Water and fertilizer play an important role in crop growth in dryland areas. It is a necessity to improve the water use efficiency (WUE) of the crop once the water resource is limited. In northern China, where there is a wide shortage of water resources, it is therefore necessary to investigate how fertilization affects the WUE of spring maize and to quantify the effects. A total of 33 published peer-reviewed papers were collected, and a meta-analysis and random forest model analysis were performed with 364 WUE comparisons, aiming to explore the effects of fertilization on the WUE of spring maize and to clarify the optimal conditions for WUE under fertilizer management. The results showed that fertilization significantly increased the WUE of spring maize by 56.72% (P < 0.01) when compared with non-fertilization. The WUE effect under the organic–inorganic fertilizer combination (MNPK) was approximately twice as high as that under inorganic fertilizer (NPK) or organic fertilizer (M). The greatest increase in WUE occurred at 0–100 kg ha⁻¹ of nitrogen application (NA). Under environmental conditions including 7 ≤ mean annual temperature in the test year (T) ≤ 10 °C, 400 ≤ mean annual precipitation in the test year (P) ≤ 600 mm, and mean altitude (A) > 1500 m, and soil conditions including 10 ≤ soil organic matter content (SOM) ≤ 14 g kg⁻¹ and available phosphorus (AP) < 5 mg kg⁻¹, the fertilization optimally enhanced the WUE of spring maize when the agronomic measures of ridge–furrow planting (RFP) and mulching film (MF) were used. The random forest model analysis indicated that the influence factors (i.e., fertilizer regimes, environmental factors, soil factors, and agronomic measures) caused 65.62% of the variation in spring maize WUE effects, while in all influence factors, fertilizer types related to fertilizer regimes caused the most variation. The initial available potassium (AK) and available nitrogen (AN) of the soil were negatively correlated to the WUE effect, indicating that fertilization imposed a better effect on the WUE of spring maize when the soil was infertile. Fertilization significantly increased the WUE of spring maize, and organic and inorganic fertilizer application provided an effective measure for the sustainable development of spring maize in northern China. After clarifying the required conditions for fertilization increasing WUE, high-efficiency water use may be achieved.

Basic soil productivity (BSP) is the productive capacity of farmland soils with their own physical and chemical properties during a specific crop season under local field management. Improving BSP as an effective way to increase or maintain crop yield, and researching its changes and potential driving factors under long-term fertilization are crucial for ensuring high and stable crop yields. In this study, the yields of BSP were simulated using the decision support system for agrotechnology transfer (DSSAT) crop model based on three long-term experiments, and its changing characteristics and driving factors were investigated under various fertilization treatments in wheat–maize rotation systems during 1991–2019. Five treatments were included: (1) unfertilized control (CK); (2) balanced mineral fertilization (NPK); (3) NPK plus manure (NPKM); (4) high dose of NPK plus manure (1.5NPKM); and (5) NPK plus crop straw (NPKS). This study found that the BSP of wheat and maize exhibited a fluctuating increase or stable change trend under four fertilization treatments at the Yangling (YL) and Zhengzhou (ZZ) sites, while a fluctuating reduction trend was observed at the Qiyang (QY) site. Compared with CK, NPKM, 1.5NPKM and NPKS significantly (p < 0.05) improved the BSP of wheat and maize at the YL and ZZ sites. The BSP of both maize and wheat under NPKM and 1.5NPKM was significantly (p < 0.05) higher than that under NPK or NPKS, whereas no statistically significant difference was found between NPK and NPKS at the QY site. The contribution rates of basic soil productivity (CBSP) of maize and wheat at the YL (41.5–60.7% and 53.0–64.3%) and ZZ sites (44.4–59.2% and 56.8–66.7%) were overall higher than that at the QY site (25.8–37.6% and 48.3–60.1%). In most cases, the difference in CBSP among different fertilization treatments was consistent with that in BSP. Moreover, a significant positive correlation was recorded between BSP and soil pH (r = 0.73, p < 0.01; r = 0.87, p < 0.01), TN (r = 0.56, p < 0.01; r = 0.62, p < 0.01) and TK content (r = 0.49, p < 0.01; r = 0.58, p < 0.01) in maize and wheat. Soil pH significantly correlated positively with the BSP of maize (R² = 0.54, p < 0.001) and wheat (R² = 0.49, p < 0.001) at the QY site, but negative correlation (R² = 0.20, p < 0.001; R² = 0.30, p < 0.001) was only found in maize at the YL and ZZ sites. The BSP of maize and wheat showed a significant negative linear correlation with MAP (R² = 0.49–0.67, p < 0.001) and MAT (R² = 0.36–0.62, p < 0.001). Random forests (RF) and variance partitioning analysis (VPA) revealed that soil properties and its interaction with the climate showed a higher explanation rate for BSP, indicating that these factors are the key drivers of BSP change. Overall, chemical fertilizers combined with manure can effectively increase BSP, while the effects of fertilizer combined with straw on BSP vary by region. The changes in BSP in wheat–maize cropping were mainly driven by both the soil and its interactions with the climate.

Soil salinization is a serious threat to agricultural productivity and organic carbon cycling; however, the mechanisms by which carbon fractions and microbial metabolism regulate organic carbon sequestration remain unclear. This research investigated the effects of four sites (S1–S4) along a salinity gradient on microbial carbon metabolism, carbon fractions (particulate organic carbon, POC; mineral-associated organic carbon, MAOC), and bacterial and fungal communities in a typical saline–alkali soil. The results showed that, as the salt–alkali level increases, microbial growth, respiration, and carbon utilization efficiency (CUE) all exhibit a gradually decreasing trend, with CUE declining by 34.0% at S4 relative to S1. CUE was strongly negatively correlated with the soil pH, EC, and soluble salts, but showed a logarithmic rise with SOC, reaching a maximum of 0.47 near 8 g kg⁻¹. Both POC and MAOC decreased alongside the salt–alkali level, the proportion of MAOC in SOC (52.04–55.02%) was significantly higher than that of POC (44.98–47.96%), and there was a significant positive linear correlation with SOC and CUE. Under high salinity, the 16S rRNA gene copy number decreased by up to 28.8%, while the ITS copy number remained stable. As the salt and alkali levels increased, bacterial Proteobacteria and Actinobacteriota showed an increasing trend, whereas fungal communities remained dominated by Ascomycota. Furthermore, correlation analyses revealed that bacterial Acidobacteriota and Proteobacteria, and fungal Basidiomycota and Chytridiomycota play crucial roles in carbon cycling. Random forest modeling (RFM) and partial least squares path modeling (PLS-PM) also revealed that saline–alkali soil properties strongly affect 16S rRNA gene abundance and microbial respiration; carbon fractions and CUE exerted a strong positive direct effect on SOC, whereas microbial respiration had a negative effect. These findings demonstrate that saline–alkali stress restricts SOC sequestration primarily by weakening microbial activity and community function, with CUE playing only an indirect role. In conclusion, enhancing the POC and MAOC pools and improving the microbial carbon turnover efficiency may be more effective strategies for mitigating carbon loss and sustaining SOC stocks in saline–alkali soils.