| Inundation gradient is one of the most notable features of estuarine tidal wetland. Study on the distributions of soil nitrogen (N) and phosphorus (P) contents along an inundation gradient plays a vital role in many important aspects such as developing our knowlege of the storage and cyclic processes of N and P in estuarine wetland, as well as better understanding the response of sea level changes in this area. However, N and P species in marsh soils of estuarine wetlands along a hydrologic gradient and their influence factors are poorly understood. In this paper, Min River Estuary, located in the southeast of China, was selected as the study area. Nine sampling sites (sites A-I) with equal interval (40 m) were established along an inundation gradient (from high tidal flat to middle tidal flat). By the field investigation and laboratory experiments, the spatial distributions of N and P in marsh soil of the Min river estuarine wetland along an inundation gradient and their influence factors (soil physical and chemical properties, vegetation and inundation frequency) were studied. The main results showed as follows:(1) The soil total nitrogen (TN) contents ranged from 1.29 to 3.52 g·kg-1 (average 1.8 g·kg-1) in Min River Estuary. The soil inorganic nitrogen (IN) was mainly composed of NH4+-N (92.64%). From site A to site I, with the inundation frequency increased, the soil TN contents significantly decreased. Soil NH4+—N contents displayed fluctuations from site A to site I in May and August, while it significantly decreased along this gradient in November. Soil NO3-N contents exhibited a trend of decrease along the inundation gratident, and the high content was observed at the border of high and middle tidal flats (except in August).(2) The soil total phosphorus (TP) contents ranged from 338 to 846 mg·kg-1 (average 660 mg·kg-1) in the Min River Estuary. Inorganic phosphorus (IP) dominated the P fractions (accounting for 57 to 81% of TP) and was mainly composed of iron-bound phosphorus (Fe-P,38%), occluded phosphorus (O-P,30%), and calcium-bound phosphorus (Ca-P,25%). The TP, IP and Fe-P contents fluctuated along the inundation gradient in May and August (except for the upper 10 cm soil in August). The high values of organic phosphorus (Org P) were occurred in the upper 20 cm soil of the high tidal flat. The contents of Ca-P and aluminum-bound phosphorus (Al-P) increased gradually from site A to site I. The O-P content was significantly higher in Phragmites australis (site B to site D) soil compared with Cyperus malaccensis soil (P< 0.05).(3) The plant biomass, microbial abundance, inundation frequency, soil total organic carbon (TOC) contents and soil Eh values significantly decreased from site A to site I, while soil total inorganic carbon (TIC) contents and soil pH values significantly increased along this gradient. Hence, these factors can be considered as the potential factors influenced on the spatial distributions of N and P contents along the inundation gradient. On the one hand, soil nitrification and denitrification were enhanced and N retention capacity of the plant was weakened (from 49.06 g·m-2 to 15.19 g·m-2) with increasing inundation frequency, which can help us explain the reduction of soil TN contents in the middle tidal flat. On the other hand, from site A to site I, the Ca-P and Al-P contents increased in the soil with increasing soil pH value and TIC content. Meanwhile, higher pH in middle tidal flat soil also could inhibit the release of Fe-P, and resulted in Fe-P contents fluctuated along the inundation gradient. The plant biomass, which was higher in high tidal flat, was the main factor resulted in higher soil Org P contents in this area. Consequently, such variations of P fractions (displayed opposing trends) leaded to soil TP contents fluctuated along the inundation gradient.This study highlighted the effect of topography and hydrology on the N and P biogeochemical cycling in estuarine tidal wetland. |
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