Long-time precipitation reduction and nitrogen deposition increase alter soil nitrogen dynamic by influencing soil bacterial communities and functional groups

The effects of precipitation reduction and nitrogen deposition increase on soil bacterial communities and functions impact soil nitrogen cycling. Seasonal changes could modify the effects of precipitation reduction and nitrogen deposition increase on bacterial communities and functions by changing soil environments and properties. Understanding soil microbial communities and the seasonal response of functions to precipitation reduction and nitrogen deposition increase may be important for the accurate prediction of changes in the soil nitrogen dynamics. Thus, a long-term field simulation experiment of nitrogen deposition increase and throughfall exclusion was established to investigate soil bacterial communities’ response to nitrogen deposition increase and/or precipitation reduction, with no nitrogen deposition increase and no precipation reduction as a control, in a temperate forest. We examined soil bacterial communities(Illumina sequencing) under different treatments during the winter, freezing-thawing cycle periods(FTCs), and growing season. The bacterial functional groups were predicted by the FAPROTAX database. The results showed that nitrogen deposition increase, precipitation reduction, the combined effect of nitrogen deposition increase and precipitation reduction, and seasonal changes significantly altered the soil bacterial community composition.Interestingly, by combining the result of a previous study in which nitrogen deposition increase increased the nitrous oxide flux in the same experimental system, the loss of soil nitrogen was increased by the decrease in denitrification and increase of nitrification bacteria under nitrogen deposition increase,while ammonification bacteria significantly increased and N-fixing bacteria significantly decreased with precipitation reduction compared to the control. In relation to seasonal changes, the aromatic-degrading, cellulolytic, and ureolytic bacteria were lowest during FTCs, which indicated that FTCs might inhibit biodegradation. Nitrification and nitrite-oxidizing bacteria increased with nitrogen deposition increase or precipitation reduction and in FTCs compared to the control or other seasons. The interaction between treatment and season significantly changed the soil bacterial communities and functions. These results highlight that nitrogen deposition increase, precipitation reduction, seasonal changes, and their interactions might directly alter bacterial communities and indirectly alter the dynamics of soil N.

Microbial Populations,Activity and Gene Abundance in Tropical Vertisols Under Intensive Chemical Farming

There are increasing concerns on the environmental impacts of intensive chemical agriculture. The effect of high agrochemical inputs used in intensive chemical farming was assessed on soil microbiological, molecular and biochemical properties in tropical Vertisols in India. Farm field sites under normal cultivation of arable crops using high inputs of fertilizers and pesticides in chili （Capsicum annum L., 5.0× dose for fertilizers and 1.5× dose for pesticides over normal inputs） and black gram （Vigna mungo L. Hepper, 2.2× dose for fertilizers and 2.3× dose for pesticides over normal inputs） were compared with adjacent sites using normal recommended doses. Organic carbon and basal respiration showed no response to high inputs of fertilizers and pesticides in soils of both crops. Labile carbon decreased by 10% in chili soils and increased by 24% in black gram soils under high input farming system. The proportion of soil labile carbon as a fraction of soil organic carbon was unaffected by high inputs. The labile carbon mineralization coefficient （qMLc） increased by 50.0% in chili soils, indicating that the soil microorganisms were under stress due to high agochemical inputs, whereas qMLc decreased by 36.4% in black gram soils. Copiotrophs increased due to high inputs in soils of both chili （63.1%） and black gram （47.1%）. Oligotrophs increased by 10.8% in black gram soils but not in chili soils. The abundance of amoA gene reduced by 39.3% in chili soils due to high inputs and increased significantly by 110.8% in black gram soils. β-Glucosidase also increased by 27.2% and 325.0%, respectively. Acid phosphatase activity reduced by 29.2% due to high inputs in chili soils and increased by 105.0% in black gram soils. The use of high agrochemical inputs thus had adverse consequences on biological health in chili but not in black gram soils. In soils cultivated with black gram, the moderating effect of cultivating legumes and their beneficial effect on soil health were evident from the increase in soil labile carbon, lower qMLc, higher amoA gene and enzyme activities. Overall results showed that cultivation of legumes permits intensive chemical farming without deteriorating soil biological health.