| Soil acidification and salinization are common and have become a serious concern in greenhouse cultivation system. They not only have direct detrimental effects on plant growth, but also tend to induce other related production problems such as nutrient imbalance, plant diseases, and insect pests that further threaten the sustainability of greenhouse production. Mechanisms and amelioration for soil acidification and salinization have been extensively investigated by soil, horticulture and environmental scientists. However, much of the previous research work was focused on either soil acidification or salinization without simultaneously considering the two processes.This study attempted to improve our understanding of the mechanisms for the coexistence of acidification and salinization in greenhouse soils. Soil samples with different years of greenhouse cultivation were collected from Shouguang, Shandong Province (northern China) and Ningbo, Zhejiang Province (eastern China) to study the acidification and salinization characteristics of the soils with different initial soil pH and salinity levels. Laboratory incubation, greenhouse pot experiment, and modeling were conducted to study the correlation between soil acidification and salinization and to assess the effects of nitrogen fertilization and cultivation age on soil acidity and salinity. In addition, quantitative PCR and high-throughput sequencing technologies were used to further investigate the mechanism of nitrification and acidification in greenhouse soils.The following points sumerize the main findings from the study:1. Acidification and salinization of soils with different initial pH under greenhouse cultivationCompared with their corresponding open-field soils, acidification and salinization of the greenhouse soils occurred in both 0-20 and 20-40 cm soil layers for the Shouguang and Ningbo soils. Soil pH decreased gradually at different rates as greenhouse cultivation time increased in the two surveyed regions, but opposite trend was observed for soil salinity (electrical conductivity, EC). It was observed that soil acidification and salinization occurred simultaneously in the greenhouse topsoils. The pH of the Shouguang greenhouse topsoils (0-20 cm) was about 1 unit lower than that of the corresponding open-field topsoils, while the average EC reached 0.70 mS cm-1, which was over the tolerance limits for most crops (EC< 0.4 mS cm-1).For the Shouguang soils, while the percentages of K+ and NO3- increased dramatically and Ca2+ and HCO3- decreased significantly after the soils were converted to greenhouse use, the correlation between soil pH and EC was significant. Stepwise multiple regression analysis further revealed that there was a significant correlation between pH and the percent of Ca2+ and HCO3-. Future research should be devoted to understanding the mechanisms and to assess if there are correlations between soil acidification and salinization in greenhouse soils with lower initial pH values.2. The influence of salt addition on pH of greenhouse soilsWhen accompanied by Cl", all cations contributed to greenhouse soil acidification to some extent. Their influence followed the order of Ca2+> NH4+> Mg2+> K+> Na+. Meanwhile, soil pH decreased with the increase of the cation concentrations, which is closely associated with ion exchange. However, among different anions, high Cl" concentration caused significant decrease of soil pH when accompanied by Na, which was attibutted to the physiological acid of Cl" and the synergy between ions. Therefore, applying NH4Cl fertilizer should be avoided in greenhouse production. Adding HCO3-increased soil pH. Introduction of ions can activate the corresponding exchange sites in the surface of the colloids and release exchangable ions into soil solution, accelerating soil secondary salinization.3. Effects of chemical nitrogen fertilizers on the acidity and salinity of greenhouse soilsBoth soil acidity and salinity increased significantly as N input increased after one season, with pH decrease ranging from 0.45 to 1.06 units and electrolytic conductivity (EC) increase ranging from 0.24 to 0.68 mS cm-1. An estimated 0.92 mol H+was produced for each mol (NO2-+NO3-)-N accumulation in soil when urea was applied. The proton loading from nitrification ranged 14.3 to 27.3 and 12.1 to 58.2 kmol H+ ha-1, respectively, for greenhouse soils with different cultivation time in Shouguang under 600 and 1,200 kg N ha-1 fertilization rates. However, the proton loading from the uptake of excess bases (EB) by lettuce was only 0.3-4.5% of that from nitrification. Moreover, the release of protons induced the direct release of base cations from soil colloids and carbonate minerals, accelerating soil salinization.4. Effects of different nitrogen fertilization methods on the acidity and salinity of greenhouse soilsSingle chemical fertilizer and chemical fertilizer combined with organic fertilizer both resulted in the increase of soil acidity and salinity in the greenhouse soils after one season following the traditional N application rate (1,200 kg ha-1). It was also found that the rates of soil acidification and salinization were higher under single chemical fertilizer (P< 0.05). Organic fertilizers inhibited the processes of soil acidification and salt accumulation to a certain extent, but excessive application could also lead to soil acidification and salinization in greenhouse soils. Nitrification caused soil acidification and its effect on soil acidity was influenced by soil initial pH value. Meanwhile, the accumulation of nitrate directly caused soil salinization in the greenhouse soils. The response of the greenhouse soils with different cultivation times to fertilization was closely associated with the soil pH buffer capacity and its nutrient contents. Absorption of NO3--N by plants can alleviate soil acidification and salinization to some extent.5. Nitrification and nitrifying community composition in the greenhouse soilsGreenhouse cultivation increased soil nutrient contents and the diversity of nitrifying microbial community. However, the abundance and diversity decreased greatly due to the increase of soil acidity and salinity after 14 years of greenhouse cultivation. Soil potential nitrification activity (PNA) showed a significant positive correlation with the abundance of ammonia-oxidizing bacteria (AOB) but not with that of ammonia-oxidizing archaea (AOA) in the topsoil (0-20 cm) when pH≥7. Soil PNA and AOB were strongly influenced by soil pH. The groups of Nitrososphaeraceae, Nitrosomonadaceae, and Nitrospiraceae, respectively, were predominant in the active AOA, AOB, and nitrite-oxidizing bacteria (NOB) communities. Nitrifying community structure was significantly correlated with soil salinity (EC), organic carbon (OC) and nitrate nitrogen (NO3--N) contents in the greenhouse soils based on Redundancy analysis (RDA). |
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