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In the soil N cycle, heterotrophic nitrification is poorly understood. Our understanding of the factors controlling soil heterotrophic nitrification requires support from investigations in the presence of plants. In this study, a series of 15N tracing pot experiments using maize (Zea mays L.) was conducted and the heterotrophic nitrification rate (O Nrec) and maize N uptake rate were estimated using the Ntrace Plant tool to explore the mechanisms that stimulate heterotrophic nitrification by plants. The results showed that the O Nrec (0.79–3.67 mg N kg−1 d−1) was much higher in the presence of maize than in the control (CK, no plants, <0.10 mg N kg−1 d−1). After the maize was removed, the O Nrec decreased significantly, becoming similar to that of CK. These results indicated that the O Nrec was stimulated by the presence of plants. The O Nrec declined rapidly to 0.16 and 0.13 mg N kg−1 d−1 after the maize was covered with a black box for 2 and 4 days (preventing photosynthesis), respectively. Meanwhile, the soil dissolved organic carbon (DOC) concentration decreased significantly after photosynthesis was prevented. Moreover, the O Nrec correlated significantly with the soil DOC content (P < 0.05). These results revealed that root exudates derived from plant photosynthesis were the key factors that altered soil organic matter, thereby accelerating heterotrophic nitrification. We also found that the maize NO 3 − uptake rate correlated significantly and positively with the O Nrec (P < 0.01), suggesting that the stimulation of heterotrophic nitrification by plants played an important role in the supply of NO 3 − to meet the N requirements of maize and microorganisms. • Heterotrophic nitrification is stimulated by the presence of plants. • Root exudates are important in heterotrophic nitrification stimulation. • Symbiotic microorganisms did not affect heterotrophic nitrification. [ABSTRACT FROM AUTHOR]

Maize (Zea mays L.) genotypes can directly affect gene expression and nitrogen (N) uptake capacity. However, the feedback between maize genotypes and soil gross N transformations, and their regulations on N uptake capacity are not well known. So, 15N tracing pot experiments were conducted in an acidic soil and an alkaline soil with six maize genotypes, i.e. Dika 007 (DK), Zhengda 999 (ZD1), Zhengdan 958 (ZD2), Jingke 968 (JK), Longdan 339 (LD), Liangyu 99 (LY). Maize N uptake and soil gross N transformation rates were quantified by the Ntrace plant tool to identify feedbacks between maize genotypes N uptake and soil N transformations and their mechanisms. Maize total N uptake rates (U TN) varied among genotypes and soils. U TN in alkaline soil was 1.1–2.1 times higher than that in acidic soil. NH 4 + and NO 3 − uptake rates (U NH4 and U NO3 , respectively) were regulated by specific N transport genes in acidic soil, supported by positive relationships between N uptake rates and relative expression of ZmNRT1.1 , ZmNRT1.2 and ZmAMT1.1A genes. U TN , and U NO3 were positively correlated to soil gross N mineralization (M), and autotrophic nitrification (O NH4) rates, respectively. However, maize inhibited M compared to treatment without maize plantation (CK) due to effects on soil microbial community, and based on random forest analysis, the declined relative abundance of Acidicaldub , Gonytrichum and Nigrospora in acidic soil, Taeniolella in alkaline soil may cause the decrease of M by the presence of maize. Meanwhile, maize inhibited O NH4 to increase NH 4 + residence time, which in turn was responsible for the positive relationship between U NH4 and O NH4. In contrast, the stimulation of heterotrophic nitrification (O Nrec) by plants improved the NO 3 − availability. The maize genotypes, such as JK, could largely enhance N uptake by regulating the expression level of N transport genes in maize and stimulating soil N transformation to produce more inorganic N (Nmin) in the different soils. Our findings show that maize genotypes play a central role in regulating these feedbacks. These results are important for maize breeding and enhancing maize production. • Maize genotypes show the significant differences in N uptake rates. • Maize N uptake rate is regulated by NH 4 + and NO 3 − transport genes in acidic soil. • Feedbacks between maize genotypes and soil N transformation affect maize N uptake. • Genotype with strong ability regulating plant-soil feedbacks can largely enhance maize N uptake. [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]