Soil Inorganic Nitrogen Content Research Articles

Plant–soil feedback drives the ‘nursing effect’ on Sitka spruce

Abstract The productivity of economically valuable focal trees in mixtures is often improved by inclusion of lower‐value nursing species, but the mechanisms underpinning such effects are poorly resolved. This gap in understanding limits the capacity to develop efficient planting strategies for forests and woodlands to contribute to net‐zero and other critical ecosystem functions. Here, we undertook a plant–soil feedback experiment to test the hypothesis that feedback effects improve the biomass of Sitka spruce [Picea sitchensis (Bong.) Carr] in soil conditioned by monocultures of heterospecific nurse species, and a mixture comprising Sitka spruce and heterospecifics, compared to soil conditioned by only Sitka spruce. Sitka spruce saplings had greater biomass in soil conditioned by Scots pine monocultures (Pinus sylvestris L.) and the mixture compared to their own soil or soil conditioned by silver birch (Betula pendula Roth). Statistical models showed that colonization of ectomycorrhizal fungi on tree roots in the feedback phase was positively related to seedling biomass, and significantly influenced plant growth strategy. Soil inorganic nitrogen concentrations were strongly affected by monocultures and mixtures in the conditioning phase, but these effects were not related to biomass of seedlings in the feedback phase. However, the positive associations among microbial biomass nitrogen, extractable ammonium in soil, and the activity of the nitrogen‐degrading enzyme N‐acetylglucosaminidase may influence seedling biomass in the longer‐term by stimulating nitrogen cycling. Synthesis and applications. Our findings show that in the crucial early phases of tree growth, plant–soil feedback plays an important role in shaping productivity of Sitka spruce and the nitrogen cycle in forest soils, the latter which may have important consequences for tree biomass in the longer‐term. Our findings demonstrate the importance of the nursing effect in the early stages of plant growth and provide a mechanistic explanation both for long‐held observations of nursing effects in forestry systems and for biodiversity effects in natural woodlands. The nursing effect was largely driven by the activity of symbiotic ectomycorrhizal fungi, and therefore it is important for practitioners to ensure nursing species monocultures or mixtures support ectomycorrhizal fungi that are compatible with high‐value target species.

Effect of Nitrification Inhibitors on Soil Nitrogen Dynamics and Microbial Community Structure in Aerated Paddy Field

The aim of this study was to clarify the effects of nitrification inhibitors on the dynamic changes in the nitrogen and microbial community in paddy soil under aerated irrigation by using the indica hybrid rice Zhongzheyou 8 as experimental material. A trial with three treatments was conducted in 2023： conventional flooding irrigation （CF）, micro-nano bubble water aeration irrigation （MB）, and micro-nano bubble water aeration irrigation combined with nitrification inhibitor （MBNI）. The effects of nitrification inhibitors on the contents of NH4+-N and NO3--N and the apparent nitrification rate of soil under aerated irrigation were analyzed, as well as the characteristics of bacterial community diversity and its correlation with soil chemical factors. The results showed that compared with those in the CF treatment, the MB and MBNI treatment significantly increased the dissolved oxygen concentration, oxidation-reduction potential, pH, and soil NH4+-N content. Compared with that under the MB treatment, the MBNI treatment increased soil pH, reduced NH4+-N content in a short time after nitrogen fertilizer application, and later increased NO3--N content. Compared with that in the CF treatment, the MB and MBNI treatments could increase the diversity and richness of soil bacteria, while the relative abundance of Proteobacteria and Nitrospirota significantly increased, and the relative abundance of Bacteroidota decreased. The MBNI treatment reduced the diversity and richness of bacteria compared with that in the MB treatment and significantly decreased the relative abundance of Nitrospirota. Correlation analysis showed that soil chemical properties were significantly correlated with bacterial community structure. In conclusion, the combined application of nitrification inhibitors under aerated irrigation can improve soil nitrogen supply capacity, increase soil inorganic nitrogen content, improve soil chemical properties, and affect microbial community diversity and richness to optimize microbial community structure.

Effects of poly‐γ‐glutamic acid addition on the transformation and distribution of soil nitrogen under different water conditions

AbstractThe impact of external poly‐γ‐glutamic acid (γ‐PGA) on soil nitrogen (N) transformation and distribution remains unclear, despite its contrasting effects on N use efficiency. Therefore, soil culture and soil column experiments were conducted using three different γ‐PGA addition rates (0%, 4% and 8% of dry soil weight, w/w) under different soil water contents (40%, 60% and 80% of field water capacity) and dry–wet cycles (0, 2, 4 and 8 times cycles; a single dry–wet cycle involved reducing soil water content from 80% to 40% of field water capacity) in sandy loam soil. The results of soil culture experiment showed that the γ‐PGA significantly increased soil –N and –N contents, as well as nitrification and transformation rates. However, these effects were observed to be influenced by both the culture time and soil water content. In addition, the results of soil column experiment showed that γ‐PGA not only significantly enhanced the soil inorganic nitrogen content within the 0–20 cm soil layer, but also improved water retention capacity. However, the differences between the γ‐PGA treatments gradually diminished with an increase in dry–wet cycle times. These results indicate that γ‐PGA addition enhanced soil inorganic N content and soil water retention by influencing soil N transformation and water distribution. However, the impact of γ‐PGA addition on soil improvement was regulated by soil water content, which should be taken into full consideration in agricultural practices.

Biochar application regulating soil inorganic nitrogen and organic carbon content in cropland in the Central Europe: a seven-year field study

Biochar incorporation into soil has shown potential, in enhancing nitrogen fertilizer (N-fertilizer) efficacy and soil organic carbon content (SOC). This study addresses a critical gap in the literature by investigating the effects of biochar addition over a seven-year period (2014–2020) on inorganic N, SOC, and pH in Haplic Luvisol. The research involved a rain-fed field experiment, with a crop rotation comprising spring barley, maize, spring wheat, and pea. Biochar, applied at the rates of 0, 10, and 20 t ha−1 in 2014, was reapplied to specific plots in 2018. Biochar was also combined with N-fertilizer at three level (N0, N1, and N2). Results showed a significant interactive influence of biochar and N-fertilizer combination on NO3− and NH4+ contents. Intriguingly, the addition of 10 t biochar ha−1 consistently decreased soil inorganic N levels across most of the examined months. Increasing biochar application rates led to a significant rise in pH, establishing a clear, negative correlation between soil pH and inorganic N content. Biochar significantly increased SOC compared to the control, particularly after the reapplication in 2018. However, this effect showed a diminishing trend over time. The study suggests that incorporating biochar treatments may enhance N-fertilizer effectiveness. However, the long-term implications of biochar application with N-fertilizer on N mineralization are specific to individual soil and biochar combinations. Except the application of 20 t ha−1 biochar at N2 in 2019, biochar did not affect the crop yields. Studied soil properties, including those influenced by biochar had nuanced impact on different aspects of crop yield.Graphical

Improved Method for Extracting Nitrites in Soil

Soil nitrite (NO2−) is an important reactive intermediate in many nitrogen transformation processes, but it is unstable under acidic conditions and may be lost as gaseous N. The canonical extraction method of soil NO2− using a potassium chloride (KCl) solution greatly underestimates its concentration. To reflect the concentration more accurately, we optimized the extraction method of soil NO2− for three agricultural soils differing in soil texture and pH, an alkalic fluvo-aquic soil and acidic Mollisol and Ultisol soils, respectively. Both extractable soil ammonium (NH4+) and nitrate (NO3−) were systematically investigated to optimize the simultaneous extraction of soil inorganic nitrogen. The effects of different extractants (deionized water (DIW), un-buffered 2 mol L−1 KCl, and pH-buffered 2 mol L−1 KCl), shaking time (10 and 30 min), and storage duration of the extracts (stored at −20 °C for 1 day, and at 4 °C for 1, 3, and 6 days) on the determination of soil inorganic nitrogen were investigated. The results showed that the un-buffered KCl extractant significantly underestimated soil NO2− concentration compared to DIW. The highest recovery of NO2− was obtained by extracting with DIW at 10 min of shaking for all three soils. Compared with DIW, the concentration of NH4+ and NO3− in soil extracted from the KCl solution increased significantly. Furthermore, the soil inorganic nitrogen content of extracts stored at 4 °C for one day was closer to the direct measurements of fresh samples than with the other storage methods. Overall, the recommended analysis method for soil NO2− was extraction by DIW, shaking for 10 min, and filtering with a 0.45 µm filter, while soil NH4+ and NO3− were extracted with a KCl solution and shaken for 30 min. The extract should be stored at 4 °C and analyzed within 24 h. Our study provides an efficient extraction method for soil NO2− and supports studies on the biogeochemical nitrogen cycle, e.g., in the investigation of soil nitrous acid (HONO) and nitric oxide (NO) emissions.

Impact of soil inorganic nitrogen on bacterial phylogeny in estuarine intertidal zones: a study of nitrogen metabolism.

Here we investigated the potential impacts of soil inorganic nitrogen (SIN) content on the phylogenetic characteristics and ecological functions of soil bacterial communities in estuarine intertidal zones in China, aiming to comprehend the response mechanism of soil microorganisms to variations in SIN content within estuarine wetlands. Our results show that SIN in estuarine areas has a significant spatiotemporal variation on spatial and seasonal scales, in this study and is significantly associated with the phylogenetic diversity and phylogenetic turnover of soil bacterial communities. In addition, the results of the metagenomic analysis showed that the relative abundance of nitrogen-cycling functional genes in bacterial communities did not differ significantly in sampling sites and seasons, and weakly correlated with SIN content. Further, the results based on structural equation modeling (SEM) analysis showed that SIN directly and significantly regulated the phylogenetic characteristics of bacterial communities, thereby indirectly affecting the potential of bacterial nitrogen metabolism. This study emphasizes the key influence of SIN variations on the phylogenetic dissimilarity in soil bacterial communities. Moreover, although there was a weak direct relationship between the functional characteristics of the bacterial nitrogen metabolism and SIN content, the spatiotemporal variation of bacterial nitrogen metabolic potential may be indirectly regulated by SIN content by influencing the phylogenetic diversity in bacterial communities. Our study unravels the pivotal mechanisms through which SIN content influences bacterial communities, thereby offering novel insights into the microbial intricacies governing nitrogen metabolism within estuaries.

Controlled-Release Nitrogen Mixed with Common Nitrogen Fertilizer Can Maintain High Yield of Rapeseed and Improve Nitrogen Utilization Efficiency.

Field experiments were conducted to study the effects of different proportions of controlled-release nitrogen fertilizer mixed with quick-acting nitrogen fertilizer on the yield and nitrogen utilization efficiency of direct-seeding rapeseed. Using a conventional nitrogen application rate of 180 kg ha-1 as a control, a total of 5 types of available nitrogen fertilizers and different proportions of controlled-release nitrogen fertilizers were mixed for fertilizer treatment. The proportion of available nitrogen fertilizer used was 135 kg ha-1, and the addition ratios of the five types of controlled-release nitrogen fertilizers were 0%, 30%, 50%, 70%, and 100%, respectively (i.e., the proportion of controlled-release nitrogen to the total nitrogen application amount). These ratios were represented as N135R0, N135R1, N135R2, N135R3, and N135R4, respectively. The results showed that there was no significant difference in the number of pods per plant, the number of seeds per pod, or the grain yield under the treatment of controlled-release nitrogen fertilizer mixed with quick-acting nitrogen fertilizer for proportions of 30-50% (N135R1~R3) when compared with the control, and a stable yield was achieved. Mixing controlled-release nitrogen fertilizer under reduced nitrogen application can significantly improve the apparent utilization rate of rapeseed nitrogen fertilizer, but it first increases and then decreases with the increase of the controlled-release nitrogen mixing ratio, reaching its highest under the N135R2 treatment. The agronomic utilization efficiency and partial productivity of nitrogen fertilizer first increased and then decreased with the increased proportion of controlled-release nitrogen, and both reached their highest utilization with the N135R2 treatment. The mixed treatment of controlled-release nitrogen did not affect soil urease activity, but significantly increased soil sucrase activity. The mixed treatment of controlled-release nitrogen also increased soil microbial biomass nitrogen and carbon content. Especially in the flowering stage, the soil microbial biomass nitrogen and carbon content was significantly higher under a controlled-release nitrogen mixing ratio of 30-50%. At the same time, it had a similar effect on soil inorganic nitrogen content. Therefore, a controlled-release nitrogen mixing treatment provided sufficient nitrogen for the key growth period of rapeseed. Under the condition of reducing the amount of nitrogen fertilizer by 25% based on the amount of nitrogen fertilizer applied to conventional rapeseed, the application of controlled-release urea mixed with common nitrogen fertilizer mixed at a ratio of 30-50% can be an effective way to maintain grain yield levels and improve nitrogen utilization efficiency.

Short term grazing increased growing-season N2O production and decreased its reduction potential by reducing the abundance and expression of nosZ clade II gene in a semi-arid steppe

Understanding nitrous oxide (N2O) production as well as reduction in response to grazing and mowing is essential for designing better management strategies to improve sustainability of grassland ecosystems. We evaluated how four years of grazing or mowing altered N2O production and reduction potential, gene abundance, and expression of microbial functional groups pertinent to N2O production in situ on a typical grassland in Inner Mongolia. In our study, we found that grazing dramatically raised soil ammonium (NH4+-N) and nitrate (NO3−-N) concentrations, AOB gene abundance and potential of N2O production through nitrification (NN2O) and denitrification (DN2O) in summer, but lessened the expression of nosZ clade II gene in all seasons. Mowing had minor effect on soil inorganic nitrogen (N) concentrations. Mowing diminished the quantity of denitrification genes (narG and nosZ), expression of nosZ and nosZ clade II genes, and DN2O concentration. The expression and abundance of nosZ clade II gene were related to DN2. These results suggested that short-term grazing could enhance N2O production potential in peak growing season, while the reduction in abundance and expression of nosZ calde II gene might be an important contributor to the enhanced N2O production of semi-arid typical steppe grasslands.

Recycled Waste Leaf Litter Pots Exhibit Excellent Biodegradability: An Experimental Analysis

The growth of the gardening kit market could result in the increased wasting of nursery pots, which are usually made of plastic. Replacing these pots with biodegradable pots made from green waste could have benefits for climate mitigation, the circular economy, and the greenness of gardening. To address this, we introduce a prototype recycled waste leaf litter (RWLL) nursery pot. Via an incubation experiment over 90 d, we examined their biodegradability and effects on microbial enzyme activity and inorganic nitrogen concentration, comparing them with commercially available biodegradable pots, namely peat–paper mixture pots (also known as Jiffypots®) and coco-coir pots. The effects of pot thickness were tested. Based on mass loss during incubation and on soil CO2 efflux, the RWLL pots exhibited excellent biodegradability, regardless of their thickness, with decomposition rates and soil CO2 efflux 1.5–6 times greater than other biodegradable pots. Biodegradability, extracellular enzyme activity, and soil inorganic nitrogen content were not affected by RWLL pot thickness or by the presence or absence of a plant in the soil. Unlike in natural ecosystems, leaf litter is treated as waste in urban green spaces, and its decomposition into soil organic matter is prevented. Creating plant pots from leaf litter enhances soil quality, reduces atmospheric carbon emissions, and satisfies the desire of gardeners for greenness.

Responses of nitrogen and phosphorus resorption of understory plants to microscale soil nutrient hetero-geneity in Chinese fir plantation.

We compared the interspecific differences in leaf nutrient resorption of two dominant understory species (Lophatherum gracile and Oplimenus unulatifolius), and analyzed the correlations between the intraspecific efficiency of leaf nutrient resorption and nutrient properties of soil and leaves in Chinese fir plantation. The results showed high soil nutrient heterogeneity in Chinese fir plantation. Soil inorganic nitrogen content and available phosphorus content varied from 8.58 to 65.29 mg·kg-1 and from 2.43 to 15.20 mg·kg-1 in the Chinese fir plantation, respectively. The soil inorganic nitrogen content in O. undulatifolius community was 1.4 times higher than that in L. gra-cile community, but there was no significant difference in soil available phosphorus content between the two communities. Both leaf nitrogen and phosphorus resorption efficiency of O. unulatifolius was significantly lower than that of L. gracile under the three measurement bases of leaf dry weight, leaf area, and lignin content. Resorption efficiency in L. gracile community expressed on leaf dry weight was lower than that expressed on leaf area and lignin content, while resorption efficiency expressed on leaf area was the lowest in O. unulatifolius community. The intraspecific resorption efficiency was significantly correlated with leaf nutrient contents, but was less correlated with soil nutrient content, and only the nitrogen resorption efficiency of L. gracile had significant positive correlation with soil inorganic nitrogen content. The results indicated that there was significant difference in the leaf nutrient resorption efficiency between the two understory species. Soil nutrient heterogeneity exerted a weak effect on the intraspecific nutrient resorption, which might be attributed to high soil nutrient availability and potential disturbance from canopy litter in Chinese fir plantation.

Effects of Pruning on Vegetation Growth and Soil Properties in Poplar Plantations

Artificial pruning is an important silvicultural practice that can produce clear wood in poplar plantations. This study focused on the growth of poplar, understory vegetation diversity and soil properties in response to different pruning intensities in poplar plantations. We implemented three different pruning treatments based on the height-to-crown base (HCB) to tree height (H) ratio in Populus deltoides ‘Nanlin 3804′ plantations: CK (no pruning), a 1/3 pruning treatment and a 1/2 pruning treatment. The poplar growth conditions, understory vegetation biodiversity and soil properties were investigated for one year after pruning. Compared with CK, the 1/2 pruning treatment significantly decreased the increment of diameter at breast height (DBHi) and stem volume increment (Vi) by 16.4% and 12.8%, respectively. Meanwhile, pruning significantly promoted understory vegetation biomass and increased the Shannon–Weiner diversity index of understory vegetation, and these variables were positively correlated with pruning intensity. The 1/2 pruning treatment significantly reduced the contents of soil nitrate nitrogen (NO3-N), total inorganic nitrogen (IN) and microbial biomass nitrogen (MBN) by 21.9%, 13.9% and 22.4%, respectively. However, the 1/3 pruning treatment had no significant influence. Pruning mainlyaffectedthe soil enzyme activity in the surface (0–10 cm) layer. The 1/3 and 1/2 pruning treatments significantly decreased soil urease activity by 20.1% and 15.0%, respectively. Furthermore, nonmetric multidimensional scaling analysis showed that the seasonal variation in soil properties was significant, and significant differences among pruning treatments were mainly observed in July and October. Redundancy analysis showed that the growth of aboveground vegetation was significantly correlated with soil properties, particularly soil IN content and urease activity. Therefore, the results highlighted that pruning could promote the growth of understory vegetation and accelerate the transformation of soil nutrients. The 1/2 pruning treatment significantly inhibited the growth of poplar in terms of DBH and V, while the 1/3 pruning treatment promoted the growth of poplar in the short term. Overall, we think that the 1/3 pruning intensity is more suitable for pruning practice.

The potential bias of nitrogen deposition effects on primary productivity and biodiversity.

Atmospheric nitrogen (N) deposition is composed of both inorganic nitrogen (IN) and organic nitrogen (ON), and these sources of N may exhibit different impacts on ecosystems. However, our understanding of the impacts of N deposition is largely based on experimental gradients of INs or more rarely ONs. Thus, the effects of N deposition on ecosystem productivity and biodiversity may be biased. We explored the differential impacts of N addition with different IN:ON ratios (0:10, 3:7, 5:5, 7:3, and 10:0) on aboveground net primary productivity (ANPP) of plant community and plant diversity in a typical temperate grassland with a long-term N addition experiment. Soil pH, litter biomass, soil IN concentration, and light penetration were measured to examine the potential mechanisms underlying species loss with N addition. Our results showed that N addition significantly increased plant community ANPP by 68.33%-105.50% and reduced species richness by 16.20%-37.99%. The IN:ON ratios showed no significant effects on plant community ANPP. However, IN-induced species richness loss was about 2.34 times of ON-induced richness loss. Soil pH was positively related to species richness, and they exhibited very similar response patterns to IN:ON ratios. It implies that soil acidification accounts for the different magnitudes of species loss with IN and ON additions. Overall, our study suggests that it might be reasonable to evaluate the effects of N deposition on plant community ANPP with either IN or ON addition. However, the evaluation of N deposition on biodiversity might be overestimated if only IN is added or underestimated if only ON is added.

Effects of Spartina alterniflora invasion and subsequent mangrove restoration on soil nitrogen mineralization in Quangang, China

The Spartina alterniflora has severely invaded the coastal mangrove ecosystems in China. Here, we attempt to evaluate the invasion effect of S. alterniflora and the restoration effect of mangrove wetlands from the perspective of the nitrogen cycle characteristics. The inorganic nitrogen content, soil nitrogen mineralization, ammonification, and nitrification rate of uninvaded mudflats with those of invaded S. alterniflora site and native Kandelia obovata site at different restoration stages were compared. The results showed that the NH4+–N was the dominant form of soil inorganic nitrogen at all sites. Owing to its higher ammonification rates, the nitrogen mineralization rates at S. alterniflora site were highest in spring and summer among all sites except for mudflat. The young mangrove (1 year and 8 years) had significantly higher soil inorganic nitrogen content but lower nitrogen mineralization than the mature mangrove, indicated the short‐term response to restoration. The S. alterniflora invasion did not enhance soil nitrogen mineralization comparing with preinvaded mudflat, but it used more NH4+–N relative to K. obovata due to its higher soil nitrogen mineralization than mangrove communities in germination and growing seasons.

Soil nitrogen functional transformation microbial genes response to biochar application in different irrigation paddy field in southern China.

Growing evidence points to the controlled irrigation (CI) and biochar application (BA) having agricultural economic value and ecological benefits, but their synergistic effect and microbial mechanism of nitrogen conversion remain unknown in paddy fields. The effects of different BA (0, 20, 40 t/hm2) on the soil nitrogen functional transformation microbial genes (nifH, AOA-amoA, AOB-amoA) in different irrigation (CI, flooding irrigation) were clarified. After one seasonal growth of paddy, the correlation between the abundance of functional genes OUT and soil nitrogen transformation environment factors during the typical growth period was analyzed. High-throughput sequencing results illustrated that the application of CC (40 t/hm2 biochar) increased the nifH genes bacterial community abundance; the abundance of dominant microorganism increased by 79.68~86.19%. Because biochar can potentially control the rates of N cycling in soil systems by adsorbing ammonia and increasing NH4+ storage, it increased soil NH4+-N and NO3--N content by 60.77% and 26.14%, improving microbial nitrogen fixation. Rare species Nitrosopumilus, Nitrosococcus, and Methylocystis appeared in biochar treatments group, which increased the diversity of microbial in paddy. The combined use of CI and BA affected soil inorganic nitrogen content, temperature (T), pH, Eh, etc., which affected urease, urea hydrolysis, and nitrogen functional transformation microorganism genes. Correlation analysis shows that soil NH4+-N, T, and Eh, respectively, are significant factors for the formation of nifH, AOA-amoA, and AOB-amoA soil bacterial communities, respectively. This study suggests that to maintain the biodiversity of soil and realize the sustainable development of rice cultivation, CI is of great importance in combination with BA.

Litter-derived nitrogen reduces methane uptake in tropical rainforest soils

Litter comprises a major nutrient source when decomposed via soil microbes and functions as subtract that limits gas exchange between soil and atmosphere, thereby restricting methane (CH4) uptake in soils. However, the impact and inherent mechanism of litter and its decomposition on CH4 uptake in soils remains unknown in forest. Therefore, to declare the mechanisms of litter input and decomposition effect on the soil CH4 flux in forest, this study performed a litter-removal experiment in a tropical rainforest, and investigated the effects of litter input and decomposition on the CH4 flux among forest ecosystems through a literature review. Cumulative annual CH4 flux was −3.30 kg CH4-C ha−1 y−1. The litter layer decreased annual accumulated CH4 uptake by 8% which greater in the rainy season than the dry season in the tropical rainforest. Litter decomposition and the input of carbon and nitrogen in litter biomass reduced CH4 uptake significantly and the difference in CH4 flux between treatment with litter and without litter was negatively associated with N derived from litter input. Based on the literature review about litter effect on soil CH4 around world forests, the effect of litter dynamics on CH4 uptake was regulated by litter-derived nitrogen input and the amount soil inorganic nitrogen content. Our results suggest that nitrogen input via litter decomposition, which increased with temperature, caused a decline in CH4 uptake by forest soils, which could weaken the contribution of the forest in mitigating global warming.

One-Time Nitrogen Fertilizer Application Using Controlled-Release Urea Ensured the Yield, Nitrogen Use Efficiencies, and Profits of Winter Wheat

One-time fertilization with controlled-released fertilizer (CRF) is a promising way for reducing labor cost, increasing nitrogen use efficiency (NUE) and alleviating environmental pollution in winter wheat (Triticum aestivum L.) cultivation. However, CRF release characteristics are related to various factors such as soil properties, temperature and precipitation, and further study is needed in developing suitable fertilizer formulas adapting to local conditions. In this study, five types of CRF were used for a one-time application in a two-year field experiment conducted at two sites with two wheat cultivars. Their effects on soil inorganic nitrogen (SIN) content, nitrogen uptake, wheat growth and grain yield were investigated. The results indicated that nitrogen supply in the CRF-60/80 treatments not only ensured the tiller differentiation at the early growth stage, but also provided adequate SIN after the jointing stage, thereby facilitating the dry matter accumulation and improving post-anthesis photosynthates accumulating in grains. When compared with conventional split fertilization, the CRF-60 and 80 treatments improved the NUE by 9.7–12.1%, and boosted farmers’ economic efficiency by 282.4–327.2 CNY ha−1. According to our research, a one-time application of CRF-60 and CRF-80 could meet the needs of the two-peak nitrogen demand of wheat in Jiangyan and Yanghzou respectively, therefore increasing NUE and having low labor costs for wheat fertilizer application.

Dissecting the combined effects of cultivar, fertilization, and irrigation on rhizosphere bacterial communities and nitrogen productivity in rice

Rice cultivars, fertilizer types, and irrigation modes can affect soil bacterial communities and thus influence nitrogen utilization by soil microorganisms and plants. However, the combined effects of these three factors on soil bacterial communities and nitrogen productivity in rice plants remain unknown. Here, we examined the response of rhizosphere bacteria and nitrogen productivity to different combinations of cultivar (japonica or indica), fertilization (organic plus chemical or chemical), and irrigation (controlled or shallow-frequent). The results demonstrated the interactive effects of cultivars with fertilizers and irrigation on rhizosphere bacterial communities, nitrogen accumulation, and grain yield. These significant interactive effects were related to differences in the response to soil environment (soil inorganic nitrogen concentration and moisture condition) between diverse rhizosphere bacteria recruited by indica and japonica. We found that rhizosphere bacterial communities recruited by indica were more active in soil fertilized with organic plus chemical nitrogen, while those recruited by japonica were suitable for living in soil fertilized with chemical nitrogen. Rhizosphere bacteria diversity positively correlated with soluble inorganic nitrogen in soil, suggesting that more diverse bacterial communities and greater contents of NH4+-N might favor nitrogen accumulation in rice plants under shallow-frequent irrigation. The combinations of cultivars, fertilizer types, and irrigation greatly affected rhizosphere bacterial communities, thus triggering a significant difference in soil inorganic nitrogen content, which could play an essential role in affecting nitrogen productivity.

Effects of biochar combined with nitrification/urease inhibitors on soil active nitrogen emissions from subtropical paddy soils

We examined the effects of biochar and urease inhibitors/nitrification inhibitors on nitrification process, ammonia and N2O emission in subtropical soil, and determined the best combination of biochar with nitrification and urease inhibitors. This work could provide a theoretical basis for the mitigation of the negative environmental risk caused by reactive nitrogen gas in the application of nitrogen fertilizer. A indoor aerobic culture test was conducted with seven treatments [urea+biochar (NB), urea+nitrification inhibitor (N+NI), urea+urease inhibitor (N+UI), urea+nitrification inhibitor+urease inhibitor (N+NIUI), urea+nitrification inhibitor+biochar (NB+NI), urea+urease inhibitor+biochar (NB+UI), urea+nitrification inhibitor+urease inhibitor+biochar (NB+NIUI)] and urea (N) as the control. The dynamics of soil inorganic nitrogen content, N2O emission and the volatility of ammonia volatilization were observed under combined application of biochar with urease inhibitor (NBPT)/nitrification inhibitor (DMPP). The results showed that:1)Compared to the control (5.11 mg N·kg-1·d-1) during the incubation period, NB treatment significantly increased therate constant of nitrification by 33.9%, and N+NI treatment significantly reduced the nitrification rate constant by 22.9%. NB treatment significantly increased the abundance of ammonia oxidizing bacteria (AOB) by 56.0%. 2) Compared with N treatment, N+NI and NB+NI treatments signi-ficantly enhanced the cumulative emission of NH3 by 49%. The N+UI treatment reduced the cumulative loss of NH3. The inhibition effect of NB+UI treatment was more significant. 3) The emission rate of N2O was highest in the first 10 days after fertilization. The N2O emission under NB treatment was the earliest, and that of N treatment was the highest (5.87 μg·kg-1·h-1). The combined application of DMPP and NBPT performed the best in reducing soil N2O emission. We estimated global warming potential (GWP) of the direct N2O and indirect N2O (NH3) emissions. Compared with N treatments, N+NI and NB+NI treatments increased the GWP by 34.8% and 40.9%, respectively. While the NB and NB+UI treatments significantly reduced the GWP by 45.9% and 60.5%, the combination of biochar and urease inhibitor had the best effect on reduction of GWP of soil active nitrogen emissions.

Reduced fertilization mitigates N2O emission and drip irrigation has no impact on N2O and NO emissions in plastic-shed vegetable production in northern China

Plastic-shed vegetable production in China creates hotspots for emission of the potent greenhouse gas nitrous oxide (N2O) and the atmospheric pollutant nitric oxide (NO). To mitigate N2O and NO emissions, determination of the predominant processes of N2O and NO generation in plastic-shed vegetable production is important. Here, we reported the findings of a 2-year experimental study on the effects of reduced fertilization and/or drip irrigation on N2O and NO emissions during plastic-shed tomato production in northern China. Five treatments were applied: 1) over fertilization and flood irrigation (conventional practice); 2) fertilization reduced by 20% and flood irrigation; 3) fertilization reduced by 20% and drip irrigation; 4) fertilization reduced by 30% and drip irrigation, and 5) control (no fertilizer input and flood irrigation). Reduced both basal and top-dressed fertilization maintained tomato yields. Compared with conventional practices (mean annual N2O and NO emissions: 18.1 ± 1.3 and 0.79 ± 0.02 kg N ha−1 yr −1, respectively), fertilization reduction by 20%–30% decreased the annual N2O emission by 21.2%–27.0% owing to lower soil inorganic nitrogen (SIN) contents under the reduced fertilization practices. Switching from flood to drip irrigation might weaken denitrification due to lower soil moisture and less wet soil area, but increased SIN contents, and thus had no significant impact on annual N2O and NO emissions. Peak N2O fluxes occurred at soil temperature 28 °C and water-filled pore space (WFPS) > 60%, were higher than those for NO, and peak NO fluxes appeared 4–6 days later than N2O fluxes, consistent with the decline in WFPS. These observations indicated that N2O and NO from alkaline plastic-shed soil may be mainly generated via heterotrophic denitrification and nitrification, respectively. Reduced fertilization and drip irrigation in plastic-shed tomato production maintained crop productivity and mitigated N2O emission. These results could be integrated into the decision-making in sustainable plastic-shed production.

Microbial Biomass Is More Important than Runoff Export in Predicting Soil Inorganic Nitrogen Concentrations Following Forest Conversion in Subtropical China

Elevated runoff export and declines in soil microbial biomass and enzyme activity following forest conversion are known to reduce soil inorganic nitrogen (N) but their relative importance remains poorly understood. To explore their relative importance, we examined soil inorganic N (NH4+ and NO3−) concentrations in relation to microbial biomass, enzyme activity, and runoff export of inorganic N in a mature secondary forest, young (five years old) Castanopsis carlessi and Cunninghamia lanceolate (Chinese fir) plantations, and forests developing through assisted natural regeneration (ANR). The surface runoff export of inorganic N was greater, but fine root biomass, soil microbial biomass, enzyme activity, and inorganic N concentrations were smaller in the young plantations than the secondary forest and the young ANR forests. Microbial biomass, enzyme activity, and runoff inorganic N export explained 84% and 82% of the variation of soil NH4+ and NO3− concentrations, respectively. Soil microbial biomass contributed 61% and 94% of the explaining power for the variation of soil NH4+ and NO3− concentrations, respectively, among the forests. Positive relationships between microbial enzyme activity and soil inorganic N concentrations were likely mediated via microbial biomass as it was highly correlated with microbial enzyme activity. Although surface runoff export can reduce soil inorganic N, the effect attenuated a few years after forest conversion. By contrast, the differences in microbial biomass persisted for a long time, leading to its dominance in regulating soil inorganic N concentrations. Our results highlight that most of the variation in soil inorganic N concentration following forest conversion was related to soil microbial biomass and that assisted natural regeneration can effectively conserve soil N.