Seasonal Dynamics Of Soil Water And Its Impact On Plants Photosynthesis Under Different Vegetation Types In Typical Karst Mountain Areas

Based on a combination of field research and laboratory incubation, our study mainly focused on the fluctuations of soil moisture in a typical desertification management region, Puding, locating in southwestern Guizhou, comprehensively studied the effect of soil moisture on the native plant photosynthesis, and further discussed the interrelationships among the soil water content, plant productivity as well as species diversity to elucidate the interaction between soil moisture and plant growth and response of plant photosynthesis to soil water fluctuations in mountainous area. The main conclusions were as follows:(1) To monitor the changes of soil nutrients under different vegetation types, soil cores of 0-15 and 15-30 cm depth were collected from the four targeted vegetation types: grassland, shrubs, secondary forest and natural forest at 4 different seasons: January, April, July and October, in Karst mountainous area of central Guizhou province, southwest China. The results showed that soil moisture and BD ranged from 11.98% to 59.43% and 0.54 to 1.54 g·cm-3 throughout the year, respectively. The SOC and TN differed greatly among different vegetation types: the annual average SOC and TN were 72.61 and 7.39 g·kg-1 in natural forest, respectively, which was significantly higher than those of secondary forest(30.33 and 2.90 g·kg-1), shrubs(19.32 and 2.04 g·kg-1) and grassland(17.75 and 1.83 g·kg-1). Among all the investigated soil nutrient indexes, Cs and Ns were significantly correlated with SOC, TN, C/N, and BD. In the present study, vegetation contributed the most to soil nutrient(74.31%), followed by the seasonal factor(14.85%). Changes of SOC, TN, Cs and Ns showed the same tendency in eachseason under different vegetation categories, which were accumulated higher in spring and autumn but lower in summer and winter.(2)Focused on the vegetation types(grassland, shrub land, secondary forest and natural forest) in the typical ecological demonstration area in Puding, we studied the soil water content, bulk density, temporal stability, soil moisture reserve and the composition of water-stable aggregates in a monthly scale lasting from September to January of the second year. The results showed that the water-stable aggregates fraction(>0.25mm) accounted for the most proportion in four vegetation types, which was higher in the secondary forest and natural forest than the grassland and the shrub land. Soil moisture of natural forest and shrub land were higher than secondary forest and grassland, while bulk density showed an opposite direction. There were significant correlations of soil moisture or bulk density under the influence of different vegetation types, time, and their interactions. Soil moisture reserve was ranked in the following order: shrub land(121.14mm)> grassland(112.86mm)> secondary forest(112.34mm)> natural forest(92.53mm). Furthermore, the results suggested that different vegetation types and precipitation were the major factors affecting the soil moisture in Karst area.(3)Taking Castanea mollissima, Sapium sebiferum(L.) Roxb., Caesalpinia crista Linn. and Imperata cylindrica(Linn.Beauv.) as the representative of plants under different vegetation types(grassland, shrub land, secondary forest and natural forest) in the typical ecological demonstration area in Puding, characterized the seasonal changes of photosynthesis and its major affecting factors. The research showed that the effect of soil moisture on plant photosynthetic rate varied across the seasons: for example, the interdependency of Pn and WCS for Imperata cylindrica(Linn.Beauv.) reached the maximun in October, but in April and July for both Caesalpinia crista Linn. and Sapium sebiferum(L.) Roxb and in April and October for Castanea mollissima, In nature, the Pn of different plant species exhibited different responses to seasonal changes, but the "noon break phenomenon" was consistently observed across 4 species in every season. Altogether, PAR and Ta were the primary environmental factors, while Tr 、Gs and WCR were the main physiological factors affecting the Pn.(4)To compare the soil moisture and plant photosynthetic characteristics of different ecological regions, samples of Huajiang demonstration area were collected from 3 different vegetation types(natural forest, secondary forest and shrub land) characterized by its representative species, Toona sinensis, Sapium sebiferum(L.) Roxb. and Rhus chinensis Mill. The research showed that soil moisture was ranked as follow: natural forest > secondary forest > shrub land, this agreed well with that from Puding; whereas the daily performance of Sapium sebiferum(L.) Roxb outcompeted the rest two species, Rhus chinensis Mill and Toona sinensis showing an intermediate and lowest behavior. Unlike soil moisture, the daily performance in Huajiang demonstration areas was higher than that of Puding; Rhus chinensis Mill. showed a good but not significant correlation between Pn and WCS in the two research areas; consistent with that from Puding, the main environmental factors that affect the Pn of plants were Pn, WCS and Ta, Tr and Gs were the main physiological factors affecting the Pn of plants in Huajiang area. It again demonstrated that PAR and Ta, Tr and Gs served as the pivotal environmental and physiological factors that affected the Pn of plants in Karst.