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Soil organic nitrogen (N) mineralization not only supports ecosystem productivity but also weakens carbon and N accumulation in soils. Recalcitrant (mainly mineral‐associated organic matter) and labile (mainly particulate organic matter) organic materials differ dramatically in nature. Yet, the patterns and drivers of recalcitrant (MNrec) and labile (MNlab) organic N mineralization rates and their consequences on ecosystem N retention are still unclear. By collecting MNrec (299 observations) and MNlab (299 observations) from 57 15N tracing studies, we found that soil pH and total N were the master factors controlling MNrec and MNlab, respectively. This was consistent with the significantly higher rates of MNrec in alkaline soils and of MNlab in natural ecosystems. Interestingly, our analysis revealed that MNrec directly stimulated microbial N immobilization and plant N uptake, while MNlab stimulated the soil gross autotrophic nitrification which discouraged ammonium immobilization and accelerated nitrate production. We also noted that MNrec was more efficient at lower precipitation and higher temperatures due to increased soil pH. In contrast, MNlab was more efficient at higher precipitation and lower temperatures due to increased soil total N. Overall, we suggest that increasing MNrec may lead to a conservative N cycle, improving the ecosystem services and functions, while increasing MNlab may stimulate the potential risk of soil N loss. [ABSTRACT FROM AUTHOR]

It is not clear how the number of 15 N-injection needles affects the estimated results of gross N transformation rates in the presence of plants. To evaluate this, a 15 N tracing study of sandy loam soil and a silty loam soil in the presence of maize (Zea mays L., cv. "Zhengdan 958") was conducted and the Ntraceplant tool was used to quantify gross N transformation rates. Our results showed that the number of 15 N-injection needles could significantly influence the estimated results of gross N transformation rates in the studied soil–plant systems. There was no discernible difference in the gross rates of N mineralization (Min), the oxidation of NH4+ to NO3− (ONH4), the oxidation of recalcitrant organic-N to NO3− (ONrec), the NH4+ and NO3− plant uptake between 4-needle and 6-needle injections for both studied soils. However, for the 1-needle and 2-needle injections in the silty loam soil, Min was significantly lower than the 4-needle and 6-needle injections. While, in sandy loam soil, Min measured with 1-needle injection was significantly higher than the 4-needle and 6-needle injections. The ONH4 and ONrec also varied for the 1-needle and 2-needle injections, compared to the 4-needle and 6-needle injections. The total plant N uptake rate declined with the increase in the number of injection needles. Based on these results and considering the simplicity of the experimental procedure, we suggest that the 4-needle injection could be used in the 15 N tracing studies conducted in pot (diameter = 6.4 cm in this study) experiments with plants, i.e., one needle for about 8 cm2. [ABSTRACT FROM AUTHOR]

A 15 N tracing study was conducted with soils from a long-term nitrogen (N) fertilization experiment to quantify the concurrent gross N transformation rates in soil and the underlying mechanisms for crop yield and N2O emission variability. The treatments were chemical fertilization (NPK, NP, PK, NK), organic fertilization (OF), half chemical/organic fertilization (HOF), and no fertilization (control, CK). The results showed that 30 years of repeated organic or chemical N applications significantly stimulated gross rates of N mineralization and autotrophic nitrification compared with CK. In addition, gross rates of N mineralization (except NP) and autotrophic nitrification (except CK, PK) in all treatments were significantly higher in 2019 (30 years) than in 2007 (18 years). However, the immobilization of mineral N decreased significantly (near to zero) after 30 years of repeated N applications compared to CK or the results of 18 years. The highest maize yields were observed in NPK, but they were not significantly different from HOF yields. The OM yields significantly increased with the duration of the experiment and reached values comparable to NPK yields after more than 12 years of repeated N applications. There was a logarithmic positive relationship between maize yields and gross N mineralization rates (p < 0.01), indicating that it was the key factor explaining the variations in yields. Long-term organic or chemical N applications significantly stimulated N2O emission rates, in line with the stimulation of gross autotrophic nitrification rates. Highest N2O emissions were associated with the organic fertilization treatments. Considering the most suitable climate-smart agricultural practice, taking into account yield, N2O emissions, and carbon sequestration, the combination of organic and chemical N fertilizers can be recommended for the region where the study was carried out. [ABSTRACT FROM AUTHOR]

Appropriate N management can improve soil fertility and favour crop growth and production. Plant activities (e.g. N uptake and root exudates) also have feedback effects on soil N transformations and influence soil N supply and retention capacity. However, the inherent mechanisms between the feedback effect intensities and soil fertilizer are not fully elucidated. Thus, we carried out 15N tracing studies with arable soils at various long-term N fertilizer applications, including unfertilized control, synthetic N fertilizer, biochar, pig manure with and without synthetic N fertilizer, crop residues with and without synthetic N fertilizer. Soil gross N transformation rates, maize N uptake rates, and rhizosphere priming effects (RPE) on soil gross N transformations were quantified to explore mechanisms of soil N dynamics. Results showed that long-term N fertilizer applications significantly stimulated soil mineral N production rates, i.e. gross rates of N mineralization (M) and autotrophic nitrification (O NH4), and further enhanced maize N uptake and yield. Negative RPE on M were observed following long-term N fertilizer applications, ranging from −6.71 to −17.44 mg N kg−1 d−1. Furthermore, the strongest negative RPE on M were found for the combinations of chemical and organic N fertilizers. RPE on O NH4 (ranging from −3.10 to −31.91 mg N kg−1 d−1) also showed a similar trend to RPE on M in different treatments. Microbial N immobilization rates (I TN) and dissimilatory NO 3 − reduction to NH 4 + (D NO3) were stimulated by the presence of maize, showing an obvious positive RPE on I TN and D NO3. Therefore, N retention capacity in the rhizosphere was enhanced via a strong negative RPE on O NH4 , and a positive RPE on I TN and D NO3 in presence of maize following long-term N fertilizer applications. The stronger capacity of soil N supply in response to appropriate N management leads to higher maize activities and stronger feedbacks between plant and soil N transformation to improve N retention in rhizosphere, suggesting that the "Matthew effect" also applies to nutrient dynamics in the rhizosphere. Our results show that the combined applications of chemical and organic N fertilizers are the best way to enhance yield, NUE and N retention, and reduce the risks of soil N losses in arable soils. • Rhizosphere priming effects on soil N dynamics at various fertility levels were investigated. • Long-term N fertilizer applications improve soil mineral N supply and yield. • Rhizosphere priming effects on soil N dynamics vary following long-term N fertilizer applications. • The 'Matthew effect' exists in plant-soil systems. • Combination of chemical and organic N fertilizer is best N management in the studied region. [ABSTRACT FROM AUTHOR]

Long-term application of high doses of N fertilizer to achieve high yields has led to a significant accumulation of soil N (largely in the form of NO 3 −) in greenhouse vegetable production (GVP). At present, it is not clear how efficiently vegetables can use the NO 3 − accumulated in soil and from which source the N taken up by vegetables in GVP originates. In this study, three greenhouse soils with various degrees of soil NO 3 − accumulation (N1

Plants with ammonium preference are able to exude biological nitrification inhibitors to reduce nitrification keeping the mineral N in NH 4 + form. The question is whether plants with a NO 3 − preference are able to stimulate nitrification to shift mineral N towards NO 3 − production to meet their NO 3 − demand. In this study we attempted to solve this conundrum by conducting 15N tracing studies in a range of soils planted with wheat (Triticum aestivum L.), a typical NO 3 −-preferring crop, to quantify the gross rates of soil N transformations and the plant N uptake rates. Gross N mineralization rates (M) were stimulated by the presence of wheat in all studied soils, improving the mineral N supply. The wheat NH 4 + uptake rates (U NH4) were significantly, positively correlated with M (p < 0.01). The wheat NO 3 − uptake rates (U NO3) were significantly higher than U NH4 confirming the NO 3 − preference of this plant. As NO 3 − production pathways we considered NH 4 + oxidation (O NH4 , the autotrophic pathway) and organic N oxidation to NO 3 −, O Nrec) in this study. The stimulations of O NH4 were only observed in three out of five soils and, except one soil, O NH4 was much lower (average 1.29 mg N kg−1 d−1) than U NO3 (average 7.66 mg N kg−1 d−1) showing that the NO 3 − supply via this pathway was insufficient to meet the plants NO 3 − demand. In these soils, O Nrec was significantly stimulated ranging from 0.86 to 5.52 mg N kg−1 d−1 and was responsible for 34%–74% of NO 3 − production during the 30 days experimental duration. Moreover, U NO3 was significantly, positively correlated with O Nrec (p < 0.05), indicating a direct link between heterotrophic nitrification and plant NO 3 − uptake. One soil (SC2) exhibited a much higher O NH4 (>8.00 mg N kg−1 d−1) and only M was stimulated by the plants presence but not heterotrophic nitrification because the NO 3 − supply via O NH4 was sufficient to meet the plant NO 3 − demand. Heterotrophic nitrification was stimulated by NO 3 − preference plants when NO 3 − supply via oxidation of NH 4 + to NO 3 − was insufficient to meet the NO 3 − requirements. • Interactions between wheat N acquisition and soil N transformations were observed. • Gross N mineralization rates were stimulated by the presence of wheat. • Heterotrophic nitrification was stimulated by wheat when autotrophic nitrification was unable to supply sufficient NO 3 −. [ABSTRACT FROM AUTHOR]