Effects of straw management and N levels on gross nitrogen transformations in fluvo-aquic soil of the North China Plain.

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 15N 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 (NO3--N, INO3) and autotrophic nitrification rates (ONH4) compared to NS. With N fertilization, AS increased gross N immobilization (Itotal), ammonium-N immobilization (NH4+-N, INH4), net NH4+-N immobilization (InetNH4) and net NH4+-N absorption rates (AnetNH4). Specifically, at N150, AS significantly increased recalcitrant organic N mineralization rate (MNrec), while significantly reducing ONH4, labile organic N mineralization (MNlab), and gross N mineralization rates (Mtotal). At N250, AnetNH4, MNlab, MNrec and ONH4 under AS were significantly higher than under NS. Nitrogen application significantly increased ONH4, Itotal and INO3 under two straw management practices, and enhanced INH4 and InetNH4 under AS. Compared to N250, N150 significantly increased INH4 and InetNH4 under AS, while decreasing Mtotal. Opposite results were observed under NS. Meanwhile, NO3--N and dissolved organic carbon (DOC) were master factors controlling immobilization, total nitrogen (TN), hydrolysable NH4+-N (HNN) and stable organic N significantly affected AnetNH4, while labile organic N were the key environmental factors affecting MNrec, 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.

The fate of fertilizer-derived phosphorus under different long-term fertilization regimes: A phosphate oxygen isotope study.

Understanding the fate of exogenous fertilizer-derived inorganic phosphorus (Pi) is essential for effective P management. Hence, this study carried out a 180-day incubation experiment with or without KH2P18O4 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 Pi pools (H2O-Pi, NaHCO3-Pi, NaOH-Pi, and HCl-Pi), soil respiration, potential phosphatase activities, and microbial biomass. Our results showed that exogenous phosphate fertilizer was immediately transformed into the H2O-Pi and NaHCO3-Pi pools and gradually partially immobilized in the HCl-Pi pool. Additionally, biotransformation plays an important role in the turnover of fertilizer-derived Pi. After the 180-day incubation, the biologically transformed H2O-Pi content was significantly (P<0.05) reduced by 63.2 % on average, with the largest reduction in PK. The NaHCO3-Pi 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-Pi 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 Pi increment in CK, OM, NPK, and PK were stored in the HCl-Pi pool, respectively. Overall, these findings provide important information on the dynamics of fertilizer-derived Pi, delivering new insights into rational phosphate fertilizer management and sustainable agricultural development.