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Straw incorporation with nitrogen (N) fertilization is crucial for enhancing soil fertility and minimizing negative environmental impacts by altering the magnitude and direction of soil N transformation processes. However, the response of soil N transformations to long-term carbon (C) and N inputs, and their primary driving factors, remain poorly understood. Thus, a 15 N tracing study was conducted to investigate the effects of straw incorporation (AS) and straw removal (NS) with N levels of 0, 150 and 250 kg N ha -1 per season (N0, N150 and N250) on gross N transformation rates in the North China Plain after 6-year trial. Results indicated that at N0, AS significantly increased soil microbial immobilization of nitrate (NO 3 - -N, I NO3 ) and autotrophic nitrification rates (O NH4 ) compared to NS. With N fertilization, AS increased gross N immobilization (I total ), ammonium-N immobilization (NH 4 + -N, I NH4 ), net NH 4 + -N immobilization (I netNH4 ) and net NH 4 + -N absorption rates (A netNH4 ). Specifically, at N150, AS significantly increased recalcitrant organic N mineralization rate (M Nrec ), while significantly reducing O NH4 , labile organic N mineralization (M Nlab ), and gross N mineralization rates (M total ). At N250, A netNH4 , M Nlab , M Nrec and O NH4 under AS were significantly higher than under NS. Nitrogen application significantly increased O NH4 , I total and I NO3 under two straw management practices, and enhanced I NH4 and I netNH4 under AS. Compared to N250, N150 significantly increased I NH4 and I netNH4 under AS, while decreasing M total . Opposite results were observed under NS. Meanwhile, NO 3 - -N and dissolved organic carbon (DOC) were master factors controlling immobilization, total nitrogen (TN), hydrolysable NH 4 + -N (HNN) and stable organic N significantly affected A netNH4 , while labile organic N were the key environmental factors affecting M Nrec , all of which positively influenced the rates of assimilation, mineralization and clay mineral adsorption. Overall, this study provides new insights into reducing N fertilization under straw incorporation by quantifying soil N transformation processes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The accumulation of nitrogen (N) from straw and fertilizer in soil effectively reduces N losses, which is vital for protecting dryland farming environments. However, the quantification of exogenous N contributions to soil organic nitrogen (SON) under different carbon (C) and N management practices in maize-wheat cropping systems remains unknown. Here, a 15 N in situ labelling experiment was conducted, based on continuous 5-year N levels (0, 150, 250 kg N ha -1 applied for each crop) and two straw management practices (NS, straw removal; AS, straw incorporation) to investigate the allocation of exogenous N to SON and its underlying accumulation mechanisms. The atom% excess in SON was determined after fractionating it into active and stable fractions by the acid hydrolysis method. Compared to NS, AS significantly increased the distribution of fertilizer N into stable SON by 168.4 %-223.6 % in the maize season, and into active and stable SON by 256.7 %-278.4 % and 142.0 %-167.6 %, respectively, in the wheat season. The content and retention rate of fertilizer N in SON were highest at the N250 and N150 levels, respectively, under both NS and AS treatments in the two crop seasons. In contrast, N addition decreased the allocation of straw N to SON, especially in the wheat season. Notably, the content and residual rate of exogenous N in SON between the N150 and N250 levels showed no significant differences. Straw incorporation exerted the most significant direct and positive impact on the immobilization of fertilizer N in the soil, whereas N application indirectly influenced straw N accumulation, primarily by altering labile C and N contents, subsequently selecting specific microbial communities. Gram-positive bacteria and actinomycetes exhibited a significant positive correlation with straw N content in SON. This study provides a new perspective on N nutrient management by quantifying exogenous N accumulation in the soil., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier B.V.)

Understanding the fate of exogenous fertilizer-derived inorganic phosphorus (P i ) is essential for effective P management. Hence, this study carried out a 180-day incubation experiment with or without KH 2 P 18 O 4 in soils with four different fertilization regimes [without fertilizer (CK), mineral P and K fertilizer (PK), mineral N, P, and K fertilizer (NPK), compost (OM)]. We analyzed the atom % excess in phosphate oxygen isotope of sequentially extracted P i pools (H 2 O-P i , NaHCO 3 -P i , NaOH-P i , and HCl-P i ), soil respiration, potential phosphatase activities, and microbial biomass. Our results showed that exogenous phosphate fertilizer was immediately transformed into the H 2 O-P i and NaHCO 3 -P i pools and gradually partially immobilized in the HCl-P i pool. Additionally, biotransformation plays an important role in the turnover of fertilizer-derived P i . After the 180-day incubation, the biologically transformed H 2 O-P i content was significantly (P<0.05) reduced by 63.2 % on average, with the largest reduction in PK. The NaHCO 3 -P i gradually increased in both CK and OM through biotic processes. However, it continuously decreased in PK and NPK, likely due to the strong adsorption and microbial fixation. The NaOH-P i fluctuated slightly in CK, NPK, and OM while gradually decreasing in PK. At the end of the incubation, 28.6 %, 37.0 %, 61.2 %, and 75.2 % of the P i increment in CK, OM, NPK, and PK were stored in the HCl-P i pool, respectively. Overall, these findings provide important information on the dynamics of fertilizer-derived P i , delivering new insights into rational phosphate fertilizer management and sustainable agricultural development., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)