In order to explore the effects of the composition and structure of soil’s dissolved organic matter (DOM) and its electron transfer capacity (ETC) on the bioavailability of the potential toxic elements chromium (Cr), lead (Pb) and cadmium (Cd) after the application of decomposed pig manure organic fertilizer, three-dimensional fluorescence spectroscopy (3D-EEMs), parallel factor analysis (PARAFAC) and electrochemical methods were used to analyze the composition characteristics of DOM in soil solution and the changes in the ETC, and the dynamic relationship between the relative content of DOM, ETC and various forms of potential toxic elements was explored by means of a Pearson correlation analysis and redundancy analysis (RDA). Among them, Cr, Pb and Cd were the elements with significant biological toxicity in farmland soil. The results indicated the following: (1) The soil DOM before and after returning the organic fertilizer to the field contained four components: UV and UVA humic-like (C1), tryptophan-like and UVA humic-like (C2), Exogenous and visible humic-like (C3) and tyrosine-like (C4). Humus-like was the main component. (2) After applying organic fertilizer, the relative contents of the DOM humus and tyrosine-like components in the soil increased by 8% and 8.73%, respectively. In this process, the DOM electron-accepting capacity (EAC) and electron-donating capacity (EDC) increased by 39.98% and 27.91%, respectively. (3) The humic-like fraction showed a highly significant positive correlation with ETC (p < 0.01), and the tyrosine-like fraction showed a significant negative correlation with ETC (p < 0.05). (4) The humus-like substance and ETC were positively correlated with the total amount, reducible state and oxidizable state of the potential toxic elements and negatively correlated with the weak acid extracted state and residue state; this showed that the humus-like components and ETC were more helpful for the transformation of the weak acid extracted state to the reducible state, oxidizable state and residue state in the interaction between the DOM components and Cr, Pb and Cd. In summary, the reasonable application of organic fertilizer could improve the relative content of DOM and ETC in soil, inhibit the biological toxicity of potential toxic elements in soil and provide a theoretical basis for the safe use of organic fertilizer.

In order to unravel the effect of aerated irrigation on soil dissolved organic matter (DOM) fluorescence characteristics, and humification degree, a randomized block experiment was conducted with three factors and a two-level design, i.e., two irrigation rates (0.6 and 1.0 times of crop evaporation pan coefficient, W1 and W2), two nitrogen application rates (225 and 300 kg hm−2, N1 and N2), and two aeration rates (15% and 0% in control treatment, A1 and A0). Fluorescence regional integration (FRI) and correlation analysis methods were used to investigate the evolution characteristics of the soil DOM fluorescence spectrum. Under aerated and conventional subsurface irrigation, soil DOM components were dominated by humic acid-like substances, fulvic acid-like substances, tryptophan-like proteins, and supplemented by tyrosine-like proteins and dissolved microbial metabolites. Soil aeration could promote the consumption of soil DOM components under low irrigation rates and accelerate the consumption of soil DOM components under high irrigation rates. The humification index of AI treatments varied from 8.47 to 9.94 during the maturity growth stage of pepper, averagely increased by 31.59% compared with the non-aeration treatment. To sum up, aerated irrigation can promote the depletion of small molecular proteins and accelerate nutrient turnover and the accumulation of big molecular proteins.

China is one of the regions with the most frequent drought disasters and serious social and economic losses. Agricultural drought loss is one the most serious natural disasters. Due to climate change, the regional agricultural drought risk assessment has always been the focus of the academic circle. This study takes Zunyi City as an example, which is the most typical City of karst landform development. The monthly precipitation data set of ground meteorological observation stations in Zunyi City from 1956 to 2020 was selected, and the drought characteristic variables were extracted by the coupled use of the precipitation anomaly percentage (Pa) index and the theory of runs. The Copula function was applied to establish the joint distribution model of characteristic variables, obtaining the drought frequency and drought return periods. Combined with the Jensen model, the agricultural drought loss rate under different drought return periods in the target year (2020) was calculated and evaluated. The results showed that the Gumbel-Hougaard copula function was suitable for the joint distribution of drought joint variables in Zunyi City. From 1956 to 2020, fewer droughts occurred in Zhengan and Wuchuan, and the most droughts took place in Fenggang, Meitan, and Yuqing. The average drought duration in each county was about 1.5 months, and the average drought severity was about 0.35 in spatial distribution. Crop loss rate caused by drought increased and the affected area expanded with the increase of drought return periods (5, 10, 20, 50, and 100 years) in temporal distribution. Meanwhile, the drought disaster was most drastic in the eastern region, followed by the south, north, west, and central area. The results were highly consistent with the historical drought in Zunyi City, which verified the validity of the model. This study could provide scientific knowledge for drought resistance and reasonable mitigation programing for the security of the regional agricultural production and the sustainability of social and economic development.

In order to investigate the effects of water-fertilizer-air-coupling drip irrigation on soil health status, including soil aeration (SA), enzyme activity (EA) and microbial biomass (MB), and its response relationship, this glasshouse experiment was conducted using tomato as the test crop, and we designed two fertilization gradients of 135 and 180 kg N·ha−1, two irrigation levels of 0.6-fold and 1.0-fold of the crop-pan coefficient, and two aeration treatments of 5 and 15 mg·L−1 for the three-factor and two-level completely randomized block experiment. The effects of soil dissolved-oxygen concentration, oxygen diffusion rate, soil respiration rate, soil urease, catalase, phosphatase activities and soil microbial biomass were systematically monitored and analyzed in the middle and at the end of crop growth. A structural equation model was used to comprehensively analyze the response relationship among relevant influencing factors. The results showed that coupled drip irrigation increased the soil’s dissolved oxygen, oxygen diffusion rate and soil respiration rate by 14.05%, 30.14% and 53.74%, respectively. Soil urease, catalase and phosphatase activities increased by 22.83%, 93.01% and 61.35%, respectively. The biomass of bacteria, fungi and actinomycetes increased by 49.06%, 50.18% and 20.39%, respectively. The results of a structural equation model analysis showed that water-fertilizer-air-coupling drip irrigation could effectively improve soil health status, and the descending order of influence was MB > EA > SA. This study provides scientific knowledge to reveal the improvement of soil health status by water-fertilizer-air-coupling drip irrigation.