Cropping systems affect paddy soil organic carbon and total nitrogen stocks (in rice-garlic and rice-fava systems) in temperate region of southern China

Spatial distribution and driving factors of soil organic carbon in the Northeast China Plain: Insights from latest monitoring data

Soil organic carbon (SOC) is a critical component of soil fertility and plays a crucial role in the global carbon cycle. Despite the widespread reports of a decrease in SOC content and stock in the Northeast China Plain in recent decades, the current status and driving factors of its content and distribution are unclear. In this study, the surface soil (0-20 cm) SOC content data of 1920 sampling points within the Northeast China Plain covering an area of 2.6 × 105 km2 were obtained based on the Land Quality Geochemical Monitoring Network established in 2018. Random forest model and correlation analysis were used to identify the main driving factors of SOC distribution. The results showed that the SOC content, soil organic carbon density (SOCD), and soil organic carbon storage (SOCS) in the Northeast China Plain were 13.48 g·kg-1, 3.45 kg·C·m-2, and 898.95 Tg, respectively. SOC content in paddy land was the highest among different land use types, which reached 18.77 g·kg-1. SOC content showed strong spatial dependence and gradually increased from southwest to northeast in the monitoring area. The results of the random forest analysis showed that the SiO2, mean annual temperature, and Fe2O3 explained 39.4 %, 18.9 %, and 12.8 % of the spatial variation of SOC, respectively. Although the SOCS (0-20 cm) in the Northeast China Plain has decreased by 8.68 % in the last 40 years compared to the Second National Soil Survey (1980), it's important to note that the SOCS has transitioned from a decreasing trend between 1980 and 2006 to an increasing trend from 2006 to 2018.This study provides important information for decision-makers on the spatiotemporal changes of SOC and its driving factors in the Northeast China Plain, which has a great significance for soil carbon sequestration and the development of management strategies to maintain soil fertility.

Responses of the maize rhizosphere soil environment to drought-flood abrupt alternation stress

Changes in the soil environment in the root zone will affect the growth, development and resistance of plants. The mechanism underlying the effect of drought and flood stress on rhizosphere bacterial diversity, soil metabolites and soil enzyme activity is not clear and needs further study. To analyze the dynamic changes in bacteria, metabolites and enzyme activities in the rhizosphere soil of maize under different drought-flood abrupt alternation (DFAA) stresses, the barrel test method was used to set up the 'sporadic light rain' to flooding (referring to trace rainfall to heavy rain) (DFAA1) group, 'continuous drought' to flooding (DFAA2) group and normal irrigation (CK) group from the jointing to the tassel flowering stage of maize. The results showed that Actinobacteria was the most dominant phylum in the two DFAA groups during the drought period and the rewatering period, and Proteobacteria was the most dominant phylum during the flooding period and the harvest period. The alpha diversity index of rhizosphere bacteria in the DFAA2 group during the flooding period was significantly lower than that in other stages, and the relative abundance of Chloroflexi was higher. The correlation analysis between the differential genera and soil metabolites of the two DFAA groups showed that the relative abundance of Paenibacillus in the DFAA1 group was higher during the drought period, and it was significantly positively correlated with the bioactive lipid metabolites. The differential SJA-15 bacterium was enriched in the DFAA2 group during the flooding period and were strongly correlated with biogenic amine metabolites. The relative abundances of Arthrobacter, Alphaproteobacteria and Brevibacillus in the DFAA2 group were higher compared with DFAA1 group from rewatering to harvest and were significantly positively correlated with hydrocarbon compounds and steroid hormone metabolites. The acid phosphatase activity of the DFAA1 group was significantly higher than that of the DFAA2 group during the flooding period. The study suggests that there is a yield compensation phenomenon in the conversion of 'continuous drought' to flooding compared with 'sporadic light rain', which is related to the improvement in the flooding tolerance of maize by the dominant bacteria Chloroflexi, bacterium SJA-15 and biogenic amine metabolites. These rhizosphere bacteria and soil metabolites may have the potential function of helping plants adapt to the DFAA environment. The study revealed the response of the maize rhizosphere soil environment to DFAA stress and provided new ideas for exploring the potential mechanism of crop yield compensation under DFAA.

Long-term manure application increased greenhouse gas emissions but had no effect on ammonia volatilization in a Northern China upland field

The impacts of manure application on soil ammonia (NH₃) volatilization and greenhouse gas (GHG) emissions are of interest for both agronomic and environmental reasons. However, how the swine manure addition affects greenhouse gas and N emissions in North China Plain wheat fields is still unknown. A long-term fertilization experiment was carried out on a maize-wheat rotation system in Northern China (Zea mays L-Triticum aestivum L.) from 1990 to 2017. The experiment included four treatments: (1) No fertilizer (CK), (2) single application of chemical fertilizers (NPK), (3) NPK plus 22.5t/ha swine manure (NPKM), (4) NPK plus 33.7t/ha swine manure (NPKM+). A short-term fertilization experiment was conducted from 2016 to 2017 using the same treatments in a field that had been abandoned for decades. The emissions of NH₃ and GHGs were measured during the wheat season from 2016 to 2017. Results showed that after long-term fertilization the wheat yields for NPKM treatment were 7105kg/ha, which were higher than NPK (3880kg/ha) and NPKM+ treatments (5518kg/ha). The wheat yields were similar after short-term fertilization (6098-6887kg/ha). The NH₃-N emission factors (EF_amm) for NPKM and NPKM+ treatments (1.1 and 1.1-1.4%, respectively) were lower than NPK treatment (2.2%) in both the long and short-term fertilization treatments. In the long- and short-term experiments the nitrous oxide (N₂O) emission factors (EF_nit) for NPKM+ treatment were 4.2% and 3.7%, respectively, which were higher than for the NPK treatment (3.5% and 2.5%, respectively) and the NPKM treatment (3.6% and 2.2%, respectively). In addition, under long and short-term fertilization, the greenhouse gas intensities for the NPKM+ treatment were 33.7 and 27.0kg CO₂-eq/kg yield, respectively, which were higher than for the NPKM treatment (22.8 and 21.1kg CO₂-eq/kg yield, respectively). These results imply that excessive swine manure application does not increase yield but increases GHG emissions.