A maize pot experiment was conducted to compare the difference of N distribution between bulk and rhizospheric soil after chemical fertilizer with or without soil straw amendment at an equivalent N rate using a ¹⁵N cross-labeling technique. Soil N pools, maize N and their ¹⁵N abundances were determined during maize growth. The urea plus straw treatment significantly (p < 0.05) increased the recovery of urea N in soil and 26.0% of straw N was assimilated by maize. Compared with urea treatment in bulk soil, urea plus straw treatment significantly (p < 0.05) increased the concentration and percentage of applied N as dissolved organic N (DON) and microbial biomass N (MBN) from milk stage to maturity, increased those as particulate organic N (PON) and mineral associated total N (MTN) throughout maize growth and decreased those as inorganic N (Inorg-N) from the eighth leaf to the silking stage. Compared with bulk soil, rhizospheric soil significantly (p < 0.05) decreased the concentration and percentage of applied N as PON and increased those as Inorg-N and MTN in both applied N treatments from the silking stage, and significantly (p < 0.05) decreased the concentration and percentage of applied N as microbial biomass N (MBN) in the urea plus straw treatment. Overall, straw N was an important N source and combined application of chemical fertilizer with straw increased soil fertility, with the rhizosphere regulating the transformation and supply of different N sources in the soil–crop system.

The objective of this study was to determine the efficiency of different plant systems in capturing deep soil nitrate (NO₃⁻) to reduce NO₃⁻ leaching in a field plot experiment using ¹⁵N labelling. The study was conducted on a calcareous alluvial soil on the North China Plains and the plant systems evaluated included alfalfa (Medicago sativa), American black poplar (Populus nigra) and cocksfoot (Dactylis).¹⁵N-labelled N fertilizer was injected to 90 cm depth to determine the recovery of ¹⁵N by the plants. With conventional water and nutrient management, the total recovery of ¹⁵N-labeled NO₃⁻-N was 23.4% by alfalfa after two consecutive growth years. The recovery was significantly higher than those by American black poplar (12.3%) and cocksfoot (11.4%). The highest proportion of soil residual ¹⁵N from the labeled fertilizer N (%Ndff) was detected around 90 cm soil depth at the time of the 1st year harvest and at 110-130 cm soil depth at time of the 2nd year harvest. Soil %Ndff in 0-80 cm depth was significantly higher in the alfalfa treatment than those in all the other treatments. The soil %Ndff below 100 cm depth was much lower in the alfalfa than those in all the other treatments. These results indicated that ¹⁵N leaching losses in the alfalfa treatment were significantly lower than by those in the black poplar and cocksfoot treatments, due to the higher root density located in nitrate labeling zone of soil profile. In conclusion, alfalfa may be used as a plant to capture deep soil NO₃⁻ left from previous crops to reduce NO₃⁻ leaching in high intensity crop cultivation systems of North China Plain.

Microbial residue nitrogen can indicate soil quality and is crucial for soil nitrogen retention and supply. However, it is still unclear how the dynamic changes in soil microbial residue nitrogen affect crop nitrogen uptake in agricultural practice. Based on a long-term wheat-maize rotation experiment with different nitrogen application rates (150 kg N ha⁻¹, 200 kg N ha⁻¹, and 300 kg N ha⁻¹), ¹⁵N-labeled nitrogen fertilizer was applied during the wheat season to track the dynamics of soil microbial residue nitrogen and its mediated fertilizer nitrogen. The results showed that nitrogen addition was beneficial to the accumulation of microbial residue nitrogen (mainly fungal microbial residue nitrogen). Its contribution rate to soil total nitrogen is 54.87–56.55%, and the fertilizer nitrogen allocated to it accounts for 27.10–47.50% of the remaining fertilizer nitrogen in the soil. Ultimately, 6.77–10.24% of the nitrogen fertilizer applied remained in the soil as microbial residue nitrogen. In addition, microbial residue nitrogen is mainly mineralized during the jointing and filling stages of wheat. In a word, the accumulation and mineralization of soil microbial residue nitrogen during the application of 200 kg N ha⁻¹ better guaranteed the uptake of nitrogen by wheat, which provided a reliable basis for guiding farmland fertility improvement and nitrogen fertilizer reduction in the North China Plain.

Intercropping with green manures is an effective practice for increasing agricultural production and reducing environmental issues. However, the effects of green manure type and intercropping patten on soil nutrient availability and microbial communities remains underexplored. In the present study, the impacts of three green manure–maize intercropping patterns on maize yield, rhizosphere nutrient availability, and soil fungal community were evaluated. Four treatments (three replicate plots for each) were involved, including a monoculture treatment (MC) as a control and three intercropping patterns as follows: maize–ryegrass (Lolium perenne L.) (IntL), maize–forage soybean (Fen Dou mulv 2, a hybrid soybean cultivar) (IntF), and maize–ryegrass–forage soybean (IntLF) intercropping. The results showed that all three intercropping patterns significantly increased maize yield and rhizosphere available phosphorus (AP) compared with MC. Intercropping shifted the dominant assembly process of the maize rhizosphere fungal community from stochastic to deterministic processes, shaping a community rich in arbuscular mycorrhizal fungi (AMF) and limited in plant pathogens, primarily Exserohilum turcicum. AP showed significant correlations with fungal community and AMF, while maize yield was negatively correlated with plant pathogens. In addition, the dual-species green manure intercropping pattern (IntLF) had the strongest positive effects on maize yield, AP content, and fungal community compared with single-species patterns (IntL and IntF). These results illustrate the advantages of planting diversification in boosting crop production by improving nutrient availability and soil health in the rhizosphere and suggest that the maize–ryegrass–forage soybean intercropping system is a potential strategy for improving soil fertility and health.

A ¹⁵ N maize pot experiment was conducted to compare the N value of fertilizer alone and fertilizer combined with straw at an equivalent N rate. The four treatments were control (CK), ¹⁵ N-urea, ¹⁵ N-urea plus straw, and ¹⁵ N-straw plus urea. Soil N pools, maize N and their ¹⁵ N abundance were determined during maize growth. At maturity 26.0% of straw N was assimilated by maize in the urea plus straw treatment. From the eighth leaf stage to maturity, urea plus straw had a significantly (P < 0.05) higher concentration and percentage of exogenous substrate N present as soil total N (TN), particulate organic N (PON), and mineral associated total N (MTN) in bulk and rhizosphere soils than the urea-only treatment. From silking to maturity in the urea plus straw treatment, rhizosphere soil significantly (P < 0.05) increased the percentage of exogenous substrate N present as inorganic N (Inorg-N) and MTN, and significantly (P < 0.05) decreased that present as PON and microbial biomass N (MBN) compared with the bulk soil. From the eighth leaf stage to maturity, rhizosphere soil significantly (P < 0.05) increased the percentage of straw N present as Inorg-N and MTN except for MTN at the silking stage, and significantly decreased (P < 0.05) that present as PON compared with the bulk soil. Overall, straw was an available N source to the crop, and the increase in straw N availability needs to be considered from the interaction of fertilization practices and the crop rhizosphere.