半月周期的潮汐对滨海湿地土壤理化性质的影响

周期性的潮汐是滨海湿地重要的水文特征,为了探讨潮汐的半月周期(包括小潮期和大潮期)对土壤理化性质的影响及可能的机制,于2009年7月中下旬测定了长江口崇明东滩湿地土壤理化性质在小潮期和大潮期交替周期内的变化规律。结果表明,与小潮期相比,由于频繁的潮水淹没,大潮期0～5 cm土壤含水量的增加量从低潮滩向高潮位依次为:44.8%、18.5%、10.9%和14.3%,5～10 cm含水量的增加量从低潮滩向高潮位依次为:19.2%、9.8%、12.6%和16.2%,10～20 cm则依次为:5.6%、6.1%、2.5%和7.3%。大潮期,从低潮滩向高潮滩增加的盐度依次为0.18、0.13、0.10和0.09 ms·cm^-1,增加的硫酸盐依次为0.32、0.21、0.16、0.13 mg·g^-1。与小潮期相比,大潮期氧化还原电位(Eh)显著降低;土壤容重、pH、可溶性有机碳和可溶性氮在大小潮期间无明显差异。此外,在潮汐的半月周期内,低潮滩土壤比高潮滩有更高淹水频率和更长淹水时间,受潮水的影响更明显,大潮期低潮滩土壤含水量、盐度和硫酸盐的增加幅度大于高潮滩,低潮滩土壤Eh降低幅度则小于高潮滩。半月周期的潮汐可以显著影响滨海湿地的部分土壤理化性质,且不同潮位的土壤性质对潮汐的响应程度不同,进而可能会对湿地植物生长和相关生态过程起到重要的调控作用。

EFFECTS OF SEMI-LUNAR TIDAL CYCLING ON SOIL PHYSICAL AND CHEMICAL PROPERTIES IN COASTAL WETLANDS

As important hydrological features of coastal wetlands, periodic tides usually present two types of hydrological cycles on different time scales, semi-diurnal and semi-lunar tidal cycles (consisting of neap and spring tide periods (NTP and STP)). Therefore, tidal effects on soil physical and chemical properties of the vegetated zone may vary greatly between tidal cycles, because of different inundation frequencies and durations. Previous studies suggested that soil properties showed no significant change across the semi-diurnal cycle, probably because these work were mainly conducted in STP when soils were inundated or water-saturated most of the time. However, there has been no study on whether or how the semi-lunar tidal cycle affects soil properties in coastal wetlands. A field study was conducted in Chongming Dongtan wetland in the Yangtze River estuary to investigate temporal variations of soil physical and chemical properties along with the transition from NTP to STP in a semi-lunar tidal cycle and to explore possible underlying mechanisms of these variations. During the cycling of semi-lunar tides, periodic neap and spring tides significantly affected soil physical and chemical properties of coastal wetlands. A major change caused by the transition between NTP and STP was in soil water conditions. Soil moisture was significantly greater in STP than in NTP, because soil was over-saturated or submerged nearly all the time during STP due to frequent tidal inundation, especially in the low tide zone. Further analysis indicated that increased moisture in top soils in STP was greater than those in subsurface and deep soils. This suppressed the diffusion of O₂ from atmosphere into soil, and existing soil O₂ was rapidly consumed during STP. Therefore, soil Eh decreased dramatically with the transition from neap to spring tides. Regression analysis also showed that soil Eh was negatively correlated with moisture (R² = 0.60, P < 0.0001). Furthermore, variations of soil Eh with that transition increased gradually from the low to the high tide zone, probably because the frequency of inundation by tidewater is greater and the duration is longer in the former zone. No significant change in soil pH values was observed between NTP and STP. Soil pH is an important factor in regulating the soil inorganic carbon (SIC) pool. SIC may account for more than 60% of soil total carbon in wetlands of the Yangtze River estuary. Periodic tidal inundation can maintain the alkaline environment (pH > 8.0) of soils, ensuring the stability of the SIC pool in the estuary. Soil conductivity and sulfate content were significantly greater in STP than in NTP because of the influence of tides, suggesting that tides transport substantial nutrients to coastal wetlands. Because of greater inundation frequency and longer duration, this exchange of salts was more efficient in the low tide zone than in the high tide zone. Consequently, compared with NTP, increase in soil conductivity and sulfate content in STP decreased gradually from the low to the high tide zone. Thus nutrient input via this mechanism may be vital in supporting high plant productivity in coastal wetlands. Positive effects of nutrient input on plant productivity were likely stronger in the low tide zone than in the high tide zone. This speculation may be confirmed by the results that plant traits of Spartina alterniflora such as aboveground plant biomass, plant height, and basal stem diameter were more advantageous in the low tide zone than in the high tide zone. In addition, soil bulk density, pH values, dissolved organic carbon (DOC) and dissolved nitrogen (DN) did not change significantly with the transition from NTP to STP, except at site S1 where only topsoil bulk density, DOC and DN were significantly lower in NTP than in STP. These findings indicate that semi-lunar tidal cycling can significantly impact soil physical and chemical properties in coastal wetlands, especially soil water, salinity and nutrient characteristics as well as redox environment, which may play important roles in regulating plant growth and relative ecological processes of coastal wetlands.