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Soil N mineralization (Nmin) rates varied spatially among cropland fields. Soil Nmin rates increased with a decreasing elevation. Soil Nmin was mainly affected by SOC, TN, and available C and N. Nmin in cropland soil should be considered when evaluating regional water pollution. Soil nitrogen mineralization (Nmin) is a key process that converts organic N into mineral N that controls soil N availability to plants. However, regional assessments of soil Nmin in cropland and its affecting factors are lacking, especially in relation to variation in elevation. In this study, a 4-week incubation experiment was implemented to measure net soil Nmin rate, gross nitrification (Nit) rate and corresponding soil abiotic properties in five field soils (A–C, maize; D, flue-cured tobacco; and E, vegetables; with elevation decreasing from A to E) from different altitudes in a typical intensive agricultural area in Dali City, Yunnan Province, China. The results showed that soil Nmin rate ranged from 0.10 to 0.17 mg·kg−1·d−1 N, with the highest value observed in field E, followed by fields D, C, B, and A, which indicated that soil Nmin and Nit rates varied between fields, decreasing with elevation. The soil Nit rate ranged from 434.2 to 827.1 µg·kg−1·h−1 N, with the highest value determined in field D, followed by those in fields E, C, B, and A. The rates of soil Nmin and Nit were positively correlated with several key soil parameters, including total soil N, dissolved organic carbon and dissolved inorganic N across all fields, which indicated that soil variables regulated soil Nmin and Nit in cropland fields. In addition, a strong positive relationship was observed between soil Nmin and Nit. These findings provide a greater understanding of the response of soil Nmin among cropland fields related to spatial variation. It is suggested that the soil Nmin from cropland should be considered in the evaluation of the N transformations at the regional scale.

An aerobic 15N microcosmic experiment was conducted to compare the inhibitory effects of the biological nitrification inhibitor (BNI), methyl 3-(4-hydroxyphenyl) propionate (MHPP) at rates of 500 and 1000 mg kg−1 with the synthetic nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) at 1% of applied NH4+, on the gross nitrification rate (n_gross) and on the abundance and community composition of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) of two contrasting soils (pH: 5.10 vs. 8.15, clay content 17.8 vs. 30.8). DMPP inhibited 56.6% of n_gross in the acidic soil and 50.3% in the calcareous soil, whereas MHPP inhibited 18.3–55.5% of n_gross in the acidic soil and 14.1–20.2% in the calcareous soil. MHPP used at the high rate showed the same inhibition on n_gross as DMPP in the acidic soil but not in the calcareous soil. DMPP and MHPP likely regulated n_gross by causing niche differentiation between AOA and AOB. Moreover, the community composition of AOB was more sensitive to nitrification inhibitor application than that of AOA, particularly in the acidic soil. However, the response of AOB community composition was less sensitive to the application of MHPP than to that of DMPP. MHPP mainly targeted Nitrosospira clusters 3a.2, 3b.2, and 9 of the AOB in the acidic soil.

This study evaluated the impacts of a nitrification inhibitor (3,4-dimethylpyrazole phosphate, DMPP) and herbicides (atrazine and glyphosate) on nitrification, gross nitrite, and nitrate (NO2−-N + NO3−-N) consumption rate, nitrous oxide (N2O) emission, and abundances of microbial functional genes related to nitrogen (N) cycling in an Australian sugarcane soil. The experiment included four treatments: blank control (CK), DMPP application (NI), atrazine application (ATR), and glyphosate application (GLY). All treatments received (NH4)2SO4 at 50 mg N kg−1 dry soil and KNO3 at 50 mg N kg−1 dry soil and were incubated initially at 55% of water holding capacity (WHC) for 7 days and subsequently at 75% WHC for another 7 days (K15NO3 with 5 atom% 15N added at the beginning of each stage). Compared with the CK treatment, DMPP application significantly decreased N2O emissions throughout the incubation, while atrazine or glyphosate application significantly inhibited N2O emissions only during the 4–7-day period. DMPP application also decreased ammonium-oxidizing bacteria (AOB) amoA gene abundances, gross NO2−-N + NO3−-N consumption rates at 55 and 75% WHC, and nirS and nirK gene abundances of denitrifiers at 75% WHC. The atrazine and glyphosate applications decreased the gross nitrification and NO2−-N + NO3−-N consumption rates, abundances of both ammonium-oxidizing archaea (AOA) and AOB amoA genes at 55 and 75% WHC, and abundances of functional genes related to different reactions of the denitrification during the incubation. These results suggested that DMPP, atrazine, and glyphosate could decrease soil gross nitrification and denitrification rates perhaps by inhibiting microbial functional gene abundances and that application of DMPP could effectively reduce N2O emissions in the sugarcane cropping soil.

Nitrification is a major pathway of N2O production in aerobic soils. Measurements and model simulations of nitrification and associated N2O emission are challenging. Here we innovatively integrated data mining and machine learning to predict nitrification rate (\\(R_nit\\)) and the fraction of nitrification as N2O emissions (\\(f_N_2O_Nit\\)). Using our global database on \\(R_nit\\) and \\(f_N_2O_Nit\\), we found that the machine-learning based stochastic gradient boosting (SGB) model outperformed three widely used process-based models in estimating \\(R_nit\\) and N2O emission from nitrification. We then applied the SGB technique for global prediction. The potential \\(R_nit\\) was driven by long-term mean annual temperature, soil C/N ratio and soil pH, whereas \\(f_N_2O_Nit\\) by mean annual precipitation, soil clay content, soil pH, soil total N. The global \\(f_N_2O_Nit\\) varied by over 200 times (0.006%–1.2%), which challenges the common practice of using a constant value in process-based models. This study provides insights into advancing process-based models for projecting N dynamics and greenhouse gas emissions using a machine learning approach.

Nitrification and nitrogen (N) immobilization are important pathways in soil N transformations, involving soil N loss and retention, respectively. The ratio of nitrification to N immobilization generally reflects the potential risk of soil N loss. However, little is known about the response of this ratio to anthropogenic carbon (C) and N inputs, but also climate and soil conditions. Here, we aimed to elucidate, for the first time, the impacts of chemical fertilizer and manure application on the ratio of gross nitrification to N immobilization by using 15 N dilution technology, based on ten long-term fertilization trials spanning multiple climatic zones in eastern China. Results showed that manure application differentially increased gross N immobilization rather than nitrification compared to the chemical fertilizer treatment, leading to manure-induced decreases in gross nitrification to N immobilization ratio ranging from 1.2 to 93% across the sites. The decreased gross nitrification to N immobilization ratio in the manure treatment was mainly due to the increased ratio of bacteria to nitrifiers abundance. Manuring was more effective for a decrease in the gross nitrification to N immobilization ratio at sites characterized by high rainfall and low soil pH, as it prevented soil pH decline thereby favoring bacterial abundance and N immobilization. Consequently, manure application resulted in a substantial increase in soil total N accumulation, facilitated by increased microbial N immobilization that promoted microbial biomass. These findings suggest that substituting manure for chemical fertilizer in the areas with high rainfall and acidic soils promisingly reduces soil N loss risk, with positive consequences for soil N retention. This knowledge highlights the potential to reconcile soil N loss and fertility improvement through optimizing regional manure management, which offers valuable insights for the development of a tailored regional fertilization management strategy.

Aims Soil fungi play an important role in decomposing soil organic matter and facilitating nutrient uptake by plants, however, the relationship between fungal community and soil biogeochemical cycling during plant invasion is poorly understood. The objective of this study was to investigate the effects of Moso bamboo ( Phyllostachys edulis ) invasion into broadleaf forests on the soil organic C (SOC) chemical composition, fungal community and mineral N production. Methods We collected soil samples in evergreen broadleaf forests, mixed bamboo-broadleaf forests and bamboo forests. Soil fungal community and SOC chemical composition were determined. Results Bamboo invasion decreased alkyl C but increased O -alkyl C contents. Soil fungal abundance (18S rRNA) was decreased, while their alpha diversity was increased by bamboo invasion. Additionally, bamboo invasion enhanced net N mineralization rate but reduced gross nitrification rate. The fungal community composition strongly correlated with alkyl C content, and alkyl C content explained 32% of the variation in the fungal abundance. Fungal community composition correlated with gross nitrification rate, with 43% of the variation in gross nitrification rate attributable to soil fungal abundance. Conclusions Changes in soil fungal community caused by bamboo invasion into broadleaf forests were closely linked to changed soil organic C chemical composition and decelerated nitrate production.

Soil moisture changes, arising from seasonal variation or from global climate changes, could influence soil nitrogen (N) transformation rates and N availability in unfertilized subtropical forests. A ¹⁵ N dilution study was carried out to investigate the effects of soil moisture change (30–90 % water-holding capacity (WHC)) on potential gross N transformation rates and N₂O and NO emissions in two contrasting (broad-leaved vs. coniferous) subtropical forest soils. Gross N mineralization rates were more sensitive to soil moisture change than gross NH₄ ⁺ immobilization rates for both forest soils. Gross nitrification rates gradually increased with increasing soil moisture in both forest soils. Thus, enhanced N availability at higher soil moisture values was attributed to increasing gross N mineralization and nitrification rates over the immobilization rate. The natural N enrichment in humid subtropical forest soils may partially be due to fast N mineralization and nitrification under relatively higher soil moisture. In broad-leaved forest soil, the high N₂O and NO emissions occurred at 30 % WHC, while the reverse was true in coniferous forest soil. Therefore, we propose that there are different mechanisms regulating N₂O and NO emissions between broad-leaved and coniferous forest soils. In coniferous forest soil, nitrification may be the primary process responsible for N₂O and NO emissions, while in broad-leaved forest soil, N₂O and NO emissions may originate from the denitrification process.

Large areas in the tropics receive elevated atmospheric nutrient inputs. Presently, little is known on how nitrogen (N) cycling in tropical montane forest soils will respond to such increased nutrient inputs. We assessed how gross rates of mineral N production (N mineralization and nitrification) and microbial N retention (NH 4 + and NO 3 − immobilization and dissimilatory NO 3 − reduction to NH 4 + [DNRA]) change with elevated N and phosphorus (P) inputs in montane forest soils at 1000-, 2000-, and 3000-m elevations in south Ecuador. At each elevation, four replicate plots (20 × 20 m each) of control, N (added at 50 kg N·ha −1 ·yr −1 ), P (added at 10 kg P·ha −1 ·yr −1 ), and combined N + P additions have been established since 2008. We measured gross N cycling rates in 2010 and 2011, using 15 N pool dilution techniques with in situ incubation of intact soil cores taken from the top 5 cm of soil. In control plots, gross soil-N cycling rates decreased with increase in elevation, and microbial N retention was tightly coupled with mineral N production. At 1000 m and 2000 m, four-year N and combined N + P additions increased gross mineral N production but decreased NH 4 + and NO 3 − immobilization and DNRA compared to the control. At 3000 m, four-year N and combined N + P additions increased gross N mineralization rates and decreased DNRA compared to the control; although NH 4 + and NO 3 − immobilization in the N and N + P plots were not different from the control, these were lower than their respective mineral N production. At all elevations, decreased microbial N retention was accompanied by decreased microbial biomass C and C:N ratio. P addition did not affect any of the soil-N cycling processes. Our results signified that four years of N addition, at a rate expected to occur at these sites, uncoupled the soil-N cycling processes, as indicated by decreased microbial N retention. This fast response of soil-N cycling processes across elevations implies that greater attention should be paid to the biological implications on montane forests of such uncoupled soil-N cycling.

Intensive management practices in largescale oil palm plantations can slow down nutrient cycling and alter other soil functions. Thus, there is a need to reduce management intensity without sacrificing productivity. The aim of our study was to investigate the effect of management practices on gross rates of soil N cycling and soil fertility. In Jambi province, Indonesia, we established a management experiment in a large-scale oil palm plantation to compare conventional practices (i.e. high fertilization rates and herbicide weeding) with reduced management intensity (i.e. reduced fertilization rates and mechanical weeding). Also, we compared the typical management zones characterizing large-scale plantations: palm circle, inter-row and frond-stacked area. After 1.5 years of this experiment, reduced and conventional management showed comparable gross soil N cycling rates; however, there were stark differences among management zones. The frond-stacked area had higher soil N cycling rates and soil fertility (high microbial biomass, extractable C, soil organic C, extractable organic N, total N and low bulk density) than inter-row and palm circle (all p ≤ 0.05). Microbial biomass was the main driver of the soil N cycle, attested by its high correlation with gross N-cycling rates (r = 0.93–0.95, p < 0.01). The correlations of microbial N with extractable C, extractable organic N, soil organic C and total N (r = 0.76–0.89, p < 0.01) suggest that microbial biomass was mainly regulated by the availability of organic matter. Mulching with senesced fronds enhanced soil microbial biomass, which promoted nutrient recycling and thereby can decrease dependency on chemical fertilizers.

Aims Long-term application of pig manure can improve soil fertility and alleviate soil acidification, but also increase nitrogen (N) losses in subtropical upland red soils. However, mechanisms driving N losses via nitrate leaching or N₂O emissions remain unknown. Herein we investigated long-term pig manure applications in upland red soils by assessing soil N transformation dynamics. Methods Pig manure was applied with or without lime over a 14-year period in four treatments: No manure (CK); Low-rate manure (LM, 150 kg N ha⁻¹ y⁻¹); High-rate manure (HM, 600 kg N ha⁻¹ y⁻¹); High-rate manure and lime (HML, 600 kg N ha⁻¹ y⁻¹ and 3000 kg Ca(OH)₂ ha⁻¹ (3y)⁻¹). ¹⁵N tracing was used to quantify gross N transformation and N dynamics. Results Prolonged manure application increased soil gross N mineralization and NH₄⁺ immobilization, although the increase was only significant for HM. Both rates were further enhanced by lime addition. Gross autotrophic nitrification also increased with increasing manure application, and further increased with lime addition. In contrast, dissimilatory NO₃⁻ reduction to NH₄⁺ (DNRA) and NO₃⁻ immobilization were negligible irrespective of manure application. Thus, NO₃⁻ produced via autotrophic nitrification was not converted to NH₄⁺ and microbial biomass N, and accumulated in soil. Gross autotrophic nitrification was positively correlated with N₂O emissions and NO₃⁻ leaching, suggesting it largely determined N losses. Conclusions Autotrophic nitrification governs N losses in upland red soils receiving repeated manure applications, and attempts to reduce N emissions or N leaching should therefore control this.