant increase in NO3 – leaching after the 3-year N fertilization experiment was observed at the site with the highest levels of NO3 – leaching (Shibecha), indicating that the susceptibility of NO3 – leaching is high in locations where N leaching was originally high.

Effect of Nitrogen Application and Microbial Fertilizer on Nitrogen Conversion Processes in Saline Farmland

The nutrient utilization rate of salinized farmland soils is low, the nitrogen loss is high, and soil salinity inhibits the hydrolysis of urea and the release of nutrients. In this work, the effect of microbial fertilizer on the nitrogen transformation characteristics and nitrogen morphology of salinized soils was studied using indoor constant temperature incubation tests with different nitrogen application rates—without (A0) and with microbial fertilizer application (A1 (15 t/ha)) or nitrogen application (N) of 0 kg/ha (N0), 97.5 kg/ha (N1), or 195 kg/ha (N2). The results show the following: (i) When no microbial fertilizer was applied, an increased nitrogen application promoted nitrogen fertilizer’s ammonification and nitrification reactions. Furthermore, the maximum net nitrification rate with the high nitrogen fertilizer application decreased; the apparent ammonification rate and net ammonification rate A0N2 increased by 26.1% and 24.6%, respectively, compared with A0N1 on the first day of incubation; the maximum net nitrification rate of A0N1 was more than that of A0N2; and A0N1 > A0N2 on day 3, while A0N2 > A0N1 on days 3 to 15. At 3 d, the nitrogen conversion process of A0N1 was dominated by the nitrification reaction, while the ammonification reaction dominated in A0N2. (ii) Microbial fertilizers significantly increased the ammonification and nitrification rates under the low N fertilizer application. The intensity of ammonification and nitrification under the low N fertilizer application was greater than that under the high N fertilizer application. The apparent ammonification rate and net ammonification rate of A1N1 increased by 60.9% and 52.6% compared with A0N1 and 21.9% and 21.7% compared with A1N2 on the first day of incubation, and the peak net nitrification rates of A1N1 and A1N2 (28.19 mg/kg d and 11.02 mg/kg d, respectively) and net nitrification rates of A1N1 and A1N2 were 113.7% higher than those of A0N1. The net nitrification rates of A1N1 and A1N2 were 82.3% and 58.6% lower than the maximum net nitrification rates on the 15th day of incubation, respectively. (iii) In saline soils, low-nitrogen microbial fertilizers led to more ammonium nitrogen in the soil, and the high-nitrogen fertilizer application resulted in higher nitrate nitrogen in the soil, leading to nitrogen leaching. Therefore, when applying microbial fertilizer, choosing the most suitable period for reduced chasing is important for the efficient use of fertilizers, the alternative role of biofertilizers, and the study of environmental pollution.

Seasonal linkages between soil nitrogen mineralization and the microbial community in broadleaf forests with Moso bamboo (Phyllostachys edulis) invasion

Plant invasions significantly alter the microbiome of the soil in terms of fungal and bacterial communities, which in turn regulates ecosystem processes and nutrient dynamics. However, it is unclear how soil microbial communities, nitrogen (N) mineralization, and their linkages respond to plant invasions over the growing season in forest ecosystems. The present study investigated the seasonal associations between the microbial composition/function and net N mineralization in evergreen broadleaf, mixed bamboo-broadleaf, and Moso bamboo (Phyllostachys edulis) forests, depicting uninvaded, moderately invaded, and heavily invaded forests, respectively. The ammonification and nitrification rates in the bamboo forest were significantly higher than those in the broadleaf and mixed bamboo-broadleaf forests during the spring season only. The forest type and seasonal variation significantly influenced the net rates of ammonification and nitrification and the abundances of bacterial apr and AOB amoA, fungal cbhI and lcc genes, as well as the microbial composition. Moreover, the partial least squares path model revealed that bamboo invasion enhanced net ammonification through increasing total N and fungal-to-bacterial ratio, and enhanced net nitrification through modifying the bacterial composition and increasing the fungal-to-bacterial ratio during spring. However, microbial parameters had no significant effect on net ammonification and nitrification during autumn. We conclude that shifts in the microbial abundance and composition following bamboo invasion facilitated soil N mineralization during spring, contributing to the rapid growth of Moso bamboo at the beginning of the growth season and its invasion into adjacent subtropical forests.

Litterfall quality modulates soil ammonium and nitrate supply through altering microbial function in bamboo encroachment of broadleaf forests

Plant invasion can change soil microbial function and nitrogen (N) supply through the input of litter with different qualities and quantities, among other factors. However, the underlying mechanism for litter quality to regulate soil N mineralization via microbial function with plant invasion remains unclear. We examined the effect of litterfall with contrasting chemical properties on soil net ammonification and nitrification rates, enzyme activity, and associated microbial function using reciprocal litterfall transplanting between a natural broadleaf forest and a Moso bamboo forest formed after the encroachment of a natural broadleaf forest. Our results showed that, on average, litterfall removal decreased ammonification rates by 33.6% but increased nitrification rates by 137.8%, while litterfall replacement increased nitrification rates by 55.9% in the natural broadleaf forest. Litterfall removal increased ammonification rates by 33.9%, while litterfall replacement increased ammonification rates by 42.1% at the end of the experiment in the bamboo forest. Structural equation modeling revealed that higher quality litterfall (higher cellulose content and lower lignin/N ratio) from the broadleaf forest enhanced net ammonification by altering fungal composition and increasing their ligninolytic capacity. However, the lower quality litterfall from the bamboo forest increased net nitrification rates through altering bacterial community and increasing urease activity. Our results indicate that the increased N supply resulting from litterfall replacement could counteract the low NH4+ availability in the bamboo forest soil, but not in the broadleaf forest soil. The contrasting modulation effects of litterfall quality on soil mineral N production would have cascading effects on bamboo encroachment and forest succession with implications for ecosystem management.

Effect of Litter Removal and Addition on Root Exudation and Associated Microbial N Transformation in a Pinus massoniana Plantation

In forest ecosystems, variations in aboveground litter input caused by global changes, substantially alter soil N cycling. In field-grown plants, few studies have directly measured root exudation rates and quantified their effects on N transformations under litter manipulation. We quantified soil N transformation rate responses to litter manipulation in a Pinus massoniana plantation, and unravelled the effect of root exudation on soil N transformations. We measured in situ P. massoniana root exudation rates as well as soil microbial biomass, soil C and N concentrations, the activities of four soil enzymes involved in soil N transformations, and net N mineralization and net nitrification rates after experimental litter removal and litter addition treatments. Litter removal and litter addition treatments had little impact on soil C and N concentrations, microbial biomass, soil enzyme (urease, hydroxylamine reductase, nitrate reductase, and nitrite reductase) activity, and net N mineralization rates. However, both litter removal and addition increased net N nitrification rates. Additionally, litter removal significantly decreased root C exudation rates (in April 2021 and annually), whereas litter addition had no significant effects on root C exudation rates across all seasons. Furthermore, root C exudation rates were positively associated with urease and nitrate reductase activities, but negatively associated with hydroxylamine reductase and nitrite reductase activities, as well as net N nitrification rate. Overall, we demonstrated that root exudates may be an important physiological adjustment by which trees respond to changes in litter input caused by global environmental changes, regulating underground N biochemical processes. Furthermore, we provide new evidence from root exudates for understanding the potential influence of litter inputs on soil N cycling. A strong correlation exists between root exudates and N transformation, shedding new light on the dynamics of rhizosphere nutrient cycling crucial for maintaining forest ecosystem stability and productivity under changing environmental conditions.

Polyethylene microplastic and soil nitrogen dynamics: Unraveling the links between functional genes, microbial communities, and transformation processes

Microplastics (MPs) have emerged as pollutants of growing concern due to their potential threat to soil ecosystems. While some studies have investigated the effects of MPs on soil nitrogen content, the underlying physicochemical and microbial driving mechanisms still need to be explored. In this study, a six-month incubation experiment was conducted with varying polyethylene MP addition rates: CK (0%, mass ratio), MP0.5 (0.5%), MP1 (1%), MP2 (2%), MP4 (4%), and MP8 (8%). The experiment aimed to examine the effects of MPs on soil nitrogen content, physicochemical properties, nitrogen cycling-related genes, microorganisms, and gross nitrogen transformation rates. The results revealed no significant changes in soil total nitrogen and dissolved total nitrogen. However, dissolved organic nitrogen significantly decreased by 16.00–54.60% following MP addition, while ammonium (NH4+-N, 45.71–271.43%) and nitrate (NO3--N, 43.15–209.54%) nitrogen and microbial biomass nitrogen (46.02–123.70%) significantly increased. Soil pH, bulk density, and soil porosity decreased after MP addition, while soil carbon contents, water-stable macroaggregates, and redox potential increased. The soil microbial community structure changed significantly, and microbial diversity increased under MP treatment. MP addition significantly altered the abundance of soil nitrogen cycling functional genes. The relative abundance of nitrogen fixation and denitrification genes decreased with increasing MP addition rates, while organic degradation and synthesis genes increased. The soil nitrogen cycling functional microbial composition shifted dramatically with increased MP addition. Networks with high addition rates (MP2 +MP4 +MP8) exhibited more total nodes, total links, negative links, node degrees, and modules but shorter average path distances and lower modularity than those with low addition rates (CK +MP0.5 +MP1), reflecting increased network complexity induced by MPs. The gross ammonification rate, NH4+-N consumption and immobilization rates, and NO3--N immobilization rate increased, while the gross nitrification rate and net nitrification rate exhibited an initial increase followed by a decrease with increasing MP addition rates, peaking at MP2. Furthermore, redundancy analysis and structural equation modeling demonstrated that soil physicochemical properties significantly affected soil nitrogen cycling genes and microorganisms, ultimately altering nitrogen content. In conclusion, polyethylene MPs promoted soil nitrogen mineralization and transformation and changed the related functional microorganism community structure, exhibiting a noticeable dose-effect relationship. This study provides deeper insight into the effects of MPs on soil nitrogen cycling.

Can the co-application of biochar and different inorganic nitrogen fertilizers repress N2O emissions in acidic soil?

The sole application of nitrogen (N) fertilizer with lower N2O emission potential or combined with biochar may help for mitigating N2O production. However, how biochar applied with various inorganic N fertilizers affected N2O emission in acidic soil remains unclear. Thus, we examined N2O emission, soil N dynamics and relating nitrifiers (i.e., ammonia-oxidizing archaea, AOA) in acidic soil. The study contained three N fertilizers (including NH4Cl, NaNO3, NH4NO3) and two biochar application rates (i.e., 0% and 0.5%). The results indicated that the alone application of NH4Cl produced more N2O. Meanwhile, the co-application of biochar and N fertilizers enhanced N2O emission as well, especially in the combined treatment of biochar and NH4NO3. Soil pH was decreased with the application of various N fertilizers, especially with NH4Cl, and the average decrease rate was 9.6%. Meanwhile, correlation analysis showed a negative relationship between N2O and pH, dramatically, which might indicate that the alteration of pH was one factor relating to N2O emission. However, there was no difference between the same N addition treatments with or without biochar on pH. Interestingly, in the combined treatment of biochar and NH4NO3, the lowest net nitrification rate and net mineralization rate appeared during days 16–23. Meanwhile, the highest emission rate of N2O in the same treatment also appeared during days 16–23. The accordance might indicate that N transformation alteration was another factor relating to N2O emissions. In addition, compared to NH4NO3 alone application, co-applied with biochar had a lower content of Nitrososphaera-AOA, which was a main contributor to nitrification. The study emphasizes the importance of using a suitable form of N fertilizers and further indicates that two factors, namely alteration of pH and N transformation rate, are related to N2O emission. Moreover, in future studies, it is necessary to explore the soil N dynamics controlled by microorganisms.

Dynamics of soil net nitrogen mineralization and controlled effect of microbial functional genes in the restoration of cold temperate forests

Soil nitrogen mineralization (Nmin) generally controls the availability and circulation of soil nitrogen (N). To better understand the mechanisms of soil Nmin and environmental protection, this study explored the dynamics of net Nmin in soils from restored forests and the effect of soil physicochemical properties (SPPs) and soil microbial mineralization genes (SMGs) on Nmin. Field studies were conducted from July 2019 to April 2020 in five representative restored forests in the mountainous region of northwestern Hebei, China (i.e., Larix principis-rupprechtii forest (LF), Betula platyphylla forest (BF), mixed forest of Larix principis-rupprechtii and Betula platyphylla (MF), Picea asperata forest (SF) and Pinus sylvestris var. mongolica forest (MPF)), and soil net nitrogen mineralization rate (Rmin), SPPs and SMGs were determined after collecting soil samples in five plots. The influence of restored forests and seasonality on Rmin was investigated to determine the characteristics of the dynamics of soil net Nmin. Principal component analysis (PCA) and variation partitioning analysis (VPA) revealed that soil properties and functional genes could explain approximately 80 % of the variation in Nmin. SMGs significantly contributed to the variation in the soil net ammonification rate (Ramm) and nitrification rate (Rnit) compared to soil properties. SMGs were strongly correlated with Ramm\\Rnit (except in LF), implying that the shifts in functional genes may control Rmin. Given the debate on the role of soil properties and N cycling genes in driving soil N transformation, this study provides new insights into the controlled effect of microbial functional genes during the early stage of forest restoration in the cold temperate zone.

Nanoattapulgite reduces the stimulation effect of nitrogen fertilizer on ammonia‐oxidizing microorganisms in salinized fluvo‐aquic soil

AbstractThe nitrification, which utilized ammonium nitrogen (NH4+‐N) as substrate, is susceptible to the bioavailability of NH4+‐N. However, it is unclear how the nitrification process responds to the decrease of NH4+‐N availability due to nanoattipulgite (NA) addition, especially in salt‐affected soils. A pot experiment was performed to study the effect of different NA rates (NA0, NA1, and NA2: 0, 25, 50 g kg−1) and nitrogen fertilizer (F) gradients (F1, F2, and F3: 54, 81, 108 mg N kg−1) on the net nitrification rate (NNR), abundance of ammonia monooxygenase (amoA) gene copies, and communities of ammonia‐oxidizing archaea (AOA) and bacteria (AOB) in a moderately salinized fluvo‐aquic soil. The concentrations of NH4+‐N and nitrate nitrogen in soils showed insignificant differences among various NA treatments. The addition of NA reduced NNR after topdressing but had no significant effect on NNR after base fertilizer. Nitrogen fertilizer could decrease the amoA gene abundance of AOA and increase the amoA gene abundance of AOB in soil. NA could maintain the relative stability of AOB amoA gene abundance. NA treatment only changed the community composition of AOB before topdressing at the F2 level but did not change the community composition of ammonia‐oxidizing microorganisms under other conditions. Overall, NA adsorbs more NH4+‐N and indirectly affects nitrification by reducing the stimulating effect of F on the activity of ammonia‐oxidizing microorganisms, which can play an important role in reducing nitrate pollution and denitrification loss from saline soil.

Nitrogen dynamics of grassland soils with differing habitat quality: high temporal resolution captures the details

AbstractExcessive nitrogen (N) input is one of the major threats for species‐rich grasslands. The ongoing deterioration of habitat quality highlights the necessity to further investigate underlying N turnover processes. Our objectives were (1) to quantify gross and net rates of mineral N production (mineralization and nitrification) and consumption in seminatural grasslands in southwest Germany, with excellent or poor habitat quality, (2) to monitor the temporal variability of these processes, and (3) to investigate differences between calcareous and decalcified soils. In 2016 and 2017, gross N turnover rates were measured using the 15N pool dilution technique in situ on four Arrhenatherion meadows in biweekly cycles between May and November. Simultaneously, net rates of mineralization and nitrification, soil temperature, and moisture were measured. The vegetation was mapped, and basic soil properties were determined. The calcareous soils showed higher gross nitrification rates compared with gross mineralization. In contrast, nitrification was inhibited in the decalcified soils, most likely due to the low pH, and mineralization was the dominant process. Both mineralization and nitrification were characterized by high temporal variability (especially the former) and short residence times of N in the corresponding pools (<2 days) at all sites. This illustrates that high temporal resolution is necessary during the growing season to detect N mineralization patterns and capture variability. Parallel determination of net N turnover rates showed almost no variability, highlighting that net rates are not suitable for drawing conclusions about actual gross turnover rates. During the growing season, the data show no clear relationship between soil temperature/soil moisture and gross N turnover rates. For future experiments, recording of microbial biomass, dissolved organic matter, and root N uptake should be considered.

Shifts in soil nitrogen availability and associated microbial drivers during stand development of Mongolian pine plantations

AbstractAfforestation on degraded lands is an effective measure to control desertification and soil erosion, but these functions are often constrained by soil nitrogen (N) availability. Moreover, soil N availability usually shifts along plantation development, yet we have a limited understanding of factors driving this shift. Here, we examined dynamics of soil N mineralization and nitrification rates along plantation development, and explored mechanisms behind these dynamics from the perspective of N‐cycling microbes. We measured needle litter quality, N‐hydrolyzing enzyme activity, biomass and community composition of soil microbes (phospholipid fatty acid analysis) and ammonia oxidizers (real‐time quantitative PCR and high‐throughput sequencing), and net N mineralization and nitrification rates along a chronosequence of Pinus sylvestris var. mongolica stands with six age classes ranging from 15‐ to 61‐year‐old in the Three‐North region of China. Results showed that stand development increased net N mineralization rate by 54%, nitrate‐N concentration by 106%, and nitrate‐N:ammonium‐N ratio from 1.09 to 2.63. Gram‐positive:gram‐negative bacterial ratio and β‐N‐acetylglucosaminidase activity initially increased and subsequently decreased along stand development with the highest values in 40‐year‐old stands, while fungal:bacterial ratio showed the opposite pattern with the lowest value in 40‐year‐old stands. These shifts in soil microbial properties were associated with age‐related changes in needle litter C:N ratio and soil pH. Net N mineralization rate was positively related to β‐N‐acetylglucosaminidase activities, but not to soil microbial biomass and community composition. Net nitrification rate was negatively correlated with ammonia‐oxidizing archaeal abundances and positively with ammonia‐oxidizing bacterial abundances. Collectively, our results indicate that soil N availability increases and N cycling accelerates with plantation growth, and suggest that microbial N recycling driven by β‐N‐acetylglucosaminidase is tightly related to soil N dynamics along stand development.

DMPP mitigates N2O and NO productions by inhibiting ammonia-oxidizing bacteria in an intensified vegetable field under different temperature and moisture regimes

Vegetable soils with high nitrogen input are major sources of nitrous oxide (N2O) and nitric oxide (NO), and incorporation of the nitrification inhibitor 3, 4-dimethylpyrazole phosphate (DMPP) into soils has been documented to effectively reduce emissions. However, the efficiency of DMPP in terms of soil N2O and NO mitigations varies greatly depending on soil temperature and moisture levels. Thus, further evaluations of DMPP efficiency in diverse environments are required to encourage widespread application. A laboratory incubation study (28 d) was established to investigate the interactive effects of DMPP, temperature (15, 25, and 35 °C), and soil moisture (55% and 80% of water-holding capacity (WHC)) on net nitrification rate, N2O and NO productions, and gene abundances of nitrifiers and denitrifiers in an intensive vegetable soil. Results showed that incubating soil with 1% DMPP led to partial inhibition of the net nitrification rate and N2O and NO productions, and the reduction percentage of N2O production was higher than that of NO production (69.3% vs. 38.2%) regardless of temperature and soil moisture conditions. The increased temperatures promoted the net nitrification rate but decreased soil N2O and NO productions. Soil moisture influenced NO production more than N2O production, decreasing with the increased moisture level (80%). The inhibitory effect of DMPP on cumulative N2O and NO productions decreased with increased temperatures at 55% WHC. Conversely, the inhibitory effect of DMPP on cumulative N2O production increased with increased temperatures at 80% WHC. Based on the correlation analyses and automatic linear modeling, the mitigation of both N2O and NO productions from the soil induced by DMPP was attributed to the decreases in ammonia-oxidizing bacteria (AOB) amoA gene abundance and NO2--N concentration. Overall, our study indicated that DMPP reduced both N2O and NO productions by regulating the associated AOB amoA gene abundance and NO2--N concentration. These findings improve our insights regarding the implications of DMPP for N2O and NO mitigations in vegetable soils under various climate scenarios.

Effects of long-term fertilizer practices on rhizosphere soil nitrogen mineralization in the double-cropping rice field.

Nitrogen (N) was an important indictor in change of soil fertility, which was closely related with N mineralization process. However, there is still need to further study on how rhizosphere soil N mineralization in paddy field response to different fertilizer management. Therefore, the influence of long-term (37-years) fertilizer regime on rhizosphere soil N mineralization, ammonification and nitrification rates, and its relationship under the double-cropping paddy field in southern of China were investigated in this study. The field experiment included following fertilizer regimes: inorganic fertilizer alone (MF), rice straw and inorganic fertilizer (RF), 30% organic manure and 70% inorganic fertilizer (OM), and no application of any fertilizer as a control (CK). The result indicated that rhizosphere soil organic carbon (SOC), total N, NO3 -N, and NH4 -N contents in paddy field with OM and RF treatments were increased. The result showed that rhizosphere soil NO2 - -N and mineral N contents with OM and RF treatments were increased, and the order of soil NO2 - -N and mineral N contents with all fertilizer treatments was showed as OM > RF > MF > CK. This result proved that soil aerobic and anaerobic N mineralization rates in paddy field with OM and RF treatments were higher than that of CK and MF treatments. Compared with MF treatment, soil ammonification rate with RF and OM treatments increased by 45.16% and 67.74%, soil nitrification rate with RF and OM treatments increased by 45.71% and 77.14%, respectively. There had significantly positively correlation between soil net mineralization, nitrification rate and SOC, total N contents. As a result, applied with rice straw and organic manure was a good measure to improve soil N mineralization in the double-cropping rice field.

Responses of Alpine Soil Nitrification and Denitrification Rates to Nitrogen Addition Gradient—The Role of Functional Genes

Increases in nitrogen (N) deposition affect the nitrification and denitrification processes that are regulated by microorganisms. With the aim of understanding what happened to the soil nitrification and denitrification rates under the N application gradients, we set a field experiment treated with N at 6 different rates (0, 2, 4, 8, 16, and 32 g N·m−2·yr−1) in 2014. We determined the physico-chemical properties, abundances and community structures of the nitrifiers and denitrifiers, the net nitrification rate (NNR) and the potential denitrification rate (PDR) of soil samples that were collected in 2020. We found that the abundances of ammonia-oxidizing bacteria amoA (AOB amoA), the sum of ammonia-oxidizing archaea amoA (AOA amoA) and AOB amoA increased with the increase in N application rate, but the abundances of nosZ decreased with that. The microorganisms that dominate the ammonia oxidation process could shift from AOA to AOB under high N application rates. Furthermore, the soil microorganisms respond to the N addition preferentially with the abundance changes rather than the community composition changes. Moreover, the NNR increased with the N input, while the decrease in the PDR was due to the decrease in the pH value caused by high N application. The results also showed that the amoA gene abundance explained most (46.3%) of the variation in the NNR. Moreover, the soil moisture and pH explained 44.0% and 27.1% of the variation in the PDR, respectively. The results demonstrated that the NNR and PDR were mainly explained by functional genes abundances and environmental factors, respectively, in alpine meadow soil under sustained N deposition.

The effects of ectomycorrhizal and saprotropic fungi on soil nitrogen mineralization differ from those of arbuscular and ericoid mycorrhizal fungi on the eastern Qinghai-Tibetan Plateau.

Interactions between soil fungi and soil environmental factors regulate soil nitrogen (N) mineralization rates on the eastern Qinghai-Tibetan Plateau. Some studies have also illuminated differences in soil N mineralization rate based on different mycorrhizal forests, but the associated effect of soil fungal functional guilds and soil environmental factors underlying this process are not well-understood. Three primary forests respectively dominated by Abies fargesii var. faxoniana (ectomycorrhizal, EcM), Cupressus chengiana (arbuscular mycorrhizal, AM) and Rhododendron phaeochrysum (ericoid mycorrhizal, ErM) trees were selected in this area. Meanwhile, soil net N mineralization rate, soil fungal composition and soil enzyme activity among these three mycorrhizal forests were studied. Our results showed that there were significant differences in the seasonal variation of soil net N mineralization rates among three mycorrhizal forests. Soil net N mineralization rate in the AM forest was faster. EcM fungi and saprotroph are the main functional guilds in these three mycorrhizal forests. Meanwhile, the relative abundances of soil fungal functional guilds, soil temperature and soil peroxidase activity could explain 85.0% in the difference of soil net ammonification rate among three mycorrhizal forests. In addition, soil temperature, soil water-filled pore space and soil ammonium content play a central role in controlling the differing soil net nitrification rate among three mycorrhizal forests. Our results suggest differences in soil net mineralization among different mycorrhizal forest types are driven mainly by soil net ammonification. Soil fungal functional guilds and temperature regulate the rate of soil net ammonification by modulating soil peroxidase activity.

Responses of soil microbial biomass nitrogen to organic fertilizer with different degrees of maturity and regu-lation to soil mineral nitrogen.

By combining the composting process with soil culture experiment, we conducted an experiment with four treatments, including conventional chemical fertilizer (CK), chemical fertilizer + compost maturity reaching 50% germination index (GI, the same below) organic fertilizer (CO1), chemical fertilizer + compost maturity reaching 80% GI organic fertilizer (CO2), chemical fertilizer + compost maturity reaching 100% GI organic ferti-lizer (CO3). We measured soil microbial biomass nitrogen (MBN), mineral nitrogen (NH4+-N, NO3--N), net nitrification rate, microbial biomass carbon (MBC), dissolved organic carbon (DOC), soil urease and soil protease, aiming to reveal the regulatory effect of soil MBN on mineral nitrogen. The results showed that organic fertilizer application significantly increased MBN and NH4+-N concentrations by 50.1%-62.4% and 109.9%-147.1%, reduced NO3--N concentration and net nitrification rate by 23.3%-46.8%, and 26.2%-51.5%, and enhanced MBC, DOC, urease and protease activities by 33.8%-69.6%, 7.4%-20.8%, 11.2%-69.0% and 9.4%-25.1%, respectively. The change ranges of CO2 and CO3 were significantly higher than CO1. Redundancy analysis (RDA) and structural equation model (SEM) results showed that the application of organic fertilizer with higher degree of maturity (GI≥80%) positively regulated soil MBC, MBN, NH4+-N, and the activities of urease and protease, but had a negative effect on soil net nitrification rate. The combined application of chemical fertilizers and high decomposed organic fertilizers could significantly increase soil MBN and NH4+-N contents, as well as soil urease and protease activities, but reduce soil net nitrification rate. To efficiently utilize organic solid wastes, it is recommended to use chemical fertilizer in combination of organic fertilizers with 80% decomposing degree in practical production to reduce the cost in both economy and time.

Soil nitrification and denitrification in Cunninghamia lanceolata plantations with different stand ages.

The variations in soil nitrification and denitrification processes, together with the abundances of functional microbes were investigated in Cunninghamia lanceolata plantations with different stand ages of 5, 8, 21, 27, and 40 years old. The results showed that the net nitrification rate fluctuated with increasing forest ages, with that of 8-year- and 27-year-old C. lanceolata plantations being significantly lower than other stand ages. The abundance of ammonia-oxidizing archaea (AOA) amoA in the 27-year-old plantation was significantly lower than that of the 40-year-old plantation, while there was no significant difference among the other stand ages. There was no significant difference in the abundance of AOB amoA gene, denitrifying functional genes or soil denitrification potential among different stand ages. The results of stepwise regression analysis showed that the abundance of AOA amoA gene was not significantly affected by soil physical and chemical properties. In addition, the abundance of AOB was positively associated with soil total carbon content and soil pH. The abundance of denitrifying functional genes including narG, nirK and nosZ increased with increasing soil pH. The abundance of nirK and nirS was influenced by soil total carbon. Stand age influenced soil net nitrification rate through the AOA amoA abundance. Moreover, soil denitrification potential was directly affected by stand age, or indirectly affected by stand age through soil microbial biomass carbon, soil pH and denitrifying gene abundance of narG and nirK. Compared with the denitrification process, soil nitrification and associated AOA amoA gene abundance were more sensitive to the development of C. lanceolata plantations. The rotation period sould be appropriately extended to reduce the risk of nitrogen losses resulting from soil nitrification.

Effects of altered precipitation patterns on soil nitrogen transformation in different landscape types during the growing season in northern China

The soil nitrogen (N) cycle and transformation rate are susceptible to precipitation regime changes in semiarid sandy ecosystems, and they play important roles in both the soil fertility and in plant interactions. However, the response mechanisms of soil N transformation to changing precipitation regimes in semiarid ecosystems are still unclear. Thus, we conducted a three-year rainfall manipulation experiment along a landscape evolution gradient from fixed dunes to dune grasslands in a semiarid sandy region of northern China during the growing season (from May to September) between 2017 and 2019. The study included a contrast (P0: natural precipitation) and four treatments in which precipitation was decreased and increased by 60 % [P(−S) and P(+S)] between May and September and decreased and increased by 100 % [(P(−J) and P(+J)] between May and June. The soil nitrate nitrogen (NO3−-N) and inorganic N concentrations significantly increased under P(−S) and P(−J) (P < 0.05) in both fixed dunes and dune grasslands during the growing season. P(−S) and P(−J) significantly increased the soil net nitrification rate (Rn) and net N mineralization rate (Rm) only in July, with Rm significantly increasing with sampling months and years (P < 0.001). The structural equation model showed that Rn and Rm did not change with water availability variation, and an increase in Rm caused by the direct effects of increased precipitation was offset by its indirect effects mediated by reducing electrical conductivity and soil NO3−-N concentration while increasing soil water content and vegetation cover. Soil properties and vegetation characteristics were the main factors affecting soil N transformation during the growing season. Our findings are vital for understanding the seasonal patterns, cumulative N transformation, and possible climate change feedback of semiarid sandy land.