General Status of Soil Acidity in South China and Plant Adaptation to Acid Soil.
Ren Fang Shen and Rong Fu Chen. State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, 71 Beijing East Road, P.O.Box 821, Nanjing, China
The acidity or alkalinity regimes of soils in China follow the rule of “acid in the south, alkaline in the north”. Generally speaking, soils in the south of Yangtse River (33o north latitude) are acidic (pH 5.5-6.5) or strongly acidic (pH<5.5), e.g. pH of Ultisol and Alfisol distributed widely in South China and Southwest China is mainly 4.5-5.5, while pH of Ultisol in Central China and East China is 5.5-6.5; the soils in the north of Yangtse River are mainly neutral (pH 6.5-7.5) or alkaline (pH >7.5). Acid soil areas are mainly located in tropical and sub-tropical regions, which cover 2.18 million km2, accounting for 22.7% of total land of China (Zhao et al., 1998). These regions are characterized by a markedly monsoon climate, with a mean annual temperature of 14-22oC and a mean annual precipitation of 1000-1600 mm (Sun et al.,1999). These superior natural conditions supply 1-2 times higher potential of crop productivity compared with that in Northeast China and North China. Ultisol in China is usually highly weathered and with low base saturation. One of the most important chemical properties of these soils is the existence of abundant iron oxides and aluminum oxides. These soils belong to variable charge soils with amphoteric surface charge, that is, positive and negative charges exist simultaneously on clay surfaces. Moreover, their surface charges change with soil pH and other conditions. Therefore, in these soils the characteristics of ion adsorption and surface acidity are amphoteric, which endow the soils the properties that can simultaneously adsorb cations and anions and can simultaneously contain exchangeable acidity and exchangeable alkalinity. Though there are many disadvantages related to soil acidity, the major factor limiting crop growth and yield is that aluminum (Al) is solubilized to the phytotoxic species Al3+, Al(OH)2+, Al(OH)2+ in acid soils. Nevertheless, there are a number of plant species and genotypes within species exhibit an inheritable tolerance to Al. A recent limited-scale investigation in Jiangxi and Zhejiang provinces in South China showed that many plants including some native plants and cultivated crops (totally 26 species) could grow well in these areas. Some crops can also grow well with local farmers' routine agricultural practices to tackle with soil acidity. Among these plants, some are Al-accumulator, e.g. tea and buckwheat; while some are not, e.g. rice etc. Earlier analyses on leaves of Rubiaceae verified the systematic and phylogenetic importance of Al accumulation (Jansen et al., 2000): Al accumulators generally are woody, relatively primitive taxa which occur in tropical forests with relatively high rainfall. Herbaceousness is possibly associated with the absence of Al accumulation. However, among all the investigated plants, whether plants accumulate Al or not showed no relation with the genus which they belong to and Al contents followed the order of old leaves > new leaves > seeds (if available) for each plant. It was reported that pH of fresh plant tissues was the most important factor controlling Al uptake, translocation and accumulation in plants (Xie et al., 2002). Al accumulators had low pH values in plant tissues and decreased rhizosphere pH and rendered Al more available for uptake. Al excluders increased pH values in the rhizosphere soil to avoid higher Al uptake by roots. That is, different plants had different strategies to adapt the acid soil. In addition, a great deal of reports had suggested that acidity in Ultisol of South China are intensified due to heavy fertilization or acid rain caused by industrial pollution and accompanied with the depletion of nutrients such as P, N, Ca, K etc. Unfortunately, this process is becoming more and more serious, which leads to an increasing threat to agricultural and natural ecosystems in these regions. Thus apart from taking measures to minimize the impact of human activities on the acidification, it is important for us to understand the processes that cause soil acidification and to adopt techniques by applying lime to neutralize the acidity. On the other hand, it is also essential to develop plants that are more tolerant of acid (Al3+) stress (more adaptive to acid soils) for maintaining and even increasing productivity on acid soils. This can be achieved by traditional breeding strategies for some species, but for others that show little natural variation in Al tolerance, genetic manipulation is a new alternative approach.