Denitrification is a key process in soil available nitrogen (N) loss. However, the effects of different water-saving irrigation systems on soil N components and denitrifying bacterial communities are still unclear. In this study, quantitative fluorescence PCR and Illumina MiSeq sequencing were used to investigate the effects of three main irrigation systems, conventional flooding irrigation (FP), shallow buried drip irrigation (DI), and mulched drip irrigation (MF), on the abundance, community composition, and diversity of soil nirK- and nirS-type denitrifying bacteria in the semi-arid area of Northeast China, and to clarify the driving factors of nirK- and nirS-type denitrifying bacterial community variations. The results showed that, compared with FP, MF significantly increased soil moisture, alkaline hydrolyzed nitrogen (AHN), nitrate nitrogen (NO₃⁻-N), non-acid hydrolyzed nitrogen (AIN), and amino sugar nitrogen (ASN), but significantly decreased the contents of ammonium nitrogen (NH₄⁺-N) and acid hydrolyzed ammonium nitrogen (AN). The irrigation system changed the relative abundance of the dominant genera of denitrifying bacteria, DI and MF significantly increased nitrate reductase (NR) and nitrite reductase (NiR) activities, and MF significantly increased the diversity of nirK- and nirS-type denitrifying bacteria but significantly decreased the richness. The community structure of nirK- and nirS-type denitrifying bacteria was significantly different among the three irrigation systems. NO₃⁻-N was the main driving factor affecting the community structure of nirS-type denitrifying bacteria, and moisture significantly affected the community structure of nirK-type denitrifying bacteria. DI and MF significantly increased the abundance of nirK- and nirS-type denitrifying bacteria and also increased the abundance ratio of nirS/nirK genes. Therefore, although DI and MF significantly increased the abundance of denitrifying microorganisms, they did not lead to an increase in the N₂O emission potential.

The accumulation of nitrite is frequently overlooked, despite the fact that nitrification is the most essential phase of the entire nitrogen (N) cycle and that nitrifying bacteria play a significant role in nitrification. At present, the effects of different N application rates on soil nitrite-oxidizing bacteria (NOB) abundance, community composition, diversity, and its main influencing factors are still unclear. In this study, five N fertilizer application rates under film mulching and a drip irrigation system were studied in the semi-arid area of Northeast China. The treatments were 0 kg ha⁻¹ (N0), 90 kg ha⁻¹ (N1), 150 kg ha⁻¹ (N2), 210 kg ha⁻¹ (N3), and 270 kg ha⁻¹ (N4). Fluorescent quantitative PCR and Illumina Miseq sequencing were used to analyze the abundance and community structure of NOB under different amounts of N application. The results showed that the increase in amounts of N application was strongly accompanied by an increase in the content of soil organic matter (SOM), total nitrogen (TN), nitrate nitrogen (NO₃⁻-N), and ammonium nitrogen (NH₄⁺-N), while the pH significantly reduced with an increase in N fertilization. N fertilization significantly increased soil nitrite oxidoreductase (NXR) activity, soil nitrification potential (PNR), and soil nitrite oxidation potential (PNO). A high N application rate significantly heightened the abundance of Nitrospira- and Nitrobacter-like NOB. N fertilizer considerably raised the Shannon index of Nitrospira-like NOB. The N application amount was the key factor affecting the community structure of Nitrospira-like NOB, and available nitrogen (AN) had the dominant influence on the community structure of Nitrospira-like NOB. N fertilizer can cause soil acidification, which affects NOB abundance and diversity. Nitrospira-like NOB may promote nitrite oxidation in different N application rates under a mulched fertigation system. The findings offered a crucial scientific foundation for further investigation into how nitrite-oxidizing bacteria respond to N fertilizer management strategies in farmland soil under film mulching drip irrigation in Northeast China.

The denitrification process is one of the important processes in the soil nitrogen (N) cycle, and is closely related to the loss of soil N fertilizer. Five treatments were included in this study: N0 (control, no N application); N90 (N application rate 90 kg ha⁻¹); N150 (N application rate 150 kg ha⁻¹); N210 (N application rate 210 kg ha⁻¹); and N270 (N application rate 270 kg ha⁻¹), to study the effect of different N application rates on the soil nirS-type denitrifying bacterial community structure, the influence of key enzyme activities during the denitrification process, and the main environmental factors affecting the variation of the denitrifying bacterial community in maize field soil under the mulched fertigation system in the semi-arid region of Northeast China. The results showed that increasing N fertilizer application significantly increased the contents of soil inorganic N and acid-hydrolyzable organic N, but significantly decreased pH. N fertilizer significantly increased nitrate reductase (NAR) activity and nitrite reductase (NIR) activity. Excessive application of N fertilizer significantly increased the nirS gene copy numbers, and, at the same time, significantly decreased the diversity of nirS-type soil denitrifying bacteria. Proteobacteria was the dominant denitrifying phylum in all treatments, and N application promoted the growth of Bradyrhizobium belonging to this phylum. The application of N fertilizer significantly changed the community structure of nirS denitrifying bacteria, and the NO₃⁻-N content was the most important factor for this observation. Soil organic matter (SOM) and non-hydrolyzable N (NHN) indirectly affected the denitrifying bacterial community structure through NAR activity and NIR activity, while soil total N (TN) and nitrate N (NO₃⁻-N) indirectly affected yield through denitrifying bacterial community structure. Although increasing N fertilizer was beneficial in increasing soil nutrients, the community structure of nirS-type denitrifying bacteria changed significantly. This is attributed to the increase in soil NO₃⁻-N accumulation caused by a large amount of N application. The results of this research provide an important scientific basis for further research on the response mechanism of farmland soil denitrifying microorganisms to different N fertilizer managements under the mulched fertigation system in the semi-arid region of Northeast China.

An appropriate drip irrigation amount and the straw return method are important ways to save water and achieve efficient maize production in semi-arid areas. A 2-year controlled field plot experiment was performed with two factors: straw return (straw removal, straw mulching) and differing drip irrigation amounts (200, 350, and 500 mm). Changes in growth, development, photosynthesis, yield, the components, and the water-use characteristics of maize under the intercropping conditions of drip irrigation amount and straw return were studied. The results showed that an increase in drip irrigation favored an increase in the net photosynthetic rate (P_n), stomatal conductance (G_s), and intercellular carbon dioxide concentration (C_i) of maize, and promoted an increase in maize plant height and leaf area index, which resulted in the accumulation of more dry matter and increased the maize yield. Compared with straw removal, straw mulching maintained a higher photosynthetic capacity at the later stages of maize growth and development under irrigations of 200 and 350 mm; the average increase in P_n over two years ranged from 4.06 to 19.19%; and good plant growth was maintained, thereby leading to the accumulation of more dry matter, with the average increase over two years ranging from 0.51 to 27.22%. Straw mulching also significantly improved water-use efficiency (WUE) at 350 mm of irrigation, with the average increase in yield over two years ranging from 4.58 to 4.83%. Overall, straw mulching had a positive impact on maize when irrigation was low, and when it was high, straw mulching did not adversely affect maize. Therefore, irrigation combined with straw mulching technology may be used to improve maize yield and WUE in semi-arid areas of Jilin Province.

The soil nitrogen (N) cycle is an essential role of the biogeochemical cycle. Bacteria play an irreplaceable part in the soil N cycle, but the impact of different N gradients on bacterial communities remains unclear. The purpose of this research was to explore the bacterial abundance, community composition, and diversity under different N application rates in a water-limited area. We investigated the bacterial abundance, diversity, community composition, and structure under five different N gradients (0, 90, 150, 210, and 270 kg ha⁻¹) using real-time quantitative PCR and high-throughput sequencing, and then explored bacterial functional groups with FAPROTAX. N application significantly affected bacterial abundance and community composition. Bacterial diversity was enhanced at low N application rates and reduced at higher N application rates. Principal coordinate analysis showed that bacterial community structure was separated into two groups between low N application rates and high N application rates; these differences in bacterial community structure may be driven by available nitrogen (AN). The results of FAPROTAX revealed that N application promoted the functions of Aerobic_nitrite_oxidation, Nitrate_reduction, and Aerobic_ammonia_oxidation, but inhibited the Nitrogen_fixation function of the bacterial community. The high N network caused the reduction of network structure stability. Our results revealed that N fertilizer driven bacterial community structure and soil nutrients were the main influential factors in the variation of bacterial community structure. We suggest that the optimal N application rate in this study may be approximately 150 kg ha⁻¹, based on the variations of soil properties and bacterial community structure in semi-arid areas.

A mulched drip fertigation system is an effective way to improve maize yield, but at present, the efficiency of nutrient delivery and water use are generally low. Therefore, this study conducted optimization field experiments to identify the main factors limiting the delivery of water and fertilizer, including regulations on nitrogen (N) fertilizer, irrigation conditions, planting density and maize varieties, in the semi-arid area of Northeast China. As part of an effort to close the yield gap for maize, an optimized system (DS) for optimal crop, nutrient and water management was designed to improve the agronomic and economic performance of maize farming in the area. The application rate of N fertilizer was 240 kg ha⁻¹; the base fertilizer was applied four times (once at the jointing stage, twice at the belling stage and once at the silking stage); the rates of application of phosphorus (P) and potassium (K) fertilizer were 90 kg P₂O₅ ha⁻¹ and 90 kg K₂O ha⁻¹, respectively; the irrigation amount was 270 mm ha⁻¹; the maize variety Fumin 985 was planted at a density of 80,000 plants ha⁻¹ in DS; the grain yield of DS reached 13.8 Mg ha⁻¹, 93% of the yield potential. DS yielded an economic benefit of 18,449 yuan ha⁻¹, which was significantly higher than the economic benefit of 13,818 yuan ha⁻¹ achieved under farmers’ practices (FP). Furthermore, the utilization rates of N, P, K, and water were significantly improved under DS. In conclusion, DS increased production potential, with high efficiency in nutrient delivery and water use and low losses of nutrients and water. The crop, fertilizer, and water management of DS provided a technological system to simultaneously improve crop production and resource-use efficiency in the semi-arid area of Northeast China.