Anthropogenic calcified paddy soils in subtropical China GONG Zi-Tong and ZHANG Gan-Lin State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China Tel: +86-25-8688 1277; Fax: +86-25-8688 1000; E-mail: ztgong@issas.ac.cn

ABSTRACT Under humid subtropical zone, especially in irrigation condition, usually zonal soils suffer strong leaching and being acid in reaction. But some paddy soils in subtropical China, different from zonal soils and local paddy soils have high content of CaCO3. This calcified paddy soils were attracted by a series pedologists with great interesting since 1940 in China. The calcified paddy soils (CPS) are distributed in the humid subtropics, such as Zhujiang delta and Guilin Prefecture of south China, where the annual precipitation is above 1500mm and the mean annual soil temperature is 15°C-20°C.That is to say the soils have udic soil moisture regime and thermic soil temperature regime. The relief of the soil is not only in alluvial plain, but also in hilly region with original soils as either zonal soils or alluvial soils. It seems no close relation between environment and formation of calcified paddy soils. The calcified paddy soils contain abundant CaCO3 as high as 100-200 g kg-1 with CaCO3/ MgCO3 ratio 10~30, which is mainly accumulated in the upper part and decreased with depth. Micromophological evidence indicates that carbonates are present mainly in the forms of carbonate concretions and cryptocrystalline interflorescences, and calcareous nodules in various sizes are commonly present and sometimes carbonate hardpan is formed. The CPS have poor productivity due to low available P, K, B, Zn, Mo and Mn and low permeabilaty as well. There are several possible sources of carbonates. Natural rainfall brings in about 135 kg ha-1 CaCO3 a years. Irrigation water, either from rivers or karst caves, carries about 600 to 1500 kg ha-1 CaCO3 per hectare yearly while artificial input by liming may amount to 1500 to 2250 kg ha-1 in the same period. Specific forming conditions for carbonate accumulation are favoring, besides the carbonate sources. The CPS have pH value of 7.5-8.0 that is suitable for deposition of CaCO3. The pH value of surface layer of CPS can reach 9~10 when burned lime implication by the farmers. Partial pressure CO2 measurement also shows that it is helpful to change CaCO3 into Ca (HCO3)2 that can be dissolved and leached out with soil solution. CO2 in CPS is less 2~7 times than that of control soils.. The measured 14C age of the typical CPS is 1675±115 a.B.P., corrected by dendrology of trees nearby, which is close to the West Jin Dynasty of China. As generally thought, soil is formed based upon its parent material under influences from climate, relief, organism and time. But sometimes human activities can effect soil formation so greatly that these might be new soil type in contrast to its natural neighbor soils.